Modern Sediment Accumulation Research Articles

Sediment and terrestrial organic carbon budgets for the offshore Ayeyarwady Delta, Myanmar: Establishing a baseline for future change

Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures and because of rapid sedimentation have the potential to impact global biogeochemical cycling. The Ayeyarwady Delta in Myanmar ranks as the world's third largest river delta in terms of sediment supply; however, modern increases in regional anthropogenic impacts risk severe alteration to sediment and TerrOC loads within this major system. By investigating modern sediment and terrestrial organic carbon (TerrOC) accumulation within the offshore Ayeyarwady Delta this study estimates baseline sediment and TerrOC budgets for this understudied mega-delta. Using 210Pb geochronology of 27 sediment cores collected from the continental shelf, we estimate that 405 (+52/-47) Mt of sediment, or ~70‐80% of fluvial sediment discharged from the Ayeyarwady and Thanlwin rivers (the main inputs to the delta), accumulates there annually. Sediment not retained on the shelf is likely partitioned between the Ayeyarwady floodplain, shoreline accretion, and minor deep-sea export. Estimates of TerrOC (based on δ13C mixing models) were coupled with modern sediment accumulation rates to determine an annual burial of 1.93 (+1.09/-0.15) Mt C on the shelf, with TerrOC burial fluxes being highest in the foreset beds of the subaqueous delta, coincident with the area of highest sediment accumulation rate. Based on estimates of the Ayeyarwady and Thanlwin rivers' TerrOC delivery, an apparent ~100% of TerrOC input is preserved on the continental shelf. However, an across shelf trend of increasing TerrOC degradation with distance offshore is also observed, indicating that while the shelf has high apparent TerrOC sequestration, carbon remineralization is also occurring prior to deposition within the subaqueous delta. Based on these conflicting outcomes, we suggest that input of TerrOC from additional sources other than the Ayeyarwady and Thanlwin rivers roughly balance the observed carbon remineralization. Main additional sources of TerrOC include the Sittang and several smaller rivers, and the Ayeyarwady delta plain below the river gauging station. As anthropogenic development within the Ayeyarwady and Thanlwin watersheds continues to increase, these sediment and TerrOC budgets provide a baseline from which future changes within the offshore Ayeyarwady Delta can be monitored.

Variability of Sediment Accumulation Rates in an Antarctic Fjord

AbstractFjords on the West Antarctic Peninsula (WAP) serve as sediment traps, preserving histories of glacial sediment supply. Regional warming trends are expected to change sediment supplies, altering water quality, depositional history, and ecosystem drivers. Our ability to assess magnitudes of these changes is limited by sparse data on modern sediment accumulation. Twelve new cores and four existing cores from Andvord Bay were used to characterize variability in sediment accumulation rates. These range from 1.5 to 7.9 mm/year (0.12 to 0.56 g·cm−2·year−1). Spatial differences and a weak down‐fjord gradient in rates suggest diverse sediment sources, including from outside the fjord. This data set provides a comprehensive assessment of sedimentation during the past century, indicating little change in rates due to recent WAP warming, and sets a benchmark for assessing climate‐related changes in sediment delivery and ecosystem drivers (e.g., burial disturbance) in the fjord over coming decades.

Organic matter compositions and loadings in soils and sediments along the Fly River, Papua New Guinea

The compositions and loadings of organic matter in soils and sediments from a diverse range of environments along the Fly River system were determined to investigate carbon transport and sequestration in this region. Soil horizons from highland sites representative of upland sources have organic carbon contents (%OC) that range from 0.3 to 25wt%, carbon:nitrogen ratios (OC/N) that range from 7 to 25mol/mol, highly negative stable carbon isotopic compositions (δ13Corg<−26‰) and variable concentrations of lignin phenols (1<LP<5mg/100mgOC). These compositions reflect inputs from local vegetation, with contributions from bedrock carbon in the deeper mineral horizons. Soils developed on the levees of active floodplains receive inputs of allochthonous materials by overbank deposition as well as autochthonous inputs from local vegetation. In the forested upper floodplain reaches, %OC contents are lower than upland soils (0.8–1.5wt%) as are OC/N ratios (9–15mol/mol) while δ13Corg (−25 to −28‰) and LP (2–6mg/100mgOC) values are comparable to upland soils. These results indicate that organic matter present in these active floodplain soils reflect local (primarily C3) vegetation inputs mixed with allochthonous organic matter derived from eroded bedrock. In the lower reaches of the floodplain, which are dominated by swamp grass vegetation, isotopic compositions were less negative (δ13Corg>−25‰) and non-woody vegetation biomarkers (cinnamyl phenols and cutin acids) more abundant relative to upper floodplain sites. Soils developed on relict Pleistocene floodplain terraces, which are typically not flooded and receive little sediment from the river, were characterized by low %OC contents (<0.6wt%), low OC/N ratios (<9mol/mol), more positive δ13Corg signatures (>−21‰) and low LP concentrations (∼3mg/100mgOC). These relict floodplain soils contain modern carbon that reflects primarily local (C3 or C4) vegetation sources. Total suspended solids collected along the river varied widely in overall concentrations (1<TSS<9000mg/L), %OC contents (0.1–60wt%), OC/N ratios (7–17mol/mol) and δ13Corg signatures (−26 to −32‰). These compositions reflect a mixture of C3 vascular plants and freshwater algae. However, little of this algal production appears to be preserved in floodplain soils. A comparison of organic carbon loadings of active floodplain soils (0.2 and 0.5mgC/m2) with previous studies of actively depositing sediments in the adjacent delta–clinoform system (0.4–0.7mgC/m2) indicates that Fly River floodplain sediments are less effective at sequestering organic carbon than deltaic sediments. Furthermore, relict Pleistocene floodplain sites with low or negligible modern sediment accumulation rates display significantly lower loadings (0.1–0.2mgC/m2). This deficit in organic carbon likely reflects mineralization of sedimentary organic carbon during long term oxidative weathering, further reducing floodplain carbon storage.

Soil erosion and sediment fluxes analysis: a watershed study of the Ni Reservoir, Spotsylvania County, VA, USA

Anthropogenic forces that alter the physical landscape are known to cause significant soil erosion, which has negative impact on surface water bodies, such as rivers, lakes/reservoirs, and coastal zones, and thus sediment control has become one of the central aspects of catchment management planning. The revised universal soil loss equation empirical model, erosion pins, and isotopic sediment core analyses were used to evaluate watershed erosion, stream bank erosion, and reservoir sediment accumulation rates for Ni Reservoir, in central Virginia. Land-use and land cover seems to be dominant control in watershed soil erosion, with barren land and human-disturbed areas contributing the most sediment, and forest and herbaceous areas contributing the least. Results show a 7% increase in human development from 2001 (14%) to 2009 (21.6%), corresponding to an increase in soil loss of 0.82Mg ha(-1)year(-1) in the same time period. (210)Pb-based sediment accumulation rates at three locations in Ni Reservoir were 1.020, 0.364, and 0.543g cm(-2)year(-1) respectively, indicating that sediment accumulation and distribution in the reservoir is influenced by reservoir configuration and significant contributions from bedload. All three locations indicate an increase in modern sediment accumulation rates. Erosion pin results show variability in stream bank erosion with values ranging from 4.7 to 11.3cm year(-1). These results indicate that urban growth and the decline in vegetative cover has increased sediment fluxes from the watershed and poses a significant threat to the long-term sustainability of the Ni Reservoir as urbanization continues to increase.

Sediment accumulation rates in subpolar fjords – Impact of post-Little Ice Age glaciers retreat, Billefjorden, Svalbard

The Little Ice Age (∼1600–1900 AD) and 20th century sediment accumulation rates in Billefjorden, a subpolar fjord on Svalbard, were reconstructed by applying 210Pb, 137Cs and AMS 14C datings. The modern sediment accumulation rate decreases from more than 0.39 cm y −1 at the fjord head to 0.08 cm y −1 close to the fjord mouth. However, during the Little Ice Age the sediments accumulated at a much lower rate of 0.02 cm y −1 in the central fjord basin. This difference is most likely related to the rapid retreat of glaciers during the 20th century, when most of them withdrew up to 2 km. The post-Little Ice Age increases in temperature and a negative glacier mass balance resulted in a larger meltwater discharge transferring substantial amounts of sediments released from the glaciers, as well as those eroded from previously stored unconsolidated glacial sediments. A comparison of data from the subpolar fjords of Svalbard suggests that the increase in the sediment accumulation rate is a common trend, and further increases might be expected if climate warming continues. The properties of the fjord sediments (grain size, IRD, coarse-fraction composition, clay mineralogy) from the Little Ice Age and the 20th century showed no distinct differences. The change in the accumulation rate may be the most evident sedimentary record of this climatic change.

Formation, morphology and preservation of high-energy carbonate lithofacies: Evolution of the cool-water Recherche Archipelago inner shelf, south-western Australia

Cool-water carbonate environments are responsible for up to one third of the carbonate sediment produced on continental shelves, and are useful modern analogues for many geologically ancient carbonate deposits. On modern and ancient high wave-energy carbonate ramps, it has been suggested that wave abrasion generally limits sediment accumulation to the outer shelf. Sediment are produced on the modern inner shelf, although few studies have investigated processes of sedimentation in these energetic environments. The Recherche Archipelago, in the west of the Great Australian Bight (GAB), was examined to better identify processes of carbonate sedimentation and preservation on extremely high energy shelves. Surficial sediments, video traverses, multibeam sonar data, cores and shallow seismics were analysed. These data reveal that the topographically varied environment of the Recherche Archipelago experienced gradual, episodic accumulation of a prograding sediment wedge, with substantially thicker deposits located on the lee side of topographic highs. Modern sediment accumulation is largely restricted to the shoreface and inner shelf, where the highly discontinuous, storm influenced blanket of Holocene sediment comprises a shore-attached quartzose sediment prism, leeward accretionary wedges and rhodolith beds above a laterally-continuous Pecten-dominated basal transgressive lag. Storm events play an important role in these environments, resulting in a mostly bare ‘shaved’ middle and outer shelf, onshore sediment transport, reworking of particles and periodic removal of sediment. The modern Recherche Archipelago is a high energy end-member of cool-water carbonate environments, in which calcareous algae dominate sediment production. Modern Esperance Bay provides a useful analogue for the interpretation of high-energy palaeoenvironments, and demonstrates that in addition to sea level change, wave energy and local geomorphology are important considerations for carbonate facies reconstructions. This study has presented new information for carbonate inner shelves that differs from classic carbonate ramp models, showing that ancient rhodolith-bearing carbonate deposits have the potential to form and be preserved in very high energy settings.

Modern sediment accumulation on the Po shelf, Adriatic Sea

Sediment accumulation over the past century on the continental shelf near the Po delta varies with distance from the most active distributary channels. Near the Pila and Goro distributaries, sediment accumulation is rapid (1–4 cm yr −1) and occurs in pulses. In these areas, the seabed is dominated by physical sedimentary structures that can be related to flood sedimentation. Between the two distributaries and in the southern portion of the dispersal system, sediment accumulation is slower (rates reach a minimum of 0.23 cm yr −1 at ∼50 km from the Pila mouth) and steady-state, reflecting more continuous dispersal of sediment during non-flood periods. Sedimentary strata in these locations are composed of finer (clayey silt), mottled sediment. The similarity in the spatial distribution of long-term (100-yr) sediment accumulation to deposition resulting from the 2000 flood event suggests that the Po shelf is flood-dominated. About half of the sediment delivered by the Po River on a 100-yr time scale can be accounted for in the seabed deposit within ∼50 km of the Pila mouth. The remaining sediment is likely transported southward by the prevailing circulation, and this sediment coalesces with inputs from the Apennine Rivers.

Changes in Sediment Accumulation Rate in An Oxbow Lake Following Late 19Th Century Clearing of Land for Agricultural Use: A 210Pb, 137Cs, and 14C Study in Mississippi, USA

Sediment cores were collected from 2 sites in the forested fringe of an oxbow lake surrounded by land that was converted from forest to agricultural use in the late 19th century. The 2 sampling areas were selected to represent areas of high (West site) and low (East site) current sediment accumulation rates, based on distance from a perennially discharging stream. Modern (post settlement and land clearing) sediment accumulation rates were calculated using 210Pb and 137Cs on bulk sediment samples from 2 cores from each site. Two additional cores were collected from each site for radiocarbon analysis of twig cellulose with the assumption that most twigs in the sediment within the forested fringe fell from overhead and are contemporaneous with the sediment. Only the West site, however, yielded sufficient identifiable twig material for analysis. Modern sediment accumulation rates based on 210Pb and 137Cs fall between 0.2–0.4 cm/yr at the East site, and 0.7–1.3 cm/yr at the West site (nearest the stream inlet), with approximate agreement between the 210Pb and 137Cs methods. Modern sediment accumulation rate based on bomb-pulse 14C activity of twigs from cores from the West site is approximately 1.0 cm/yr, in agreement with the 210Pb and 137Cs results. Historic sediment accumulation rates were estimated at the West site using twigs from deeper intervals with pre-bomb 14C activity. Sediment covering approximately 1000 yr of pre-settlement sediment accumulation exhibited evidence of minor bioturbation or in-washing of reworked material, but with a clearly lower accumulation rate of less than 0.1 cm/yr.

Brazil: Cadam — kaolin

Field surveys of the 350-km shoreline adjacent to the Amazon river mouth reveal three distinct types: erosional mud, accretionary sand, and accretionary mud. Formation of these zones is controlled by the delivery of Amazon suspended sediment, mediated by the hydrodynamic regime. Erosional mud shorelines extend from Rio Araguari (near the Amazon river mouth) northwestward 280 km to 3.5 °N (near Cabo Cassipore). Shoreface (<5 m water depth) retreat annually yields 1–4 × 106 tons of fine-grained sediment deposited during an earlier phase of shoreface progradation. Sandbodies up to 5 m thick overlie erosional mud shorefaces for 10–30 km downdrift of small rivers. The sand is supplied from these rivers and released by shoreline retreat of the river-mouth areas. Amazon River sand is restricted in the coastal zone from its mouth north to the Cabo Norte shoal area, where it mixes with sand carried by Rio Araguari from the Guiana Highlands.North of 3.5 °N, mud aggradation and progradation is taking place on underconsolidated, low-gradient tidal flats backed by mangrove swamps. 210Pb and 14C geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid (0.24–2.0 cm/yr) landward of the 2-m isobath, supplied from a thick (50–150 cm) seasonal surface layer. Shoreface progradation is episodic, separated by decadal hiatuses. Fine-grained suspended-sediment flux from the Amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats. Shore-normal tidal currents and solitary waves rework the surface mud layer, preferentially transporting available sand landward into the mangrove fringe, and producing very fine-grained accumulation on the tidal flat (10–12 φ mean grain size). Lateral accretion of features 10–100 km long, termed mudcapes, produces tens of kilometers of seaward coastal-plain addition along the northern coast. Similar features are identified downdrift along the Guianas coast as far as the Orinoco River (1600 km). The northern Amapa shoreface deposits are a locus of modern sediment accumulation, which progrades over subaqueous deltaic strata.

Diagenesis of carbon and nutrients and benthic exchange in sediments of the Northern Adriatic Sea

The characteristics of diagenesis and benthic biogeochemical cycling were studied at six stations in the Northern Adriatic during September, 1988. The objectives of this work were to quantify the mechanisms responsible for mass transport and to establish the stoichiometry of reactions involving carbon and nutrients. Stations were chosen to include sites near the Po delta that have rapid sediment accumulation, sites south of the Po delta that lie beneath its nutrient-rich plume but beyond the region of rapid accumulation, and one site further offshore in a zone with little or no modern sediment accumulation. Benthic flux measurements of oxygen, TCO , ammonia, alkalinity, phosphate, silicate and radon were made at five of the 2 study sites. Synthesis of this data shows that fluxes of oxygen and nutrients are similar at both high and low accumulation rate sites, but that the offshore site has fluxes that are at least two times smaller for all parameters except silica and radon. Cores were collected, and analyses of solid phases organic carbon, nitrogen, sulfur, total and organic phosphorus, and 210 . . Pb and pore waters alkalinity, pH, ammonia, silica, calcium, iron, and phosphate were carried out. Calculations indicated that pore waters were near equilibrium or slightly supersaturated with calcite, supersaturated with apatite, and undersaturated with vivianite. Pore water profiles of TCO and silica were used to calculate diffusive fluxes across the 2 sediment-water interface. Comparison of these calculated fluxes to in situ flux measurements indicated solute transport at the rapid accumulation sites is dominated by diffusive fluxes, while at the other sites about half is accomplished by irrigation; the contrast reflects the abundance of macrofauna. Rate constants for organic matter degradation were estimated and reflect the presence of fractions with mean lifetimes from a few months to several years. Diagenetic stoichiometry is dominated by degradation of organic carbon, which accounts for about 95% of the TCO flux. Ultimately, oxygen is the 2 principal terminal electron acceptor, although ferric iron, nitrate and sulfate must be important intermediates. Carbonate dissolution accounts for the remaining 5% of the TCO flux. The average CrN ratio of degrading organic material derived 2 from pore water profiles of TCO and ammonia is 5 2; this is 30-50% of the ratio measured in solid phases,

Manzala lagoon, Nile delta, Egypt: modern sediment accumulation based on radioactive tracers

This study was undertaken to determine whether recent anthropogenic changes in the Nile basin have affected the modern rate of sediment accumulation in the Nile delta. Excess 210Pb, 137Cs, and 239,240Pu were used to develop a sediment chronology for a core from central Manzala lagoon, the delta sector which has had the highest average rate of sediment accumulation during the Holocene (to about 0.7 cm year–1). Excess 210Pb was detected in the top 32 cm of the core, yielding an accumulation rate of 1.2 cm year–1, higher than the mean rate for the Holocene. A high 137Cs/239,240Pu ratio requires a reactor source (possibly Chernobyl) for these nuclides. Low concentrations of excess 210Pb and weapons-fallout nuclides precluded recognition of changes in sediment accumulation rate in Manzala lagoon during this century and may limit the use of tracer radionuclides for modern sediment chronology in the Nile delta.

Sediment and heavy metal accumulation in the Cauvery basin

Eleven cores were collected from the Cauvery basin. Radiometric dates were used to determine modern sediment accumulation rates. Sediment accumulation rates ranged from 0.4 to 4 mm yr−. Heavy metal concentration decreases with the increase of depth. The heavy metal concentrations at certain depths are attributed to the irregular input of metals and their remobilization. Heavy metal accumulations have been computed using sediment accumulation rates, and accumulation rates show an additional anthropogenic input of metals and sediments in the recent past. Factor analysis and correlation analysis show the diverse source and accumulation mechanism influencing the metal distribution in the basin.

Rates and mechanisms of shoreface progradation and retreat downdrift of the Amazon river mouth

Field surveys of the 350-km shoreline adjacent to the Amazon river mouth reveal three distinct types: erosional mud, accretionary sand, and accretionary mud. Formation of these zones is controlled by the delivery of Amazon suspended sediment, mediated by the hydrodynamic regime. Erosional mud shorelines extend from Rio Araguari (near the Amazon river mouth) northwestward 280 km to 3.5 °N (near Cabo Cassipore). Shoreface (<5 m water depth) retreat annually yields 1–4 × 10 6 tons of fine-grained sediment deposited during an earlier phase of shoreface progradation. Sandbodies up to 5 m thick overlie erosional mud shorefaces for 10–30 km downdrift of small rivers. The sand is supplied from these rivers and released by shoreline retreat of the river-mouth areas. Amazon River sand is restricted in the coastal zone from its mouth north to the Cabo Norte shoal area, where it mixes with sand carried by Rio Araguari from the Guiana Highlands. North of 3.5 °N, mud aggradation and progradation is taking place on underconsolidated, low-gradient tidal flats backed by mangrove swamps. 210Pb and 14C geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid (0.24–2.0 cm/yr) landward of the 2-m isobath, supplied from a thick (50–150 cm) seasonal surface layer. Shoreface progradation is episodic, separated by decadal hiatuses. Fine-grained suspended-sediment flux from the Amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats. Shore-normal tidal currents and solitary waves rework the surface mud layer, preferentially transporting available sand landward into the mangrove fringe, and producing very fine-grained accumulation on the tidal flat (10–12 φ mean grain size). Lateral accretion of features 10–100 km long, termed mudcapes, produces tens of kilometers of seaward coastal-plain addition along the northern coast. Similar features are identified downdrift along the Guianas coast as far as the Orinoco River (1600 km). The northern Amapa shoreface deposits are a locus of modern sediment accumulation, which progrades over subaqueous deltaic strata.

Nutrient profiles in the pore water of a deltaic lagoon: Methodological considerations and evaluation of benthic fluxes

Several aspects of the early diagenesis of major nutrients (silica, ammonia, phosphorus) and iron, based on sediment and porewater studies, were examined in Sacca di Scardovari (Po Delta, Italy). Radionuclide measurements ( 210Pb, 137Cs, 134Cs) were carried out in order to calculate modern sediment accumulation and mixing rates. The pore water obtained from two different sampling techniques is also compared. One method involves direct sampling through the use of dialysis samplers; the second one obtains samples through the centrifugation of sediment cores. The two methods compare well for all the elements considered, except for Fe values in the topmost centimetres of the sediment and for NH 4 in the lowest 15 cm. These differences can be explained by alterations of porewater composition during the manipulation of core samples and by spatial heterogeneity. The distributions obtained for the different ionic species are typical of anoxic sedimentary environments, rich in organic matter. Evaluation of silica, ammonia and phosphorus fluxes at the water-sediment interface is conducted by applying a one-dimensional model: the sediment is considered as a two-layer system and a steady-state diagenesis is assumed. The model takes into account molecular diffusion and irrigation in a cumulative coefficient D 1 or distinguishes irrigation effects in a separate term, I. The two schemes furnish different flux values, and explanations for these differences are proposed.

Modern sediment accumulation and strata formation on the Amazon continental shelf

Formation of fine-scale ( < 50 cm ) strata on the Amazon continental shelf was investigated using radiographic and radiochemical (e.g. 210Pb) techniques. Three distinct sedimentary environments are found within the inner shelf ( < 60 m water depth) mud deposit: (1) interbedded mud and sand in a corridor extending from the river mouth across the inner shelf; (2) faintly laminated muds on both flanks of the corridor; and (3) bioturbated muds comprising the remainder of the deposit. High rates of sediment accumulation ( > 2 cm/ yr ) are found in the region of interbedded mud and sand, and in the regions of faintly laminated muds. Accumulation rates in the region of bioturbated muds are relatively low (0.1–2 cm/yr), and an inverse relationship is observed between accumulation rate and extent of sedimentary structure alteration by benthic fauna. A first-order sediment budget indicates that sediment accumulation on the Amazon shelf (between the Brazil-French Guiana border and the Para River) is nearly equal to estimates of sediment discharged by the Amazon River ( 9 × 10 8 tons/yr ). The model of Amazon shelf sedimentation most consistent with the radiographic and radiochemical data suggests that the inner shelf mud deposit is a subaqueous delta prograding seaward over relict basal sands and that a turbulent jet emanates from the river mouth with competence (during some flood periods) to carry sand as bedload across the inner shelf. The presence of cross-bedded sands in the corridor of interbedded mud and sand supports the hypothesis of a turbulent jet. The faintly laminated muds probably represent suspended sediment rapidly deposited on both sides of the jet.

Geochemical Evidence for Modern Sediment Accumulation on the Continental Shelf off Southern New England

ABSTRACT An area of fine-grained sediment approximately 170 km 74 km in size, located in water depths between 60 m and 150 m, south of Martha's Vineyard, Mass., is a site of modern sediment deposition. The 14C ages systematically increase with sediment depth from about 1,300 years B.P. at the surface to 8,000-10,000 years B.P. at the depth of maximum core penetration. The old age for the surface sediments probably results from a combination of deposition of old carbon and faunal mixing. In the finest sediments, the sedimentation rates were approximately 130 cm/1,000 yrs when deposition began and have decreased to about 25 cm/1,000 yrs. The decreasing sedimentation rate reflects a diminishing source of fine sediments, which presuma ly came from the Georges Bank and Nantucket Shoals area. Inventories of excess 210Pb in undisturbed cores average 70 dpm/cm2 (disintegrations per minute per square centimeter), more than two times higher than the flux of 210Pb from the atmosphere and from 226Ra decay in the overlying water. This additional influx of 210Pb either must be with new fine-grained sediment material or from solutions that are stripped of their 210Pb by particulates in the bottom nepheloid layer. Stable Pb concentrations in surface sediments are about 28 ppm, as much as two times higher than concentrations at depth. The high accumulation rates, 210Pb inventories, and trace-metal profiles imply that this area is a modern sink for fine-grained sediments and for pollutants associated with particulate matter in the water column. To our knowledge, this is the only site of present-day natural deposition on the Continental Shelf off the eastern United States, exclusive of the Gulf of Maine. Because the net currents on the outer half of this Continental Shelf flow from northeast to southwest, this fine-grained deposit may receive its sediments and possible contaminants from the Nantucket Shoals and Georges Bank regions.

The use of Pb-210 geochronology as a sedimentological tool: Application to the Washington continental shelf

In the past, marine sedimentologists have been unable to quantitatively investigate modern sediment accumulation on continental shelves, but recent development of Pb-210 geochronology provides a tool to overcome this limitation. It is used in this study to examine the accumulation of modern Columbia River sediments on the Washington continental shelf. Pb-210 profiles in Washington shelf sediments reveal three characteristic regions: a homogeneous surface layer (about 10 cm thick) where sediments are actively mixed by physical and biological processes, a region where Pb-210 activities decrease logarithmically with depth in the sediment, and a lower region of background activities. The surface mixed layer generally reflects erosion by wave and current activity in inner shelf sediments (shallower than about 60 m), and biological mixing in mid and outer shelf sediments. The region of logarithmic activities (i.e., radioactive decay) in the Pb-210 profiles provides the rate of sediment accumulation, which is on the order of mm/yr. The predominant depositional feature is a mid-shelf silt deposit in which accumulation rates progressively decrease north-northwestward away from the Columbia River. These results are consistent with other sedimentological observations, and indicate that Pb-210 geochronology not only allows determination of modern sediment accumulation rates, but also provides additional insight into processes affecting accumulation. Pb-210 geochronology can be an important sedimentological tool for future studies of continental shelf sedimentation in areas undergoing modern sediment accumulation.