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234Th/238U disequilibrium in near-shore sediment: Particle reworking and diagenetic time scales
Abstract
The distribution of 234Th(t1/2 = 24.1days) in excess of its parent238U in the upper layers of near-shore sediment makes possible the evaluation of short-term sediment reworking and diagenetic rates.234Th has a maximum residence time in Long Island Sound water of 1.4 days. Seasonal measurement of234Th/238U disequilibrium in sediment at a single station in central Long Island Sound demonstrates rapid particle reworking and high234Thxs (>1 dpm/g) in the upper 4 cm of sediment with slower, irregular reworking and low234Thxs to at least 12 cm. The rate of rapid particle reworking varies seasonally and is highest in the fall. The rapidly mixed zone is characterized by steep gradients in sediment chemistry implying fast reactions spanned by234Th decay time scales.238U is depleted in the upper mixed zone and shows addition in reducing sediment at depth.
... The data in this synthesis come from multiple studies and were acquired primarily from sites throughout Long Island Sound but with an emphasis on the Central Basin and stations along the W-E axis (Figures 1, 2). Sediment box cores or pipe cores were retrieved by divers (1974-19791992-1993Aller and Cochran, 1976;Gerino et al., 1998), or obtained remotely using either a Soutar box corer for surface sediment sampling or a gravity corer for deeper radiochemical measurements [1989Long Island Sound Study (LISS)]. These techniques ensured retention of an undisturbed sediment surface, confirmed by inspection. ...
... Particle-reactive 234 Th is produced continually in the water column by the decay of dissolved 238 U, and provides a natural tracer of particle dynamics and the fate of labile C org in LIS (Aller and Cochran, 1976;. The production of 234 Th varies with 238 U activity, and thus directly with salinity ( 238 U = 2.45 × (S/35) dpm L −1 ; where S = salinity). ...
... Vertical gradients of 234 Th xs in deposits can be used to derive particle mixing coefficients, D B , and to estimate other modes of particle transport (e.g., nonlocal; Figure 6) (Aller and Cochran, 1976). In the present cases, D B was estimated assuming exponential decreases of 234 Th xs with depth, and optimal fits to the integrated activities measured over finite sampling intervals (per . ...
Bioturbation promotes priming and total remineralization of sedimentary organic matter (Corg) in multiple ways. A primary local mode is the injection of reactive Corg from the water column, surface sediment, and mucus secretions into deposits. During feeding, burrowing, and construction activities by benthic fauna, labile substrates are brought into close association with more refractory material over a wide range of time scales, geometries, and depths, enhancing decomposition of the less reactive components (priming). One measure of these local interactions is the particle mixing coefficient, DB, which can be estimated from the averaged penetration of particle-reactive radionuclides into deposits. Patterns of DB in Long Island Sound, an estuarine system with well-defined sources of naturally occurring radionuclides, show consistent positive correlations between DB and total inventories of excess ²³⁴Th (t1/2 = 24 days) and ²¹⁰Pb (t1/2 = 22 years) at local and basin scales. These correlations, maintained seasonally in the case of ²³⁴Th, demonstrate not only the penetration of plankton-derived, reactive Corg into deeper regions of deposits during bioturbation over monthly (∼5–10 cm) to decadal timescales (∼20–100 cm) but also the enhanced capture of labile substrates from the water column across basin scales into bioturbated patches as the intensity of reworking increases. In Long Island Sound, sedimentary Chl-a distributions and benthic nutrient regeneration (e.g., NH4⁺ fluxes) reflect these particle exchange processes. Basin and regional scale capture of labile substrates into bioturbated deposits can be generally demonstrated, for example, along the highly productive Cape Hatteras continental margin. Thus, total and net remineralization necessarily increase with the biogenic enhancement of the quantity of labile particulate substrate in deposits. This capture, intermixing, and close association of reactive and refractory substrates (reductant blending), and thus the optimization of priming potential, represent important, often overlooked, pathways by which bioturbation generates biogeochemical conditions conducive to maximum efficiency of remineralization.
... For example, 7 Be deposition from the atmosphere depends on seasonality of both precipitation and atmospheric inventory [18]. Scavenging of 234 Th from the water column is more likely under high suspended sediment concentrations, such as during elevated river discharges or high wave energy, which both vary seasonally and with weather [6,19]. Sediment bed surface activities of 7 Be in continental shelf environments range from 1 to 10 disintegrations per minute per gram of sediment (dpm g −1 ) and for 234 Th range from 10 to 80 dpm g −1 [5,6,19,20]. ...
... Scavenging of 234 Th from the water column is more likely under high suspended sediment concentrations, such as during elevated river discharges or high wave energy, which both vary seasonally and with weather [6,19]. Sediment bed surface activities of 7 Be in continental shelf environments range from 1 to 10 disintegrations per minute per gram of sediment (dpm g −1 ) and for 234 Th range from 10 to 80 dpm g −1 [5,6,19,20]. Specific activities of suspended sediments range from 0 to 50 dpm g −1 for 7 Be and 15 to 115 dpm g −1 for 234 Th under near oceanic salinities [2,3]. ...
... This approach was equivalent to assuming that the activity of any sediment in suspension immediately equilibrates to the level driven by local seawater activity. This assumes that equilibration in the model occurs over much shorter timescales than the timescales of typical resuspension events and natural removal of 234 Th by sediment, which occurs on the order of days and is dependent on the amount of sediment resuspension [3,19,43]. Once sediment carrying 234 Th enters the sediment bed, activities begin to decay. Through these treatments of the activities of 7 Be and 234 Th, we can characterize their behavior in a coastal ocean environment with river input and episodic resuspension. ...
Geochronologies derived from sediment cores in coastal locations are often used to infer event bed characteristics such as deposit thicknesses and accumulation rates. Such studies commonly use naturally occurring, short-lived radioisotopes, such as Beryllium-7 (7Be) and Thorium-234 (234Th), to study depositional and post-depositional processes. These radioisotope activities, however, are not generally represented in sediment transport models that characterize coastal flood and storm deposition with grain size patterns and deposit thicknesses. We modified the Community Sediment Transport Modeling System (CSTMS) to account for reactive tracers and used this capability to represent the behavior of these short-lived radioisotopes on the sediment bed. This paper describes the model and presents results from a set of idealized, one-dimensional (vertical) test cases. The model configuration represented fluvial deposition followed by periods of episodic storm resuspension. Sensitivity tests explored the influence on seabed radioisotope profiles by the intensities of bioturbation and wave resuspension and the thickness of fluvial deposits. The intensity of biodiffusion affected the persistence of fluvial event beds as evidenced by 7Be. Both resuspension and biodiffusion increased the modeled seabed inventory of 234Th. A thick fluvial deposit increased the seabed inventory of 7Be and 234Th but mixing over time greatly reduced the difference in inventory of 234Th in fluvial deposits of different thicknesses.
... steady-state conditions is calculated using the equation (Aller and Cochran, 1976;Nittrouer et al., 1984): ...
... In April 1998, the two inshore stations exhibited high 7 Be inventories and greater depth of 7 Be penetration than other seasons (Fig. 5). Logarithmically decreasing profiles observed in short-period radiotracers are typically interpreted as being the product of biological mixing (Aller and Cochran, 1976). Although calculated mixing rates (D b in Table 2) are moderately high and would appear to coincide with the period of spring population increase in benthic biology, mixing rates are an order of magnitude lower in the two offshore stations. ...
... For the analyses of 234 Th, separate sediment samples (~4 g) were dried and leached with a hot solution of 6 N HCl + H 2 O 2 for 3 times. Subsequently, the leachate was purified following the classic ion-exchange chemistry protocol (Aller and Cochran, 1976). Thorium-234 in the eluate was coprecipitated with a MnO 2 suspension and determined by counting its daughter 234m Pa in a gas-flow proportional low-level beta counter (GM-25-5, RISØ National Laboratory, Denmark). ...
... The association of RN released during the Chernobyl accident to colloidal material in surface water was also found (Matshunaga, 2004). The residence times of 234 Th and 228 Th with respect to removal onto particles in estuarine and nearshore waters can be of the order of days (Aller and Cochran, 1976;Santschi, 1979Santschi, , 1980. Recent work (Baskaran, 1992) has shown that a significant portion of Th nuclides is associated with colloidal material in the surface waters of the continental margin regions of the Gulf of Mexico. ...
Colloids are ubiquitous in nature in the form of solid or liquid or gas. The presence of colloids is significant in environments; hence the study has attained its significance. The current study focuses on groundwater colloids quantifying by filtration method in four different sizes and assessment of requirement of filter membranes for the same. As the result of filtering in four different size membrane we could have get four size ranges of colloids. Two coastal bore wells were selected for this study along the southeast coast of India. The results show that higher accumulation of colloids will increases the number filter membrane due to blocking of pores in the membranes. Compared to these bore wells Kalpakkam Township has higher accumulation of colloids than Anupuram Township and consequently it needs more number of filter membrane to filter the groundwater sample. In both locations 0.8µm size fraction got the least accumulation of colloids and conceding least number of filter membrane for filtration.
... Understanding what controls the exchange of chemicals and nutrients between the sediment and overlying water reservoirs is a key aspect of marine geochemical studies (Broecker and Peng, 1982). Naturally occurring radionuclides in the uranium and thorium decay chains are powerful tools for the investigation of mass transfer processes taking place at the sediment-water boundary (Krishnaswamy et al., 1971;Koide et al., 1973;Aller and Cochran, 1976;Tamborski et al., 2022). In particular, the advent of the 224 Ra/ 228 Th isotope pair in the past decade has provided a new way to study solute exchange across the sediment-water interface in the coastal ocean (Cai et al., 2012(Cai et al., , 2014. ...
We describe a new method suitable for the precise and accurate determination of 226Ra in porewater and sediment samples using a single-collector sector field ICP-MS (ThermoFisher Element XR) equipped with an Apex-Q desolvation device and a high-sensitivity Jet-X interface. In combination with 230Th measurements in parallel sediment samples, this method allows precise and accurate quantification of the 226Ra/230Th disequilibria in surface sediment cores, thereby enabling the use of this isotope pair as a tracer of solute transfer across the sediment-water interface in the deep ocean. The method integrates a step of isotope dilution with 228Ra as an internal spike, a pre-concentration of Ra and Ba by MnO2 precipitation, and an efficient separation of Ra from other undesirable elements using a cation exchange resin and a Triskem Sr-spec resin. With the inclusion of one or two additional cation resin columns and the use of up to 16 bed-volumes of a lower molarity (1.7 M) HCl eluent, our procedure eliminates the complicated matrix effects persistently encountered in previous studies, and provides a highly purified solution suitable for 226Ra measurement using an Element XR ICP-MS apparatus. Consequently, we are able to determine the activity of 226Ra in ~20-50 ml of porewater or 100 mg of sediment with an internal precision of ~1.0% and an accuracy of ~99.2%. The precise measurements of porewater and solid phase 226Ra in a sediment core from the North Pacific Ocean allowed the distribution coefficient (Kd) of 226Ra to be constrained tightly within a range of 4700-11600 ml g-1. Moreover, with the aid of a one-dimensional exchange model, the combination of the 226Ra and 230Th measurements allowed us to estimate a 226Ra flux of 1140±20 dpm m-2 y-1 from the sediment core.
... The particles then may settle and deposit at the sediment/water interface (Rink and Thompson, 2016;Smoak and Patchineelam, 1999). The 234 Th xs thus can be used as a geochronological tool where sediments are deposited rapidly without mixing Demaster et al., 1985;Larson et al., 2018) or an indicator of surface sediment mixing (e.g., bioturbation) in the estuaries, the coastal ocean, and the deep sea (Aller and Kirk Cochran, 1976;Fuller et al., 1999;McClintic et al., 2008;Pope et al., 1996;Santschi et al., 2001;Schmidt et al., 2007;Smoak and Patchineelam, 1999;Yeager et al., 2004). For example, 234 Th has been applied to determine the sedimentation rates (4.4 cm month −1 ) of continental shelf deposits in the East China Sea on a 100-day scale (Demaster et al., 1985;McKee et al., 1983). ...
Microplastic (<5mm) accumulation in water environments is an emerging concern issue, it can be absorbed throughout the food chain. Filter organisms are the most susceptible, so they have the potential to be an environmental sentinel. Thus, tests were carried out with three mussels species in order to investigate the filtration and prevalence of microplastic in them. The investigated bivalves have shown different forms of microplastics in soft tissues, feces and pseudofeces. However, the identification of polymer blends is still tricky. This outcome has indicated the potential of these bivalves to be used as bioindicators associated with this contaminant in environments.
... The particles then may settle and deposit at the sediment/water interface (Rink and Thompson, 2016;Smoak and Patchineelam, 1999). The 234 Th xs thus can be used as a geochronological tool where sediments are deposited rapidly without mixing Demaster et al., 1985;Larson et al., 2018) or an indicator of surface sediment mixing (e.g., bioturbation) in the estuaries, the coastal ocean, and the deep sea (Aller and Kirk Cochran, 1976;Fuller et al., 1999;McClintic et al., 2008;Pope et al., 1996;Santschi et al., 2001;Schmidt et al., 2007;Smoak and Patchineelam, 1999;Yeager et al., 2004). For example, 234 Th has been applied to determine the sedimentation rates (4.4 cm month −1 ) of continental shelf deposits in the East China Sea on a 100-day scale (Demaster et al., 1985;McKee et al., 1983). ...
Dating methodologies for Quaternary marine sediments play increasingly important roles in the reconstruction
of paleoenvironments and paleoclimate in (paleo)oceanography. Previous reviews or studies have focused
mainly on one or two methodologies, and their applications in one specific environment. With the continuing
technological and methodological advances in different methods over the past few decades, an up-to-date comparison
of the pros and cons of each dating methodology is needed to clearly understand their applications inmarine
geoscience research. In this review,we first briefly summarized the common methods of absolute dating and
relative dating. These are (1) radioisotope dating with different half-lives using natural nuclides of 234Th, 210Pb,
230Th, and 226Ra, cosmogenic nuclides of 7Be, 14C, 10Be, 32Si, 26Al, 36Cl and 21Ne, and the artificial radionuclides
of 137Cs, 239, 240Pu, 241Am and 129I that have been induced by atmospheric nuclear tests, accidents in nuclear
plants, and discharges of radioactive wastes; (2) radiation exposure dating of luminescence and electron paramagnetic
resonance (ESR) dating; and (3) stratigraphic dating of δ18O and paleomagnetic sequence. Applications
and limitations fromthemarine terraces, estuaries, to hadal trenches have been summarized to each technique in
the study of Quaternary marine geoscience extending from the Anthropocene through the Pleistocene. Finally,
we introduced some emerging event dating methods, namely the arrivals of microplastics, mercury isotopes,
and organic pollutant deposition that all appeared after the industrial resolution in our now changing oceaninfluenced by acidification, global warming, and anthropogenic activities. We ended by discussing future perspectives
for reliable and high-resolution chronology by interdisciplinarymethods including computer programming
to better understand the natural geological evolution and predict the future changes in earth science.
... This could be explained by the fact that 234 Th is rapidly scavenged from the water column by particles upon falling to the ocean floor. Thereafter, these particles are reworked into the sediment by both biogenic and physical activity (Aller and Kirk Cochran, 1976). 232 Th was determined with an assumption that 232 Th was in equilibrium with its decay daughters 228 Th and 228 Ra. ...
Baseline study of natural (⁷Be, ²¹⁰Pb, ²²⁶Ra, ²³⁴Th, ²²⁸Ra, ⁴⁰K) and anthropogenic (¹³⁷Cs) radionuclides was carried out in two cores collected from Sabaki River estuary (Kenya, Indian Ocean). There was no exponential decrease of excess ²¹⁰Pb down the cores, which did not allow dating and determination of heavy metal pollution history. The use of ¹³⁷Cs as a time marker was not possible due to its low fallout rates in East Africa. The short-lived radioisotope ⁷Be, a tracer of river floods, confirmed 2018 flood in Sabaki River estuary. Heavy metal concentration in the two cores showed nonsystematic trends with depth. Only Pb concentration in Sabaki River estuary was higher than the background levels. Application of the “Environmental Risk from Ionising Contaminants Assessment and management” (ERICA) tool confirmed that the potential dose rates to biota from the sediment radioactivity concentrations are unlikely to pose appreciable ecological risks.
... 假设 [41] ，开展基于 210 Pb-CFCS模式的沉积速率研究， ...
Radionuclides are widely used as tracers in oceanography. As the final fate of many substances, marine sediment is mainly concerned from three perspectives: source identification, sedimentation rate, and sediment resuspension. In the present study, Uranium-series radionuclides (210Pb, 226Ra, 234Th, and 238U) were applied in several sea regions (nine typical coral reefs in the South China Sea, Weizhou Island, Pearl River Estuary, Arctic Ocean, and Southern Ocean) to study the processes of source identification, sedimentation rate, and sediment resuspension. Firstly, activity ratio of 226Ra to 238U was found to be extremely low (<0.1) in marine sediment of coral reef regions relative to activity ratio (0.5~1.0) of other marine sediments and could be used to identify distinct sources of marine sediments in the coral reef regions in addition to other geochemical tools (Al, Ti, and REE). Secondly, the sedimentation rate (3.7±0.6 mm/a) was calculated for a sediment core in coral reefs near the Weizhou Island via excess 210Pb (Constant Flux Constant Sedimentation Model, CFCS model) and was lower than most results (5 mm/a~96 mm/a) in other coastal areas of China. Finally, residual β activity of particulate 234Th (RAP234) was proposed for tracking marine sediment resuspension. The RAP234 was successfully applied in the Arctic Ocean, South China Sea, and Southern Ocean. Overall, the successful applications of these radioactive tracers provided potential tools in addition to the ongoing toolbox used in tracing marine sedimentary processes.
... Dates obtained from one age-dating method (e.g., 137 Cs) can be used to validate the dates derived by another method (e.g., 210 Pb). Chillrud et al. (2003), Fuller, van Geen, Baskaran, and Anima (1999), and Aller and Cochran (1976) are examples of studies based on radioisotope analysis of sediment cores. ...
The Navy has more than 200 contaminated sediment sites, with a projected remediation cost of $1.3 billion. Space and Naval Warfare Systems Center San Diego (SSC San Diego) developed this guide to ensure that sediment investigations and remedial actions are successful and cost effective. It provides the latest guidance on evaluating sediment transport at contaminated sediment sites, and describes how to use sediment transport information to support sediment management decisions. When Navy Remedial Project Managers (RPMs) and their technical support staff do not adequately characterize or predict sediment transport at a contaminated site, the range of potential response actions can be limited because technical defensibility is inadequate. As contaminated sediment site investigations move into the Feasibility Study phase, a lack of accurate and defensible information regarding sediment transport and sediment deposition patterns can potentially lead to selection of unnecessary removal or treatment actions, potentially costing the Navy millions of dollars. Alternatively , the failure to contain or remove contaminated sediments that may be subject to destabilizing hydrodynamic events may lead to larger contamination footprints, movement of contamination off site, and potentially increased future cleanup costs. Little practical guidance has been available for performing a sediment transport assessment at a contaminated sediment site. This guide provides Navy RPMs and their technical support staff with practical guidance on planning and conducting sediment transport evaluations. It identifies and reviews methods and tools that can be used to characterize sediment transport, and provides a framework that can be used to more clearly identify the types of measurements and data analysis methods that can be used at a contaminated sediment site. It also provides guidance on how the results of a well-designed sediment transport evaluation can be used to develop management decisions for contaminated sediment sites. Regulatory and stakeholder acceptance of sediment management decisions will be facilitated by using sound science and engineering principles and targeted, consensus-based data collection efforts. The framework developed in this report has been applied at three demonstration sites: Hunters Point Shipyard in San Francisco, CA; Bremerton Naval Complex in Puget Sound, WA; and Naval Station Newport in Newport, RI. Various technologies and data analysis methods identified in this guide were applied at the sites, and results were used to develop a detailed conceptual site model (CSM) that could be used to support the development and selection of the most cost-effective and environmentally sound remediation scenarios for the sites. A case study report for each of these demonstration sites is provided in the appendices. Demonstration site results were used to refine the general approach for characterizing sediment transport presented in this guide. Many Navy sediment sites are located in areas of relatively low hydrodynamic energy such as rivers, bays, and estuaries, where sediments and contaminants tend to accumulate over time. In some cases, the original source(s) of contamination have been eliminated, reduced, or controlled as environmental management practices have improved over the past 30 years. At some sites, the deposition of newer, relatively clean sediment on top of more contaminated sediment has resulted in burial of contamination. iv The most common management questions associated with these sites are as follows: • Could erosion of the sediment bed lead to the exposure of buried contamination? • Will sediment transport lead to the redistribution of contamination within the site, or movement of contamination off site? • Will natural processes lead to the burial and isolation of contamination by relatively clean sediment? • If a site is actively remediated, could sediment transport lead to the recontamination of the site? This guide focuses on the collection and analysis of data needed to address these primary questions. A combination of regional and historical data, site-specific measurements, empirical data evaluation methods, and numerical modeling techniques can be used to characterize sediment transport at a given site. Empirical approaches are particularly useful for characterizing the past and present effects of sediment transport; however, numerical models are more useful for predicting the effects of future events and sediment deposition patterns. The appropriate method(s) and tool(s) should be selected and used on a site-specific basis to qualitatively and/or quantitatively characterize sediment transport, and assess the viability of various remedial options. The approach for a given site depends on the size and complexity of the site, the CSM, the specific site objectives, and the available resources. This guide presents an overview of sediment transport processes and their relative importance in various site settings. It also describes the sedimentary environments found at most Navy contaminated sediment sites. This background information lays the groundwork for understanding the Tier 1 and Tier 2 evaluation approaches. It discusses the compilation of available data, development of a CSM for sediment transport, and formulation of site-specific sediment management questions and study objectives. Tier 1 data needs and data analysis methods are also presented. In discussing Tier 2 Evaluation, this guide presents the data needs and data analysis methods for a Tier 2 sediment transport evaluation. It then applies this information to site management and describes how the results of a sediment transport evaluation can be used to support sediment management decisions for a site. Appendices to the document include a compilation of information on the various tools and technologies that can be used in the Tier 1 and Tier 2 sediment transport evaluations (Appendix A). Supporting information for the available tools and technologies includes a description of the technology, applicability, advantages and limitations, level of development, and relative cost. The case study reports for the site demonstrations are provided in the other appendices.
... sediment residence time and transport in coastal and estuarine Systems (Dibb and Rice, 1989;Sommerfield et al., 1999;Taylor et al., 2013). Another highly particle-reactive element, , with Kd up to -105-106 (Guo et al., 1995;IAEA, 2004;Baskaran and Swarzenski, 2007), is produced from the decay of dissolved 238U and is commonly present in excess (ex) of its parent 238U in coastal suspended matter and bottom sediments (Aller and Cochran, 1976). 238U concentrations in rivers and oceans vary generally linearly with salinity (Skwarzec, 1995). ...
Be and ²³⁴Thex activities were determined in sediment cores off the Rhône River mouth (Gulf of Lions), in order to trace the initial transport and deposition of riverine suspended particulate matter (SPM) and evaluate the impact of flood events through 7 cruises carried out between 2007 and 2008. Consistently high ⁷Be and ²³⁴Thex inventories of 2000–3000 mBq cm⁻² and 3000–5000 mBq cm⁻², respectively, were observed within a ~5 km radius off the Rhône River mouth. Their spatial distributions showed a gradual decrease with increasing distance from the Rhône River mouth, and the decrease in ⁷Be was more pronounced than that of ²³⁴Thex, indicating that recent riverine SPM is rapidly deposited in the area located near the river mouth. This area is also characterized by high accumulation rates determined using ¹³⁷Cs or ²¹⁰Pbex. Both ⁷Be and ²³⁴Thex inventories increased in 2008 compared to 2007, and are correlated to the cumulated SPM flux for normal and flood discharge. Moreover, the ⁷Be/²³⁴Thex inventory ratio appears to be a potential tracer to identify the dominant influence of recently deposited particles between terrestrial and marine waters. This ratio provides an effective tool to assess river and marine influence: Zone I at a distance inferior to 3.0 km, with ⁷Be/²³⁴Thex inventory ratio over 0.50 (surface area near river mouth ~ 7 km²) is dominated by riverine influence; in contrast, Zone III at a distance superior to 8.5 km, with ⁷Be/²³⁴Thex inventory ratio less than 0.10 (surface area off river mouth beyond 150 km²) is predominantly under a marine influence. In between, an intermediate area (Zone II at a distance between 3.0 and 8.5 km, with ⁷Be/²³⁴Thex inventory ratios between 0.10 and 0.50) displays a mixed influence. This zoning could help in further understanding the spreading of particle-reactive contaminants and its initial sedimentary deposition in the Gulf of Lions.
... Modeling of fluvial inputs to the Sound suggests that about 45% of the total sediment supply has a source in the neighboring coastal ocean (Kim & Bokuniewicz, 1991), with the majority of muddy sediment accumulating in the western and central sound (Kim & Bokuniewicz, 1991;Latimer et al., 2014). The top few centimeters to decimeters of deposits in the sound are rapidly reworked by bioturbation and are underlain by a slower reworked layer down to 1-2 m (Aller & Cochran, 1976;Goldhaber et al., 1977;Benninger et al., 1979). Figure S1) in water column depths between 11 and 17 m. ...
Burial of biogenic silica (bSitotal) in high sedimentation rate continental margins remains highly uncertain. Cosmogenic 32Si (t1/2~140y) can be used to trace the fates of bSitotal post deposition, including as opal (bSiopal) and diagenetically altered opal (bSialtered); the latter dominantly authigenic clay (bSiclay). To determine the magnitude and form of bSitotal storage in coastal sediments, conventional operational leaches targeting bSiopal and bSialtered (including bSiclay) were modified for large scale samples necessary for measurement of 32Si. 32Si activity was used to estimate total biogenic silica burial (bSitotal = bSiopal+bSialtered) in several depositional settings: Gulf of Papua, Gulf of Mexico, Long Island Sound, and in the previously studied Amazon-Guianas deltaic system. In subtropical and temperate regions, 32Si was detected in both traditional biogenic silica leaches (bSiopal) and residual authigenic clays. Traditional bSiopal and modified operational leaches designed to target the most reactive authigenic silicates (~bSialtered) consistently underestimate authigenic clay formation (bSiclay) and thus the magnitude of bSitotal burial in temperate coastal zones and subtropical deltas by 2–4 fold. In tropical deltas, 32Si activities in the residua≳l fraction after removal of bSiopal demonstrate rapid and almost complete alteration of initial bSiopal to new forms, most likely bSiclay. Globally, 4.5–4.9 Tmol/y Si may be trapped in marine nearshore deposits as rapidly formed clay (bSiclay), ≳100% of the “missing silica sink” in the marine silica budget.
... If we assume that the long-term sedimentation rate obtained from the 14 C method is real, and the high sedimentation rate obtained using 210 Pb xs and 239,240 Pu are due to dynamic mixing, we can estimate the sediment mixing coefficient and compare it to values reported in other aquatic systems. Advective and diffusive mixing are often combined to produce a mixing coefficient that is analogues to eddy diffusion (Aller and Cochran, 1976;Nozaki et al., 1977;Guinasso and Schink, 1975;Officer, 1981;DeMaster and Cochran, 1982;Cliffton et al., 1995). In such cases, the vertical distribution of 210 Pb xs (A) is given by: ...
... Mixing rates in the MCZ, OZ, and AZ ranged from 0.7 (C9) to 9.6 (C10) cm 2 yr −1 for 234 Th ex -derived estimates and from 0.06 (C1) to 3.7 (C10) cm 2 yr −1 for 210 Pb ex -derived estimates (Table 2). At a given location, 234 Th ex -derived estimates were generally higher as they reflect processes occurring in the uppermost active sediment layers (Aller and Cochran, 1976). Rates in the SCZ and NCZ suggested intense mixing, with full core 210 Pb ex -derived estimates starting at 11 cm 2 yr −1 and the majority of rates being unquantifiable due to the vertical 210 Pb ex profiles. ...
Quantifying the amount of cesium incorporated into marine
sediments as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP)
accident has proven challenging due to the limited multi-core sampling from
within the 30 km zone around the facility; the inherent spatial
heterogeneities in ocean sediments; and the potential for inventory
fluctuations due to physical, biological, and chemical processes. Using
210Pb, 234Th, 137Cs, and 134Cs profiles from 20 sediment
cores, coastal sediment inventories were reevaluated. A 137Cs sediment
inventory of 100 ± 50 TBq was found for an area of 55 000 km2
using cores from this study and a total of 130 ± 60 TBq using an
additional 181 samples. These inventories represent less than 1% of the
estimated 15–30 PBq of cesium released during the FDNPP disaster. The time
needed for surface sediment activities (0 to 3 cm) at the 20 locations to be
reduced by 50% via sediment mixing was estimated to range from 0.4 to
26 yr. Due to the observed variability in mixing rates, grain size, and
inventories, additional cores are needed to improve these estimates and
capture the full extent of cesium penetration into the shallow coastal
sediments, which was deeper than 14 cm for all cores retrieved from water
depths less than 150 m.
... 234 Th and 210 Pb) are useful tracers to determine sediment budgets in coastal marine systems (Feng et al., 1999;Giffin and Corbett, 2003) because their well-characterized sources permit construction of mass balances and their strong association with particles permits them to be used to evaluate fluxes of sediment transport and redistribution. The short-lived radionuclide 234 Th (half-life ¼ 24.1 days) is useful in studying processes that operate over seasonal time scales (Aller and Cochran, 1976;Aller et al., 1980), whereas longer-lived 210 Pb (half-life ¼ 22.3 years) is more applicable for investigating processes on longer time-scales up to about 100 years (Appleby and Oldfield, 1992;Corbett et al., 2006Corbett et al., , 2007. Dominant sources of these radionuclides to coastal waters include in situ production from dissolved 238 U in the case of 234 Th and atmospheric input in the case of 210 Pb; both 234 Th and 210 Pb are rapidly scavenged onto particles and subsequently track the fate of the particles (Baskaran and Santchi, 1993;Baskaran and Swarzenski, 2007). ...
The sediment budget of Jamaica Bay (New York, USA) has been determined using the natural particle-reactive radionuclides 234Th and 210Pb. Inventories of excess thorium-234 (234Thxs, half-life = 24.1 d) were measured in bottom sediments of the Bay during four cruises from September 2004 to July 2006. The mean bay-wide inventory for the four sampling periods ranged from 3.5 to 5.0 dpm cm−2, four to six times that expected from 234Th production in the overlying water column. The presence of dissolved 234Th and a high specific activity of 234Thxs on particles at the bay inlet (∼30 dpm/g) indicated that both dissolved and particulate 234Th could be imported into the bay from the ocean. Based on these observations, a mass balance of 234Th yields an annual input of ∼39 ± 14 × 1010 g sediment into the bay. Mass accumulation rates determined from profiles of excess 210Pb (half-life = 22.3 y) in sediment cores require annual sediment import of 7.4 ± 4.5 × 1010 g. Both radionuclides indicate that there is considerable marine-derived sediment import to Jamaica Bay, consistent with earlier work using 210Pb. Such sediment input may be important in sustaining longer-term accretion rates of salt marshes in the bay.
... The mines in the Tamar catchment were worked for copper, arsenic, lead, manganese, silver, iron ore, tin, tungsten, fluorite, zinc, gold and barytes (Barton, 1967(Barton, , 1980Johns, 1986). In estuaries with high concentrations of dissolved organic matter (DOM), it has been shown the abundance of particle-reactive nucleotides of these metals such as, 7 Be and 210 Pb (Aller and Cochran, 1976;Benninger, 1978;Benninger et al., 1979;McKee et al., 1986;Moore, 1992;Santschi et al., 1979), tends to be relatively high (Benoit et al., 1994;Guentzel et al., 1996). ...
... According toRhoads (1967), biogenic reworking is the result of sed iment ingestion, manipulation (e.g. by tube construction) and displacement as the animal passes through the sediment or over the sediment surface. Aspects of particle reworking have been intensively studied by, for example, Rhoads (1974),Guinasso & Schink (1975),Aller & Cochran (1976), Yingst & Rhoads (1980) and J. N.Smith & Schafer (198 4). The use of appropriate particle tracers, such as radioactive isotopes (e.g. ...
Burrowing macrofauna may, by activities such as burrowing and feeding ('reworking'), redistribute organic matter deposited or produced at the surface layer of aquatic sediments. Infauna) ventilation of water through burrows is responsible for an active transport of solutes into and out of the sediment. The metabolism of benthic communities (measured as oxygen uptake and carbon dioxide release) is often increased 1.5-3.0 times by the presence of burrowing macrofauna. Most of this is due to a general stimulation of heterotrophic microorganisms in the sediment and only to a minor degree caused by macrofaunal respiration. Accordingly, high microbial and catalysing enzyme activity is usually associated with burrow structures and faecal casts. Both aerobic and anaerobic decomposition processes can be stimulated by bioturbating macrofauna. The oxic zone of coastal sediments is usually restricted to the upper few millimetres. Burrowing macrofauna, however, may expand the area of this zone significantly by burrow construction and ventilating activities. It is estimated that the amphipod Corophium volutator may increase the volume of oxic sediment by 150%. Such improved supply of oxygen may enhance the aerobic decomposition of organic matter. Sulphate reduction, which is the quantitatively most important anaerobic decomposition process in marine sediments, may be increased 3-5 times in the presence of burrowing animals. Nitrate reduction (denitrification) is similarly stimulated by bioturbation. Thus, nereid burrows are estimated to account for 50-80% of the bulk sediment denitrification rate. Ventilation-mediated influx of electron acceptors (sulphate and nitrate) into, and simultaneous removal of reduced metabolites (sulphide and ammonium) from, the sediment are thought to be responsible for the elevated metabolic activity. Remineralization and release of inorganic nitrogen in the sediment are important for the primary producers. The efflux of ammonium to the water column is often increased several times by bioturbating fauna.
... At Richardson Bay, mixing coefficients for surface sediment were estimated from 234 Thr 238 U Ž . 234 Ž disequilibria Aller and Cochran, 1976 . Th half-. ...
... This must be ascribed to a balance of decay and penetration of 234 Th ex into deeper sediment subsequent to its deposition over the seafloor. With a steady-state assumption, sediment mixing rates can be inferred from the depth profiles of 234 Th ex (e.g., Aller and Cochran, 1976). ...
The 224Ra/228Th disequilibrium that was recently observed in coastal sediments has been proven to be an excellent proxy for tracing the benthic processes that regulate solute transfer across the sediment–water interface. In order to better utilize this proxy, there is a need to understand the reaction kinetics of 224Ra in sediments. In this study, depth profiles of 224Ra and 228Th in bulk sediments were collected along a transect in the Pearl River Estuary (PRE). Together with bulk sediment measurements, dissolved 224Ra, dissolved inorganic carbon (DIC), and nutrients (NO2− + NO3−, NH4+) in pore water and in the overlying waters were also determined. A marked deficit of 224Ra with respect to 228Th with large spatial variations was observed in the PRE sediments. By use of a diagenetic model for the distributions of dissolved and adsorbed 224Ra in sediments, we infer that adsorption removes 224Ra from aqueous phase at a rate of 0.1 ± 1.1–2000 ± 400 d−1. In addition, adsorption of 224Ra exhibits a rate sequence of oxic freshwater > anoxic freshwater > anoxic brackish water, probably reflecting the effect of the redox conditions and ionic strength on the adsorption–desorption kinetics of 224Ra.
The Hudson River Estuary is a comprehensive look at the physical, chemical, biological and environmental management issues that are important to our understanding of the Hudson River. Chapters cover the entire range of fields necessary to understanding the workings of the Hudson River estuary; the physics, bedrock geological setting and sedimentological processes of the estuary; ecosystem-level processes and biological interactions; and environmental issues such as fisheries, toxic substances, and the effect of nutrient input from densely populated areas. This 2006 book places special emphasis on important issues specific to the Hudson, such as the effect of power plants and high concentrations of PCBs. The chapters are written by specialists at a level that is accessible to students, teachers and the interested layperson. The Hudson River Estuary is a fascinating scientific biography of a major estuary, with relevance to the study of any similar natural system in the world.
The Hudson River Estuary is a comprehensive look at the physical, chemical, biological and environmental management issues that are important to our understanding of the Hudson River. Chapters cover the entire range of fields necessary to understanding the workings of the Hudson River estuary; the physics, bedrock geological setting and sedimentological processes of the estuary; ecosystem-level processes and biological interactions; and environmental issues such as fisheries, toxic substances, and the effect of nutrient input from densely populated areas. This 2006 book places special emphasis on important issues specific to the Hudson, such as the effect of power plants and high concentrations of PCBs. The chapters are written by specialists at a level that is accessible to students, teachers and the interested layperson. The Hudson River Estuary is a fascinating scientific biography of a major estuary, with relevance to the study of any similar natural system in the world.
Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel ²²⁴Ra/²²⁸Th disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4⁺ were relatively low, ranging from 6 to 59 mmol O2 m⁻² d⁻¹ and from 1.6 to 13 mmol N m⁻² d⁻¹, respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4⁺ surged to 44-690 mmol O2 m⁻² d⁻¹ and 22-58 mmol N m⁻² d⁻¹, respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment-water interaction. Importantly, sediment oxygen consumption rates (FO2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that FO2and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas.
Particle dynamics play significant roles in sedimentological and geomorphological processes as well as biogeochemical cycles in estuaries. We collected a suite of water samples from the North Passage of the Changjiang (Yangtze) Estuary during different tidal conditions in summer 2009 to observe variations in the particulate ²³⁴Th, ⁷Be, ²¹⁰Pb, and ¹³⁷Cs activities as well as in particle transport, deposition, and resuspension. The mean activities of ⁷Be and ²¹⁰Pbex in surface water during low tide were lower than those during high tide. The residence times of ⁷Be (1.1–3.4 d), ²³⁴Th (4.7–28 d), and ²¹⁰Pb (7.9–34 d) in the North Passage increased with the increase of salinity and suspended particulate matter. The estimated results, based on a two‐dimensional model of ⁷Be, ²³⁴Th, and ²¹⁰Pb, showed that sedimentary particles in the North Passage experienced net deposition during neap tides whereas those in more seaward areas of the North Passage exhibited net resuspension during spring tides. The ⁷Be/²¹⁰Pbex activity ratio in seaward area of the North Passage was lower during spring tide than during neap tide but both were close to values in surface sediments beyond the estuary. Using ⁷Be/²¹⁰Pbex and ¹³⁷Cs/²¹⁰Pbex activity ratios as tracers, we deduced that most of particles in the North Passage were not derived from the direct deposition of Changjiang input particles but were transported from sediment deposited offshore. These results emphasized the important contributions of offshore inputs to sediments in this channel, which should be considered when conducting channel dredging of other estuarine systems.
Lead-210 is one of the natural radionuclides belonging to the uranium series and has a halflife of 22.3 y. It has been used to measure sedimentation rates of lake and marine sediments. The calculations of ages are achieved on the basis of CRS model or CIC model depending on the source of Pb-210 fluxes. Surface sediments are disturbed and mixed by biological (bioturbation) and physical turbulence, and this process is incorporated into the model as a diffusion equation. The sedimentation process and mixing process cannot be distinguished by Pb-210 profiles, so much attention should be paid to the observation of cores. The profiles of other nuclides, anthropogenic elements and organics would help better understanding. Computers are often used for calculations and effective models are required for the estimation of the ancient sedimentary environment.
Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed ²²⁴Ra/²²⁸Th disequilibrium approach to quantify PEX fluxes of ²²⁴Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column ²²⁴Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of ²²⁴Ra in the sediment column, we demonstrated that PEX fluxes of ²²⁴Ra were highly variable, both temporally and spatially, and can change by 1–2 orders of magnitude in our study area. Moreover, we identified a strong correlation between ²²⁴Ra-based irrigation rate and ²³⁴Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment-water interface. Total PEX flux of ²²⁴Ra (in 10¹⁰ dpm d⁻¹) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of ²²⁴Ra (in 10¹⁰ dpm d⁻¹) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of ²²⁴Ra by the ratio of the concentration gradients of component/²²⁴Ra at the sediment-water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH4⁺, NO3⁻, and H4SiO4) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of ²²⁴Ra by the DIC (nutrients)/²²⁴Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO3⁻ in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.
Variability of bioturbation on different spatial scales was revealed through a survey at 6 stations in the southwestern Baltic Sea with different sediment types, salinities and macrozoobenthic communities. At each station, 6 sampling locations were investigated with 4 cores each (24 cores per station). The cores were analyzed for vertical chlorophyll (chl) profiles, which were modeled with both a local (tracer distribution decreasing exponentially with depth indicative of diffusive transport, DB) and a non-local (presence of subsurface maximum of the tracer, injection flux J and ingestion rate r) mixing model developed by Soetaert et al. (1996; J Mar Res 54: 1207-1227). Degradation of chl was determined experimentally by an incubation of fresh sediment under anoxic, dark conditions and provided decay constants kD of 0.01 d⁻¹ for mud and 0.02 d⁻¹ for sand. Mixing depths reach 7.1 ± 1.6 cm at stations in the west (except Lübeck Bay, LB), 2 cm deeper than at stations in the east, which reach 5.2 ± 1.7 cm (in cluding LB), mainly depending on the macrozoobenthic community present. Bioturbation intensities indicate high variability between closely located sampling sites as well as across the southern Baltic Sea, and depend on the food supply from the water column. Stations indicate a difference in local mixing (DB) of a factor of 20 and in non-local processes (J) of 6. Non-local transports account for 33 to 50% of the investigated area in the west and for 70 to 100% in the east. The statistical description of the results indicates the necessity of high sampling effort when using chl as a particle tracer.
Uranium (U) concentrations and isotopic compositions were measured in filtered water samples from the upper Rhône River and Lake Geneva. Uranium concentrations are generally high and variable in the Rhône (0.75–8.9 μg/l). The 234U/238U activity ratio (AU ratio) is about 1. The high U content and low AU ratio are probably related to the presence of mineralized zones and U-enriched granites in the Rhône watershed. Uranium from phosphate fertilizers contributes only a small part of the total U load to the Rhône, but it may increase the U concentrations seasonally. Uranium concentrations in Lake Geneva are nearly constant in time and space. The possible application of 234Th to study sedimentologic and geochemical processes in the lake is suggested.
Two naturally-occurring uranium isotopes (235U,238U) and one thorium isotope (232Th) occur in nature and are unusual in two respects. The first is that these isotopes are naturally radioactive, decaying mainly by alpha emission via a whole chain of daughter isotopes, the ultimate stable end products being 207Pb, 206Pb and 208Pb respectively. The second unusual feature is that during neutron irradiation, they undergo nuclear fission, 235U predominantly in a thermal neutron flux and 238U, 232Th in a fast neutron flux. These properties have led to the development of a number of techniques specific for uranium and thorium, including the following.
A central goal of ecotoxicology is the development of sufficient ecological understanding to enable the accurate prediction of the behaviour and effects of contaminants in the environment. Progress toward this goal has been slow. We believe future advancement requires increased emphasis on interdisciplinary studies which comprehensively investigate chemical fate and effect at spatial and temporal scales relevant to the natural systems of interest (Forbes and Forbes, 1994). Building a bridge from the laboratory to the field will require improved extrapolation models incorporating a more complete understanding of the relationship between ecosystem structure and function than presently exists (Forbes and Forbes, 1993)
Water samples were collected in a detailed depth profile (32 samples) of Framvaren Fjord (water depth: 183 m) and examined for dissolved (< 0.22 µm) and particulate (> 0.22 µm) phase 234Th (t1/2 = 24.1 days) concentrations. The particulate thorium fraction is relatively constant (50–60% of total) in the oxic surface waters of Framvaren Fjord. Below the oxic-anoxic interface (approximately 18 m), the particulate thorium fraction increases dramatically, reaching a maximum (96%) at 24 m, coincident with the maximum in total suspended matter concentrations (TSM). In this environment, the incorporation of thorium into the particulate phase seems to be more closely tied to the presence of bacteria than to the formation of particulate Fe and Mn oxyhydroxide phases. Distribution coefficients (Kd) for thorium in surface waters are 106–107 L/kg, typical for low particulate marine environments. Below the TSM maxima, Kd values are much lower (104–105 L/kg) and remain consistently low in the anoxic bottom waters. These are among the lowest Kd values for thorium ever observed for a marine environment. Equilibrium and kinetic approaches to quantifying the flux of thorium into the particulate phase indicate that, in the anoxic bottom waters of Framvaren Fjord, thorium sorption reactions become less dominant relative to reverse reactions such as desorption and particulate remineralization.
An experimental method has been developed to study material and radioactivity fluxes at the sediment-water interface and in the sedimentary column. This method has been applied in the Gulf of Fos, an area that is affected by deposits from the river Rhone, and where biodeposition products, resulting from the presence of intensive mussel cultures, can induce a concentration of trace elements at the sediment-water interface.
Sediment surface materials were labelled with a mixture of radionuclides (Cerium-144, Cobalt-60 and Cesium-137), in experimental cores filled either with sediment containing in situ fauna or with defaunated sediment.
The coupling of this mixture with luminophores — inert colored sediment particles — enabled us to measure radionuclide flux in both solute fraction and solid fraction. At the sediment-water interface, the tracer balance indicates that migrations into deeper sediment are estimated to be until 25 times greater in presence of macrofauna, depending on the tracer examined. Bioturbation may equally enhance exportation to the water column, to a factor ranging from 1.5 to 2.0. During a period of 14 days, in presence of macrofauna, we observed a migration of radionuclides to a maximum depth of 11 cm. A similar distribution pattern of luminophores at the same sediment depths indicates the preponderance of particle reworking in migration.
By studying sediment depth profiles of radioisotopes having different sources and half-lives we differentiate between steady-state accretion and mixing. The effect of 7Be and 137Cs source variability on the sediment-depth profiles of these isotopes is discussed. Mixing rates throughout any estuarine sediment deposit cannot be assumed to be constant. At sites identified as ‘mixed’ we investigate different mixing components and boundary conditions and in the upper estuary, we quantify bulk sediment movement.
The complexity of Long Island Sound’s physical oceanography, coupled with its natural geology, and its history of anthropogenic inputs exerts strong controls on the chemistry of both the water column and sediments of Long Island Sound (LIS). These, in turn, affect and are further affected by the organisms that live in these environments.
The natural radionuclides Th-231 and Be-7 have known sources in estuaries and strongly and rapidly associate with particles. They are thus good tracers of the transport and sources of particles in the suspended sediment reservoir, and, by implication, of particle-associated contaminants such as heavy metals. In the Hudson River estuary, Th-234 is produced from decay of dissolved U-238, which varies linearly with salinity, while Be-7 is added directly to the estuary from the atmosphere. The Th-234/ Be-7 activity ratio eliminates variations in radionuclide specific activities that result from changes in sediment grain size or composition. Sampling in the Hudson under conditions of low and high river flow shows that Th-234/Be-7 in suspended sediment increases from low salinity to higher salinity waters at all sampling times. The Th/Be activity ratio may thus be used as a tracer of where particles are labeled with these radionuclides in the estuary and of the relative importance of resuspension and particle transport through the estuary. The:distribution of heavy metals in the suspended sediment reservoir can be compared with that of Th-234/ Be-7 by normalizing the metals to Fe. Iron and Al are well correlated in suspended sediments of the Hudson, indicating that either element can be used to normalize grain size effects on specific concentrations of metals. Ratios of Ag and Pb to Fe in suspended particles generally increase down-estuary at all sampling times, whereas Cu, Zn and Cd-to-Fe ratios-show more scatter and less consistency in trends. Both source(s) and chemical behavior affect the metal/Fe ratios. The strongest positive correlation with Th-234/ Be-7 is seen for Ag/Fe, reflecting the dominance of lower estuary sources of this trace metal to the Hudson. Relationships between Pb/Fe, Cu/Fe and Th-234/ B-7 are less strong, reflecting multiple sources of these elements. No correlation is seen for Cd/Fe with (234)Tn/Be-7, indicating either a source up-river or release of Cd as its K-d decreases with increasing salinity.
The Hudson River Estuary is a comprehensive look at the physical, chemical, biological and environmental management issues that are important to our understanding of the Hudson River. Chapters cover the entire range of fields necessary to understanding the workings of the Hudson River estuary; the physics, bedrock geological setting and sedimentological processes of the estuary; ecosystem-level processes and biological interactions; and environmental issues such as fisheries, toxic substances, and the effect of nutrient input from densely populated areas. This 2006 book places special emphasis on important issues specific to the Hudson, such as the effect of power plants and high concentrations of PCBs. The chapters are written by specialists at a level that is accessible to students, teachers and the interested layperson. The Hudson River Estuary is a fascinating scientific biography of a major estuary, with relevance to the study of any similar natural system in the world.
In this chapter the effects of macrobenthos on chemical changes in sediments occurring during and after burial (i.e., chemical diagenesis) will be considered. The importance of sediments to the biogeochemical cycling of materials is well known (Mortimer, 1941, 1942, 1971; Lee, 1970). Freshwater sediments act as both a source and a sink for biologically important materials such as phosphorus, carbon, nitrogen, sulfur, and silicon. Furthermore, sediments are known to play an active role in regulating cycles of trace metals, radionuclides, and xenobiotics (Jones and Bowser, 1978). Because of this, knowledge of the chemical diagenesis of sediments is essential to an understanding of materials cycling in freshwater environments.
The purpose of this chapter is to examine existing mathematical models of important chemical, physical, and biological effects of organisms on sediments. The objectives and nature of the models will be discussed, the mathematical solution techniques will be identified, and the advantages and disadvantages of each type of model will highlighted.
: The construction and irrigation of permanent tube-dwellings by sedentary infaunal benthos result in complex patterns of chemical reactions and diffusion gradients in sedimentary deposits. These patterns are reflected in the three-dimensional distribution of pore water constituents, diagenetically mobile solid phases, biogeochemically important microorganisms, and meiobenthos. The quantitative effect of burrows on transport-reaction processes in sediments is analogous to the effect of roots on terrestrial soils. In contrast to factors which cause heterogeneity, mobile infauna rapidly homogenize sediment and presumably simplify internal gradients. The activities of both mobile and sedentary benthos increase the rates of certain metabolic reactions, such as sulfate reduction, which take place below the sediment-water interface. These same activities increase the flux of dissolved material between sediment and the overlying water to a greater degree than that accounted for by one-dimensional molecular diffusion. Increased exchange rates may obscure the presence of some reactions in sediments by preventing the depletion of reactants or buildup of reaction products. Because biogenic influences on particle and fluid transport in sedimentary deposits reflect the types of organisms present, bottom areas which are inhabited by macrobenthos of varying mobility and feeding type differ in their chemical characteristics. In environments having a large seasonal temperature range, biogenic control of sedimentary chemistry is replaced by abiogenic reaction-diffusion controls during winter periods.
During the production of crude oil, condensates, or natural gas, water that is brought to the surface with the product stream is removed. The separated produced water (or oilfield brine) is reinjected down a well, either for disposal or enhanced recovery of hydrocarbons, or as is the case of the vast majority of production from the northern Gulf of Mexico region, is discharged into surface waters of the ocean or coastal areas.
Radioactive nuclides present in the environment have proved very valuable for the introduction of time parameter in a variety of earth science problems. Their utility in the field of oceanography, meteorology, hydrology, and related fields has been well documented (Eriksson, 1962; Lal, 1963; Sheppard, 1963; Broecker, 1963, 1965; Lal and Suess, 1968; Lal, 1969; Machta, 1974; Fitch et al., 1974; Goldberg and Bruland, 1974; Oeschger and Gugelmann, 1974; Burton, 1976; Ku, 1976; Turekian and Cochran, 1977). The application of radionuclides for deciphering the time element in limnological processes is still in its infancy The past decade has witnessed a surge of activity in this field, especially in limnochronology and gas exchange across the water-air interface. In this chapter we will discuss the application of radionuclides for determining the chronology of sediments and other deposits forming from contemporary lakes and those collected from glacial and “fossil” lakes. The sedimentary record of contemporary lakes contains information on the recent past of the environment, whereas those from the glacial and “fossil” lakes can be used to reconstruct the long-term history of the earth’s past, especially its climatic changes.
This paper summarizes the results of detailed field studies of C,N,P and S cycling in the anoxic sediments of a small coastal lagoon, Cape Lookout Bight (CLB), located on the Outer Banks barrier island chain of North Carolina (U.S.A.). These results are utilized to illustrate the efficiency of the principal processes controlling the biogeochemical cycling of each element, including factors leading to their differential release to overlying waters and preservation via burial. The Cape Lookout site is particularly well suited for studies of elemental cycling in organic-rich sediments because of the steady-state nature of annual sediment accumulation occurring since the mid-1960’s and the relative lack of physical disturbances via bioturbation or resuspension of the sediment column. Comparison of Cape Lookout results with those from other sedimentary environments leads to important insights concerning how the biogeochemical cycles of C, N, P and S interact.
The qualitative and quantitative evaluation of bioturbation is important in understanding the impact of the latter on marine ecosystems. Luminophores, colored sand grains, visible under UV light, were used as tracers for a bioturbation study in the Baltic Sea. Distribution of luminophores added to sediment cores was followed over a period of one month, the cores being kept in a laboratory temperature bath in order to evaluate particle bioturbation rates. Reconstituted cores, populated artificially with single animal species, were also used to identify important qualitative differences in sediment displacement by a deposit feeding bivalve (Macoma calcarea) and a carnivorous wandering polychaete (Nephtys caeca). Use of luminophores as tracers appears to be an effective technique, very simple in principle, whose applications could be greatly extended if an automated method for counting of tracers could be developed.
Continental margins are valuable for many reasons, including the rich record of Earth history that they contain. A comprehensive understanding about the fate of fluvial sediment requires knowledge that transcends time-scales ranging from particle transport to deep burial. Insights are presented for margins in general, with a focus on a tectonically active margin (northern California) and a passive margin (New Jersey). Formation of continental-margin strata begins with sediment delivery to the seabed. Physical and biological reworking alters this sediment before it is preserved by burial, and has an impact upon its dispersal to more distal locations. The seabed develops strength as it consolidates, but failure can occur and lead to sediment redistribution through high-concentration gravity flows. Processes ranging from sediment delivery to gravity flows create morphological features that give shape to continental-margin surfaces. With burial, these surfaces may become seismic reflectors, which are observed in the subsurface as stratigraphy and are used to interpret the history of formative processes. Observations document sedimentary processes and strata on a particular margin, but numerical models and laboratory experimentation are necessary to provide a quantitative basis for extrapolation of these processes and strata in time and space.
The effects of benthic organisms on the physical properties of granular substrata are well documented. The range of effects has been presented in H. B. Moore (1931, 1939), Schwartz (1932), Dapples (1942), D. G. Moore and Scruton (1957), McMaster (1967), Rhoads (1974), Rowe (1974), Powell (1974), Richards and Park (1976), Myers (1977a,b), Self and Jumars (1978), Lee and Swartz (1980), and Carney (1981). These papers relate the effects of benthic species to changes in grain size, sorting, fabric, water content, compaction, shear strength, and bottom stability. Those autecologic parameters that appear to be most highly correlated with physical modifications of sediments include: method of feeding, feeding selectivity, feeding level relative to the sediment—water interface, degree of mobility, organism size and population density, burrowing depth, and, if the organism is a tube dweller, the density, spacing, and length of tubes.
This contribution provides an overview of the use of radionuclides in aquatic sciences to study trace element-suspended particle and sediment-water interactions in the ocean and in lakes. Radionuclide involvement in geochemical processes is evaluated. The critical processes include the role of plankton and colloids in the vertical transport of stable and radioactive trace elements, the migration of radionuclides to and from horizontal boundaries, and the remobilization of solutes and particles from sediments. The matching of the time scale of the geochemical process to be studied by the radioactive half-life of the nuclide or by the other geophysical processes involved (such as the mixing or flow of water or the movement of particles) is of primary importance. The use of multiple tracers with either different radioactive decay times in the field, or with different chemical properties in experimental systems, is a very promising holistic approach that may constrain the effect of interrelated processes. These approaches are applicable to many current problems in limnological and oceanographic research.
The introduction provides a brief overview of the publication. The chapter brings together the current status of applications of U- and Th-series nuclides in studies of aquatic systems. It presents a summary of the broad applications of U- and Th-series nuclides to this field, particularly major advances brought about by new concepts and developments in measurement, and modeling during the past 20 years. The studies of U and Th minerals led to the discovery of radioactivity. Within a couple of decades of the discovery of radioactivity, many of the members of the U- and Th-decay series had been isolated from minerals and their decay properties established. This led to the finding that the U and Th series were made of a number of radioactive isotopes of many elements with widely different chemical properties and half-lives. In addition to differences in chemical properties, the very phenomenon of radioactive decay can also contribute to radioactive disequilibrium among U- and Th-series nuclides. The application of U- and Th-series nuclides to investigate aqueous processes began nearly a century ago, soon after the discovery of radioactivity. The discovery of 234Th–238U and 228Th–228Ra disequilibrium in surface waters and 210Pb–226Ra in the deep sea has provided a better understanding of scavenging processes in the ocean and their time-scales. These findings also opened new areas of research on trace element scavenging and on the use of 234Th and 210Pb as proxies to determine fluxes of biogeochemically important elements through the water column. Studies of the applications of U- and Th-series nuclides in the atmosphere and freshwater systems have also progressed considerably during this period.
It is perhaps ironic that the editors of this book would emphasize in their own contribution that certain patterns of biotic interactions found in modern soft-bottom communities are not likely to be preserved in the fossil record and that the interpretation of some preserved patterns is problematical. But this is our conclusion with respect to successions on soft bottoms. After examining both live and dead shelled faunas of nearshore clastic facies, we also conclude that the areal distribution of fossil species cannot be used to establish with certainty the dominant controls of the distribution of living fauna. But these conclusions are provisional, and more subtle and clever analysis may eventually vitiate our pessimism and better explain the causes for some distributional patterns.
In temperate marine environments the reworking of muddy sediments by deposit-feeding organisms may result in an unstable bottom whose properties are not suitable for suspension feeders. This kind of exclusion or inhibition of one trophic group as a consequence of the activities of another is termed trophic group amensalism. The amensalism principle has recently been applied to the distribution of tropical benthos in a Florida estuary. In this study, the authors investigated the tropical benthos of a typical back-reef lagoon, Discovery Bay, Jamaica. Emphasis was placed on the different animal-sediment relations, the stability of bottom sediments and the possibility of amensalistic interaction between different groups of bottom organisms. The results of this study and their implications for the reef community are discussed here, and are compared with examples from the temperate benthic environment. Refs.
Two sampling stations were established on the top and edge of a newly
dumped dredge spoil deposit in 12 and 18 meters of water in central Long Island
Sound. These stations were sampled periodically from June 1974 to April, 1975.
The spoil pile represents over 10$sup 6$M$sup 3$ of organic-rich sand, silt, and
clay, dumped between October 1973 and April 1974. A control station was
established 5.5 km NW from the dump in 14 meters of water. The colonization of
the dump surface may be divided into three stages: Stage I (June-July) represents
initial recruitment of shallow burrowing surface deposit feeders, suspension
feeders, and meiofauna. Stage II (August-November) is a phase of exponential
recruitment of stage I populations and new recruitment of deeper-feeding infauna.
Stage III (December-April) is a period of leveling-off in population densities.
The control station shows relative constant standing crops of deep-feeding
deposit feeders over the sampling period. Late stage II and stage III abundances
and diversities on the dump exceed those at the control station. Habitat
modification related to the three colonization stages are: Stage I--Fecal pellet
production starts and the redox potential discontinuity (RPD) is depressed to
about one centimeter by feeding, bioturbation, and respiration activities. Stage
II--The surficial layer of pellets experiences some destruction by meiofaunal
grazing. The RPD is depressed to 2-3 cm and pore water profiles in SO$sub 4$$sup
2-$ and NH$sub 4$ approach constant values to a depth of 3-6 cm. Sediment water
content increases to 2-3 cm. The seafloor is bound by microbial exudates. Stage
III - The pelletal surface decays, the RPD rebounds to a depth of 1-2 cm. Pore
water profiles are diffusion-altered relicts of late summer irrigation activity.
Microbial binding decreases. Several hypotheses are stated regarding the
potential importance of these habitat modifications to the colonization sequence.
(auth)
Biological mixing in deep-sea sediments is described in terms of a time-dependent eddy diffusion model where mixing takes place to a depth L at constant eddy diffusivity D. The differential equation that describes this model has been solved for an impulse source of tracer delivered to the plane surface that forms the top of the mixed layer. The solution then serves as a Green's function, which can be used to determine the distribution of tracer in depth and in time for a surface input of tracer specified as any arbitrary function of time. The characteristic properties of the solution are dependent on the dimensionless parameter D/Lnu, where nu is the sedimentation rate. If D/Lnu is greater than 10, the surface layer becomes homogeneous, and the model is identical to the homogeneous layer model proposed by Berger and Heath (1968). If D/Lnu is less than 0.1, little mixing can take place before the sediments are buried, and so the surface concentration propagates downward into the sediments with little dispersion. For all values of D/Lnu the weighted mean depth of the concentration distribution is the depth at which an impuse source would be found in the sediment if no mixing had taken place. The microtekite data of Glass (1969, 1972) and Glass et al. (1973) indicate that abyssal sediments are mixed from the surface to a maximum mixing depth that ranges between 17 and 40 cm below the surface. Mixing occurs at rates between 1 and 100 cm2 kyr-1. Higher mixing rates may occur nearer the surface, but microtekite distributions cannot be used to estimate these rates in the presence of the deeper, slower mixing. Estimates for D based on dimensional analysis of sediment reworking rates for nearshore organisms (103-106 cm2 kyr-1) are used to predict abyssal mixing rates between 1 and 103 cm2 kyr-1 by invoking the assumption that mixing is proportional to biomass. Plutonium distributions in deep-sea sediments (Nshkin and Bowen, 1973) indicate abyssal mixing rates ranging from 100 to 400 cm2 kyr-1.
Deposit-feeding and suspension-feeding benthos in Buzzards Bay, Massachusetts, show marked spatial separation; suspension feeders are largely confined to sandy or firm mud bottoms while deposit feeders attain high densities on soft muddy substrata. Food source and bottom stability have been investigated as potential factors effecting this trophic-group separation. Between October 4, 1967 and August 22, 1969, observations were made at 11 stations in Buzzards Bay, Massachusetts, along two widely separated transects over bottoms ranging in texture from silt to fine and medium sand. Water depths at these stations ranged from 3 m to 20 m. Scuba divers made many of the field observations and collected most of the samples. This study included sampling of benthic macrofauna, taking bottom photographs, analyzing sedimentary structures, texture, organic content and water content of the sediments, and measuring both water currents and suspended sediment above the bottom. Laboratory experiments were also carried out to determine differential resuspension between burrowed and unburrowed muds. Intensive reworking of the upper few centimeters of a mud bottom by deposit feeders produces a fluid fecal-rich surface that is easily resuspended by low-velocity tidal currents. We suggest that the physical instability of this fecal surface tends to: (i) clog the filtering structures of suspension-feeding organisms, (ii) bury newly settled larvae or discourage the settling of suspension-feeding larvae, and (iii) prevent sessile epifauna from attaching to an unstable mud bottom. Thus suspension feeders are unable to successfully populate all areas of the bottom where a suspended food source is available, especially in areas where mud bottoms are intensively reworked by deposit feeders. Modification of the benthic environment by deposit feeders, resulting in the exclusion of many suspension feeders and sessile epifauna, is an example of trophic group amensalism. This biotic relationship appears to be important in shaping trophic-group distributions in embayments and basins on continental shelves.
The naturally occurring radioactive isotope of lead, Pb-210, provides a valuable tracer for the behavior of heavy metals in the soil-stream-estuary system. Since it is continuously produced only as a member of the U-238 decay series, it is free from the problems of environmental or analytical contamination so often encountered in stable heavy metal studies. In addition, because of its half-life of about 22 years it is useful not only as a tracer but also as a dating tool to monitor events of the past 100 years in various repositories.
IN the Cambrian black shales of the Oslo area, there is a considerable concentration of uranium, analyses values ranging from 25 to 180 gm./ton. (These and the following uranium analyses were made at the Norwegian Defence Research Institution, which has given permission for their publication.)
The activity of the short-lived 234Th (half-life = 24 days) is found to be in marked radioactive disequilibrium with respect to the concentration of its parent. 238U in the top hundred meters of the sea water column; values above and below secular equilibrium exist. The mechanisms responsible for the observed distribution of 234Th are discussed and their rates are evaluated.
Yoldia limatula is a deposit-feeding pelecypod. In Buzzards Bay, Massachusetts, and Long Island Sound it comprises less than 10 percent (by number) of the total bottom fauna, yet is probably capable of entirely reworking the sediment. Yoldia reworks the sediment by ejecting it several centimeters into the water from its excurrent siphon. Measurements of hourly reworking rates were made under controlled temperature conditions. Extrapolations of yearly reworking rates were determined to be 5-6 liters per square meter per year in Buzzards Bay and 23-51 liters per square meter per year in Long Island Sound. This study suggests that great numbers of organisms may not be required to account for extensive sediment reworking.
From the data of 25 deep-sea cores it is possible to show that the U/Th ratio of sediments is nearly constant at a low CaCO3-content. This result and reflections about some other points have led to the assumption that U and Th are being transported into the sediment through the intermediary of minerals. During the transport U (preferably 234U), Ra, and some Th are leached by ocean water. This model provides for a possibility to interpret the activity ratio 234U/238U < 1 in deep-sea sediments with low carbonate content as well as the activity ratio 228Th/232Th ~ 15 in ocean water. Likewise a part of the 226Ra excess (compared with the Io in solution) may be caused by that leaching.
Sediments from five rivers in the eastern and southern United States have been divided into several size fractions and analyzed for thorium and uranium isotope concentrations. The238U decay series exhibits disequilibrium as a result of weathering. The234U/238U activity ratio is less than unity in sediment derived from well-leached soils and greater than unity in sediment containing relatively large amounts of organic material from organic-rich surface layers of soil profiles.230Th exists in excess of equilibrium amounts relative to its parent234U in the fine fraction of most of the sediments analyzed. The thorium concentration and230Th excess are significantly higher in the 2-0.2μ and less than 0.2μ size fractions than in the 2–20μ fractions, indicating adsorption by clays, complexing with sesquioxides, or possible concentration in resistates. Unsupported230Th is being added to deep-sea sediments at a maximum rate of 0.5 dpm/cm2/1000 y; therefore, detrital material is not a significant source of excess230Th in deep-sea sediments.
Translation of Uran-234 by J. Sehmorak. The following subjects are
discussed: /sup 234/U and other natural radioactive isotopes, fractionation of
heavy radioactive elements in nature, fractionation of radioactive isotopes, /sup
234/U in nuclear geochemistry, /sup 234/U in uranium minerals, /sup 234/U in
continental waters and in quaternary deposits, and /sup 234/U in the ocean.
(LK)
The234Th/238U activity ratios in the near-bottom waters at a station in the South Pacific have been measured. The activity ratios are close to the secular equilibrium value, ranging between 0.9 and 1.13 (± 8%), suggesting that the rate of removal of234Th by bottom-water scavenging processes at this station is slow compared to its rate of radioactive decay. The mean234U/238U activity ratios in these waters is 1.14 ± 0.02, the same as the reported values for the world oceans.
Organic-rich sediments and coexisting phosphorites from the continental shelf off South West Africa have been analysed for uranium and thiorum by alpha-spectrometry. The uranium concentrations in the sediments range from 10 to 55 ppm, with an isotopic composition close to that of sea-water, indicating that uranium is passing into the sediments at the present time. The phosphorites occur in the sediments as thin unconsolidated laminae and as lithified nodules and pellets, with uranium contents ranging from 79 to 158 ppm. Based on the uranium isotopic composition, only the unconsolidated phosphorite laminae are recent, while the lithified nodules and pellets, with 234U/238U and 230Th/234U ratios close to radio-active equilibrium, appear to be inherited from a previous period of phosphorite deposition. Deposition of uranium appears to take place predominantly by incorporation into carbonate fluorapatite growing authigenically within the sediment. Uranium accumulation rates, computed from 14C-dated sections of the sediment cores, and using only uranium values with modern isotopic composition, range from 232 to 765 μg/cm2 per 1,000 years. These results stress the importance of organic-rich sediments containing authigenic phosphorite beneath areas of high organic productivity as a major sink for uranium in the ocean.
Deep sea hemipelagic sediments from the east Pacific show an oxidized upper zone of variable thickness and a reduced zone below. The geochemistry of a core from this region was studied in detail. The Eh of the upper zone is close to + 100 mV, while in the lower zone it reaches about −400 mV. Reducing conditions in the lower section of the core are related to the oxidation of organic matter in the sediment, as indicated by the gradual decrease with depth of the concentration of nitrogen in the upper part of the core. Various elements, such as Mn, Ni, Co, P and La, are enriched in the upper oxidized zone, while Cr, V, U and S are enriched in the lower reduced zone. Postdepositional mobility of the elements in question, mainly by diffusion in the interstitial solutions, can explain their distribution in the core. Redox reactions can account directly for the mobilization of Mn, Ni, Co, Cr, V and U, indirectly for that of P and La. Fe and Cu do not migrate significantly, since they are immobilized as sulfides in the reduced zone. Th appears not to be affected by diagenetical mobility. Some of the consequences of the Postdepositional mobility of elements in deep sea sediments are: 1.(1) The average elemental content of deep sea sediment, generally estimated from measurements of samples from the upper part of the sediment column, may be affected by errors; namely, an overestimation in the case of Mn, Ni, Co, P and La; an underestimation in the case of V, Cr and U.2.(2) Postdepositional redistribution of U may introduce errors in age determinations of sediments by the and methods.3.(3) Some geographical variations in the composition of Fe-Mn nodules can be explained by postdepositional mobility of Mn, Ni and Co.
Acoustic reflection profiles and bottom sampling are used to measure the volume of sediments accumulated in Long Island Sound. There is present 1.0 × 1010m3 of sediment of which 5.3 × 109m3 is marine mud and 4.9 × 108m3 is probably of pre-marine, lacustrine origin. The balance consists of reworked sand derived from glacial drift. The acoustically determined sub-bottom structure of the Sound and available sea level data indicate that the Sound basin was occupied by a large lake for at least 6000 years and has been an arm of the sea since 8000 years b.p. The volume of lacustrine sediment is accounted for by direct riverine input over 6000 years but the volume of marine mud present substantially exceeds the riverine supply over 8000 years. The Sound is shown to act as a trap for sediments originating on the continental shelf.
The activity ratio of228Th/228Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of228Ra in surface sea water this result provides a basis for the prediction of the distribution of “reactive” pollutants released to the surface sea from coastal areas.
Uranium concentrations in a large number of marine sediment samples of different types with world-wide spatial distribution have been determined using the rapid, precise and nondestructive technique of counting the delayed neutrons emitted during U235 fission induced with thermal neutrons. Several interesting relationships were apparent. 1.(1) A direct proportionality was observed between percentage of organic carbon and uranium in sediments deposited in an anoxic environment in the Pettaquamscutt River in Rhode Island with concentrations ranging from 7 per cent organic carbon and 7 ppm uranium to 14 per cent organic carbon and 30 ppm uranium. A similar relationship was found in cores of sediments deposited on the Sigsbee Knolls in the Gulf of Mexico.2.(2) For manganese nodules a direct relationship can be seen between uranium and calcium concentrations and both decrease with increasing depth of deposition. For nodules from 4500 m in the Pacific, concentrations are 3 ppm uranium and 0.3 per cent calcium compared with 14 ppm uranium and 1.5 per cent calcium at 1000 m.3.(3) Relatively high uranium concentrations were observed in carbonates deposited in the deepest parts of the Gulf of Mexico, with the >88 μ carbonate fraction in Sigsbee Knoll cores having as much as 1.20 ppm.A model to explain the observed variations must include uranium enrichment in near shore environments via an anoxic pathway, followed by redeposition in a deep ocean environment with dilution either by low-uranium-bearing foraminiferal or silicious oozes or, along the continental margins, dilution with high-uranium-bearing carbonate sands.
Anaerobic sediments from the upper continental slope in the eastern Pacific, the Gulf of California, and a Norwegian Fjord have been analyzed for their total uranium content and 234U/238U ratios by alpha-spectrometry. The uranium concentrations range from 4.8 to 39 ppm and are thus significantly higher than the average uranium content of 3 ppm in deep sea clays, stream sediments and soils. The 234U/238U ratios approach the sea water value of 1.15 in samples with high uranium content. These results indicate that uranium is being removed from sea water in near shore areas under certain conditions. If such conditions prevail in only 0.4% of the total area covered by the ocean, an amount of uranium equal to that supplied by streams (assuming 0.04 μg/ℓ) can be removed from the ocean, on the basis of measured accumulation rates in the Gulf of California, off southern California, and in other areas. The effect of Pleistocene sea level fluctuations as a controlling factor in determining the concentration of uranium in sea water is diminished by the occurrence of extensive areas of uranium deposition below 200 m and by the very long residence time of uranium in the ocean.
234Th produced from 238U within sea water was found to be in radioactive disequilibrium with respect to its progenitor nuclide 238U in the surface layer of the ocean. The median value for activity ratio is 0.80 in the upper 200 m layer. A box-model calculation gives a removal residence time of thorium of about 0.38 yr. This suggests that 234Th is scavenged from the surface layer by the uptake of thorium by biota.
The distribution and rates of accumulation of Mo in marine deposits have been determined and compared with the same parameters for U and Mn. High concentrations of Mo are associated both with oxidizing environments represented by the presence of ferro-manganese oxide-rich sediments (where Mo/U ∼- 3) and with reducing environments (where Mo/U is about unity). The supply of Mo by streams is more than adequate to balance the measured removal rate in normal deep-sea deposits and no submarine volcanic ‘emanations’ need be involved. On an ocean-wide basis, 4 · 3 μg Mo/cm2/1000 yr is supplied in solution by streams. Of this, 2·0 μg Mo/cm2/1000 yr is removed in deep-sea sediments and manganese nodules. The remaining 2·3 μg Mo/cm2/1000 yr is probably removed in primarily (but not exclusively) near-shore reducing sediments. The average Mo accumulation rate in these environments is about 1000 μg Mo/cm2/1000 yr; thus only 0·23 per cent of the world ocean area need be such reducing sites.
Thesis--Columbia University. Appendices A, B, and D are reprints from journals. Bibliography: leaves 144-157.
Thesis--Yale. Includes bibliographical references (leaves 190-198). Microfilm of typescript.
1.1. Rhythmic activity has been observed from the polychaetes Clymenella torquata, C. zonalis and C. mucosa. No such activity was observed in Petaloproctus socialis or Branchioasychis americana.2.2. The difference in rhythmic activity can be correlated with the ecological niche. Clymenella populations live in anoxic sediments; P. socialis live in coarse sediments. B. americana has filamentous gills.3.3. The volume of sediment turned over by C. torquata kept at 26°C was 1 ml/day.4.4. The oxyfen consumption of C. torquata is equivalent to 1 cal/day. The food consumed has a caloric value of 10 cal/day.
The rate of ejection of alpha-recoil thorium-234 into solution from the surface of zircon sand gives an alpha-recoil range of 550 angstroms. The alpha-recoil thorium-234 atoms ejected into the groundwater may supply excess uranium-234. In pelagic sediments, ejected alpha-recoil thorium-234 may contribute to the supply of mobile uranium-234 in the sedimentary column.
Kinetic models for the early diagenesis of nitrogen, sulfur, phosphorus, and silicon in anoxic marine sediments
- Berner
Oceanography of Long Island Sound, 1952–1954, II. Physical oceanography
- Riley