Article

Accumulation rates and sources of sediments and organic carbon on the Palos Verdes shelf based on radioisotopic tracers

Authors:

Peter H. Santschi

Texas A&M University - Galveston

Laodong Guo

University of Wisconsin - Milwaukee

Mead A Allison

Tulane University

We report here bioturbation and sediment accumulation rates determined from replicate sediment cores at four different

Potential Use of the Benthic Foraminifers Bulimina denudata and Eggerelloides advenus in Marine Sediment Toxicity Testing

Article

Full-text available

Mar 2021

Mary McGann

The benthic foraminifers Bulimina denudata and Eggerelloides advenus are commonly abundant in offshore regions in the Pacific Ocean, especially in waste-discharge sites. The relationship between their abundance and standard macrofaunal sediment toxicity tests (amphipod survival and sea urchin fertilization) as well as sediment chemistry analyte measurements were determined for sediments collected in 1997 in Santa Monica Bay, California, USA, an area impacted by historical sewage input from the Hyperion Outfall primarily since the late 1950s. Very few surface samples proved to be contaminated based on either toxicity or chemistry tests and the abundance of B. denudata did not correlate with any of these. The abundance of E. advenus also did not correlate with toxicity, but positively correlated with total solids and negatively correlated with arsenic, beryllium, chromium, lead, mercury, nickel, zinc, iron, and TOC. In contrast, several downcore samples proved to be contaminated as indicated by both toxicity and chemistry data. The abundance of B.denudata positively correlated with amphipod survival and negatively correlated with arsenic, cadmium, unionized ammonia, and TOC; E. advenus negatively correlated with sea urchin fertilization success as well as beryllium, cadmium, and total PCBs. As B. denudata and E. advenus are tolerant of polluted sediments and their relative abundances appear to track those of macrofaunal toxicity tests, their use as cost- and time-effective marine sediment toxicity tests may have validity and should be further investigated.

Millennial‐Scale Age Offsets Within Fossil Assemblages: Result of Bioturbation Below the Taphonomic Active Zone and Out‐of‐Phase Production

Article

Full-text available

Jun 2019

Oceanographic and evolutionary inferences based on fossil assemblages can be obscured by age offsets among co‐occurring shells (i.e., time averaging). To identify the contributions of sedimentation, mixing, durability, and production to within‐ and between‐species age offsets, we analyze downcore changes in the age‐frequency distributions of two bivalves on the California shelf. Within‐species age offsets are ~50–2,000 years for Parvilucina and ~2,000–4,000 years for Nuculana and between‐species offsets are 1,000–4,000 years within the 10‐ to 25‐cm‐thick stratigraphic units. Shells within the top 20–24 cm of the seabed are age‐homogeneous, defining the thickness of the surface completely‐mixed layer (SML), and have strongly right‐skewed age‐frequency distributions, indicating fast shell disintegration. The SML thus coincides with the taphonomic active zone and extends below the redoxcline at ~10 cm. Shells >2,000–3,000 years old occurring within the SML have been exhumed from subsurface shell‐rich units rich where disintegration is negligible (sequestration zone, SZ). Burrowers (callianassid shrimps) penetrate 40–50 cm below the seafloor into this SZ. The millennial offsets within each increment result from the advection of old shells from the SZ, combined with an out‐of‐phase change in species production. Age unmixing reveals that Parvilucina was abundant during the transgressive phase, rare during the highstand phase, and increased steeply in the twentieth century in response to wastewater. Nuculana was abundant during the highstand phase and has declined over the past two centuries. This sequestration‐exhumation dynamic accentuates age offsets by allowing both the persistence of shells below the SML and their later admixing with younger shells within the SML.

Sediment accumulation and mixing in the Penobscot River and estuary, Maine

Article

Apr 2018

Sediment accumulation on the Southern California Bight continental margin during the twentieth century

Article

Jan 2009

Sediment discharged into the portion of the Southern California Bight extending from Santa Barbara to Dana Point enters a complex system of semi-isolated coastal cells, narrow continental shelves, submarine canyons, and offshore basins. On both the Santa Monica and San Pedro margins, 210Pb accumulation rates decrease in an offshore direction (from ̃0.5 g cm-2yr-1 to 0.02 g cm-2yr -1), in concert with a fining in sediment grain size (from 4.5φ to 8.5φ), suggesting that offshore transport of wave-resuspended material occurs as relatively dilute nepheloid layers and that hemiplegic sedimentation dominates the supply of sediment to the outer shelf, slope, and basins. Together, these areas are effectively sequestering up to 100% of the annual fluvial input. In contrast to the Santa Monica margin, which does not display evidence of mass wasting as an important process of sediment delivery and redistribution, the San Pedro margin does provide numerous examples of failures and mass wasting, suggesting that intraslope sediment redistribution may play a more important role there. Basin deposits in both areas exhibit evidence of turbidites tentatively associated with both major floods and earthquakes, sourced from either the Redondo Canyon (San Pedro Basin) or Dume Canyon (Santa Monica Basin). On the Palos Verdes shelf, sediment-accumulation rates decrease along and across the shelf away from the White's Point outfall, which has been a major source of contaminants to the shelf deposits. Accumulation rates prior to the construction of the outfall were ̃0.2 g cm-2yr-1 and increased 1.5-3.7 times during peak discharges from the outfall in 1971. The distal rate of accumulation has decreased by ̃50%, from 0.63 g cm -2yr-1 during the period 1971-1992 to 0.29 g cm -2yr-1 during the period 1992-2003. The proximal rate of accumulation, however, has only decreased ̃10%, from 0.83 g cm -2yr-1 during the period 1971-1992 to 0.73 g cm -2yr-1 during the period 1992-2003. Effluent-affected sediment layers on the Palos Verdes shelf can be identified in seabed profiles of naturally occurring 238U, which is sequestered in reducing sediments. The Santa Clara River shelf, just north and west of the Santa Monica and San Pedro margins, is fine-grained and flood-dominated. Core profiles of excess 210Pb from sites covering the extent of documented major flood deposition exhibit evidence of rapidly deposited sediment up to 25 cm thick. These beds are developing in an active depocenter in water depths of 30-50 m at an average rate of 0.72 g cm-2yr-1. Budget calculations for annual and 50-yr timescale sediment storage on this shelf shows that 20%-30% of the sediment discharge is retained on the shelf, leaving 70%-80% to be redistributed to the outer shelf, slope, Santa Barbara Basin, and Santa Monica Basin.

Event-Driven Deltaic Sedimentation on a Low-Gradient, Low-Energy Shelf: The Brazos River Subaqueous Delta, Northwestern Gulf of Mexico

Article

Jul 2014
MAR GEOL

Recent organic carbon accumulation (∼100 years) along the Cabo Frio, Brazil upwelling region

Article

Jan 2013
CONT SHELF RES

Transport and diagenesis of trace metals and organic matter in Palos Verdes shelf sediments affected by a wastewater outfall

Article

Feb 2001
MAR CHEM

Particles from the Whites Point/JWPCP outfalls operated by the Los Angeles County Sanitation District (LACSD) have been discharged onto the Palos Verdes (PV) shelf, Southern California, since the late 1930s. Since the early 1950s, they have made a significant contribution to the sedimentary deposits on the shelf. In order to study the transport and diagenesis of organic carbon (OC) and associated trace metals, replicate sediment cores were collected during 1996 and 1997 at four different sites at the ∼60 m isobath on the PV shelf, and analyzed for OC, Ag, Al, Cd, Cr, Cu, Mn, Ni, Pb, and Zn. We conclude from these results that a significant fraction of OC and associated heavy metals were transported laterally on silt particles from shallower environments. Cross-shelf transport of sediments caused multiple peaks in measured profiles of OC and trace metals at site 6C, 2 km away from the outfall. The same mechanism is likely to contribute to a concentration decrease that is smaller than that expected from decreases from the Whites Point outfall emissions. Based on Pb/OC ratios in sediments, deposited in 1971, and comparisons to the outfall from the same year, we estimate that 50±10% of the OC deposited in the early 1970s, now buried at 30–50 cm depth, had oxidized since that time, implying a half-life of about 26 years for the outfall-OC, as an upper limit. The average OC oxidation rate at peak depth (about 2 mg C cm−2 year−1) is, however, only about 10% of the present-day OC accumulation rate (20 mg C cm−2 year−1), which itself is adding not much more than 1% per year to the post-1950s OC inventory (∼1500 mg cm−2). We furthermore estimate that the OC inventory in PV shelf sediments in 1971 was equivalent to about 35% of that emitted by the outfall. OC and trace metal inventories did not decrease in the period 1981 to 1997, contrary to those of other contaminants such as DDTs and PCBs.

100 Years of benthic foraminiferal history on the inner Texas shelf inferred from fauna and stable isotopes: Preliminary results from two cores

Article

Apr 2012
CONT SHELF RES

Coastal regions, such as the Texas–Louisiana shelf, are subject to seasonal hypoxia that strongly depends on the magnitude of freshwater discharge from local and regional river systems. We have determined benthic foraminiferal fauna and isotopic compositions in two 210-Pb dated box cores (BR4 and BR5) to examine the evidence for nearshore hypoxia and freshwater discharge on the Texas shelf during the last 100 years. The 210 Pb chronologies of both cores reveal sedimentation rates of 0.2 and 0.1 cm yr À 1 , translating to $ 60 and $ 90 year records. The fauna of both cores were almost exclusively composed of Ammonia parkinsoniana and Elphidium excavatum, indicating euryhaline ambient waters. The Ammonia–Elphidium (A–E) index, a qualitative measure of low oxygen conditions, shows an increase from values between 20 and 50 to near 100 in both cores, suggesting low oxygen conditions between 1960 and the core top. Between 1950 and 1960 (9–10 cm), low A–E values in BR4 coincide with high d 18 O and d 13 C values greater than 0% and À 1% respectively. This event corresponds to severe drought (the Texas Drought of Record) over the Brazos River drainage basin and considerably reduced river discharge from 1948 to 1957. High A–E values prior to this event imply low-oxygen conditions were prevalent prior to anthropogenic exacerbation of Louisiana shelf hypoxia and at least since the dredging of a new Brazos River delta in 1929. Elphidium excavatum d 13 C values are very low (À 4%) and indicative of significant vital effect. The d 13 C values of A. parkinsoniana average À 3% and exhibit little variability, most likely reflecting pore waters influenced by aerobic and anaerobic respiration. The association of lowered Brazos River discharge with more oxygenated shelf bottom waters suggests Brazos River discharge and shelf hypoxia are linked, but the influence of Mississippi– Atchafalaya discharge can also contribute to shelf stratification.

Sediment accumulation and radionuclide inventories (239,240Pu, 210Pb and 234Th) in the northern Gulf of Mexico, as influenced by organic matter and macrofaunal density

Article

Nov 2004
MAR CHEM

Six cores were collected from the Northern Gulf of Mexico (GOM) as part of the “Deepwater Program: Northern Gulf of Mexico Continental Slope Habitats and Benthic Ecology” (DGoMB). These cores were collected from water depths ranging from ≈700 to 3500 m and processed for radiochemical assays to determine particle reworking (bioturbation) and sedimentation rates in these sediments, pursuant to the research objective to investigate biological, chemical, and physical processes that control the environmental setting for GOM benthic fauna. Bioturbation rates were quantitatively derived from 234Thxs profiles, with 234Thxs penetration depths ranging between 0.5 and 4 cm, and bioturbation coefficient values (Db), ranging from ≈2 to 30 cm2/year. 234Thxs data were also used to determine the shallow depth limit for particle mixing. Significant values of 239,240Pu were found over the 3–15 cm depth range, without any pronounced peak activities suitable for sediment dating, indicating periodic and nonlocal mixing events. Sedimentation rates (S) were therefore calculated from 210Pbxs profiles, assuming steady state conditions, and using the constant flux-constant sedimentation (CF-CS) model. However, only 210Pbxs data below the 239,240Pu penetration depth were used for the purpose of determining an upper limit of S. The range of apparent sedimentation rates determined by this approach for these stations is 0.04–0.44 cm/year, after approximately 1000 m depth, sedimentation becomes essentially constant at ≈0.08 cm/year. To the best of our knowledge, there are no other published sedimentation rates for the GOM outside the immediate area of the Mississippi River Delta region. However, estimates of sedimentation rates presented here fall in line with rates from similar continental margin marine settings.

Box Coring Artifacts in Sediments Affected by a Waste Water Outfall

Article

May 2001
MAR POLLUT BULL

Variations in porosity of surface sediments are often the major cause of sediment loss during gravity and box coring. Sediments with a high content of organic matter usually have higher porosity, and thus, lower resistance (strain) towards mechanical disturbance. Here, we demonstrate that box coring artifacts (i.e. sediment loss and core shortening) can be produced in sediments from the Palos Verdes (PV) shelf, which in the past had received relatively high loads of organic carbon (OC) enriched particulate matter originating from the Whites Point outfall that had created a high porosity layer at depth. This has been overlooked as a possibility for obtaining low estimates of sediment and pollutant accumulation rates. Since any such sediment loss during coring can lead to serious underestimates of sedimentation rates, our results here may have important implications for any attempts at reconstructing pollutant fluxes and histories in these coastal marine sediments.

Mercury inputs and redistribution in the Penobscot River and estuary, Maine

Article

Dec 2017

We examined total mercury (Hg) distributions in sediments from the Penobscot River and estuary, Maine, a site of extensive Hg releases from HoltraChem (1967-2000). Our objectives were to quantify: (1) bottom sediment Hg inventories (upper ~1m; 50-100 y); (2) sediment accumulation rates; and (3) contemporary Hg fluxes to bottom sediments; by sampling the Penobscot River (PBR), Mendall Marsh (MM), the Orland River (OR) and the Penobscot estuary (ES). Hg was rapidly distributed here, and the cumulative total (9.28 metric tons) associated with sediments system-wide was within the range released (6-12 metric tons). Evidence of sediment/Hg remobilization was observed in cores primarily from the PBR, and to a lesser extent the ES, whereas cores from MM, most of the OR, the ES, and half from the PBR exhibited sharp peaks in Hg concentrations at depth, followed by gradual decreases towards the surface. Based on background PBR sediment Hg concentrations (100ngg-1), "elevated" (300ngg-1), or "highly elevated" (600ngg-1) Hg concentrations in sediments, and resulting inventories, we assessed impact levels ("elevated"≥270, or "highly elevated"≥540mgm-2). 71% of PBR stations had "elevated", and 29% had "highly elevated" Hg inventories; 45% of MM stations had "elevated", and 27% had "highly elevated" inventories; 80% of OR stations had "elevated" inventories only; and 17% of ES stations had "elevated" inventories only. Most "highly elevated" stations were located within 8km of HoltraChem, in MM, in the PBR, and in the OR. Near-surface sediments in the OR, PBR and MM were all "highly elevated", while those in the ES were "elevated", on average. Mean Hg fluxes to bottom sediments were greatest in the OR (554), followed by the PBR (469), then MM (452), and finally the ES (204ngcm-2y-1).

Determining Rates of Sediment Accumulation on the Mekong Shelf: Timescales, Steady-State Assumptions, and Radiochemical Tracers

Article

Jun 2017
CONT SHELF RES

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess ²¹⁰Pb distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3 m, and stations sampled topset, foreset, and bottomset beds (water depths 7–21 m). Apparent excess ²¹⁰Pb sediment accumulation rates ranged from >10 cm/y (no down-core decrease of excess activity over 300 cm core length) near the Song Hau river mouth, to 1–3 cm/y in topset and foreset beds within 20 to 50 km of the river mouth, to rates as low as 0.4 cm/y in cores from bottomset beds. The ²¹⁰Pb sediment accumulation rates yield an overall sediment burial rate of 6.1 x10¹³ g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 x10¹⁴ g/y; based on our ²¹⁰Pb and seismic data and ²¹⁰Pb data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3–1.6 x10¹⁴ g/y) from the literature. The inventory of excess ²¹⁰Pb in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon ¹⁴C ages were measured on 10 cores from the proximal Mekong delta and compared to ²¹⁰Pb sediment accumulation rates in the same core. The ²¹⁰Pb accumulation rates in all 10 cores were considered to be more robust and accurate than the ¹⁴C geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the δ¹³C value of the seabed organic carbon.

Historical record and fluxes of DDTs at the Palos Verdes Shelf Superfund site, California

Article

Jan 2017
SCI TOTAL ENVIRON

Marine sediments at many locations in the world are contaminated with a wide range of persistent organic pollutants. The Palos Verdes Shelf (PVS) is located in the ocean off the coast of Los Angeles, California and has been listed as a Superfund site by the US EPA since 1997, because of heavy contamination of DDTs and PCBs. However, little is known about the historical trend in the deposition of DDTs as a result of decades-long discharge of wastewater effluents. In this study, sediment cores were taken from the PVS site and determined for DDT and its metabolites including DDE and DDD (denoted as DDTs). Individual DDTs were found in the majority (95%) of the samples analyzed. The highest ∑ DDT concentrations were found in three cores along the 60-meter isobath with geometric means of 31300, 7490, and 5010 ng/g dw and medians of 82400, 17300, and 5200 ng/g dw, respectively. Among DDT congeners, p,p′-DDE, o,p′-DDE and p,p′-DDD were predominant, contributing to approximately 54%, 27%, and 14% of the ΣDDTs in sediment. The vertical profiles of concentrations of contaminants in the sediment cores were examined. For most of the cores, a steady increase in the concentrations of DDTs during 1940s to 1980s was observed, while the concentrations declined gradually toward the surficial layers. On the basis of the mass flux of DDTs calculated and the area of the PVS Superfund site, we estimated the total deposition amount of DDTs in sediment and the deposition amount of ΣDDTs in this region during 1947–1971 was 132 tons, which was fairly close to what was reported in earlier studies for industrial wastewater discharge in the PVS site (870–1450 tons). Our findings suggest that the elevated levels of DDTs in sediment from the PVS site are linked to the discharge of these contaminants between the 1940s–1980s.

Analytical solutions of the tracer-conservation equation in sediments, reviewed from the com- paction potential theory: A unifying approach for radiometric dating models

Article

Full-text available

J.M. Abril

The 210 Pb dating method is being widely applied to study recent (up to 130-150 years) accumulation rates of sediments, peats and other natural deposits. Diiculties arise when diierent models can be equally used to explain the ob-served speciic activity versus depth prooles but with contradictory results. Then the simultaneous study of man-made fallout radionuclides (with a peak in atmospheric deposition in 1963) can provide a valuable additional test for the 210 Pb chronology. These radionuclides often remain, at least partially, in the top sediment zone under active compaction and mixing eeects, what makes diicult to establish a chronology. The theory of a compaction energy poten-tial allows a new writing of the diierential equations for the conservation of solids and the particle-associated tracers. This provides a comprehensive treat-ment of mixing and compaction with unsteady radionuclide inputs. This work is concerned with the analytical solutions of this reviewed equations under steady-state bulk density prooles, providing an unifying approach for radio-metric dating models. In this way, the most known dating models for 210 Pb arise as particular solutions of the steady-state problem, with analytical solu-tions including compaction. The application of Laplace transforms allows to solve the time-dependent problems from the known solutions of the steady-state case. Analytical solutions are presented for several dating models with time-dependent inputs. This serves, also, to discuss some constrains on the ap-plication of the direct identiication of time-markers as an alternative dating tool.

Sedimentary Impacts of Anthropogenic Alterations on the Yeongsan Estuary, South Korea

Article

Full-text available

Sep 2014
MAR GEOL

The use of tracers to evaluate the importance of bioturbation in remobilising contaminants in Baltic sediments

Article

Jan 2006

Large areas of the bottom sediments of the Baltic Sea are temporarily or permanently anoxic. These sediments are also an important sink for a variety of contaminants. Reoxygenation of bottom waters allows recolonisation by benthic infauna, which may have important implications for the fate of buried contaminants. This study used tracers to experimentally examine the role of bioturbation by benthic infauna in transporting sediment-associated contaminants in the Baltic Sea. Three different tracer methods were used in two experiments, using three key Baltic macrofaunal species: the amphipod crustacean Monoporeia affinis; the Baltic clam Macoma baltica; and the priapulid worm Halicryptus spinulosus. In the first experiment, a reoxygenation–recolonisation scenario was recreated in the laboratory, using hypoxic sediment cores collected in the field, to determine if there was remobilisation of buried 137Cs from the Chernobyl nuclear accident in 1986. The potential for the infauna to bury newly settled surface contamination was also investigated, using a fluorescent particle tracer. In the second experiment, artificially-created radiolabelled tracer layers (14C and 51Cr) were used to quantify both upward and downward movements of organic matter and sediment-associated contaminants by bioturbation.

Carbon isotopes and iodine concentrations in a Mississippi River delta core recording land use, sediment transport, and dam building in the river’s drainage basin

Article

May 2007
MAR ENVIRON RES

Sedimentary material from coastal and nearshore areas in the Mississippi Delta region are comprised of different organic carbon sources with diverse ages that require isotopic and elemental records for resolving the various sources of plant residues. Carbon isotopic ((13)C, (14)C) values were used to differentiate contributions from plants using the C3, C4, and/or CAM (crassulacean acid metabolism) carbon fixation pathways., and iodine concentrations indicated that wetland plant residues are a significant source of organic carbon in a sediment core from the Mississippi River delta region collected at a 60 m water depth. This sediment core had been extensively described in Oktay et al. [Oktay, S.D., Santschi, P.H., Moran, J.E., Sharma, P., 2000. The (129)Iodine Bomb Pulse Recorded in Mississippi River delta Sediments: Results from Isotopes of I, Pu, Cs, Pb, and C. Geochim. Cosmochim. Acta 64 (6), 989-996.] and significantly, includes unique features that had not previously been seen in the marine environment. These special features include a plutonium isotopic close-in fallout record that indicates a purely terrestrial source for these sediment particles and the elements associated with it, and a distinct iodine isotopic peak (as well as peaks for plutonium and cesium isotopes) that indicate little bioturbation in this core. Our carbon isotopic and iodine data can thus be compared to published records of changes in drainage basin land use, river hydrology, and hydrodynamic sorting of suspended particles to elucidate if these changes are reflected in nearshore sediments. This comparison suggests a significant contribution for organic carbon (OC) from C4 plants to these sediments during the 1950's to early 1960's. Relative older carbon isotopes, and episodically high iodine concentrations (up to 34 ppm) were observed during this time period that (1) indicate sediment deposition that is coincident with the times of major hydrological changes induced from dam and levee building in both the upper and lower reaches of the Mississippi River drainage basin, and (2) suggest episodic organic carbon deposition from wetland plant residues.

A downcore increase in time averaging is the null expectation from the transit of death assemblages through a mixed layer

Article

Full-text available

Jan 2023

Understanding how time averaging changes during burial is essential for using Holocene and Anthropocene cores to analyze ecosystem change, given the many ways in which time averaging affects biodiversity measures. Here, we use transition-rate matrices to explore how the extent of time averaging changes downcore as shells transit through a taphonomically complex mixed layer into permanently buried historical layers: this is a null model, without any temporal changes in rates of sedimentation or bioturbation, to contrast with downcore patterns that might be produced by human activity. Assuming stochastic burial and exhumation movements of shells between increments within the mixed layer and stochastic disintegration within increments, we find that almost all combinations of net sedimentation, mixing, and disintegration produce a downcore increase in time averaging (interquartile range [IQR] of shell ages), this trend is typically associated with a decrease in kurtosis and skewness and by a shift from right-skewed to symmetrical age distributions. A downcore increase in time averaging is thus the null expectation wherever bioturbation generates an internally structured mixed layer (i.e., a surface, well-mixed layer is underlain by an incompletely mixed layer): under these conditions, shells are mixed throughout the entire mixed layer at a slower rate than they are buried below it by sedimentation. This downcore trend created by mixing is further amplified by the downcore decline in disintegration rate. We find that transition-rate matrices accurately reproduce the downcore changes in IQR, skewness, and kurtosis observed in bivalve assemblages from the southern California shelf. The right-skewed shell age-frequency distributions typical of surface death assemblages—the focus of most actualistic research—might be fossilized under exceptional conditions of episodic anoxia or sudden burial. However, such right-skewed assemblages will typically not survive transit through the surface mixed layer into subsurface historical layers: they are geologically transient. The deep-time fossil record will be dominated instead by the more time-averaged assemblages with weakly skewed age distributions that form in the lower parts of the mixed layer.

Compound-specific radiocarbon reveals sources and land–sea transport of polycyclic aromatic hydrocarbons in an urban estuary

Article

Jun 2021
WATER RES

As typical chemical indicators of the Anthropocene, polycyclic aromatic hydrocarbons (PAHs) and their environmental behavior in urban estuaries can reveal the influence of anthropogenic activities on coastal zones worldwide. In contrast to conventional approaches based on concentration datasets, we provide a compound-specific radiocarbon (14C) perspective to quantitatively evaluate the sources and land‒sea transport of PAHs in an estuarine‒coastal surficial sedimentary system impacted by anthropogenic activities and coastal currents. Compound-specific 14C of PAHs and their 14C end-member mixing models showed that 67−73% of fluoranthene and pyrene and 76−80% of five- and six-ring PAHs in the Jiulong River Estuary (JRE, China) originated from fossil fuels (e.g., coal, oil spill, and petroleum-related emissions). In the adjacent Western Taiwan Strait (WTS), the contributions of fossil fuel to these PAH groups were higher at 74−79% and 84−87%, respectively. Furthermore, as a significant biomarker for source allocation of terrigenous organic matter, perylene, a typical five-ring PAH, and its land‒sea transport from the basin through the JRE and finally to the WTS was quantitatively evaluated based on the 14C transport models. In the JRE, fluvial erosions and anthropogenic emissions affected the 14C signature of perylene (Δ14Cperylene, -535 ± 5‰) with contributions of > 38% and < 62%, respectively. From the JRE to the WTS, the decreased Δ14Cperylene (-735 ± 4‰) could be attributed to the long‒range transport of “ocean current-driven” perylene (-919 ± 53‰) with a contribution of 53 ± 8%. This compound-specific 14C approach and PAH transport model help provide a valuable reference for accurately quantifying land‒sea transport and burial of organic pollutants in estuarine‒coastal sedimentary systems.

Gauging benthic recovery from 20th-century pollution on the southern California continental shelf using bivalves from sediment cores

Article

Apr 2019

Urbanization of coasts creates stresses on adjacent marine communities, but the full impact is seldom known owing to scarce historical records. Paleoecological analysis of sediment cores can be a powerful means of reconstructing baseline benthic communities, but is particularly challenging for continental shelves where dead-shell assemblages are prone to greater time-averaging than in natural sediment sinks such as deltas, coastal bays, and estuaries. We compared temporal changes in the composition of bivalve shell assemblages collected from box cores on the Palos Verdes (southern California, USA) shelf to a 40 yr time series of annually collected living benthic assemblages in the same area in order to calibrate bivalve core assemblages to known changes in community composition during wastewater remediation. Older (pre-1970) core assemblages were then used to reveal the nature of bivalve communities from the early 20 th century and the extent to which present-day communities match, i.e. have recovered to, early urban baselines. Deep bioturbation and only moderate sedimentation rates (0.2 cm yr ⁻¹ ) damp the magnitude and rapidity of changes in core assemblage composition. Despite the geological complexity, bivalve core assemblages (1) detect known late 20 th century dynamics in broad outline, (2) reveal the undocumented rise of chemosymbiont-bearing bivalves during the early 20 th century, and (3) establish that the present-day community is largely but not fully recovered to its pre-effluent, early urban (1900−1930) baseline. Thus, cores capture the nature, timing, and duration of macrobenthic response to 20 th century wastewater, validating this approach for shelf settings with scarce or no historical data.

Sediment dynamic equilibrium, a key for assessing a coastal anthropogenic disturbance using geochemical tracers: Application to the eastern part of the Bay of Seine

Article

Feb 2019
CONT SHELF RES

Geochemical studies are becoming more and more frequent in the context of the increasing pressure of human activities on marine coastal ecosystems and represent an appropriate tool to assess anthropogenic disturbances. Moreover, it is difficult to find discriminant markers. The eastern part of the Bay of Seine (English Channel) is highly impacted by the presence of harbour activities, fishing and sediment extraction. Dredged sediment from the Grand Port Maritime du Havre (GPMH) are deposited at the subtidal Octeville site, in the northeastern part of the mouth of the Seine estuary (mixed sediment area). To understand natural and anthropogenic sedimentary mechanisms in this area, a geochemical and sedimentological study was conducted at the beginning of 2016. A dense sampling campaign including 179 stations was carried out between Cap de La Hève and Cap d'Antifer. For comparison, sampling was carried out in the harbour (13 samples in basins strongly or very weakly dredged), in the dredged grab itself and in the Seine estuary sediment (one station in the brackish zone and two stations in the river freshwater zone). Elemental compositions were determined by X-Ray Fluorescence spectrometry and infrared spectrometry. Using PCA (Principal Component Analysis), seven constituents were selected (Si, As, S, Pb, Rb, organic Br and TOC) to determine the area influenced by dumped spoil and the sediment transport directions. Sediment areas in dynamic 2 equilibrium display a TOC gradient perpendicular to the coastline, linked to granulometric variations due to a combination of the swell and tidal currents. In the study area, dredged sediment are finer grained and have undergone changes due to the influence of diagenetic processes characteristic of the harbour environment. As a result, these sediment are enriched in sulphides, Pb, TOC and Rb, which allows us to highlight the in-situ deposited sediment spoil. Dumped sediment and the area subject to their influence are clearly identified since they locally disrupt the natural dynamic equilibrium state.

Sources and burial fluxes of soot black carbon in sediments on the Mackenzie, Chukchi, and Bering Shelves

Article

Mar 2018
CONT SHELF RES

Reconstruction of Recent Sedimentary Processes in a Carbonate Platform (Gulf of Batabano, Cuba) Using Environmental Radiotracers

Article

Full-text available

Feb 2016

Recent (past 100 years) sedimentary processes in the highly dynamic Gulf of Batabano (Cuba, Caribbean Sea) were investigated through the analyses of environmental radionuclides (e.g., 210Pb, 226Ra, 137Cs, 239,240Pu, and 14C) in nine sediment cores. We evaluated the mean mass accumulation rates (MARs) and the surface mixed layers (SMLs) in each sediment core. Based on these results, three sedimentary environments were identified in the study region. In the central zone, the sediments were mainly composed of carbonate transported from the southern area and showed elevated mass accumulation rates (MAR, 0.11–0.23 g cm−2 year−1) and relatively deep surface mixed layers (SML, 14–16 cm). The southwestern zone was characterized by lower MAR (0.05–0.08 g cm−2 year−1) and thinner SML (7–8 cm). In both areas, the long sediment mixing times in the SMLs (of 45–61 years) smoothed out the sedimentary records. The coastline sedimentary environments were characterized by higher MAR (0.30–0.57 g cm−2 year−1) and the sedimentary records displayed clear signatures of extreme climatic events such as the intensive rains in 1999 reported for La Coloma and the hurricanes Lili and Isodore in 2002. Our study shows that the application of the 210Pb sediment dating method in dynamic costal zones is a challenging task but still may provide important information regarding sedimentation and mixing processes in the ecosystem.

Estudio de los flujos atmosféricos de 210Pb, 7Be, 137Cs, 40K y Hg en Cienfuegos entre el 2010 y el 2011

Research

Full-text available

Aug 2015

Yasser Morera Gómez

Tesis de grado de Yasser Morera Gómez

Influence of hypoxia on biogenic structure in sediments on the Louisiana continental shelf

Article

Full-text available

Jul 2015

Sedimentation Pulse in the NE Gulf of Mexico following the 2010 DWH Blowout

Article

Full-text available

Jul 2015
PLOS ONE

The objective of this study was to investigate the impacts of the Deepwater Horizon (DWH) oil discharge at the seafloor as recorded in bottom sediments of the DeSoto Canyon region in the northeastern Gulf of Mexico. Through a close coupling of sedimentological, geochemical, and biological approaches, multiple independent lines of evidence from 11 sites sampled in November/December 2010 revealed that the upper ~1 cm depth interval is distinct from underlying sediments and results indicate that particles originated at the sea surface. Consistent dissimilarities in grain size over the surficial ~1 cm of sediments correspond to excess 234Th depths, which indicates a lack of vertical mixing (bioturbation), suggesting the entire layer was deposited within a 4-5 month period. Further, a time series from four deep-sea sites sampled up to three additional times over the following two years revealed that excess 234Th depths, accumulation rates, and 234Th inventories decreased rapidly, within a few to several months after initial coring. The interpretation of a rapid sedimentation pulse is corroborated by stratification in solid phase Mn, which is linked to diagenesis and redox change, and the dramatic decrease in benthic formanifera density that was recorded in surficial sediments. Results are consistent with a brief depositional pulse that was also reported in previous studies of sediments, and marine snow formation in surface waters closer to the wellhead during the summer and fall of 2010. Although sediment input from the Mississippi River and advective transport may influence sedimentation on the seafloor in the DeSoto Canyon region, we conclude based on multidisciplinary evidence that the sedimentation pulse in late 2010 is the product of marine snow formation and is likely linked to the DWH discharge.

Record of Anthropogenic Impact on the Western Irish Sea Mud Belt

Article

Jun 2015

Tesis Yasser Gomez

Research

Full-text available

Jun 2015

Carlos M Alonso-Hernandez

Estudio de los flujos atmosféricos de 210Pb, 7Be, 137Cs, 40K y Hg en Cienfuegos entre el 2010 y el 2011.

Modern accumulation rates and sources of organic carbon in the NE Gulf of Cádiz (SW Iberian Peninsula)

Article

Aug 2015

Organic carbon (OC) content, elemental (C/N) and isotopic (δ 13C) composition of organic matter (OM) constrain relative contributions from both marine and terrestrial sources to modern sediments in the NE Gulf of Cádiz (GoC) shelf. C/N and (δ 13C) indicate a transition from a dominantly marine to a terrestrial input of OM deposited in Bay of Cádiz and the Guadalquivir prodelta. OC and mass accumulation rates (MARs, based on bulk density and 210Pb-derived sediment MAR) suggest that labile OM from primary productivity accounts for the low OC content and burial rates in sediments in the NE GoC shelf.

The Influence of a Salt Wedge Intrusion on Fluvial Suspended Sediment and the Implications for Sediment Transport to the Adjacent Coastal Ocean: A Study of the Lower Brazos River TX, USA

Article

Full-text available

Nov 2014
MAR GEOL

Heavy Metals in Urban Channel Sediments of Aveiro City, Portugal

Article

Full-text available

The quantification of six metals: aluminum (Al), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn), alongside with determination of organic carbon (OC) and calculations of C:N ratios was performed from surface sediments collected in urban channels of Aveiro City. The city is located along the shores of the Ria de Aveiro, a coastal lagoon, and until recently the city's channels and some of its oldest areas were tidally submerged by tidal floods. These events alongside with the impacts associated with urban development have contributed to create accumulation of metals in the sediments. Organic enrichment is generalized in the inner city channels whereas the opposite occurs in the outer channels. Significant correlations were established for the original dataset between Al and Cr > Zn > Cd (R 2 = 0.79, 0.74, and 0.69, respectively), but not for Cu or Pb (R 2 = 0.25, and 0.005, respectively), pointing to the prevalence of point sources of these two metals. Regressions of Metal/Al were used as the dataset normalizer allowing simultaneously the establishment of mathematical geochemical backgrounds and the consequent calculation of Enrichment Factors (EF). It was concluded that enrichment occurs in the inner city channels which export metals that are readily dispersed by tidal action.

Temporal variability of Δ14C, δ13C, and C/N in sinking particulate organic matter at a deep time series station in the northeast Pacific Ocean

Article

Full-text available

Dec 2004

A 6-year time series of Δ14C, δ13C, and C/N measurements in deep sinking particulate organic matter (POM) is presented for an abyssal site, Station M in the northeast Pacific Ocean. The Δ14C values revealed that sinking POM at 3450 m depth (650 m above bottom) contained old carbon despite its presumed short transit time in the water column. The isotopic and chemical properties of the sinking POM varied with time and appear to be controlled by more than one major process. In 1993, 1994, and late 1996, isotopic signatures and C/N molar ratios indicate negligible or vertically homogeneous influence of resuspended particles from the bottom or particles laterally transported from the margin to the study site. However, during early 1995 and 1998, Δ14C values were lower than those during other periods and C/N values at three deep depths were not equal, indicating that the study site was influenced by resuspended sediments more severely than during other periods. During mid-1995 to mid-1996, δ13C values decreased abruptly while Δ14C values increased slightly, and C/N values were extremely high (up to ˜80) at 50 and 600 m above bottom; these results suggest input of degraded, modern, terrestrial organic matter. The periods of anomalous isotopic signatures, as well as vertically heterogeneous C/N values [, 2001], were correlated with high discharge periods of California rivers with a time lag of 2 to 4 months. The correlation suggests that regional meteorological events are important in controlling the biogeochemical properties of particles at Station M by varying the intensity of resuspension and transport of organic matter from the continental margin.

Effects of Oil from the 2010 Macondo Well Blowout on Marsh Foraminifera of Mississippi and Louisiana, USA

Article

Full-text available

Aug 2013
ENVIRON SCI TECHNOL

Foraminifera responded to both heavy and light oiling of marshes relative to unoiled control sites by changes to both standing stock and depth of habitation (DOH) in sediment following the 2010 Macondo well blowout. Push cores were taken from the middle marsh at sites classified as unoiled, lightly oiled, and heavily oiled based on concentrations of total polycyclic aromatic hydrocarbons ([TPAH]). Cores were sliced and stained with rose Bengal to detect live specimens of foraminifera. Short-term, sediment-mixing depths were determined using the penetration depths of excess (234)Th, and sedimentary organic carbon and carbonate were measured to distinguish depositional environments. Marsh foraminifera reacted to the highest oil concentration (5,000-18,000 ng/g of TPAH) by reducing standing stock and shortening the DOH compared with the control sites. At a second, less heavily oiled site, foraminifera responded with a shallower DOH, but with a boom in standing stock. Deformed, dead foraminifera occurred in all heavily oiled cores-but not elsewhere. Live foraminifera responded with a population boom at lightly oiled sites with [TPAH] near 1,100 ng/g. Changes in standing stock and DOH with [TPAH] suggest disturbance to the marsh food web, apparently due to oil pollution, and support the use of foraminifera as sentinel species.

Temporal Variability of delta13C and Delta14C in Sinking POC at a Deep Time-Series Station in the Northeast Pacific Ocean

Article

Full-text available

delta13C and Delta14C were measured on sinking particulate organic carbon (POC) collected at station M in the Northeast Pacific (at 3450 m depth, 650 m above bottom, 34°50'N, 123°00'W). Data for 6 years are presented with other properties such as organic carbon flux, C to N mole ratios. delta13C values help us understand processes that change the properties of sinking POC such as primary productivity in surface water and input of terrestrial organic carbon. Delta14C values provide complimentary information such as incorporation of older carbon from dissolved organic carbon (DOC) because their Delta14C signatures are different. delta13C values of sinking POC vary by about 2.5 0/00 with the median value of -22.20/00 . In general they seem to show a correlation with organic carbon flux, hence the primary productivity in surface water. Delta14C values vary between -40 and +40 0/00 , and the variability does not seem to be controlled by one major process. The correlations between carbon isotope ratios and other parameters such as primary productivity, incorporation of DOC or suspended POC, input of terrestrial organic carbon, and river discharge from the California coast, will be discussed in detail.

Reconstruction of sedimentation and pollution in Havana Bay, Cuba

Article

Full-text available

Sep 2011

Since 1998 the highly polluted Havana Bay ecosystem has been the subject of a mitigation program. In order to determine whether pollution-reduction strategies were effective, we have evaluated the historical trends of pollution recorded in sediments of the Bay. A sediment core was dated radiometrically using natural and artificial fallout radionuclides. An irregularity in the 210 Pb record was caused by an episode of accelerated sedimentation. This episode was dated to occur in 1982, a year coincident with the heaviest rains reported in Havana over the XX century. Peaks of mass accumulation rates (MAR) were associated with hurricanes and intensive rains. In the past 60 years, these maxima are related to strong El NiñoNi˜Niño periods, which are known to increase rainfall in the north Caribbean region. We observed a steady increase of pollution (mainly Pb, Zn, Sn, and Hg) since the beginning of the century to the mid 90s, with enrichment factors as high as 6. MAR and pollution decreased rapidly after the mid 90s, although some trace metal levels remain high. This reduction was due to the integrated coastal zone management program introduced in the late 90s, which dismissed catchment erosion and pollution.

Organic matter diagenesis in the sediments of the San Pedro Shelf along a transect affected by sewage effluent

Article

Apr 2002
CONT SHELF RES

A study was conducted to examine the rate of organic matter degradation within the sediments adjacent to the Whites Point Los Angeles County sewage outfall system on the San Pedro Shelf. Benthic chamber deployments were made at three stations in a transect away from the outfall pipe during three cruise periods (October 1990, February and October 1991). The fluxes of phosphate, silicate and radon-222 showed the most significant difference (factor of 3) between stations proximal and distal to the sewage effluent outfall pipe; the fluxes of nitrate, ammonia, alkalinity and TCO2 showed some gradient (a factor of 1.5–2) and the uptake of oxygen showed no variability between sites. Carbon oxidation in these sediments is driven primarily by net oxygen consumption and secondarily by net sulfate reduction. Net sulfate reduction accounts for about 30% of the carbon oxidation near the outfall pipe and 10–15% at the distal sites. Measurements of radon-222 fluxes and radon emanation rates from sediments indicate that the intensity of bio-irrigation is greater by a factor of 2 at sites away from the outfall pipe, but also shows that bio-irrigation does take place at the site adjacent to the pipe. The total amount of particulate organic carbon (POC) oxidized in the sediments around the outfall is ∼2×107g C/d. An estimate of carbon burial in this region is 4×107g C/d. The outfall system could supply ∼3×107g C/d of which 12–20% is estimated to have been deposited in the region. Hence, effluent-derived POC, although it may be a part of the total organic carbon pool undergoing diagenesis and burial on the sea floor, is not likely the only source of carbon to this system. Primary productivity in the surface ocean could supply 9–13×107g C/d to the sea floor. A balanced carbon budget requires that half the primary production is exported to the sea floor, although this budget does not account for other sources of POC to the region.

The Behavior of U- and Th-series Nuclides in the Estuarine Environment

Article

Full-text available

Jan 2003

Peter Wolfgang Swarzenski

Rivers carry the products of continental weathering, and continuously supply the oceans with a broad range of chemical constituents. This erosional signature is, however, uniquely moderated by biogeochemical processing within estuaries. Estuaries are commonly described as complex filters at land-sea margins, where significant transformations can occur due to strong physico-chemical gradients. These changes differ for different classes of elements, and can vary widely depending on the geographic location. U- and Th-series nuclides include a range of elements with vastly different characteristics and behaviors within such environments, and the isotopic systematics provide methods for investigating the transport of these nuclides and other analog species across estuaries and into the coastal ocean. There are numerous types and definitions of estuaries (see Dyer 1973). In this paper, it is simply considered to be a region where freshwater and seawater mix, to emphasize the chemical focus of the issues involved here. This region can either be contained within a river channel or extend onto the shelf, with effects often extending well into an adjoining ocean basin (Fairbridge 1980). These are complex ecological, biogeochemical, and hydrodynamic systems, with strong gradients in the concentrations and composition of micro- and macro particulate matter as well as dissolved organic and inorganic species. Particle-reactive radionuclides and trace elements being transported across estuaries are subject to partitioning between the dissolved, colloidal and particulate phases. The variable chemical nature of the U- and Th-series radionuclides is evident in their distributions in an estuarine environment. Such radionuclides can effectively serve as tracers to identify the sources, fate and transport of particles and colloids as well as other pollutants that behave similarly to these nuclides. These nuclides also have a wide range of half-lives that can be used to examine processes over various time scales. For example, Th and Ra daughter isotopes have …

Atmospheric lead fallout over the last century recorded in Gulf of Lions sediments (Mediterranean Sea)

Article

Nov 2006
MAR POLLUT BULL

Six marine sediment cores from the Gulf of Lions continental slope (700–1700 m water depth) were analyzed for stable lead isotopes and 210 Pb geochronology in order to reconstruct lead atmospheric fallout pattern during the last century. The detrital lead contribution is 25 lg g À1 and the mean sediment anthropogenic inventory is 110 ± 7 lg cm À2 , a little bit higher than atmospheric deposition estimate. Anthropogenic lead accumulation in sediments peaked in early 1970s (1973 ± 2) in agreement with lead emissions features. For the period 1986–1997, the sediment signal also reflect the decrease of atmospheric lead described by independent atmospheric fallout inves-tigations. The anthropogenic Pb deposition in the late 1990s was similar to the 1950s deposition, attesting thus of the output of European environmental policies.

An evaluation of mobile mud dynamics in the Mississippi River deltaic region

Article

Aug 2004
MAR GEOL

Large rivers are the primary interface between terrestrial and ocean environments. A relatively small number of rivers account for a disproportionate amount of the freshwater and suspended materials that are delivered to the coastal ocean. Sediment delivery to these coastal systems plays a key role in the global carbon cycle since deltas and continental shelves are considered to be the main repositories of organic carbon in marine sediments. Particulate material in these environments are typically deposited and resuspended several times before permanent accumulation or transport off the shelf. This sediment cycling is an important component influencing biogeochemical processes that occur in coastal environments. During two cruises in April and October 2000 on the shelf adjacent to the Mississippi River, water and sediment samples were collected for analysis of suspended solids and particle reactive radionuclides (²¹⁰Pb, ¹³⁷Cs, ⁷Be and ²³⁴Th) to evaluate the transport and fate of terrestrial and marine material. A comparison of the distribution of these tracers provides insight about the pathways and residence times of particulate materials on the shelf. Inventories of these short-lived radiotracers showed variations of more than two orders of magnitude, indicating dramatic variations in sediment deposition between sampling events.

The influence of porosity gradients on mixing coefficients in sediments

Article

May 2007
GEOCHIM COSMOCHIM AC

The surface layer of aquatic sediments is a zone characterized by both porosity gradients and intensive mixing. In the standard approach, porosity gradients are ignored when estimating mixing intensity. Here, model formulations with both constant and varying porosity are contrasted to estimate mixing coefficients Db from tracer depth profiles. Complementing the well-known exponential solu-tion of the constant-porosity model, we present a general solution to the variable-porosity model in terms of hypergeometric functions. When using these models in a forward way, the tracer activities predicted by the variable-porosity model are higher than those generated by the constant-porosity model. Similarly, when inverse modelling, Db values estimated by the variable-porosity model are systematically higher than those derived from the constant-porosity model. Still, differences in D b values remain relatively small. When applying both mixing models to excess 210Pb data profiles from slope sediments, a maximal difference of 30% is obtained between Db values, the average deviation being 16%. A systematic exploration of parameter space predicts a maximal underestimation of 60% when deriving Db values from the constant-porosity mixing model. Given the uncertainty imposed by other model assumptions underlying the diffusive mixing model, the influence of porosity gradients on Db values must be classified as rather modest. Hence, the current mixing coefficient database is not biased by the constant porosity approximation.

The relative importance of terrestrial versus marine sediment sources to the Nueces-Corpus Christi Estuary, Texas: An isotopic approach

Article

Jun 2006

Determining sources of sediment to coastal systems is an important and complex problem that figures prominently in a myriad of geological, geomorphological, geochemical, and biological processes. Lithogenic (226Ra,228Ra,228Th,230Th,232Th) and fallout (137Cs,210Pb) isotopes were employed in conjunction with sedimentological methods to determine rates of sedimentation in the Nueces Delta and Nueces-Corpus Christi Estuary and to assess the relative importance of marine versus terrestrial sediment sources to the estuary. Similarity of lithogenic isotope ratios in surface sediments throughout the system precluded a numeric approach to discerning the importance of each of the two large scale sediment sources (terrestrial and marine). A stepwise, graphical examination of discrete lithogenic isotope activity concentrations shows more promise. Terrestrial, marine, and bay sediment means for226Ra versus232Th,226Ra versus230Th, and228Ra versus232Th show that terrestrial and marine sediment sources have different signatures, despite having similar grain size distributions (sands), and that sediment deposited in Nueces and Corpus Christi Bays are indistinguishable from the terrestrial component. Supporting evidence is provided by thorium isotopes,230Th versus232Th,228Th versus232Th, and228Th versus230Th. Nueces Delta sedimentation (0.09–0.53 g cm−2 yr−1) shows a subtle gradient, with rates generally lower in the west and progressively higher moving east, likely reflecting contrasts in land use and topography. Nueces Bay cores differ from those in Corpus Christi Bay in that sands comprise a larger percentage of their composition, and they are mixed over greater depth, most likely due to geographic and physiographic effects. Sediment accumulation rates consistently decrease over the first 20 km from the Nueces River and become constant after that, implying that the river is the most significant source of sediment to the estuary. The interpretation of sediment supply to this estuary as dominated by terrestrial inputs is based on three complimentary sets of data: sediment grain size distributions, discrete lithogenic isotope data (Ra versus Th and Th versus Th), and sediment accumulation rates for both Nueces and Corpus Christi Bays.

Reconstruction of metal pollution and recent sedimentation processes in Havana Bay (Cuba): A tool for coastal ecosystem management

Article

Nov 2011

Since 1998 the highly polluted Havana Bay ecosystem has been the subject of a mitigation program. In order to determine whether pollution-reduction strategies were effective, we have evaluated the historical trends of pollution recorded in sediments of the Bay. A sediment core was dated radiometrically using natural and artificial fallout radionuclides. An irregularity in the (210)Pb record was caused by an episode of accelerated sedimentation. This episode was dated to occur in 1982, a year coincident with the heaviest rains reported in Havana over the XX century. Peaks of mass accumulation rates (MAR) were associated with hurricanes and intensive rains. In the past 60 years, these maxima are related to strong El Niño periods, which are known to increase rainfall in the north Caribbean region. We observed a steady increase of pollution (mainly Pb, Zn, Sn, and Hg) since the beginning of the century to the mid 90 s, with enrichment factors as high as 6. MAR and pollution decreased rapidly after the mid 90 s, although some trace metal levels remain high. This reduction was due to the integrated coastal zone management program introduced in the late 90 s, which dismissed catchment erosion and pollution.

Anthropogenic metal contamination and sapropel imprints in deep Mediterranean sediments

Article

Full-text available

Mar 2011

Sediment cores from the deep Balearic basin and the Cretan Sea provide evidence for the accumulation of Cd, Pd and Zn in the top few centimeters of the abyssal Mediterranean sea-bottom. In both cores, 206Pb/207Pb profiles confirm this anthropogenic impact with less radiogenic imprints toward surface sediments. The similarity between excess 210Pb accumulated in the top core and the 210Pb flux suggests that top core metal inventories reasonably reflect long-term atmospheric deposition to the open Mediterranean. Pb inventory in the western core for the past 100 years represents 20-30% of sediment coastal inventories, suggesting that long-term atmospheric deposition determined from coastal areas has to be used cautiously for mass balance calculations in the open Mediterranean. In the deeper section of both cores, Al normalized trace metal profiles suggest diagenetic remobilization of Fe, Mn, Cu and, to a lesser extent, Pb that likely corresponds to sapropel event S1.

Constraints on the use of 137Cs as a time-maker to support CRS and SIT chronologies

Article

Jun 2004
ENVIRON POLLUT

J.M. Abril

CRS and SIT are two (210)Pb-based models widely used in the radiometric dating of recent sediments. (210)Pb chronologies should be validated using at least one independent tracer, such as (137)Cs. This paper demonstrates that simple methods based on the identification of (137)Cs fallout peaks cannot provide a definitive support for CRS and SIT chronologies. Two main arguments will support this assertion: Firstly, the (137)Cs time-marks cannot support a CRS or SIT chronology if the derived sedimentation rates cannot explain the whole (137)Cs activity profile without postulating mixing. Secondly, the support by the (137)Cs time-marks for a given CRS or SIT chronology cannot be considered as definitive if other dating models can equally explain the whole set of data, thereby producing a different chronology. Several case studies selected from the literature are used to support the present discussion.

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

Article

Jan 2008

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234ThXS, the sediment-mixing coefficient (Db) was 4.3 ± 1.8 cm2 y−1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates (Sa), estimated using 137Cs and 210PbXS, were 0.16 ± 0.02 g cm−2 y−1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m−2 d−1, of which ∼10% or 2.9 ± 0.44 mmol C m−2 d−1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O2 were 26 ± 3.8 mmol m−2 d−1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m−2 d−1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m−2 d−1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m−2 d−1. The flux of DOC out of the sediments (DOCefflux) was 5.6 ± 1.3 mmol C m−2 d−1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.

Groundwater transport of tritium and krypton 85 from a nuclear detonation cavity

Article

Jul 2003

Underground nuclear detonations at the Nevada Test Site provide a unique opportunity to study the hydrologic transport of radionuclides in the field. At the Cambric experiment a pumped well 91 m from the detonation cavity was sampled regularly over 16 years and recovered 94% of the tritium but only 42% of the expected 85Kr, both following decay correction. In addition, the elution curves were different for these two ideal tracers of groundwater movement. Modeling is used to determine the fate of the missing 85Kr based on the phenomenology of underground nuclear detonations and the specific hydrogeology of the Cambric site. At Cambric, large amounts of carbon dioxide created by the detonation caused the upward migration of 85Kr through the collapsing chimney and into the unsaturated zone. A numerical model simulated tritium and 85Kr transport to the pumped well using tritium data to determine regional anisotropy in hydraulic conductivity and reduced hydraulic conductivity in the cavity region. The calibrated model reproduces the 85Kr breakthrough data when emplacement of 85Kr by the upward migration of carbon dioxide is included. Nuclear detonations provide long-term tracer tests in the subsurface, but knowledge of the spatial distribution of sources at this specific site is essential in understanding the transport of these two radionuclide tracers.

A high-resolution geophysical investigation of sediment distribution controlled by catchment size and tides in a multi-basin turbid outwash fjord: Simpson Bay, Prince William Sound, Alaska

Article

Feb 2008

Surficial sediment distribution within Simpson Bay is a function of antecedent bedrock and recently deposited glacial geology, as well as active physical processes both within Simpson Bay and Prince William Sound (PWS). Simpson Bay is a turbid, outwash fjord located in northeastern PWS, Alaska. Freshwater from heavy precipitation, and the melting of high alpine glaciers enter the bay through bay head rivers and small shoreline creeks. The catchment has a high watershed/basin surface area ratio (∼8:1), and easily erodible bedrock that contribute to high sediment loads. The system can be divided into three discrete basins, each with specific morphologic and circulatory characters. Side scan sonar, swath bathymetry, and seismic profiles reveal that bathymetric highs are areas of outcropping glacial surfaces. High backscatter coupled with surface grab samples reveal these surfaces to be composed of coarse sediment and bedrock outcrops. Bathymetric lows are areas of low backscatter, and grab samples reveal these areas to be ponded deposits of organic-rich estuarine muds. The data provide evidence of terminal morainal bank systems, and glacial grounding line deposits at the mouth of the bay and rocky outcrops were identified as subsurface extensions of aerial rocky promontories. Radioisotope analyses of short cores reveal that the bay has an average accumulation rate of approx. 0.5 cm year−1, but that this varies in function of the watershed/basin surface area ratios of the different basins. The interaction of tidal currents and sediment source drives sediment distribution in Simpson Bay. Hydrographic data reveal high spatial variability in surface and bottom currents throughout the bay. Subsurface currents are tide dominated, but generally weak (5–20 cm s−1), while faster currents are found along shorelines, outcrops, and bathymetric highs. Bathymetric data reveal steep slopes with little to no modern sediment throughout the bay, suggesting lack of deposition due to tidal currents.

Mercury accumulation and attenuation at a rapidly forming delta with a point source of mining waste

Article

Sep 2009
SCI TOTAL ENVIRON

The Walker Creek intertidal delta of Tomales Bay, California is impacted by a former mercury mine within the watershed. Eleven short sediment cores (10 cm length) collected from the delta found monomethylmercury (MMHg) concentrations ranging from 0.3 to 11.4 ng/g (dry wt.), with lower concentrations occurring at the vegetated marsh and upstream channel locations. Algal mats common to the delta's sediment surface had MMHg concentrations ranging from 7.5 to 31.5 ng/g, and the top 1 cm of sediment directly under the mats had two times greater MMHg concentrations compared to adjacent locations without algal covering. Spatial trends in resident biota reflect enhanced MMHg uptake at the delta compared to other bay locations. Eighteen sediment cores, 1 to 2 m deep, collected from the 1.2 km2 delta provide an estimate of a total mercury (Hg) inventory of 2500 ± 500 kg. Sediment Hg concentrations ranged from pre-mining background conditions of approximately 0.1 μg/g to a post-mining maximum of 5 μg/g. Sediment accumulation rates were determined from three sediment cores using measured differences of 137Cs activity. We estimate a pre-mining Hg accumulation of less than 20 kg/yr, and a period of maximum Hg accumulation in the 1970s and 1980s with loading rates greater than 50 kg/yr, corresponding to the failure of a tailings dam at the mine site. At the time of sampling (2003) over 40 kg/yr of Hg was still accumulating at the delta, indicating limited recovery. We attribute observed spatial evolution of elevated Hg levels to ongoing inputs and sediment re-working, and estimate the inventory of the anthropogenic fraction of total Hg to be at least 1500 ± 300 kg. We suggest ongoing sediment inputs and methylation at the deltaic surface support enhanced mercury levels for resident biota and transfer to higher trophic levels throughout the Bay.

Marine Chemistry in the Coastal Environment

Article

Sep 1977

T.M. Church

Depositional history of sediments near a major submarine municipal waste water outfall system

Article

Jan 1996

R.P. Eganhouse

Nitrous oxide in the sea off Southern California

Article

Jan 1982

Holocene Sediment Budgets for the Basins of the California Continental Borderland

Article

Jan 1985

Pollutant Inputs and Distributions off Southern California

Chapter

Jun 1975

Municipal wastewaters are the dominant known source of numerous trace contaminants to the Southern California Bight. Annual inputs of mercury, DDT, and PCB via this route are an order of magnitude larger than those from direct industrial discharges and storm runoff. Aerial precipitation is also a major source of chlorinated hydrocarbons. Bottom sediments around the largest outfall system are highly contaminated by various trace elements and DDT residues. Corresponding contamination of benthic organisms occurs for DDT but not the trace elements studied. Seawater collected from the mouths of three important harbors contained copper and the chlorinated hydrocarbons at the part per billion and part per trillion level, respectively.

Bottom Currents And Sediment Transport on San Pedro Shelf, California

Article

Jan 1985

GEOPROBE (Geological Processes Bottom Environmental) tripods were used to measure bottom currents, pressure, and light transmission and scattering and to obtain time-series photographs of the sea floor at depths of 23m and 67m on San Pedro shelf. Wave-generated bottom currents and bottom-pressure variations were sampled at hourly intervals. The wave currents maintained relatively high concentrations of sediment in suspension near the bottom over the inner shelf (< 25 m), and this material (principally silt and clay) was transported offshore by the weak mean flow. Approximately 50% of this material was deposited as the bottom orbital velocities decreased to subthreshold values (approximately 10-15 cm/s). The observed movement of fine sediment across the inner shelf can account for a portion of the mud content of the modern silty sands on the central shelf and on the outer shelf. However, it is clear that the sand fractions, which constitute > 70% of the central shelf substrate, must be transported during high-energy winter storms.-from Authors

Sediment transport and Hg recovery in Lavaca Bay, as evaluated from radionuclide and Hg distributions

Article

Jan 1999

The Flux of Black Carbon to Surface Sediments on the New England Continental Shelf

Article

Feb 1998
GEOCHIM COSMOCHIM AC

Concentrations and 210Pb-derived fluxes of Black Carbon (BC) were deduced in recent ocean margin sediments off northeastern USA to investigate the role of this carbon form in the biogeochemical cycle of this element. BC concentrations in contemporary open shelf sediments ranged between 0.11 and 1.7 mg/gdw. Temporal trends of BC in cores, as well as the pattern of chemical markers of combustion sources, were consistent with anthropogenic fossil fuel combustion dominating the BC input to these sediments in recent times. BC fluxes of 1–2 g m−2 yr−1 suggest that the northeastern USA shelf sink is of the same magnitude as estimates of the BC production from fossil fuel and biomass burning in the upwind NE USA source region. Identification of about 10% of total organic carbon as BC has implications for sedimentary carbon preservation, as well as several other aspects of the global biogeochemical cycle of carbon.

Article

Sep 1998

Although it is well-known that concentrations of an-thropogenic radionuclides and organochlorine compounds in aquatic systems have decreased since their widespread release has stopped in the United States, the magnitude and variability of rates of decrease are not well-known. Paleolimnological studies of reservoirs provide a tool for evaluating these long-term trends in riverine systems. Rates of decrease from the 1960s to the 1990s of 137 Cs, PCBs, and total DDT in dated sediment cores from 11 reservoirs in the eastern and central United States were modeled using first-order rate models. Mean half-times of 10.0 ((2.5), 9.5 ((2.2), and 13 ((5.8) yr for decay-corrected 137 Cs, PCBs, and total DDT, respectively, are surprisingly similar. Similar rates of decrease in a few reservoirs are also demonstrated for chlordane and lead. Conceptual and simple mathematical models relating two soil distributions of 137 Cs to trends in the cores provide insight into differences in trends between watersheds with different land uses and suggest that trends are controlled by erosion, transport, mixing, and deposition of sediments. These results, supported by similar trends reported for other settings and environmental media, could provide an estimate of the decadal response time of riverine systems to changes in the regulation of other persistent hydrophobic or particle-reactive contaminants.

Radionuclides in aquatic environments

Article

Dec 1989
Int J Radiat Appl Instrum C Radiat Phys Chem

Studies on environmental radioactivity in Finland, STL-A34, 43-63

Depositional history of recent sediments from San Pedro Shelf, California: Reconstruction using elemental abundance, isotopic composition and molecular markers

Article

Jun 1988
MAR CHEM

Sediments deposited near and impacted by a major wastewater outfall system in southern California were examined for their organic content and composition. A combination of elemental (% organic carbon (OC), % organic nitrogen (ON)), stable isotopic (σ13C, σ15N) and molecular analyses were used to reconstruct the depositional history of these Recent sediments. In the immediate vicinity of the outfalls, the variations in the bulk properties (% OC; % ON; ; σ15N) of the sedimentary organic matter were closely correlated and appeared to reflect changing inputs of sewage particulates. This allowed the successful application of a two-source mixing model, whereby the fraction of waste-derived organic matter in the sediments was estimated. With increasing distance from the point of waste discharge, however, the validity of the model became tenuous owing to inputs from other sources, changing depositional environments and other factors. Source-specific molecular markers were used to corroborate predictions based on the mixing model and to establish sediment chronology. Key marker compounds used in this study include: (i) the high molecular weight normal alkanes derived from terrigenous plant debris; (ii) 17α (H), 18α (H), 21β (H) -28,30-bisnorhopane, a pentacyclic triterpane characteristic of California oil seeps and shales; (iii) DDT + metabolites; (iv) the long-chain alkylbenzenes present in munipical wastes as a result of detergent use.

Climate Change and the Episodicity of Sediment Flux of Small California Rivers

Article

May 1999

We studied the streamflow and sediment Bur characteristics of the 20 largest streams entering the Pacific Ocean along the central and southern California coast, extending for 750 km from Monterey Bay to just south of the U.S./ Mexico border. Drainage basins ranged in area from 120 to 10,800 km(2), with headwater elevations ranging from 460 to 3770 m. Annual streamflow ranged from 0 to a maximum of 1 x 10(9) m(3)/yr for the Santa Clara River in 1969, with an associated suspended sediment flux of 46 x 10(6) ton. Trend analyses confirm that El Nino/Southern Oscillation-induced climate changes recur on a multidecadal time scale in general agreement with the Pacific/North American climate pattern: a dry climate extending from 1944 to about 1968 and a wet climate extending from about 1969 to the present. The dry period is characterized by consistently low annual river sediment flux. The wet period has a mean annual suspended sediment flux about five times greater, caused by strong El Nino events that produce floods with an average recurrence of ca. 5 yr. The sediment flux of the rivers during the three major flood years averages 27 times greater than the annual flux during the previous dry climate. The effects of climate change are superimposed on erodibility associated with basin geology. The sediment yield of the faulted, overturned Cenozoic sediments of the Transverse Ranges is many times greater than that of the Coast Ranges and Peninsular Ranges. Thus, the abrupt transition from dry climate to wet climate in 1969 brought a suspended sediment flux of 100 million tons to the ocean edge of the Santa Barbara Channel from the rivers of the Transverse Range, an amount greater than their total flux during the preceding 25-yr dry period. These alternating dry to wet decadal scale changes in climate are natural cycles that have profound effects on fluvial morphology, engineering structures, and the supply of sediment and associated agricultural chemicals to the ocean.

Rapid and widespread dispersal of flood sediment on the Northern California margin

Article

Feb 1997
GEOLOGY

The dispersal of flood sediment from small river systems is a poorly studied, yet potentially important aspect of active continental-margin sedimentation. In January 1995, during a flood with a 30 yr return period, the Eel River (northern California) delivered an estimated 25 ± 3 × 106 t (metric tons) of tine-grained (<62 μm) sediment to the ocean. The flood formed a distinct layer on the sea bed that was centered on the 70 m isobath, extended for 30 km along shelf and 8 km across shelf, and was as thick as 8.5 cm, but contained only 6 × 106 t of sediment. Thus, 75% of the flood-derived sediment did not form a recount/able deposit, but was instead rapidly and widely dispersed over the continental margin. Stratigraphic models of, and compilations of sediment flux to, active continental margins need to take the dispersive nature of small river systems into account.

Holocene climate and deep ocean circulation changes: Evidence from accelerator mass spectrometer radiocarbon dated Argentine Basin (SW Atlantic) mudwaves

Article

Dec 1994

The first radiocarbon chronology for sediments of the Argentine basin has been determined using accelerator mass spectrometer (AMS) analyses of 54 total organic carbon samples from four box and two piston cores collected from the downstream and upstream sides of two central Argentine Basin mudwaves. Throughout the Holocene, sediment from the geomorphically defined upstream side of each wave accumulated at rates of 30 to 105 cm/1000 years. Sediments from the downstream side of each wave accumulated at rates of 2 to 10 cm/1000 years in the late and early Holocene, while the mid Holocene is characterized by sedimentation rates less than 1.0 cm/1000 years. During the mid-Holocene, increased aridity reduced chemical weathering and the flow of the rivers draining to the continental shelf, causing a concomitant decrease in fine-grained terrigenous input to the basin as evidenced by decreased sedimentation rates, lower N/C ratios, and depleted δ13Corg values. It is estimated that all of the organic carbon deposited in the central basin during the mid-Holocene was of a marine origin. During the late and early Holocene, however, approximately 35% of the organic carbon deposited was of terrestrial origin. Bottom water flow speeds in the late Holocene were estimated using a lee-wave model and found to average 14 cm s−1. This estimate is comparable to 10 cm s−1 mean and 15–20 cm s−1 maximum flow speeds measured by current meters deployed within the basin. Flow speeds in the Argentine Basin were 10% higher than today from 8000 to 2000 B.P., and are consistent with a general invigoration of thermohaline circulation that began between 9000 and 8000 B.P. It is proposed that the introduction of warm, salty Indian Ocean water into the northern North Atlantic at 9000 B.P. was the mechanism that provided the excess salt needed to stabilize the North Atlantic Deep Water thermohaline circulation system in its present mode.

Transport and diagenesis of trace metals and organic matter in Palos Verdes shelf sediments affected by a wastewater outfall

Article

Feb 2001
MAR CHEM

Carbon and nitrogen determination of carbonate-containing solid

Article

May 1984

Two methods are described for determination of organic carbon, inorganic carbon, and total nitrogen in sediments, sediment trap materials, and plankton. Both methods discriminate organic and inorganic carbon by acidification, avoid losses of acid-soluble organic and inorganic matter, and utilize an automated CHN analyzer for all elemental determinations. Short-term precisions for organic carbon and total nitrogen are about +-1% of the measured value with blank levels less than 1.5 microgrammes for each element.-Author

The fallout rate of PB-210 on the western coast of the United States

Article

Dec 1983

The deposition rate of atmospheric Pb-210 has been measured during a one year period using plastic funnels as collectors. Observed rates were 0.15 dpm/sq cm per yr at a site in Palo Alto, California and 0.21 dpm/sq cm per yr at a site in Los Angeles. The Palo Alto value agrees well with a long-term average of 0.14 dpm/sq cm per yr, obtained from a nearby salt marsh core. These rates are only one-third of those previously estimated using global models for Pb-210 fallout, and indicate the need to consider both longitude and precipitation as factors controlling fallout rates. More than 75 percent of the Pb-210 fallout occurs as wet deposition at the Los Angeles site.

Modeling Variability in 210 Pb and Sediment Fluxes Near the Whites Point Outfalls, Palos Verdes Shelf, California

Article

Sep 1999

Many researchers have had difficulty interpreting sediment data collected from the Palos Verdes Shelf, southern California. Factors that have been difficult to reconcile include the distribution of Â²Â¹Â°Pb and metals, the depth and extent of bioturbation, and the rate of sedimentation. This paper presents a simple model that includes these elements and simulates the flux of Â²Â¹Â°Pb, sediment, and metals to the sea floor near the Whites Point wastewater outfalls. The model uses known particle and metals emission rates from the outfalls and Â²Â¹Â°Pb fluxes to the sediments that vary in proportion to the flux of sediment mass to the sea floor. Model-predicted metals and Â²Â¹Â°Pb concentration profiles in the sediments agree well with data from cores collected at three locations on the Palos Verdes Shelf between 1972 and 1997. The implication of the model results is that Â²Â¹Â°Pb fluxes to the sediments in this area have varied greatly over the past 60 years. The model suggests that subsurface Â²Â¹Â°Pb maxima and uniform Â²Â¹Â°Pb concentrations to depths within the sediments of roughly 30 cm have resulted from time-variable Â²Â¹Â°Pb fluxes to the sediments and relatively shallow bioturbation and that natural sedimentation rates are relatively high.

Chemical oceanography and geochemistry

Article

Jan 1993

Petrology of Sedimentary Rocks: Austin

Article

Jan 1968

R. L. Folk

Computer Simulation of Ddt Distribution in Palos Verdes Shelf Sediments

Article

Feb 1989

Prior to the imposition of effective source-control measures in 1970, large quantities of DDT were discharged to the Los Angeles County municipal sewer system and, subsequently, to the Pacific Ocean. A fraction of this material accumulated among sediments of the Palos Verdes shelf. While the bulk of the DDT lies 10-40 cm below the sediment surface, its fate may be affected by future wastewater-treatment decisions at the Joint Water Pollution Control Plant (JWPCP), Los Angeles County's 385-mgd treatment plant. To assess the impact of impending JWPCP secondary treatment requirements on shelf sediment quality (processes that potentially affect the distribution of chemical tracers among those sediments, including background sedimentation rate), contributions from effluent-related solids, sediment mixing, and diffusive transport were incorporated in mathematical models. Model projections of surface-sediment quality at the most heavily contaminated sites are sensitive to projected effluent solids concentrations, and thus treatmentlevel decisions at JWPCP. Full secondary treatment projections of surface-sediment DDT concentration are 75% higher than those corresponding to continuation of present treatment practice.

Predicting Organic Accumulation in Sediments Near Marine Outfalls

Article

Feb 1990

Kevin J. Farley

The impacts of wastewater discharges in coastal waters are largely exhibited in sediment composition changes. Processes controlling the accumulation of organic material in sediments near submerged sewage outfalls are described. A simplified model is then formulated, for predicting particle deposition and organic accumulation in surface sediments. The model is based on mathematical descriptions of coastal transport, particle dynamics, and organic carbon cycles and includes a second-order kinetic description for the coagulation and settling of sewage particles and natural organic material. Sample calculations are presented demonstrating the importance of coagulation and settling behavior and tidal motion in determining the pronounced changes in deposition and sediment composition near outfalls. Model applications are also presented for the Orange County and Los Angeles County outfalls using predetermined modeling coefficients. Results compare quite well to field observations for both outfalls demonstrating the capabilities of the model in predicting deposition and accumulation of organic material in surface sediments near sewage outfalls in deep coastal waters.

TIC, TOC, DIC, DOC, PIC, POC — unique aspects in the preparation of oceanographic samples for 14C-AMS

Article

Jun 1994
NUCL INSTRUM METH B

The radiocarbon content of discrete carbon pools (total (T), dissolved (D), and particulate (P) inorganic (I) and organic (O) carbon (C)) is a useful tracer of carbon cycling within the modern and past ocean. The isolation of different carbon pools in the ocean environment and conversion to CO2 presents unique analytical problems for the radiocarbon chemist. In general, isolation and preparation of inorganic carbon presents few problems; dissolved carbon is easily extracted by acidifying the sample and stripping with an inert gas. Carbon is also readily isolated from particulate carbonate samples; in this case, CO2 is prepared by hydrolysis of the substrate with phosphoric acid. The isolation and preparation of organic carbon presents a much greater problem. Dissolved organic carbon (DOC) must first be isolated from DIC and then oxidized in the presence of very high salt concentrations. We present results from a closed-tube combustion method in which the DIC-free seawater is evaporated to dryness, transferred to a clean combustion tube, and oxidized overnight at 550°C. Combustion of total organic carbon (TOC) in sediments with a high inorganic carbon content is also difficult. Removal of CaCO3 with acid leaves severely deliquescent salts which, if not thoroughly dried, cause combustion tubes to explode. Removal of the salts by rinsing can also remove significant amounts of organic matter. Finally, we present results from a local coastal region.

Spatial variation in short-term (234Th) sediment bioturbation intensity along an organic-carbon gradient

Article

Jul 1996

Natural and human-induced spatial gradients provide a useful vehicle with which to better understand diverse marine processes. On the Palos Verdes margin (S. California), historical and ongoing waste-water discharge has created an along-shelf gradient in organic C and total N, as well as various trace metals and other pollutants (e.g., DDT). To better understand the impact of such pollution on bioturbation and to develop a more general understanding of the controlling factors of sediment bioturbation intensity, a series of stations representing severely, moderately and negligibly impacted sediments was studied. Vertical profiles of the naturally occurring radionuclide, 234Th, as well as the abundance and species composition of macrofauna were measured from box cores collected at three sites during July 1992. During a March 1993 cruise, radionuclide profiles were collected at an additional eleven sites on the margin. Excess 234Th profiles are, in general, consistent with a steady-state model that balances vertical biodiffusive mixing with radioactive decay. Biodiffusivities determined from the 234Th profiles yield a spatial pattern in which sediments near the outfall are mixed at intensities of 10 cm2/yr, and bioturbation intensities are five times as rapid at sites 57 km from the outfall. Average mixing intensities are between these extremes (28 cm2/yr) at a nearby unimpacted site. Despite the overall consistency of this pattern the reasons behind it remain unclear. Structural aspects of the macrofauna either do not vary between the three intensively studied stations (e.g., depth distribution, size) or do so in a manner that would suggest an opposite effect on the biodiffusivity (e.g., abundance). There is also little variability in trophic groupings along the enrichment gradient. Behavioral modifications, such as: (1) sublethal pollution effects caused by elevated contaminant (e.g., organic carbon and DDT) concentrations, and (2) inhibition by a tube-building polychaete, Mediomastus sp., are postulated to suppress mixing intensities near the outfall. The results of this study suggest that, at least in shallow-water settings, the general controls of bioturbation intensity are still poorly understood.

Extractable organic matter in municipal wastewaters. I. Petroleum hydrocarbons: Temporal variations and mass emission rates to the ocean

Article

Mar 1982

Samples of final effluent were collected from the major municipal wastewater discharges in southern California during 1979 and analyzed for total, aliphatic, and aromatic hydrocarbons. Mean total hydrocarbon contents of those effluents having received mainly primary treatment ranged from 6.1 to 16.3 mg/L, whereas sludge concentrations were approximately 340 mg/L. The vast majority of these hydrocarbons are associated with particulate matter; however, their composition was virtually identical for all effluents. The temporal variations of wastewater total hydrocarbon concentrations (approx.13-37%) appear to be independent of sampling frequency. From these data, southern California's treatment plants are estimated to discharge more than 17.4 X 10Â³ metric tons of hydrocarbons/yr or 4.9 g/(capita day) to coastal waters. This is approximately double the input rate due to surface runoff in this region. Moreover, it represents nearly 6% of the worldwide input of wastewater-borne petroleum to the ocean. Problems associated with global input assessments are discussed, and a correlative means of estimating wastewater hydrocarbon concentrations is presented.

The self-cleaning capacity of surface waters after radioactive fallout. Evidence from European waters after Chernobyl, 1986-1988

Article

Apr 1990

Radionuclide fallout from the burning Chernobyl reactor provided a pulsed input of {sup 137}Cs to surface waters and watersheds of Europe. Radionuclide analyses of surface waters indicated that initial rates of decrease of {sup 137}Cs concentrations in contaminated rivers were on the order of 0.125 day{sup {minus}1}, which was found to be consistent with the size of the mobile inventory in the watersheds (i.e., {approximately}1% of total) and with the initial dilution rate (i.e., {approximately} 0.5 m{sup {minus}1}) in river water. Analysis of {sup 134}Cs and {sup 137}Cs in waters from five different lakes in Switzerland and of settling particles collected in sediment traps from one of the lakes, Lake Zurich, revealed relatively fast whole-lake removal rates. Residence times of {sup 137}Cs in the five study lakes ranged from 5 to 21 months. Horizontal boundaries in this lake appeared to have acted first as sinks of Chernobyl {sup 137}Cs from the upper water column and later as sources of {sup 137}Cs to deeper parts of Lake Zurich. Rates of adsorption/desorption of {sup 137}Cs associated with settling particles, when compared in subsurface waters to those of uptake/release by other processes occurring in the lake, were found to be small.

Isotopic signatures (14C, 13C, 15N) as tracers of sources and cycling of soluble and particulate organic matter in the Santa Monica Basin, California

Article

Dec 1992
PROG OCEANOGR

Measurements of Δ14C, δ13C and δ15N are reported for dissolved (plus colloidal), suspended and sinking particulate, and total sedimentary organic matter in the Santa Monica Basin (mid-basin and shelf sites) on CaBS cruises 1, 3, 4, 7, 8 and 10. These isotopic signatures were indicative of the following processes occurring within the basin regime: (1) terrestrial inputs of organic matter to the sinking and suspended particulate organic matter were of the order of 10% or less, and as high as 25% for the sedimentary organic matter; (2) Δ14C values of the UV-oxidizable dissolved organic matter below 5m were similar to those measured in open ocean waters, while the Δ14C values in the suspended, and, to a lesser degree, in the sinking organic matter decreased markedly with depth. This latter decrease was primarily attributed to episodic resuspension of shelf and slope sedimentary organic matter, and secondarily to natural and/or anthropogenic petroleum inputs; and (3) the isotopic signatures of the UV-oxidizable dissolved organic matter, coupled with total dissolved carbon and amino acid and carbohydrate concentrations were strikingly similar in the deep basin and at an open-ocean site, suggesting a common history for the subsurface (>300m) and deep water at both sites.In addition, total mass and organic carbon and nitrogen fluxes from five particle trap deployments are described in detail. Mass fluxes increased with depth, especially on the shelf, suggesting that particle input from the basin slopes may reach the mid-basin site.We conclude that there is minimal perturbation of all organic phases in the basin from terrestrial sources, and that the properties of the UV-oxidizable dissolved organic matter are not greatly influenced by particles of local origin.

Be-7 as a tracer of flood sedimentation on the northern California continental margin

Article

Mar 1999

Sediment inventories of the cosmogenic radionuclide Be (t "53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of Th and Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995—1997 monitoring period, Be was present (2—35 dpm cm\) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of Be in shelf sediments after the floods. Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redis-tributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimen-tary systems of the northern California continental margin. Ratios of the Pb sediment-accumulation rate (100 yr average) to the Be deposition rate (1—2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3—30 depositional events complete the most recent stratigraphic record. This observation is consis-tent with the magnitude and frequency of fluvial sediment input, as Eel River floods with return periods of 3—33 yr (3% of the time of record) have supplied '80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

Evaluating the mobility of 137 Cs, 239+ 240 Pu and 210 Pb from their distributions in laminated lake sediments

Article

Jan 1995

Post-depositional mobility of137Cs,239+240Pu and210Pb was assessed in six small lake basins by comparing sedimentary nuclide profiles with their known fallout history. Laminae couplets, when present, were determined to be varves because the137Cs and239+240Pu 1963 fallout peaks are present in laminae couplets corresponding to years 1962–1964. There is no evidence of mobility of210Pb, because 1) mass accumulation rates based on210Pb agree with those based on137Cs and239+240Pu peak depths and with those based on varve counts, and 2)210Pb ages agree with varve ages. Significant mobility of137Cs is evident from the penetration of137Cs to depths 15–20 cm deeper than239+240Pu. Deep penetration of137Cs in spite of a sharp gradient below the peak is interpreted by a numerical model to suggest that137Cs is present in two distinct forms in these sediments, 67–82% as an immobile form and 18–33% reversibly adsorbed with a K d of approximately 5000. The profiles can be interpreted equally well assuming a small portionof the total137Cs was present as an extremely mobile phase (K d 5000) in the months to years following peak fallout, slowly becoming more strongly adsorbed. High NH 4 + concentrations in porewaters may enhance diffusion of the mobile form of137Cs, but not of the immobile form of137Cs that defines the sharp gradient. Mobility of137Cs is likely also enhanced by the low clay content and the high porosity of these sediments. Thus the first detection of137Cs in the sediments cannot automatically be assumed to correspond to a date of 1952 (initial testing of thermonuclear weapons), although the depth of the peak can be assumed to correspond to 1963 (the year of maximum fallout from testing of thermonuclear weapons).239+240Pu is a more reliable sediment chronometer than137Cs because it is significantly less mobile.

Long-term changes in the benthic community on the coastal shelf of Palos Verdes, Southern California

Article

Jan 1986

Between 1972 and 1982, both wastewater discharge and natural perturbations played important roles in directing marine benthic community structure on the Palos Verdes Shelf in Southern California, USA. Community succession was traced along a gradient of eleven 60 m-depth stations extending from the submarine outfalls. Spatial and temporal biological patterns were identified via direct gradient, clustering and principal-coordinates analyses. Species associations which occupied sites distant from the outfalls in the early 1970s gradually became established closer to the diffusers during the decade. The areal extent of outfall impacts shrank, reflecting both improvements in effluent quality and co-occurring beneficial natural events, specifically the short-term settlement of large numbers of the echiuran Listriolobus pelodes.

The effects of waste disposal on the coastal waters of Southern California

Article

Apr 1982

Willard Bascom

Trata sobre los efectos ecologicos que ha tenido la disposicion de aguas residuales en la costa de California del sur, Estados Unidos. Senala el equipo usado para la investigacion de los efectos, identifica las areas afectadas, e indica los cambios ocurridos en cuanto a peces, algas, bacterias y virus

Contaminant dispersal on the Palos Verdes continental margin II. Estimates of the biodiffusion coefficient, DB, from composition of the benthic infaunal community

Article

Jan 1996
SCI TOTAL ENVIRON

Sea bed mixing has generally been modeled as a one-dimensional, vertically diffusive process in which a biodiffusion coefficient is estimated by fitting regression lines to vertical profiles of tracer concentrations, typically radioisotopes. In this paper, we describe an alternative approach to deriving time- and space-dependent biodiffusion coefficients in the sea bed, in which the coefficient is estimated directly from the composition and distribution of the benthic infaunal community. We have used a random walk model to describe diffusion driven by benthic organisms. The model is applied to an infaunal population sampled in 13 cores from the 60-m isobath, each sliced into 10 segments. The biodiffusion coefficient, DB, is evaluated for a species by means of this model as the product of characteristic length and velocity scales, L2T−1. The mixing frequency, T−1, is the product of the estimated velocity of sediment displacement and mean cross-sectional area, multiplied by the number of individuals in the core segment, and divided by the volume of the segment. The areal coefficient, L2, is equal to the mean cross-sectional area of the species. The aggregate DB curve is the sum of the species curves. Much of the bioturbation is due to the behavior of a few large species which are incompletely sampled due to the limited core volume. Mixing values for these species are redistributed through all of the core segments by probabilistic methods. The mixing coefficients computed in this manner are considered relative in view of uncertainty in determining sediment velocity, but correspond closely to values estimated independently from DDT gradients. The computations indicate that bioturbation extends through the effluent deposit in the Palos Verdes shelf, into the zone contaminated by DDT.

The 129Iodine bomb pulse recorded in Mississippi River Delta sediments: Results from isotopes of I, Pu, Cs, Pb, and C

Article

Mar 2000
GEOCHIM COSMOCHIM AC

129I (t1/2 = 1.56 × 107 yr) has both natural as well as anthropogenic sources. Anthropogenic sources from nuclear reprocessing discharges and bomb test fallout have completely overwhelmed the natural signal on the surface of the earth in the last 50 years. However, the transfer functions in and out of environmental compartments are not well known due to temporal variations in the sources of 129I and to a lack of knowledge regarding the forms of iodine. From a vertical profile of 129I/127I ratios in sediments located in the Mississippi Delta region in approximately 60 meters water depth, the 129I input function to this region was reconstructed. Dates in the core were assigned based on the plutonium peak at 20 cm depth (assumed to have been deposited in 1963) and the excess 210Pb profile in the same depth interval, and below that, based on the steadily decreasing 240Pu/239Pu ratios from a ratio of 0.18 at 22 cm to 0.05 at 57 cm depth, the 1953 horizon. These low 240Pu/239Pu values are attributed to low yield, close-in, tropospherically transported bomb fallout produced from the Nevada test site in the early 1950s, which had a value of about 0.035, and strongly suggest a terrestrial source for Pu isotopes.

Characterisation of domestic and industrial sewage in Southern California coastal sediments using nitrogen, carbon, sulphur and uranium tracers

Article

Jul 1980
MAR ENVIRON RES

A degradation-mixing model has been developed to aid in the interpretation of geochemical processes occurring in sewage-contaminated marine sediment near San Pedro, California. The nitrogen isotope ratio () is shown to be an effective tracer of sewage discharge-on the San Pedro Shelf. Isotopic fractionation of during release of amino compounds or ammonia (as a consequence of bacterial degradation of organic detritus) appears to be negligible. The nitrogen isotope ratio, therefore, may be considered a conservative component for tracing the source of organic matter deposited in marine sediment.Uranium enrichment from seawater is shown not to occur in the highly reducing sewage-contaminated sediments. The content of uranium in the effluent particulates (18 ppm) is large compared with the content in the uncontaminated sediment (3 to 5 ppm). This allows the content of uranium to also be used as a tool for tracing the deposition of sewage particles in marine sediment. Uranium and nitrogen are shown to be incorporated in the organic fraction of sewage effluent and are released during bacterial degradation of the organic matter. Cadmium and sulphur are shown not to be mobilised during sewage deposition and degradation. The stable isotope ratio of sulphur () is used to demonstrate that sulphur enrichment occurs in the sediment by in situ reduction of seawater sulphate. The data summarised by Morel et al. (1975) are presented and discussed in accordance with the above model.

Contaminant dispersal on the Palos Verdes continental margin: I. Sediments and biota near a major California wastewater discharge

Article

Jan 1996
SCI TOTAL ENVIRON

Discharge of treated wastewaters, over five decades, to the Palos Verdes shelf has produced a sediment mound that contains historically discharged contaminants such as DDT and PCBs. Since 1971, emissions of effluent solids and contaminants have decreased dramatically. Surface sediment quality has improved. Benthic and epibenthic organisms have concomitantly increased in range and diversity. However, the partly buried reservoir of historically discharged contaminants continues to be available to the food chain. Moreover, invertebrates and fish disturb this reservoir via locomotion, feeding, respiration and habitat formation. Vertical distribution studies at four Palos Verdes sites have shown that a variety of organisms live in sediments to depths of at least 35 cm. They penetrate throughout the zone of greatest contamination. Annelids dominate, and mollusks are usually second most common. Approximately 60–80% of the biomass and 95–98% of the organisms occupy the top 10 cm. However, important bioturbators also live in deeper sediments. The faunal composition and behavior have been parameterized in a manner that supports numerical modeling. The Palos Verdes sediment contaminant reservoir is being disturbed by bioadvection, biodiffusion, and physical processes. Sediment-bound contaminants are being biodiffused up from the subsurface to upper sediments, where they periodically undergo resuspension and redistribution.

Radionuclide concentrations, fluxes, and residence times at Santa Monica and San Pedro Basins

Article

Dec 1992
PROG OCEANOGR

Our participation in the California Basin Study (CaBS) in 1986–1988 has produced a radionuclide data base that allows us to trace the particle and water movement in the Santa Monica and the San Pedro Basins. These data enable us to calculate the radionuclide inventories in the three compartments of the basin, the water column, the settling particles, and the sediments. We have also determined the fluxes of several radionuclides associated with the sinking particles, the residence time in the water column, and the rate of deposition in the sediments. The biogeochemical environment of the Santa Monica Basin is uniquely suited to the application of tracer techniques to study the sedimentation processes.

Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

Article

Oct 1995
CHEM GEOL

The accumulation rate of sediments in the Sabine-Neches estuary, near Beaumont, Texas, was studied in four sediment cores using radioisotopes 210Pb and 239,240Pu. Due to very low and variable activities of unsupported 210Pb in the sediments, the sedimentation rates could not be reliably established using the 210Pb dating method. However, 239,240Pu profiles showed distinct peaks, corresponding to the maximum fallout in 1963, and using these Pu peaks the average sedimentation rates were estimated at 4–5 mm yr−1 in the upper and lower estuary.Various assumptions that are commonly used in the 210Pb dating method were tested. It was found that the 226Ra activities were in equilibrium with 238U, and the possible loss of 222Rn was judged to be insignificant. The lack of an exponential profile of excess 210Pb with depth did not appear to be a direct result of sediment mixing, as the post-depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas. A significant positive correlation between the amount of < 63-μm fraction sediments and the total 210Pb activity was observed in the surface sediments. While the hydraulic residence time was relativelt short in this estuary (∼ 10 days), the average total residence time of 210Pb was ∼ 10 days. Between 50% and 94% of the total 210Pb in the water column was found in the < 0.5-μm fraction, resulting in low Kd-values. The measured inventories of excess 210Pb and 239,240Pu in estuarine sediments were only ∼ 10–34% and ∼ 19–50%, respectively, of the total expected inventories. It appears that the estuarine retention and sediment inventory of these radionuclides, and by analogy, other particle reactive metals, are a function of the hydraulic residence time, grain size, and possibly complexation of these nuclides with DOC.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

Article

Jan 1996
SCI TOTAL ENVIRON

A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′-DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.

The geological setting of Santa Monica and San Pedro Basins, California Continental Borderland

Article

Dec 1992
PROG OCEANOGR

D. S. Gorsline

The California Continental Borderland's present configuration dates from about 4 to 5 × 10⁶ years before present (BP) and is the most recent of several configuration of the southern California margin that have evolved after the North America plate over-rode the East Pacific Rise about 30 × 10⁶ years ago. The present morphology is a series of two to three northwest-southeast trending rows of depressions separated by banks and insular ridges. Two inner basins, Santa Monica and San Pedro, have been the site for the California Basin Study (CaBs), funded by the US Department of Energy.

Removal of thorium-234 by scavenging in the bottom nepheloid layer of the ocean

Article

Mar 1989
EARTH PLANET SC LETT

Concentrations of234Th were measured in deep-sea bottom waters to assess the extent of chemical scavenging near the ocean floor. At five stations in the eastern tropical Pacific, where no appreciable bottom nepheloid layer (BNL) was observed, total234Th was in secular equilibrium with its parent238U, confirming earlier published results. In contrast, samples from a well-developed BNL in the western North Atlantic showed a significant depletion of234Th due to its removal by scavenging. The largest depletions were observed in the two samples closest to the bottom (25 and 64 m above bottom) and amounted to 19% of the equilibrium value. The results can be used to test models that have been proposed to explain the existence of a BNL. It is concluded that only a model which includes local or nearby resuspension of sediment can account for the observations. From a steady-state model that assumes local resuspension, the average residence time of resuspended particles in the BNL is estimated to be 25 days.

Spatial and temporal distribution of contaminated, effluent-affected sediment on the Palos Verdes margin, southern California

Article

Apr 2002
CONT SHELF RES

A sedimentary deposit on the continental margin near the Palos Verdes Peninsula, California is comprised of sewage effluent and geologic materials and is contaminated with metals, pesticides (including DDT and associated compounds), and PCBs. The deposit was mapped with subbottom acoustic profilers, and sediment cores were analyzed for geochemical and physical properties to determine the volume of the deposit and the distribution and mass of contaminants. Mapping showed that the deposit ranges up to 60-cm thick, has a total volume exceeding 9 million m3, and covers over 40 km2. Virtually the entire effluent-affected deposit is contaminated with DDT and PCBs. Nearly half of the area of the deposit lies on the continental slope, but 70–75% of the volume of the deposit and total mass of DDT reside on the continental shelf. Analysis of data collected biennially since 1981 by the Sanitation Districts of Los Angeles County show that the mass of DDT has apparently decreased at some stations but has remained essentially constant at others. Temporal changes in mass per unit area of DDT are not statistically significant (at the 90% confidence level) at the most contaminated locations over a 16-yr period. The results of this mapping effort were used as a basis for modeling efforts described elsewhere in this issue.

Depositional history of organic contaminants on the Palos Verdes Shelf, California

Article

Jun 2000
MAR CHEM

During more than 60 years, sedimentation on the Palos Verdes Shelf has been dominated by time-varying inputs of municipal wastewater from the Los Angeles County Sanitation Districts (LACSD) and debris from the Portuguese Bend Landslide (PBL). The present study examines the depositional history of wastewater-derived organic contaminants at a site approximately 6–8 km downcurrent from the outfall system. Sediments at this location are impacted by contributions from both sources, but the relative influence of the sources has changed over time. Two classes of hydrophobic organic contaminants (chlorinated hydrocarbons, long-chain alkylbenzenes) were determined in sediment cores collected in 1981 and 1992. Using molecular stratigraphy, we determined average sedimentation rates (cm/year) and mass accumulation rates (g cm−2 year−1) for the following periods: 1955–1965, 1965–1971, 1971–1981 and 1981–1992. The results show that sedimentation and mass accumulation rates increased from 1955 to 1971 and decreased from 1971 to 1981. These trends are consistent with historical information on the emission of suspended solids from the outfall system, indicating that the discharge of wastes dominated sedimentation at the site during this period. In the 1980s and early 1990s, however, mass accumulation rates increased in spite of continually decreasing emissions of wastewater solids. Several lines of evidence indicate that this increase was due to mobilization of debris from the PBL during and after unusually strong winter storms in the 1980s. As a result, heavily contaminated sediments deposited during the years of greatest waste emissions (i.e. 1950–1970) have been buried to greater sub-bottom depths, thereby reducing their availability for mobilization to the overlying water column. These results highlight the dynamic nature of sedimentation in contaminated coastal ecosystems and its importance to the long-term fate and effects of toxic substances.

Sedimentary sources of old high molecular weight dissolved organic carbon from the ocean margin benthic nepheloid layer

Article

Feb 2000
GEOCHIM COSMOCHIM AC

Average 14C ages of dissolved organic carbon (DOC) in the ocean are 3–6,000 years, and are influenced by old DOC from continental margins. However, sources of DOC from terrestrial, autochthonous, and sedimentary organic carbon seem to be too young to be responsible for the old DOC observed in the ocean. Since colloidal organic carbon (COC, i.e., high molecular weight DOC), which is chemically very similar to that of bulk DOC, can be effectively isolated from seawater using cross-flow ultrafiltration, it can hold clues to sources and pathways of DOC turnover in the ocean. Radiocarbon measurements on COC in the water column and benthic nepheloid layer (BNL) from two continental margin areas (the Middle Atlantic Bight and the Gulf of Mexico) and controlled laboratory experiments were carried out to study sources of old DOC in the ocean margin areas.

Modern accumulation rates and a sediment budget for the Eel shelf: A flood-dominated depositional environment

Article

Feb 1999
MAR GEOL

The northern California continental margin is periodically impacted by geologically significant storms, which have a marked influence on terrigenous sediment supply, flood deposition, and long-term accumulation of fine-grained sediment on the Eel shelf. Accumulation of Eel River muds on the adjacent shelf was investigated using 210Pb and 137Cs geochronologies, in order to understand the fate of sediment discharged by the Eel River and to relate patterns of net sediment accumulation (100-yr time scale) to sediment dynamics. 210Pb data demonstrate that modern accumulation of river mud occurs from the 50-m isobath seaward. Across-shelf accumulation rates decrease from maximum mid-shelf values of 0.6–1.7 g cm−2 yr−1 to values of 0.2–0.4 g cm−2 yr−1 at the shelf break, with a spatially weighted mean of 0.5 g cm−2 yr−1 (0.4 cm/yr) for the entire shelf. 210Pbxs sediment-depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity, which are produced by flood deposition. In some cores, particular 210Pbxs activity intervals may be associated with major Eel River floods of 1955, 1964, and 1974. It is postulated that, because of the coincidence of high-river-flow events and southerly winds during cyclonic winter storms, net northward transport allows for preferential deposition of fine-grained sediment north of the river mouth. Over the past ∼100 years, fluvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter, evident by both the spatial distribution of 210Pb accumulation rates and by clay-rich flood layers partially preserved in shelf deposits. A fine-grained sediment budget for the dispersal system, based on hydrological data and 210Pb geochronologies, demonstrates that a maximum of ∼20% (3×109 kg/yr) of the mean annual supply of fluvial mud (14×109 kg/yr) is trapped on the shelf. The results of this study demonstrate that: (1) short-term sedimentation processes associated with floods can influence sediment accumulation on longer time scales; and (2) a major fraction of fine-grained sediment supplied to tectonically active margins by flood-prone mountainous rivers bypasses narrow continental shelves.

Isotopic and elemental characterization of colloidal organic matter from the Chesapeake Bay and Galveston Bay

Article

Dec 1997
MAR CHEM

In order to investigate sources and turnover rates of dissolved organic matter from Chesapeake Bay and Galveston Bay, colloidal organic matter (COM) was isolated using cross-flow ultrafiltration and subsequently characterized for its elemental (C, N, and S) and isotopic (13C and 14C) composition. Distributions of dissolved organic carbon (DOC) in Chesapeake Bay showed a non-systematic variation, while in Galveston Bay, a non-conservative behavior of DOC with source inputs in the low salinity region was observed. Results of size fractionation of total organic carbon (TOC) revealed that, on average, paniculate organic carbon (POC) comprised ~ 12% and 39% of the TOC pool in Galveston Bay and the Chesapeake Bay, respectively. Colloidal organic carbon (COC) between 1 kDa and 0.2 μm (COC1) constituted ~ 53% and 35%, respectively, with 6–7% of TOC in the HMW fraction (10 kDa to 0.2 μm, COC10), and only ~ 34% and 25%, respectively, of the TOC in the < 1 kDa dissolved fraction. Values of Δ14C and ratios of COM, in general, decreased from river to coastal seawater whereas δ13C values increased with increasing salinity, indicating that organic carbon sources changed from more terrestrial components to phytoplankton-derived sources during estuarine mixing. The distinct isotopic signature and elemental composition of riverine and estuarine COM also suggest that most riverine HMW COM could be removed or decomposed rapidly within the estuary. The fact that values of ratios increased from particulate to HMW to medium MW COM suggest that reactivities of organic matter decrease with reducing size. While Δ14C values of COM1 were generally equivalent to contemporary ages, they were consistently lower for the COM10. Lower Δ14C values and lower ratios in the COM10 than in the COM1 suggest that most of the estuarine HMW COM is from older and more proteinaceous sources within the estuaries. We hypothesize that resuspended sedimentary organic matter or recycled older DOM is likely the source for COM10.

234Th:238U disequilibria in the Gulf of Mexico: The importance of organic matter and particle concentration

Article

Mar 1996
CONT SHELF RES

We have measured the concentrations of dissolved and particulate234Th, suspended particulate matter, dissolved organic carbon, nutrients and a suite of pigments along a transect in the shelf and slope waters of the Gulf of Mexico. The deep water station samples were collected in the presence (summer 1992) and absence (spring 1992) of a warm-core ring. In the summer, the station was at the periphery of a warm-core ring containing more oligotrophic, Caribbean water, while in the spring, only Gulf water was present. The234Th deficiency, which extends throughout a 1500 m water column in spring, indicates active lateral exchange of dissolved and particulate species. This study provides insight into the role of particles for234Th scavenging and differences in the scavenging intensities in the presence and absence of a warm-core ring. In addition, the possibility that the major carrier phase of thorium cycling could be dissolved organic carbon is suggested from the correlation between234Th (dissolved and total) and dissolved organic carbon concentrations. The average particle residence time for the whole water column in spring, seven days, was significantly shorter than that for summer, 21 days. This longer residence time in the summer is attributed to the presence of a warm-core ring which resulted in lower particle, DOC, and nutrient concentrations. Values of scavenging and particle residence times and234Th fluxes, and the significance of the correlations of these values with concentrations of particle mass and various pigments are discussed.

Pu in coastal marine environments

Article

Dec 1980
EARTH PLANET SC LETT

Analysis of water samples from the New York Bight area and Narragansett Bay reveals that a small fraction of the total Pu (probably Pu (III + IV) species) is continuously removed to the sediments at a rate similar to that of the particle-reactive isotope228Th. A more “soluble” Pu species appears to be released at times from the sediments to the water column in these nearshore regions. Sediments in shallow areas of the New York Bight south of Rhode Island and Narragansett Bay have high Pu inventories and relatively deep penetration of this element, although the net sediment accumulation rate is generally low (<0.03 g/cm2 yr). The high Pu inventories can be explained if both sediment resuspension and sediment mixing are assumed to be the major controlling factors for the effective transfer of Pu from the water column to the sediments. By simultaneous modelling of the depth distribution of three tracers which operate on vastly different time scales:234Th (half-life 24 days),210Pb (half-life 22 years) and239,240Pu (introduced into the environment during the past 30 years), bioturbation rates ranging from 4 to 32 cm2/yr in the surface mixed layer (5–10 cm thick) and from 0.3 to 2.5 cm2/yr in the layer below (up to 40 cm thick) and net sediment accumulation rates of approximately zero to 0.14 g/cm2 yr were calculated for these areas.

Effect of deposit feeders on migration of 137Cs in lake sediments

Article

Feb 1979
EARTH PLANET SC LETT

Illite clay particles with adsorbed137Cs were added as a submillimeter layer to the surface of silt-clay sediments contained in rectangular Plexiglas cells stored in a temperature-regulated aquarium, in order to trace the effect of the oligochaete, Tubifex tubifex, and the amphipod, Pontoporeia hoyi, on mass redistribution near the sediment-water interface. A well-collimated NaI gamma detector scanned each sediment column (∼10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a cell with tubificids (∼5 × 104 m−2), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 cm/day (20°C). When labeled particles entered the feeding zone,137Cs reappeared in surface sediments creating a bimodal activity profile. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (∼1.6 × 104 m−2) uniform activity developed rapidly (∼17 days) down to a well-defined depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm2/yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of137Cs indicated an effective molecular diffusion coefficient of 0.02 cm2/yr.

Dispersion of sediment DDTs in the coastal ocean off Southern California

Article

May 1999
SCI TOTAL ENVIRON

The masses of DDT compounds (DDTs) in surface sediments of the Palos Verdes Shelf (PVS) and Santa Monica Bay (SMB) have declined over the last two decades, following the ban on DDT production in 1970. This mass reduction could result from a number of biological and physical processes, including biodegradation and/or dispersal away from the sites. We integrated existing data with our new data of DDTs from different compartments in the coastal zones off southern California to assess the importance of the dispersal mechanism. The synthesis of the data indicated that: (1) historically deposited DDTs have been remobilized upward in the sediment column; (2) DDTs have been resuspended into the water column; and (3) sewage-derived DDTs have been redeposited into distant areas. Resuspension of DDTs from contaminated sediments was evident from the close correlation between the DDT concentrations in the water column and surface sediment at three locations with different DDT levels. The current distribution patterns for linear alkylbenzenes and DDTs in surface sediments at SMB were also suggestive of dispersal of DDTs. The distribution of DDTs in the surface sediments exhibiting a gradient from the outfalls to offshore and the general spatial distribution pattern in the basins precluded the possibility of either aerial fallout or surface runoff as being an important source of DDTs. These results are consistent with the hypothesis that resuspended DDTs in the discharge zones are being dispersed to distant areas. The percent of DDEs in total DDTs was uniformly high (∼90%) in the PVS and SMB sediments, but varied widely in sediments of the Santa Monica and San Pedro Basins. The percent DDEs were particularly low (as low as 10%) in certain subsurface sections of the sediments near two dumpsites containing DDT wastes (from prior to 1970) comprising of low proportions of DDE. However, the top-layers of the basin sediments contained DDT residues with high %DDEs similar to that of sediments on the PVS, suggesting a common source from the historic DDTs in the wastewater discharges. The available data is insufficient to confirm the possibility of anaerobic degradation of DDEs in the sediment cores investigated which could also result in the mass reduction of DDTs in the post-1970 sediments.

History of Trace Metal Pollution in Sabine-Neches Estuary, Beaumont, Texas

Article

Jun 1995

Sabine-Neches Estuary, near Beaumont, TX, receives wastewater effluents from over 160 industrial and municipal treatment plants. The concentrations of trace metals (Go, Cr, Cu, Ni, Pb, and Zn), Al, Fe, Mn, and organic carbon were determined in four dated sediment cores. A reliable geochronology and reconstruction of the history of trace metal inputs of these sediments was possible because the Pu-239,Pu-240 profiles closely trace the bomb fallout history into the environment. Down-core variations of aluminum-normalized enrichment factors for these metals demonstrate that the sediments of this estuary have remained relatively 'pristine' with respect to trace metal contamination since 1860. While the concentrations of Pb and Zn at various depths in the sediment column are slightly enriched, Co, Cr, Cu, and Ni are depleted. The sedimentary and biogenic particles that are presently being deposited are also depleted in trace metals. Lack of strong enrichment for trace metals like Cu can be attributed to the short residence time of water, low salinity conditions, and possibly strong complexation of these metals with organic matter.

Accumulation rates and sources of sediments and organic carbon on the Palos Verdes shelf based on radioisotopic tracers

Abstract

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