Isotopic Composition Of Sediments Research Articles

The Condor seamount at Mid-Atlantic Ridge as a supplementary source of trace and rare earth elements to the sediments

The Condor Seamount rises from seabed to 180m water depth, being located 10 nautical miles southwest of the island of Faial, Azores Archipelago at the Mid-Atlantic Ridge (MAR). The vertical distribution of major, minor, trace and rare earth elements (REE) and Pb isotopes was studied in four sediment cores: one from the top of the Condor Seamount (200m, MC9), two from the seamount base (1400m, MC2 and MC4), and one from a deep area (1900m, MC8). Sediments from the top of the Condor were composed by coarser particles being the fine fraction lower than 1%. Conversely the other sediments were constituted by 51–92% of fine particles (<63μm). Individual fragments of volcanic material (>2mm) were found at several depths of the cores sampled at the base of the seamount. The core collected in the top of the Condor showed higher carbonate content (76–86%) compared with the other cores (41–64%). The chemical compositions of MC2 and MC4 point to an enhancement of V, Cr, Co, Ni and Fe concentrations. Lower concentrations in MC8 hypothesis that Condor seamount constitutes a supplementary source of trace elements. The most plausible explanation for the enhancement found in sediments of the seamount base is the weathering of slopes with volcanic activities, which supply particles with higher element concentrations than pelagic sediments. This hypothesis is corroborated by REE data, showing increased chondrite normalized ratios in MC2 and MC4. Moreover, the REE pattern found in those cores was comparable to that existing in volcanic material with Light REE enrichment in comparison to Heavy REE. These results indicate a substantial contribution of particles derived from volcanic activities to sediments settled in the vicinity of the Condor Seamount. It is argued the potential use of REE in sediments from this region as tracers of volcanic activities. Depth profiles of 206Pb/207Pb and 206Pb/208Pb showed lower ratios in the first 8cm sediment layers, reflecting atmospheric input of anthropogenic Pb in the last century. On the basis of Pb profiles it is proposed a baseline Pb concentration of 3.6±0.2μgg−1 for pelagic sediments of the region with an isotopic signature of 206Pb/207Pb=1.227±0.003 and 206Pb/208Pb=0.492±0.001 signature. The isotope plots of 206Pb/207Pb versus 208Pb/206Pb showed a linear trend indicating the mixing between more radiogenic pre-industrial end-members and less radiogenic anthropogenic lead. The Pb isotope composition of sediments from the Condor area falls closer to North Atlantic Sediment Line. Sediments showed a 206Pb/204Pb signature closer to the basalts of the Capelo volcanic complex than from Mid-Ocean Ridge Basalts (MORB), which suggests the contribution of similar geological formations to sedimentary material.

Records of Neogene seawater chemistry and diagenesis in deep-sea carbonate sediments and pore fluids

Deep-sea pore fluids are potential archives of ancient seawater chemistry. However, the primary signal recorded in pore fluids is often overprinted by diagenetic processes. Recent studies have suggested that depth profiles of Mg concentration in deep-sea carbonate pore fluids are best explained by a rapid rise in seawater Mg over the last 10–20Myr. To explore this possibility we measured the Mg isotopic composition of pore fluids and carbonate sediments from Ocean Drilling Program (ODP) site 807. Whereas the concentration of Mg in the pore fluid declines with depth, the isotopic composition of Mg in the pore fluid increases from −0.78‰ near the sediment–water interface to −0.15‰ at 778mbsf. The Mg isotopic composition of the sediment, with few important exceptions, does not change with depth and has an average δ26Mg value of −4.72‰. We reproduce the observed changes in sediment and pore-fluid Mg isotope values using a numerical model that incorporates Mg, Ca and Sr cycling and satisfies existing pore-fluid Ca isotope and Sr data. Our model shows that the observed trends in magnesium concentrations and isotopes are best explained as a combination of two processes: a secular rise in the seawater Mg over the Neogene and the recrystallization of low-Mg biogenic carbonate to a higher-Mg diagenetic calcite. These results indicate that burial recrystallization will add Mg to pelagic carbonate sediments, leading to an overestimation of paleo-temperatures from measured Mg/Ca ratios. The Mg isotopic composition of foraminiferal calcite appears to be only slightly altered by recrystallization making it possible to reconstruct the Mg isotopic composition of seawater through time.

Trace metals in sediments and aquatic plants from the Xiangjiang River, China

Purpose The metal concentrations and Pb isotopic composition in sediments and plants from the Xiangjiang River, China, were investigated to understand the contamination and potential toxicity of metals in sediments; to determine the accumulation and distribution of metals in plant tissues; and to trace the possible pollution source of Pb in sediments and plants.

Characterisation of n-alkanes and their hydrogen isotopic composition in sediments from Lake Qinghai, China

The distributions of n-alkanes and their hydrogen isotopic composition (δD) in surface and core sediments from the saline Qinghai Lake were measured to assess whether or not biological source information was recorded in the δD values of n-alkanes. The results indicate that the n-alkane distributions between shallow water surface and core sediments were similar, and closer to those of terrestrial herbaceous plants from the Qinghai Lake surrounding areas, rather than the aquatic plants living in the lake. The n-alkanes in the surface and core sediments had similar mean δD values, ranging from −185‰ to −133‰ and −163‰ to −142‰, respectively. The mean δD values of n-alkanes in the sediments showed that the even n-alkanes were heavier in D compared with the odd homologues.

Tracing fluid–rock reaction and hydrothermal circulation at the Saldanha hydrothermal field

The Saldanha hydrothermal field is positioned on the top of a seamount located in a non-transform offset (NTO5) on the Mid-Atlantic Ridge (MAR). This hydrothermal system was first described as a low-temperature diffuse field, driven by peridotite–seawater reactions following the detection of high concentrations of CH 4 and H 2 in the water column, and the occurrence of serpentinite outcrops in the vent area. We have studied the geochemistry and isotopic composition of sediment and rock samples collected across the area and show that hydrothermal circulation at Saldanha is complex and spatially variable, comprising areas of low-temperature diffuse flow but also more focused higher-temperature venting zones. While most sediment samples have an isotopic composition that is similar to normal pelagic sediments, one core (SCD7) show significant hydrothermal influence, sulphide mineralization, non-radiogenic Pb and radiogenic Nd isotope ratios and positive Eu anomalies. This is best explained by mineral precipitation from high-temperature hydrothermal fluids that have circulated through mafic rocks. The host rock lithology and alteration is also highly variable and comprises both fresh basalts, serpentinites and hydrothermally altered rocks (metabasalts, metagabbros and steatites). Serpentinites have REE patterns and ε Nd(0) values that fall between seawater and mantle peridotite reference values, resulting from extensive interaction of seawater with the original peridotite. This process was probably favoured by the deeply penetrating and long-lived faults occurring at this NTO. Steatites have a positive Eu anomaly and non-radiogenic Pb isotopic values. These signatures, together with the sulphide mineralisation and the extensive Si input necessary for steatization of serpentinites, imply that higher-temperature hydrothermal fluids reacted with gabbroic intrusions at depth. The more hydrothermally altered sediment and rock samples appear to be associated with the Saldanha fault network that promotes a more focused fluid flow and thus enhances hydrothermal alteration within a region of low-temperature diffuse flow.

Monsoon control over erosion patterns in the Western Himalaya: possible feed-back into the tectonic evolution

Abstract The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative ε Nd and higher 87 Sr/ 86 Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ 13 C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. Supplementary material: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412

Century-long record of Mo isotopic composition in sediments of a seasonally anoxic estuary (Chesapeake Bay)

A double-spike method was used to obtain Mo isotope data for sediments and waters of the seasonally anoxic Chesapeake Bay, and its primary tributary, the Susquehanna River. The dissolved Mo distribution in the estuary is non-conservative, reflecting minor Mo loss to the sediments, although removal of Mo to the sediments does not have a large influence on the isotopic composition of the water column. The δ 98Mo of dissolved Mo in most of the estuary is dominated by seawater. Six samples with salinity > 15 have an average δ 98Mo = + 2.17‰ (± 0.12), which agrees well with a δ 98Mo value for the CASS-4 seawater standard of + 2.23‰. A single sample of Susquehanna River water has a δ 98Mo of + 1.02‰, consistent with recent findings of positive δ 98Mo in rivers worldwide. Susquehanna river sediments, in contrast, have δ 98Mo ∼ − 0.1‰. The difference between the river water and sediment values implies that isotopic fractionation occurs within the river basin. The δ 98Mo values for estuarine sediments are offset from values in the overlying water. Most samples deposited before 1925 have δ 98Mo less than 0‰, similar to the Susquehanna sediments. Subsequently, there is an increase in the variability of δ 98Mo, with values ranging up to + 0.8‰. The transition to increased variability coincides with the onset of authigenic Mo deposition, which was previously attributed to escalating summertime anoxia. Authigenic Mo concentrations correlate poorly with δ 98Mo in core samples, suggesting that independent mechanisms influence the two parameters. Authigenic Mo concentrations may be controlled by shifting pore water H 2S levels, while δ 98Mo may be primarily affected by annual variations in Mn refluxing.

Using sequential extraction techniques to assess the partitioning of plutonium and neptunium-237 from multiple sources in sediments from the Ob River (Siberia)

Sequential extraction techniques have been developed to assess partitioning of anthropogenic radionuclides ( 240Pu, 239Pu, and 237Np), originating from a variety of sources, as identified by using bulk sediment isotopic composition. Sediments were leached sequentially with a series of six chemical treatments designed to approximate different environmental processes that may occur or to selectively extract trace metals contained in different solid phases of the sediments (i.e., exchangeable, reducible, carbonate, organic, acid leachable, and refractory). Results indicate the majority of Pu and Np is similarly distributed within many of the extracted fractions, with the largest percentage (66–97%) of both elements being observed in sediments treated with buffered citrate dithionite (CDB), which targets easily reduced constituents such as Mn and Fe hydrous oxides. While these results do indicate an association of Pu and Np with redox sensitive elements, the environmental implications are unclear given that the CDB treatment is more extreme than naturally occurring conditions. Minor amounts of Np partition differently from Pu in sediments. The NH 4-acetate treatment, which is designed to liberate trace metals that are loosely adsorbed onto the surfaces of sedimentary materials such as hydrated iron oxides and humic substances, or present at exchangeable sites in clay minerals, mobilized ∼12% of the total Np while Pu levels were below detection. The H 2O 2 treatment, which is designed to liberate trace metals bound to organic matter, mobilized ∼8 and ∼1% of Np and Pu, respectively. These results indicate that a minor portion of the total Np may be affected by environmental conditions that have little or no effect on Pu. Between 7 and 24% of the Pu was observed in treatments designed to liberate Pu and Np that are tightly bound to lithogenic phases or refractory silicates. The 240Pu/ 239Pu observed in accessible and refractory fractions ranged between 0.11 and 0.18. With the exception of one sample, the 240Pu/ 239Pu isotope ratios measured in the refractory fractions are essentially identical having a mean ratio value of 0.123 ± 0.001 (1 σ). The 240Pu/ 239Pu ratios provide isotopic evidence that suggests a portion of non-fallout contamination has a refractory nature. The presence of similarly low 240Pu/ 239Pu ratios in refractory fractions of sediments from the Ob and Irtysh Rivers suggests the existence of a source of refractory Pu which is consistent with refractory “hot particles” derived from surface tests at the Semipalitinsk test site.

Carbon cycling in Lake Erie during cultural eutrophication over the last century inferred from the stable carbon isotope composition of sediments

A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon and calcium carbonate (δ13CTOC and \( \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} \)) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO3). δ13CTOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient loading. CaCO3 and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO3 concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature and invasive mussels are also potential controlling factors. \( \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} \) values show a positive correlation with δ13CTOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for \( \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} \) values during eutrophication. However, a negative correlation between \( \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} \) and δ13CTOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and seasonal differences in productivity.

Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering

The combined use of Lu–Hf and Sm–Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf–Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (ε Nd ~ − 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by ε Hf values (from − 1.1 to + 1.3) far more radiogenic than associated sediments (from − 7.1 to − 12.0) and turbidite sands (from − 27.2 to − 31.6). ε Hf values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between ε Hf of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf–Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the ‘seawater array’ (i.e. the correlation defined by deep-sea Fe–Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the ‘seawater array’, which we refer to as the ‘zircon-free sediment array’ (ε Hf = 0.91 ε Nd + 3.10). Finally, we show that the Hf–Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (ε Hf = 1.80 ε Nd + 2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust.

Nd and Sr isotopic compositions of sediments from the Yellow and Yangtze Rivers: Implications for partitioning tectonic terranes and crust weathering of the Central and Southeast China

New Nd and Sr isotope data are presented in this paper for sediments from the Yellow and Yangtze River drainage basins. The average 143Nd/144Nd isotope compositions of fine-grained sediments from two drainage basins seem similar. The T DM Nd ages of sediments from the two drainage basins are relatively uniform but exhibit subtle differences. This reflects the different underlying bedrocks, in association with the unique tectonic terranes that comprise central and southeastern China, including the North China Block, the Yangtze Block, the South China Block, the Tibet Plateau and the Qinling-Dabie Orogenic Belt. In contrast, there is an obvious difference in the 87Sr/86Sr ratios between fine-grained sediments of the Yellow and Yangtze Rivers, which actually reflects an increase in chemical weathering intensity from northwestern to southeastern China.

Precise determination of cadmium and lead isotopic compositions in river sediments

A method for the accurate determination of Cd and Pb isotope compositions in sediment samples is presented. Separation of Cd and Pb was designed by using an anionic exchange chromatographic procedure. Measurements of Cd isotopic compositions were carried out by multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), by using standard-sample bracketing technology for mass bias correction and Pb isotopic ratios were determined by thermal ionization mass spectrometry (TIMS). The factors that affect the accurate and precise Cd isotope compositions analysis, such as instrumental mass fractionation and isobaric interferences, were carefully evaluated and corrected. The Cd isotopic results were reported relative to an internal Cd solution and expressed as the δ 114/110Cd. Five Cd reference solutions and one Pb standard were repeatedly measured in order to assess the accuracy of the measurements. Uncertainties obtained were estimated to be lesser than 0.11‰ (2s) for the δ 114/110Cd value. Analytical uncertainties in 2s for Pb isotopic ratios were better than 0.5‰. The method has been successfully applied to the investigation of Cd and Pb isotope compositions in sediment samples collected from North River in south China.

Spatial distributions of organic carbon and nitrogen and their isotopic compositions in sediments of the Changjiang Estuary and its adjacent sea area

The spatial distribution patterns of total organic carbon and total nitrogen show significant correlations with currents of the East China Sea Shelf. Corresponding to distributions of these currents, the study area could be divided into four different parts. Total organic carbon, total nitrogen, and organic carbon and nitrogen stable isotopes in sediments show linear correlations with mean grain size, respectively, thus “grain size effect” is an important factor that influences their distributions. C/N ratios can reflect source information of organic matter to a certain degree. In contrast, nitrogen stable isotope shows different spatial distribution patterns with C/N and organic carbon stable isotope, according to their relationships and regional distributions. The highest contribution (up to 50%) of terrestrial organic carbon appears near the Changjiang Estuary with isolines projecting towards northeast, indicating the influence of the Changjiang dilution water. Terrestrial particulate organic matter suffers from effects of diagenesis, benthos and incessant inputting of dead organic matter of plankton, after depositing in seabed. Therefore, the contribution of terrestrial organic carbon to particulate organic matter is obviously greater than that to organic matter in sediments in the same place.

Changes in vegetation and marine environments in the eastern Mediterranean (Rhodes, Greece) during the Early and Middle Pleistocene

Pollen records, marine faunal associations and stable isotope compositions of sediments from Rhodes, Greece, have been determined to track environmental changes in the eastern Mediterranean during the Early and Middle Pleistocene. A detailed chronostratigraphic curve, based on magnetostratigraphic data, was obtained by correlating pollen spectra with the Mediterranean oxygen isotopic curve of Ocean Drilling Program Site 975. Five sedimentary sequences that correspond to marine oxygen isotope stage (MIS) 31–27 and to MIS 20–17 have been investigated in the confined Tsampika microbasin. High-amplitude Pinus variations confirm glacio-eustatic changes deduced from changes in marine faunal associations and sedimentary depositional environments. Reflecting climatic cycles identified in the marine carbonate oxygen isotope record, eight vegetation successions (characterized by the dominance first of mesothermic elements, then of mid- and high-altitude elements with Pinus , and ending with maxima in herb and steppe elements) have been documented. Most of them were probably driven by changes in insolation occurring in high northern latitudes (obliquity impact) during the late Early Pleistocene and early Middle Pleistocene.

Nursery fidelity, food web interactions and primary sources of nutrition of the juveniles of Solea solea and S. senegalensis in the Tagus estuary (Portugal): A stable isotope approach

Stable carbon and nitrogen isotopes were used to assess site fidelity of Solea solea and Solea senegalensis juveniles, to investigate food web interactions and to determine the dominant nutrient pathways in two nursery areas in the Tagus estuary, Portugal. Samples of water from the main sources and from the nursery areas and respective saltmarsh creeks were collected for isotope analysis, as well as sediment, benthic microalgae, saltmarsh halophytes, S. solea, S. senegalensis and its main prey, Nereis diversicolor, Scrobicularia plana and Corophium spp. While site fidelity was high in 0-group juveniles, it was lower for 1-group juveniles, possibly due to an increase in mobility and energy demands with increasing size. Analysis of the food web revealed a complex net of relations. Particulate organic matter from the freshwater sources, from each nursery's waters and saltmarsh creeks presented similar isotopic composition. Sediment isotopic composition and saltmarsh halophytes also did not differentiate the two areas. All components of the food web from the benthic microalgae upwards were isotopically different between the nursery areas. These components were always more enriched in δ 13C and δ 15N at the lower nursery area than at the nursery located upstream, appearing as if there were two parallel trophic chains with little trophic interaction between each other. A mixture of carbon and nitrogen sources is probably being incorporated into the food web. The lower nursery area is more dependent upon an isotopically enriched energy pathway, composed of marine particulate organic matter, marine benthic microalgae and detritus of the C 4 saltmarsh halophyte Spartina maritima. The two nursery areas present a different level of dependence upon the freshwater and marine energy pathways, due to hydrological features, which should be taken into account for S. solea and S. senegalensis fisheries and habitat management.

Rare earth element geochemistry and Nd isotope composition of stream sediments, south Han River drainage basin, Korea

Chemical analyses of parent rock, soil and stream sediments were done to determine to what extent the river sediments retain the geochemical signature of the parent rocks. The study area consists mainly of Mesozoic biotite granite and Precambrian biotite gneiss, located in the central Korean peninsula. The drainage system consists of tributaries of the South Han River, such as Bokhacheon, Shinduncheon and Yanghwacheon. Stream sediments were collected every March during 2004–2006. The sediments collected were divided into coarse (gravel to sand size) and fine (silt to clay size). More than 90% is coarse sediment. The major element composition of the coarse and fine sediments are not identical to the weathering trend of granite. However, molar proportions of Al 2O 3, CaO+Na 2O, and K 2O in the sediments show that they were derived from granitoids rather than from biotite gneiss. The total Fe 2O 3, CaO, MgO, Na 2O, and MnO contents and REE concentrations of the fine sediments are higher than those of the coarse sediments. In a weathering profile from the granite and gneiss area, there was no change of REE distribution pattern from the fresh rock to soil. Compared to biotite granite-normalized REE pattern for the sediment, both coarse and fine sediments have relatively flattened REE patterns, and all the coarse sediment have positive Eu anomalies, whereas in the fine sediments negative Eu anomalies prevail. Nd isotopic composition of the sediments is similar to that of the biotite granite rather than the biotite gneiss. Stream sediments in the Icheon area were derived from the granite. Nd isotope geochemistry as well as REE geochemistry is useful tool for tracing the provenance of the river sediments.

Large‐scale drainage capture and surface uplift in eastern Tibet–SW China before 24 Ma inferred from sediments of the Hanoi Basin, Vietnam

Current models of drainage evolution suggest that the non‐dendritic patterns seen in rivers in SE Asia reflect progressive capture of headwaters away from the Red River during and as a result of surface uplift of Eastern Asia. Mass balancing of eroded and deposited rock volumes demonstrates that the Red River catchment must have been much larger in the past. In addition, the Nd isotope composition of sediments from the Hanoi Basin, Vietnam, interpreted as paleo‐Red River sediments, shows rapid change during the Oligocene, before ∼24 Ma. We interpret this change to reflect large‐scale drainage capture away from the Red River, possibly involving loss of the middle Yangtze River. Reorganization was triggered by regional tilting of the region towards the east. This study constrains initial surface uplift in eastern Tibet and southwestern China to be no later than 24 Ma, well before major surface uplift and gorge incision after 13 Ma.

Investigating Pu and U isotopic compositions in sediments: a case study in Lake Obuchi, Rokkasho Village, Japan using sector-field ICP-MS and ICP-QMS

The objectives of the present work were to study isotope ratios and the inventory of plutonium and uranium isotope compositions in sediments from Lake Obuchi, which is in the vicinity of several nuclear fuel facilities in Rokkasho, Japan. Pu and its isotopes were determined using sector-field ICP-MS and U and its isotopes were determined with ICP-QMS after separation and purification with a combination of ion-exchange and extraction chromatography. The observed (240)Pu/(239)Pu atom ratio (0.186 +/- 0.016) was similar to that of global fallout, indicating that the possible early tropospheric fallout Pu did not deliver Pu from the Pacific Proving Ground to areas above 40 degrees N. The previously reported higher Pu inventory in the deep water area of Lake Obuchi could be attributed to the lateral transportation of Pu deposited in the shallow area which resulted from the migration of deposited global fallout Pu from the land into the lake by river runoff and from the Pacific Ocean by tide movement and sea water scavenging, as well as from direct soil input by winds. The (235)U/(238)U atom ratios ranged from 0.00723 to 0.00732, indicating the natural origin of U in the sediments. The average (234)U/(238)U activity ratio of 1.11 in a sediment core indicated a significant sea water U contribution. No evidence was found for the release of U containing wastes from the nearby nuclear facilities. These results will serve as a reference baseline on the levels of Pu and U in the studied site so that any further contamination from the spent nuclear fuel reprocessing plants, the radioactive waste disposal and storage facilities, and the uranium enrichment plant can be identified, and the impact of future release can be rapidly assessed.

Nitrogen geochemistry of subducting sediments: New results from the Izu‐Bonin‐Mariana margin and insights regarding global nitrogen subduction

Toward understanding of the subduction mass balance in the Izu‐Bonin‐Mariana (IBM) convergent margin, we present an inventory of N and C concentrations and isotopic compositions in sediments obtained on Ocean Drilling Program (ODP) Legs 129 and 185. Samples from Sites 1149, 800, 801, and 802 contain 5 to 661 ppm total N (organic, inorganic combined) with δ15NAir of −0.2 to +8.2‰ (all δ15N values &lt;+2.5‰ from Site 800). At Site 1149, N content is higher in clay‐rich layers and lower in chert and carbonate layers, and δ15N shows a distinct down‐section decrease from 0 to 120 mbsf (near +8.0 at shallow levels to near +4.0‰). Reduced‐C concentration ranges from 0.02 to 0.5 wt.%, with δ13CVPDB of −28.1 to −21.7‰. The down‐section decreases in δ15N and N concentration (and variations in concentrations and δ13C of reduced C, and Creduced/N) at Site 1149 could help reconcile differences between δ15N values of modern deep‐sea sediments from near the sediment‐water interface and values for forearc metasedimentary rocks. At Site 1149, negative shifts in δ15N, from marine organic values (up to ∼+8‰) toward lower values approaching those for the metasedimentary rocks (+1 to +3‰), are most likely caused by complex diagenetic processes, conceivably with minor effects of changes in productivity and differing proportions of marine and terrestrial organic matter. However, the forearc metamorphic suites (e.g., Franciscan Complex) are known to have been deposited nearer continents, and their lower δ15N at least partly reflects larger proportions of lower‐δ15N terrestrial organic matter. Subduction at the Izu‐Bonin (IB) margin, of a sediment section like that at Site 1149, would deliver an approximate annual subduction flux of 2.5 × 106 g of N and 1.4 × 107 g of reduced C per linear kilometer of trench, with average δ15N of +5.0‰ and δ13C of −24‰. Incorporating the larger C flux of 9.2 × 108 g/yr/linear‐km in carbonate‐rich layers of 1149B (average δ13C = +2.3‰) provides a total C flux of 9.3 × 108 g/yr/linear‐km (δ13C = +1.9‰). Once subducted, sediments are shifted to higher δ15N by N loss during devolatilization, with magnitudes of the shifts depending on the thermal evolution of the margin.

Recent changes in platinum group element concentrations and osmium isotopic composition in sediments from an urban lake.

Automobile catalyst emissions have resulted in the occurrence of elevated Pt, Pd, and Rh concentration in the urban and roadside environment. We investigate the chronology of platinum group elements (PGE) accumulation in dated sediments from an urban lake near Boston, MA. Chronological profiles demonstrate that Pt, Pd, and Rh concentrations increased following the introduction in catalysts with accumulation rates 6-16 times larger in 1992-2002 than prior to the introduction of catalysts. Ratios of these elements closely match their ratios in catalysts, providing further evidence of an automobile source. Iridium and Ru accumulation in sediments also increased following the introduction of catalysts, and while past Os contamination is associated with leather tanning, recent changes in the isotopic composition of Os indicate another anthropogenic source for this element. The PGE have similar geochemical properties and are difficult to separate from one another, and therefore, we suggest that automobile catalyst emissions also result in increasing environmental concentrations of Ir, Ru, and Os, which occur as impurities in catalysts. An automobile catalyst source of Ir and Os is supported by elevated concentrations of these elements in a tunnel dust sample.