Plutonium, 241Am and 137Cs ratios,inventories and vertical profiles in Washington and Oregon continental shelf sediments

Continental shelf sediments from nine locations off Washington and Oregon have ^239,240Pu inventories which average 8.0 ± 2.6 mCi/km². The Columbia River and seawaters advecting over the shelf supply Pu which is removed to underlying sediments, principally through scavenging by inorganic paniculate matter. Mass balance calculations argue that less than 20 percent of the advected Pu need be scavenged from the water column to balance river input and total shelf sediment inventories. The percentage of the Pu removed through scavenging is consistent with observed participate concentrations in shelf waters and published sediment/water distribution coefficients.No marked separation of Pu from ¹³⁷Cs is observed with depth in Pacific shelf sediments as has been reported in Atlantic coastal sediments. This interocean distinctness can be explained by differences in particle mixing and downward diffusion of Cs in sediments of varying porosities. The transuranic inventories and Pu/Cs ratios in the Pacific sediments do not support the hypothesis of Livingston and Bowen that Pu is remobilized within the sediment column by ‘complexone’ formation with (principally) organic substances.Excess ²¹⁰Pb/^239,240Pu inventory ratios in eight representative cores from the Washington shelf average 100 ± 19, even though absolute values of both inventories vary by much larger factors. This reasonably constant ratio, for a given water depth, permits estimation of total Pu inventories and prediction of sites of unusual Pu accumulation from data on the more easily measured natural radionuclide.     

Geochemistry of sterols in sediments from Black Sea and the southwest African shelf and slope

Surface sediment samples from the shelf and continental slope off southwest Africa and sediment cores from the deepest part of the Black Sea were analyzed for sterols. Because the organic matter in these anoxic sediments is relatively well-preserved, the input from source organisms in the water column is important in controlling sterol distribution patterns. The sterol distribution on the Namibian shelf is complex, probably because of the great spatial and temporal variability of biological productivity caused by seasonal upwelling and changes in oxygen concentration. The Black Sea, perhaps because of greater physical stability of the water column, has sterol distributions which can be explained by microbial activity or chemical processes acting on a constant input of organic carbon from surface production.     

210Pb activities in and fluxes to sediments of the Washington continental slope and shelf

Surface sediments of the Washington coast have ²¹⁰Pb activites which average 104 ± 48dpm/g for submarine canyon and slope regimes and 18 ± 12dpm/g for the continental shelf regime. ²¹⁰Pb sedimentary fluxes are also higher in canyons, averaging 18 ± 13dpm/cm² per yr, compared to 5.2 ± 3.1 dpm/cm² per yr for slope and 4.8 ± 1.8dpm/cm² per year for shelf regions. These ²¹⁰Pb activities and fluxes are 2–7 times greater than those reported for other coastal regions. Inputs from the atmosphere and the Columbia River are not sufficient to supply the ²¹⁰Pb, but advection of seawater containing dissolved ²¹⁰Pb produced in situ from ²²⁶Ra provides an input several times larger than the sedimentary fluxes. The sedimentary ²¹⁰Pb flux is limited by scavenging reactions rather than by supply of dissolved ²¹⁰Pb.Calculations of maximum biological uptake and fluxes of ²¹⁰Pb and ‘selective’ chemical leaching experiments all show that the primary scavenging processes are due to hydrous Mn and Fe oxides rather than biological phases. The pattern of higher ²¹⁰Pb depositional fluxes in canyons than in nearby open slope areas of comparable water depth is most reasonably explained by enhanced scavenging of dissolved ²¹⁰Pb near the sea floor, rather than by processes operating throughout the water column. Relatively rapid removal of dissolved ²¹⁰Pb from the near bottom nepheloid layer to slope and canyon sediments is shown by its mean residence time of less than two years in this layer.     

Hydrocarbons in western North Atlantic surface sediments

High concentrations of hydrocarbons (500–3000 μg/g dry weight) together with a complex and wide molecular weight range of composition, and low ¹⁴C activity of δ¹⁴C = ?833 ± 50%. strongly indicate that fossil fuel hydrocarbons make up the bulk of hydrocarbons in and near the New York Bight dump site areas. The composition and concentrations of hydrocarbons in Hudson Channel sediments suggest some transport of dump site hydrocarbons to areas in the channel midway to the edge of the continental shelf. Sediments at 16 stations in other areas of the continental shelf, continental slope and abyssal plain have hydrocarbon concentrations and compositions indicating that no more than 1 μg/g dry weight of fossil fuel hydrocarbons are present.N-alkanes from land sources were among the most predominant hydrocarbons in all samples except those in the New York Bight region. Their presence in abyssal plain surface sediments documents transport of some land derived organic matter to these deep ocean areas. Two 25-carbon cycloalkenes are among the more predominant hydrocarbons in continental shelf surface sediments except for the dump site areas. These two compounds are present in lower concentrations in slope sediments and are not detected in abyssal plain sediments.     

Diagenetic state and source characterization of marine sediments from the inner continental shelf of the Gulf of Cádiz (SW Spain), constrained by terrigenous biomarkers

Surface sediments from the Gulf of Cádiz (GoC) were analyzed by alkaline CuO oxidation, in order to estimate the contribution of terrigenous organic matter (TOM) to the inner continental shelf of the southwest Iberian Peninsula. The parallel analysis of sediment samples from the two most important rivers draining to this coastal area (i.e. Guadiana River and Tinto–Odiel fluvial system) provided fundamental information regarding local terrestrial sources. Relatively constant intensive lignin parameters (S:V = 1.0 ± 0.1 and C:V = 0.22 ± 0.04) and high values of the lignin phenol vegetation index (LPVI = 155 ± 43) indicated that non-woody angiosperm tissues constitute the dominant component of vascular plant material reaching the shelf sediments. The NW to SE decreasing isotopic (¹³C) and molecular (Λ₈) signatures found among the sediments, coinciding with the Guadiana delivery plume, suggest that this river is the main terrestrial source in the inner GoC shelf. Slightly elevated values of degradation indicative ratios ([Ad:Al]_V = 0.41 ± 0.10; [Ad:Al])_S = 0.34 ± 0.07; [3,5-Bd:V] = 0.14 ± 0.05; P:[V + S] = 0.24 ± 0.09) suggested the alteration state of the shelf sediments. The two fold higher ratios of the river sediments (Guadiana: [Ad:Al]_V = 0.82 ± 0.08; [Ad:Al]_S = 0.84 ± 0.03; Tinto–Odiel: [Ad:Al]_V = 0.86 ± 0.12; [Ad:Al]_S = 0.83 ± 0.013) and the increasing degradation trend observed outward in the shelf, lead us to consider preferential sorption processes, instead of in situ diagenesis, to affect the degradation signature of the shelf sediments. Preferentially solubilized degraded OM is more likely to be sorbed and stabilized prior to transport to the marine system, showing an apparently more advanced degradation state. The use of the 3,5-Bd:V ratio in conjunction with (Ad:Al)_V revealed a composition continuum of the sedimentary OM ranging from fresh plant materials to highly altered soil humic constituents. Elemental and molecular analyses show a land to sea gradient by a NW to SE decrease of the terrestrial influence, accounting for larger terrestrial inputs (TOM: 71–98%) in those sediments near the Guadiana mouth, and predominantly autochthonous composition (TOM: 42–50%) in those located offshore. This work utilizes lignin derived biomarkers to determine the contribution of terrigenous OM delivered to this poorly described coastal area from regional rivers. Within a context of increasing international efforts to better understand the global C cycling, this study illustrates the relevance of using the alkaline CuO oxidation approach to evaluate C budgets and continental influence in river dominated ocean margins.     

Hydrocarbon gas in sediment from the shelf,slope, and basin of the Bering Sea

Hydrocarbon gases (methane, ethane, ethene, propane, propene, isobutane and n-butane) are present in low concentrations in the top two meters of sediment from the shelf, slope and basin of the Bering Sea. Methane is most abundant and its concentration increases with depth in the sediment. Average concentrations of methane recovered from these sediments are between about 4000 and 5000 nl per 1 of interstitial water. Ethane, ethene, propane and propene are present in almost all samples, but the concentrations of these gases are about two orders of magnitude lower than the concentration of methane. The average ratios of ethane to ethene are usually greater than one in shelf sediment, about one in slope sediment, and usually less than one in basin sediments. These hydrocarbon gases are probably derived from low-temperature chemical and biochemical processes operating at or near the sea-floor. At one location on the shelf, hydrocarbon gases suggest the possibility of petroleum at depth.     

The sedimentary cycle of the boron isotopes

Marine sediments contain two isotopically distinct components: a non-desorbable fraction with δ¹¹B between −4.3 and +2.8 per mil (n = 10) and a desorbable component with δ¹¹B between 13.9 and 15.8 per mil (n = 6). The adsorption coefficient, K, for the uptake of B from seawater by fluvial suspended material (Mississippi) has an experimentally determined value of 1.54 ± 0.05. The associated isotope fractionation factor, α, is 0.974 ± 0.003. Empirical values for K, determined from the analysis of marine sediments (n = 6), were between 3 and 4. The amount of B adsorbed onto fluvial suspended material during estuarine mixing is about 9 × 10⁹ moles/yr. Based on pore water data, adsorption experiments and comparative size fraction analyses (suspended sediments from the Mississippi River vs. bottom deposits from the Delta) there is no evidence for the incorporation of B into detrital sediments at low temperature. However, the high B content of Bauer Deep metalliferous sediments must be due to incorporation of seawater B during formation of authigenic silicates. The δ¹¹B of these minerals is 2 ± 3 per mil.Hydrothermally altered sediments from DSDP Hole 477, Guaymas Basin, Gulf of California have variable B contents, from 33 ppm, similar to the unaltered detritus, to 1.2 ppm in the recrystallized material. The latter has δ¹¹B as low as −9.0 per mil, substantially lower than the unaltered material. Hydrothermal solutions collected from vents adjacent to the hole have boron contents elevated by about a factor of 4 relative to seawater and δ¹¹B between 16.5 and 23.2 per mil. The B mobilized from the sediments is isotopically fractionated with the fluids being preferentially enriched in the heavy isotope.Analysis of an oxisol profile developed over granite in the Guyana Shield showed that boron is partially mobilized during weathering with an isotopic offset of 2.5 per mil between bedrock and soil.The observed enrichment of B in shales relative to igneous rocks does not occur during weathering or during exposure to seawater at low temperature. Incorporation occurs only during burial diagenesis at temperatures greater than about 60°C. However, the enrichments cannot be attained during a single cycle of primary weathering and burial but must be cumulative over many cycles.     

Organofacies and paleoenvironment of the Oligocene Maikop series of Angeharan (eastern Azerbaijan)

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene times, contains prolific source rocks with primarily Type II organic matter. Paleontological analyses of dinoflagellate cysts revealed a Lower to Upper Oligocene age for the investigated succession near Angeharan. A major contribution of aquatic organisms (diatoms, green algae, dinoflagellates, chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids and δ¹³C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the Upper Oligocene units of the Maikop Formation from Angeharan mountains. The terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the Shamakhy–Gobustan area.High primary bioproductivity resulted in a stratified water column and the accumulation of organic matter rich sediments in the Lower Oligocene units of the Maikop Formation. Organic carbon accumulation during this period occurred in a permanently (salinity-) stratified, mesohaline environment with free H₂S in the water column. This is indicated by low pristane/phytane ratios of all sediments (varying from 0.37–0.69), lower methylated-(trimethyltridecyl)chromans ratio in the lower units and their higher contents of aryl isoprenoids and highly branched isoprenoid thiophenes. Subsequently, the depositional environment changed to normal marine conditions with oxygen deficient bottom water. The retreat of the chemocline towards the sediment–water interface and enhanced oxic respiration of OM during deposition of the Upper Oligocene Maikop sediments is proposed.Parallel depth trends in δ¹³C of total OM, n-alkanes, isoprenoids and steranes argue for changes in the regional carbon cycle, associated with the changing environmental conditions. Increased remineralisation of OM in a more oxygenated water column is suggested to result in low TOC and hydrocarbon contents, as well as ¹⁵N enriched total nitrogen of the Upper Oligocene units.     

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate,³⁵S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O₂ bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.     

A potential nitrogen sink discovered in the oxygenated Chukchi Shelf waters of the Arctic

The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on ¹⁵N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8 ± 0.4 to 75.9 ± 8.7 nmol N₂ L^?1 h^?1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N year^?1, which compensates for the difference between sediment cores incubation (~ 3 Tg-N year^?1) and geochemical estimation based on N deficit relative to phosphorous (~ 16 Tg-N year^?1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.     

http://www.sciencedirect.com/science/article/pii/S1674987112001260

Archaea have unique glycerol dialkyl glycerol tetraether (GDGT) lipids that can be used to develop paleotemperature proxies such as TEX₈₆. This research is to validate proposed GDGT-proxies for paleotemperature determination in the South China Sea (SCS). Samples were collected from core-top sediments (0–5 cm) in the northern SCS. Total lipids were extracted to obtain core GDGTs, which were identified and quantified using liquid chromatography-mass spectrometry (LC-MS). The abundance of isoprenoidal GDGTs (iGDGTs) ranged from 271.5 ng/g dry sediment to 1266.3 ng/g dry sediment, whereas the branched GDGTs (bGDGTs), supposedly derived from terrestrial sources, ranged from 22.2 ng/g dry sediment to 56.7 ng/g dry sediment. The TEX₈₆-derived sea surface temperatures ranged from 20.9 °C in the coast (water depth < 160 m) to 27.9 °C offshore (water depth > 1000 m). TEX₈₆-derived temperatures near shore (<160 m water depth) averaged 23.1 ± 2.5 °C (n = 4), which were close to the satellite-derived winter mean sea surface temperature (average 22.6 ± 1.0 °C, n = 4); whereas the TEX₈₆-derived temperatures offshore averaged 27.4 ± 0.3 °C (n = 7) and were consistent with the satellite mean annual sea surface temperature (average 26.8 ± 0.4 °C, n = 7). These results suggest that TEX₈₆ may record the sea surface mean annual temperature in the open ocean, while it likely records winter sea surface temperature in the shallower water.     

Pathways of manganese in an open estuarine system

The distribution of Mn was examined in the bottom sediments and water column (suspended paniculate matter) of the Laurentian Trough. Gulf of St. Lawrence. A characteristic profile of Mn with depth in the sediment consisted of a Mn-enriched surface oxidized zone, less than 20 mm thick, and a Mn-depleted subsurface reducing zone. A subsurface Mn maximum occurred within the oxidized zone. Below this maximum the concentration dropped sharply to nearly constant residual levels in the reducing zone. The accumulating estuarine sediments are deficient in Mn compared to the river input of suspended matter and are definitely not the ultimate sink for manganese. Manganese escapes from the sediment by diffusion and resuspension, forming Mn-enriched, fine-grained particles which are flushed out in the estuarine circulation. 5.0 × 10⁹gyr^?1 of Mn, or 50% more than the river input of dissolved Mn. are exported to the open ocean. In spite of the efficient mobilization and export of Mn, the quantity exported is a small fraction (0.2%) of the total flux to the deep-sea sediments. This is related to the low levels of paniculate matter transported by the St. Lawrence River. The export phénomenon, however, is probably true of many coastal regions of muddy sediments and thus has interesting implications for the oceanic budget of Mn.     

Hydrocarbons in 60 northeast Gulf of Mexico shelf sediments: a preliminary survey

Hydrocarbon results from gas chromatography of 60 recent sediment and 10 benthic algae samples delineate two distinct shelf environments in the northeastern Gulf of Mexico.Sediments off Florida (shell hashes and sands) have moderate amounts of lipids/total sediment (average 113ppm ± 80%) but low hydrocarbon levels (average 3.06 ppm ± 41%). Aliphatic hydrocarbons are dominated by a series of branched or cyclic, unsaturated C₂₅ isomers. The major n-alkane is n-C₁₇. The n-alkane and isoprenoid patterns are consistent with a marine hydrocarbon source.Sediments closer to the Mississippi River (silts and clays) contain large amounts of lipids (average 232 ppm ± 53%) and hydrocarbons (average 11.7 ppm ± 55%) to total sediment. Aliphatic hydrocarbons are mainly odd carbon number high molecular weight n-alkanes, indicating a terrigenous hydrocarbon source. Isoprenoids are present in greater abundance than in sediments off Florida (n-C₁₇/ pristane and n-C₁₈/phytane ratios ~2to 3). Relatively large amounts of n-C₁₆, together with an even distribution of n-alkanes in the range C₁₄–C₂₀ and a substantial unresolved envelope all point to a fossil fuel input to the Mississippi samples.Samples off the Alabama coast show intermediate characteristics.     

Transformation of particulate organic matter at the water-bottom boundary in the Russian Arctic seas: Evidence from isotope and radioisotope data

Complex biogeochemical studies including the determination of isotopic composition of C_org in both suspended particulate matter and surface horizon (0–1 cm) of sediments (more than 260 determinations of δ¹³C-C_org) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi. The aim of this study is to elucidate causes that change the isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that the isotopic composition of C_org in sediments from seas with a high river runoff (White, Kara, and East Siberian) does not inherit the isotopic composition of C_org in particles precipitating from the water column, but is enriched in heavy ¹³C. Seas with a low river runoff (Barents and Chukchi) show insignificant difference between the value of δ¹³C-C_org in both suspended load and sediment because of a low content of the isotopically light allochthonous organic matter (OM) in particulates. Complex biogeochemical studies with radioisotope tracers (¹⁴CO₂, ³⁵S, and ¹⁴CH₄) revealed the existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents the mass influx of products of OM decomposition into water column, as well as reduces the influx of a part of OM contained in the suspended particulate matter from water into sediment.     

Transport of particulate organic carbon by the Mississippi River and its fate in the Gulf of Mexico

This study was designed to determine the amount of particulate organic carbon (POC) introduced to the Gulf of Mexico by the Mississippi River and assess the influence of POC inputs on the development of hypoxia and burial of organic carbon on the Louisiana continental shelf. Samples of suspended sediment and supporting hydrographic data were collected from the river and >50 sites on the adjacent shelf. Suspended particles collected in the river averaged 1.8±0.3% organic carbon. Because of this uniformity, POC values (in μmol l^?1) correlated well with concentrations of total suspended matter. Net transport of total organic carbon by the Mississippi-Atchafalaya River system averaged 0.48×10¹² moles y^?1 with 66% of the total organic carbon carried as POC. Concentrations of POC decreased from as high as 600 μmol l^?1 in the river to <0.8 μmol l^?1 in offshore waters. In contrast, the organic carbon fraction of the suspended matter increased from <2% of the total mass in the river to >35% along the shelf at ≥10 km from the river mouth. River flow was a dominant factor in controlling particle and POC distributions; however, time-series data showed that tides and weather fronts can influence particle movement and POC concentrations. Values for apparent oxygen utilization (AOU) increased from ～60 μmol l^?1 to >200 μmol l^?1 along the shelf on approach to the region of chronic hypoxia. Short-term increases in AOU were related to transport of more particle-rich waters. Sediments buried on the shelf contained less organic carbon than incoming river particles. Orgamic carbon and δ¹³C values for shelf sediments indicated 3 that large amounts of both terrigenous and marine organic carbon are being decomposed in shelf waters and sediments to fuel observed hypoxia.     

Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic-Atlantic gateways

The neodymium concentration, C_Nd, and isotopic composition, ε_Nd, in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (C_Nd = 15.5 pmol/kg and ε_Nd = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C_Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the ε_Nd is shifted to lower values (−11.8).In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C_Nd and ε_Nd do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin.Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C_Nd in the surface waters above 150 m, and a higher ε_Nd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic.     

Isotopic variability of organic carbon in a phytoplankton-based,temperate estuary

An isotopic survey was made of organic carbon in phytoplankton, sediments, Zooplankton, larval fish, and benthic fauna from Narragansett Bay and the Marine Ecosystems Research Laboratory, Rhode Island; the results quantify the extent of variability in a phytoplankton-based ecosystem and elucidate some of its causes. Carbon from primary producers (phytoplankton) varied with taxon and size, ranging from ?20.3 ± 0.6%. (mean ± 1 s.d.) for diatoms (primarily Skeletonema costatum) to ?22.2 ± 0.6%. for nanoplankton (primarily microflagellates and non-motile ultraplankton). Planktonic isotope ratios varied little with either water temperature (0 to 20°C) or degree of preservation (up to 2-year aerobic diagenesis in sea water). Isotopically, sediments from East and West Passages of the bay were homogeneous with location and depth, with a mean (?21.8 ± 0.6%.) similar to a mixture of carbon from diatoms and nanoplankton. Providence River sediments reflected terrigenous and anthropogenic carbon (sewage) in their isotopic ratios (?24.2 ± 0.7%.). Ratios of macrozooplankton (> 150 μm) were statistically separable from those of concurrently collected phytoplankton, being, on average, 0.5 to 0.6%. more positive. Secondary consumers in the water column (shrimp and larval fish) were 2.4%. heavier than diatoms. Thirty-four taxa of benthic fauna had relatively positive isotope ratios (?18.1 ± 1.5%.) which may indicate preferential use of carbon originally from diatoms rather than nanoplankton. The wide range of benthic ratios (?22.7 to ?14.9%.) resulted from both intraspecific variability (mean range = 3%.) and the variety of trophic positions occupied. Some of the intraspecific variability could be related to size. Among species, the isotope ratios increased from meiofauna (?19.5 ± 0.4%.) to macrofaunal non-carnivores (?18.6 ± 1.3%.) and carnivores (?16.6 ± 0.8%.).     

Uranium geochemistry on the Amazon shelf: Evidence for uranium release from bottom sediments

In Amazon-shelf waters, as salinity increases to 36.5 × 10⁻³, dissolved uranium activities increase to a maximum of 4.60 dpm 1⁻¹. This value is much higher than the open-ocean value (2.50 dpm 1⁻¹), indicating a source of dissolved uranium to shelf waters in addition to that supplied from open-ocean and riverine waters. Uranium activities are much lower for surface sediments in the Amazon-shelf seabed (mean: 0.69 ±.09 dpm g⁻¹) than for suspended sediments in the Amazon River (1.82 dpm g⁻¹). Data suggest that the loss of particulate uranium from riverine sediments (and the consequent input of dissolved uranium to shelf waters) is probably the result of uranium desorption from the ferric-oxyhydroxide coatings on sediment particles, and/or uranium release by mobilization of the ferric oxyhydroxides. The total flux (i.e., riverine flux plus desorbed-remobilized particulate flux) of dissolved ²³⁸U from the Amazon shelf (about 1.2 × 10¹⁵ dpm yr⁻¹) constitutes about 15% of uranium input to the world ocean, commensurate to the Amazon River's contribution to world river-water discharge (approximately 18%). Measurement of only the riverine flux of dissolved ²³⁸U underestimates, by a factor of about 5, the flux of dissolved²³⁸U from the Amazon shelf to the open ocean.     

The lignin geochemistry of marine sediments from the southern Washington coast

Lignin oxidation products and stable carbon isotope distributions are used to investigate the sources, transport, and chemical stability of land-derived organic matter in dated cores of modern sediment from the southern Washington State continental shelf and slope. There is no evidence for significant chemical alteration of lignin compounds in these sediments for time periods of up to 400 yr. Gymnosperm woods and nonwoody angiosperm tissues account for most of the land-derived organic matter in the deposits. These land plant remains have an average δ¹³C of approximately ?25.5% and are concentrated in a narrow band of silty sediment which extends northward from the Columbia River mouth along the mid-shelf. Marine organic matter having an approximate δ¹³C of ?21.5%, strongly predominates in most other shelf and slope environments. Net fluxes of land-derived organic matter into the surface 5 cm of the cores vary directly with sediment accumulation rates. Net fluxes of marine organic material into the surface sediments are highest in environments which favor the preservation of organic matter, but correspond to less than 1% of the primary productivity in the overlying waters.     

Changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the Holocene: evidence from stable carbon isotopes

Stable carbon isotope ratios in the organic fraction of surface sediments from the Laptev Sea shelf were analyzed in order to study the modern distribution pattern of terrestrial organic matter. The '¹³C_org signature of the surface sediments range from -26.6‰ near the coastal margin to -22.8‰ in the north towards the outer shelf. Characterizing the possible sources of organic matter by their '¹³C_org signature reveals that the terrestrial influence reaches further north in the eastern than in the western Laptev Sea. Downcore records of the '¹³C_org, measured on three AMS ¹⁴C-dated cores from water depths between 46 and 77 m, specify the spatial and temporal changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the past 12.7 ka. The major depositional changes of terrestrial organic matter occurred between 11 and 7 ka and comprised the main phase of the southward retreat of the coastline and of the river depocenters due to the postglacial sea level rise.