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THE CONTINENTAL MARGIN OFF THE ATLANTIC COAST OF THE UNITED STATES: CARBONATE IN SEDIMENTS, NOVA SCOTIA TO HUDSON CANYON1
Abstract
SUMMARYA joint survey by the Woods Hole Oceanographic Institution and the U. S. Geological Survey, begun in 1962, aims at the geologic exploration of the Atlantic continental margin. This report describes the distribution of carbonate in the surface sediments between Nova Scotia and Hudson Canyon. Samples of surface sediment on a 10-mile grid reveal that over more than 90% of its area the shelf sediment contains less than 5% calcium carbonate. These low values are due to low production by aquatic organisms, a consequence of low average water temperatures. A few locales on the shelf have high carbonate concentration owing to a prolific sessile or nearly sessile fauna. Seaward of the shelf concentrations of calcium carbonate increase regularly with depth to about 30% on the upper continental rise; this increase reflects a greater relative abundance of planktonic Foraminifera due to a decrease in the rate of deposition of land derived detritus. Limestone outcrops are very rare in the area.
... competition, predation) are likely to regulate the survival or growth of benthic organisms, especially pre-recruits (post-larvae and juveniles). This study focuses on large-size (> 10 mm) organisms, some of which may present biotic interactions with sea scallops, in order to define the spatial variability of the trophic niche occupied by scallops. Hulsemann (1967). Schlee & Pratt (1970), Schlee (1973, Diclunson & Wigley (1981), Twichell (1983, Theroux & Grosslein (1987), Twichell et a1 (1987, and Fader (pers. comm.), and from observations of the sediment retained in the collecting box 23 and 29. 1989, within the area outlined inFig. 1. The bottom sampling gear used was the AQUAREVE 111 (Applicat ...
... settling and survival of juveniles cvlll also determine to a large extent the distributional patterns of the adults, since most of the sampled megafaunal species are poorly motile. Biogenlc hottoms were identified as a specific biosedimentary category, because of their richness and particular megafauna (several species were exclusive to this unit). Hulsemann (1967) also distinguished the northeastern corner from the rest of the Bank, because of higher level of calcium carbonate in ...
Faunal assemblages of benthic megainvertebrates from Georges Bank (NW Atlantic) commercial scallop beds were defined from video-monitored sled-dredge samples. The distribution, diversity, abundance, and biomass of the megafauna were studied in relation to water depth and sediment types. A total of 140 species of megainvertebrates representing 10 phyla were identified, from which epibenthic taxa accounted for ca 76%. Molluscs, crustaceans, annelids, and echinoderms were best represented, with bivalves ranking first in abundance (55%) and biomass (86%). Significant changes of species diversity, total abundance, and total biomass were found according to sediment type, but not according to depth (except for total biomass). No clear bathymetric pattern was observed, although the mean number of species, mean total density (below 80 m), and mean total biomass (below 60 m) decreased with increasing depth in the deepest zone. Maximum megafaunal richness was found in biogenic bottoms while minimum value occurred in sand dunes. Total biomass and total density were consistently dominated by a small number of taxa. Three bivalve species (Spisula solidissima, Arctica islandica, Placopecten magellanicus) made up to 71% of total biomass overall, while 14 species (predominantly Anomia spp., Arctica islandica, and Ophiopholis aculeata) accounted for 70% of total density. Six megafaunal associations related to 2 major assemblages (biogenic sand-gravel and sand-shell fauna) were defined from multivariate analyses. Sediment type, tidal current speed, turbulent mixing, and food availability appear to be the major distribution-regulating factors of the megabenthos in the 55 to 105 m depth range. Density-dependent predator-prey relationships were the main biological associations shown by megainvertebrates.
... Gulf of Maine sediments, where terrigenous input dominates pelagic input, range from 1 to 5% calcite, the majority of which consists of coccoliths ( Mayer et al., 1988). Over the adjacent slope, at 1000}3000 m depth, coccolith-dominated calcite makes up 10 to 30% of the sediments (Hulsemann, 1967). This is due in part to the decrease in terrigenous sediments, but it is also likely that CaCO is exported from the Gulf into deeper water. ...
Gulf of Maine carbon budgets have not included estimates of calcification rates and the flux of calcite to the sediments, processes which are thought to rival organic production in terms of carbon ultimately buried in the sediments. Measurements of inorganic (calcification) and organic (photosynthetic) carbon production were made in March, June, and November of 1996 throughout the Gulf of Maine and Georges Bank. Photosynthetic rates ranged from 1.3–182mgCm−3d−1, and calcification rates from 0–9.3mgCm−3d−1, for all depths and locations sampled. June calcite production integrated over the euphotic zone (based on 17 profiles of 6 depths) averaged 5% of total carbon production, or 26mgCm−2d−1. Calcite (inorganic C) production in June was >10% of total C production over deeper areas such as Wilkinson Basin, the Northeast Channel, and the shelf break. This ratio was lowest (1.3%) in tidally mixed, high-nutrient regions near Cape Sable and the Bay of Fundy, where diatoms were abundant and euphotic zone nitrate concentrations exceeded 2.2μM. The turnover time of calcite particles in the water column, estimated from calcite production rates and suspended calcite concentrations, averaged 11.8 days in June and nearly 200 days in November, when calcite standing-stocks were high and calcification rates relatively low. Advective loss of calcite from the Gulf before settling is likely with long turnover times. Yearly carbon production for the Gulf of Maine was estimated at 182gm−2 organic C and 3.7gm−2 inorganic C, in the absence of an E. huxleyi bloom. If 1% of the organic carbon produced were buried in sediments, and 50% of the inorganic carbon, the result would be an approximately equal amount of each deposited in Gulf sediments. Inorganic carbon production by coccolithophores may therefore be an important contributor to Gulf and slope sediments, even during the non-bloom conditions studied here.
ABSTRACI': Surface sediments from the continental shelf area off Indus delta were analysed for their
textural characteristics and carbonate content. The sediments are largely silt, silty clay and clayey silty
sand. Sandy fraction is dominant in the outer region with relatively high carbonate content. The study
shows that distribution of carbonate in sediments off Indus delta continental shelf is controlled by the
dilution of terrigenous material and its distance from source area.
The common belief currently shared by many geoscientists concerning the climatic interpretation of limestones is that a warm-water environment is essential. This concept is not necessarily true because the rate and extent of terrigenous sediment dilution, rather than water temperature, is the primary factor determining whether or not a limestone forms at nearshore or continental shelf depths. Because carbonate productivity is lowest in cold climates, however, CaCO3 abundance and the thickness of carbonate accumulations tend to be least at high latitudes. In this regard present-day continental shelves and beaches offer a poor model for comparing cold-water and warm-water carbonates because of the generally emergent continental tectonic framework, recent eustatic sea-level changes, and the presence of ice caps at the modern poles.Typically, the influence of climate on non-reef continental shelf and beach environments cannot be clearly distinguished by the presence or absence of major taxonomic groups. Faunal diversity and equitability are more sensitive in this regard. The absence of shelf-depth inorganic carbonate precipitates, micrite envelopes, and peloids may also point to the cold-water origin of a rock. Skeletal mineralogy and oxygen isotopes of certain unrecrystallized carbonates may be good paleoclimatic indicators; however, trace elements and physical-textural attributes of the carbonate fraction are probably temperature insensitive.Previous studies of high-latitude continental shelves have concentrated merely on the abundance of calcareous material and there is seemingly a disproportionate amount of information with respect to low-latitude carbonate studies. Further research on cold-water carbonates may open up new avenues for alternative paleoenvironmental and paleoclimatic interpretations.
The study described examines the possibility of determining hydrogen in slags, including oxyfluoride slags, through reductive fusion in a stream of carrier gas, using pulsed heating. Composition of the gas phase was determined by calculating thermodynamic characteristics of reactions involving carbon, water, and calcium fluoride in a range of 1000-3000 degree K.
The circulation pattern proposed by Bigelow (1927) included a major current of water flowing southward off the southern side of Georges Bank. This pattern is not consistent with recent long-term current measurements that show that the water there moves westward along the bank isobaths. The measurements made by Bigelow are reviewed to show that a Gulf Stream ring was located south of the bank during his observations and that the ring was likely entraining water off the Georges Bank, consistent with Bigelow's observations. The works of Henry B. Bigelow [Bigelow, 1926, 1927; Big,low and Welsh, 1925-] form a foundation upon which much of our knowledge of the physical oceanography and the biology of the Gulf of Maine-Georges Bank region is based. With very little data he described a complex oceanographic regime in considerable detail. Observations over the intervening half century have shown that the major aspects of Bigelow's descriptions were correct and still applicable today. In one area--the circulation along the southern side of Georges Bank--recent measurements, however, are contrary to Bigel
During 1963, 260 paired photographs and large bottom samples were taken on the continental shelf and slope off northeastern United States. The photographs revealed surface characteristics of the sediments and natural attitudes of benthic animals; the samples retrieved specimens for geological and biological examination and identification.
Samples are the best source materials for making textural studies of sediments consisting mostly of sand, silt, or clay, but photographs are better than samples for bottoms too bouldery for proper sampling. Ripple marks and other surface irregularities revealed by the photographs supplement textural studies of the samples in deducing effects of wave movements and currents upon bottom materials.
For biological studies, the photographs yield much information on relationships of the benthic fauna to bottom materials, but they fail to provide useful data on biomasses. Biomasses and accurate taxonomic identifications are far better made upon samples. Thus, a combination of photographs and accompanying samples provides maximum information for biological as well as geological purposes.
A grab sample from a depth of 1675 m at a point south of Cape Cod contains early Eocene planktonic Foraminifera and is correlated with the Globorotalia rex zone of Trinidad. The assemblage indicates a depth comparable to that existing today. Regional relations suggest that the Cretaceous and Eocene deposits deepen to the west toward New Jersey.Two mollusk-bearing blocks dredged from the northern side of Georges Bank are correlative with the Miocene Yorktown Formation. Rocks from two other stations are probably Miocene. Benthonic Foraminifera in one sample indicate deposition in cool temperate waters of less than 60 m depth.
An inexpensive instrument built largely from standard laboratory items serves for rapid gasometric determination of carbonate content in sediments. Results obtained with reagent grade powder samples are in close agreement with theoretical values. Error estimations are given for variations of temperature, pressure, and sample weight.
A sample of salt-marsh peat from a depth of 59 m at the northwestern margin of Georges Bank has a radiocarbon age of 11,000 + 350 years. No greater depth or age of salt-marsh peat lying exposed on the ocean floor elsewhere in the world are known to the authors. The age corresponds well with the date of ice retreat from the Gulf of Maine and with the ages of oyster and other shells that mark the advance of the ocean across the bank and the ad- jacent continental shelf.
Shells of long-dead Crassostrea virginica are reported at 71 stations in depths of 14 to 82 meters. The depths exceed those of the estuaries where the species flourishes.
Radiocarbon measurements indicate that the oysters were alive 8000 to 11,000 years ago. It is concluded that the oysters lived
in lagoons or estuaries which became submerged when the sea level rose at the end of the latest glacial epoch.
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