Isotopic Composition Of Sediments Research Articles

Iron isotope biogeochemistry of Neoproterozoic marine shales

Iron isotopes have been widely applied to investigate the redox evolution of Earth’s surface environments. However, it is still unclear whether iron cycling in the water column or during diagenesis represents the major control on the iron isotope composition of sediments and sedimentary rocks. Interpretation of isotopic data in terms of oceanic redox conditions is only possible if water column processes dominate the isotopic composition, whereas redox interpretations are less straightforward if diagenetic iron cycling controls the isotopic composition. In the latter scenario, iron isotope data is more directly related to microbial processes such as dissimilatory iron reduction. Here we present bulk rock iron isotope data from late Proterozoic marine shales from Svalbard, northwestern Canada, and Siberia, to better understand the controls on iron isotope fractionation in late Proterozoic marine environments. Bulk shales span a δ56Fe range from −0.45‰ to +1.04‰. Although δ56Fe values show significant variation within individual stratigraphic units, their mean value is closer to that of bulk crust and hydrothermal iron in samples post-dating the ca. 717–660Ma Sturtian glaciation compared to older samples. After correcting for the highly reactive iron content in our samples based on iron speciation data, more than 90% of the calculated δ56Fe compositions of highly reactive iron falls in the range from ca. −0.8‰ to +3‰. An isotope mass-balance model indicates that diagenetic iron cycling can only change the isotopic composition of highly reactive iron by <1‰, suggesting that water column processes, namely the degree of oxidation of the ferrous seawater iron reservoir, control the isotopic composition of highly reactive iron. Considering a long-term decrease in the isotopic composition of the iron source to the dissolved seawater Fe(II) reservoir to be unlikely, we offer two possible explanations for the Neoproterozoic δ56Fe trend. First, a decreasing supply of Fe(II) to the ferrous seawater iron reservoir could have caused the reservoir to decrease in size, allowing a higher degree of partial oxidation, irrespective of increasing environmental oxygen levels. Alternatively, increasing oxygen levels would have led to a higher proportion of Fe(II) being oxidized, without decreasing the initial size of the ferrous seawater iron pool. We consider the latter explanation as the most likely. According to this hypothesis, the δ56Fe record reflects the redox evolution of Earth’s surface environments. δ56Fe values in pre-Sturtian samples significantly heavier than bulk crust and hydrothermal iron imply partial oxidation of a ferrous seawater iron reservoir. In contrast, mean δ56Fe values closer to that of hydrothermal iron in post-Sturtian shales reflects oxidation of a larger proportion of the ferrous seawater iron reservoir, and by inference, higher environmental oxygen levels. Nevertheless, significant iron isotopic variation in post-Sturtian shales suggest redox heterogeneity and possibly a dominantly anoxic deep ocean, consistent with results from recent studies using iron speciation and redox sensitive trace metals. However, the interpretation of generally increasing environmental oxygen levels after the Sturtian glaciation highlights the need to better understand the sensitivity of different redox proxies to incremental changes in oxygen levels to enable us to reconcile results from different paleoredox proxies.

North American origin of “pink–white” layers at the Mendeleev Ridge (Arctic Ocean): New insights from lead and neodymium isotope composition of detrital sediment component

Dolomite–rich layers of distinct pinkish colour are used as lithostratigraphic markers in the Amerasian Basin of the Arctic Ocean. However, origin of dolomite in these sediment units has not been studied in detail. In this study, lead (Pb) and neodymium (Nd) isotope composition of detrital clay–size fraction from different lithofacies were investigated in core PS72/340-5 recovered at the eastern flank of the Mendeleev Ridge. Prior to the geochemical analyses, grain–size distribution was analyzed in order to minimize the grain–size effect on the provenance signature. For provenance discrimination, results of isotope measurements were compared with marine surface sediment data and values for the circum–Arctic subaerial provinces. Late Quaternary sediment supply variability was analyzed using the mixing model constrained by two tracers: 207Pb/206Pb and εNd. Variations of sediment isotopic composition are inferred to result from mixing of volcanic and plutonic components. Usage of Pb isotopic ratios alone does not allow distinction between these two types of sources. Results confirm that over the last estimated ca. 200ka dolomite–rich pink layers at the Mendeleev Ridge were deposited during events probably associated with intensified iceberg transport from North America. In general, however, sedimentation was mostly controlled by terrigenous input from the Chukchi and East Siberian Seas, whereas sediment supply from the Laptev Sea area remained less important and relatively constant at the studied location.

Tracing of anthropogenic zinc sources in coastal environments using stable isotope composition

The use of zinc isotopes to trace anthropogenic sources in coastal areas has been tested in this study. We determined the stable isotopic composition of zinc in sediment cores, suspended particulate matter (SPM) and rocks collected at the Sepetiba Bay (southeastern Brazil), an estuarine lagoon heavily impacted by metallurgic activities. These isotopic signatures were compared with those from willemite ore, which represent the main mineral refined by the major industrial source of zinc. The aim was to test if this tracer system enables to identify sources and sinks of anthropogenic zinc and to reconstruct the temporal and spatial evolution of zinc contamination. The zinc isotopic compositions (expressed using the δ66Zn notation relative to the JMC 3-0749-L solution) showed significant variations in the sediment cores, the SPM, and willemite ore minerals, ranging between −0.01 and +1.15‰. Spatial and temporal analysis of sediments samples fit well in a model of mixing involving three main end-members: i) Terrestrial background (δ66ZnJMC=+0.28±0.12‰, 2σ); ii) marine detrital material (δ66ZnJMC=+0.45±0.03‰, 2σ); and iii) a major anthropogenic source associated with electroplating wastes released into the bay (δ66ZnJMC=+0.86±0.15‰, 2σ). Sediment cores collected in the mud flats showed high correlation between δ66Zn and zinc enrichment factors, suggesting good preservation of the isotopic records of natural and anthropogenic sources. The sediment core sampled from a mangrove wetland located in a zone impacted by the metallurgy presented levels of zinc up to 4% (sediment dry weight) and preserved the isotopic signatures of electroplating wastes, despite evidences that post depositional processes slightly changed the isotopic signatures in some layers from this core toward heavier δ66ZnJMC values (above +1.0‰). A two component mixing model suggests contributions of this major anthropogenic Zn source up to nearly 80% during periods of electroplating activities. Our work suggests that Zn isotope compositions of sediments are reliable tracers of anthropogenic sources and, therefore, can be useful to improve environmental monitoring efforts in coastal systems.

Stable isotopes and digital elevation models to study nutrient inputs in high-arctic lakes

Five major factors control nutrient and organic matter inputs in high-Arctic lakes, all potentially affected by climate change: ice cover; run-off from the watershed; aquatic and terrestrial primary productivity; guano deposition from birds. Quantifying these controls is a key first step to understand what combination of factors underlies the biological productivity in Arctic lakes and drives their ecological response to climate change. Based on C and N elemental content and stable isotope analysis in sediments belonging to three lakes in North Spitsbergen (Svalbard), Digital Elevation Models and drainage maps, we propose an integrated approach for the analysis of nutrient and organic matter inputs in lakes and the role of catchment hydro-geomorphology in determining inter-lake differences in the isotopic composition of sediments. Given its high run-off and large catchment, organic deposits in Tvillingvatnet were dominated by terrestrial inputs, whereas inputs were mainly of aquatic origin in Storvatnet, a lowland lake characterised by low potential run-off. In Kolhamna, organic deposits seemed to be dominated by inputs from birds. Isotopic signatures were similar between samples within each lake, representing precise tracers for studies on the effect of climate change on biogeochemical cycles in lakes. The presented approach proved to be an effective research pathway for the identification of factors underlying nutrient and organic matter inputs within each water body, as well as for the modelling of expected changes in nutrient content associated with changes in isotopic composition of sediments.

Source rocks of Carboniferous-Lower Cretaceous terrigenous sediments of the northeastern Siberian Platform: results of Sm-Nd isotope-geochemical studies

The first Sm-Nd isotope studies of the Carboniferous-Early Cretaceous clastic rocks of the northeastern Siberian Platform have been carried out. Variation in the isotope composition of sediments within this time interval has been determined and interpreted. The high £Nd(t) values for Carboniferous-Permian sediments (from -11 to -2) testify to the large contribution of the products of erosion of island-arc and juvenile complexes localized in the Taimyr-Severnaya Zemlya fold-thrust belt in the Carboniferous. The positive £Nd(t) values for Triassic sandstones suggest erosion of the coeval igneous rocks of the trap association. The Upper Jurassic and, particularly, Cretaceous sediments of the Lena-Anabar depression and Verkhoyansk foreland basin are characterized by extremely negative £Nd(t) values (from -15 to -19), which is evidence for erosion of mature continental crust. The Carboniferous-Triassic complexes of the Verkhoyansk fold-thrust belt, which show higher £Nd(t) values, cannot have been a source of detritus for Cretaceous sandstones. The sediments filling the Verkhoyansk foreland basin accumulated as a result of the decomposition of homogenized distal source rocks with the isotope characteristics of the old crust rather than the breakup of the mountain range rising to the east. Salients of the crystalline basement of the Siberian Platform, such as the Aldan Shield, might have been these source provinces.

Environmental Hf–Nd isotopic decoupling in World river clays

The hafnium and neodymium radiogenic isotope systems behave differently during Earth surface processes, causing a wide dispersion of Hf and Nd isotopic compositions in sediments and other sedimentary rocks. The decoupling between Hf and Nd isotopes in sediments is generally attributed to a combination of preferential sorting of zircon during sediment transport and incongruent weathering processes on continents. In this study, we analysed size-fractions of sediment samples collected near the mouth of 53 rivers worldwide to better understand the factors controlling the distribution of Hf and Nd isotopes in sediments. Our results for rivers draining old cratonic areas and volcanic provinces demonstrate that both granite and basalt weathering can lead to significant grain-size dependent Hf isotopic variability. While silt-size fractions mainly plot along the Terrestrial Array, World river clays are systematically shifted towards more radiogenic Hf isotopic compositions, defining together with published data a new Clay Array (εHf=0.78×εNd+5.23). The Hf–Nd isotope decoupling observed in volcanogenic sediments is best explained by selective alteration of Lu-rich mineral phases (e.g. olivine) and preferential enrichment of resistant unradiogenic minerals, such as spinel and ilmenite, in silt fractions. We also show that the extent to which World river clays deviate from the Clay Array (ΔεHf clay) is not linked to the presence of zircons. Instead, it correlates positively with weathering indices and climatic parameters (temperature, rainfall) of the corresponding drainage basins. Overall, these findings demonstrate that the distribution of Hf–Nd isotopes in clay-size sediments is related to a large extent to weathering conditions on continents, although the precise mechanisms controlling this relationship remain unclear. We finally propose that the Hf–Nd isotope pair proxy could be used in palaeoenvironmental studies to provide semi-quantitative information on past climates.

Sources and fate of mercury pollution in Almadén mining district (Spain): Evidences from mercury isotopic compositions in sediments and lichens

Variations in mercury (Hg) isotopic compositions have been scarcely investigated until now in the Almadén mining district (Spain), which is one of the most impacted Hg areas worldwide. In this work, we explore and compare Hg isotopic signatures in sediments and lichens from Almadén mining district and its surroundings in order to identify and trace Hg aquatic and atmospheric contamination sources. No statistically significant mass independent fractionation was observed in sediments, while negative Δ201Hg values from −0.12 to −0.21‰ (2SD = 0.06‰) were found in lichens. A large range of δ202Hg values were reported in sediments, from −1.86 ± 0.21‰ in La Serena Reservoir sites far away from the pollution sources to δ202Hg values close to zero in sediments directly influenced by Almadén mining district, whereas lichens presented δ202Hg values from −1.95 to −0.40‰ (2SD = 0.15‰). A dilution or mixing trend in Hg isotope signatures versus the distance to the mine was found in sediments along the Valdeazogues River-La Serena Reservoir system and in lichens. This suggests that Hg isotope fingerprints in these samples are providing a direct assessment of Hg inputs and exposure from the mining district, and potential information on diffuse atmospheric contamination and/or geochemical alteration processes in less contaminated sites over the entire hydrosystem. This study confirms the applicability of Hg isotope signatures in lichens and sediments as an effective and complementary tool for tracing aquatic and atmospheric Hg contamination sources and a better constraint of the spatial and temporal fate of Hg released by recent or ancient mining activities.

Anthropogenic Pb input into Bohai Bay, China: Evidence from stable Pb isotopic compositions in sediments

To investigate the source of Pb within Bohai Bay, Pb concentrations and Pb isotopic compositions (204Pb, 206Pb, 207Pb, and 208Pb) of surface sediments in this area were determined. The Pb concentration in this bay varied widely from 6.9 to 39.2µg/g (average: 21.8±7.8µg/g), and the Pb isotopic compositions ranged from 0.8338 to 0.8864 (average: 2.0997±0.0180) for 208Pb/206Pb and from 2.0797 to 2.1531 (average: 0.8477±0.0135) for 207Pb/206Pb, presenting in three distinct clusters. The Pb isotopic ratios of sediments from the northeastern (NE zone) and northwestern (NW zone) coastal areas were significantly influenced by anthropogenic sources such as coal combustion and automobile emission. In sediments from the central and southern Bohai Bay (C–S zone); however, Pb mainly originated from the Yellow River catchment, as a result of lithogenic sediment (from rock weathering) accumulation. The Pb isotopic ratios further indicate that, apart from riverine inputs, the neighboring large-scale ports and aerosols significantly contributed to the anthropogenic Pb contained in these sediments. Pb contamination in the Haihe and Luanhe river mouths as well as in the regions near ports is also suggested from anthropogenic enrichment factors. As cities and ports continue to develop around Bohai Bay, a long-term extensive sewage monitoring program is highly recommended.

The nitrogen isotope composition of sediments from the proto‐North Atlantic during Oceanic Anoxic Event 2

AbstractSediment records of the stable isotopic composition of N (δ15N) show light δ15N values at several sites in the proto‐North Atlantic during Oceanic Anoxic Event 2 (OAE 2) at the Cenomanian‐Turonian transition (∼94 Ma). The low δ15N during the event is generally attributed to an increase in N2 fixation and incomplete uptake of for phytoplankton growth. A compilation of all reliable data for the proto‐North Atlantic during OAE 2 demonstrates that the most pronounced negative shift in δ15N from pre‐OAE 2 to OAE 2 occurs in the open ocean but with δ15N never lower than −3‰. Using a box model of N cycling for the proto‐North Atlantic during OAE 2, we show that N2 fixation is a major contributor to the δ15N signal, especially in the open ocean. Incomplete uptake of for phytoplankton growth is important in regions dominated by downwelling, with lateral transport of acting as a major source. In the southern proto‐North Atlantic, where bottom waters were euxinic, the light δ15N signature is largely explained by upwelling of . Our study provides an overview of regional differences in δ15N in the proto‐North Atlantic and highlights the role of lateral exchange of water and nutrients, in addition to local biogeochemical processes, in determining δ15N values of OAE 2 sediments.

Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period

To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average δ238U of most samples is 0.24±0.16‰ (2SD; relative to standard CRM145), which is slightly higher than the average δ238U of 0.02±0.12‰ for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled δ238U value of 0.2‰ for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238U is preferentially removed to euxinic sediments compared to 235U, expanded ocean anoxia will deplete seawater of 238U relative to 235U, ultimately leading to deposition of ORM with low δ238U. Hence, the high δ238U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago.The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H2S]aq >11μM) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV, δ98Mo approaches the modern seawater value of 2.34±0.10‰. High δ98Mo points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O2-rich compared to O2-deficient marine environments. However, the δ98Mo value for most Member IV ORM is near 0‰ (relative to standard NIST SRM 3134=0.25‰), suggesting extensive anoxia. The low δ98Mo is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high δ238U. Hence, we propose that the low δ98Mo of most Member IV ORM was fractionated from contemporaneous seawater. Possible mechanisms driving this isotope fractionation include: (1) inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater-like δ98Mo signature in sediments or (2) delivery of isotopically light Mo to sediments via a particulate Fe–Mn oxyhydroxide shuttle.A compilation of Mo isotope data from euxinic ORM suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late Neoproterozoic and early Paleozoic time. Hence, Member IV does not capture irreversible deep ocean oxygenation. Instead, complex ocean redox variations likely marked the transition from O2-deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans.

Uranium and molybdenum isotope systematics in modern euxinic basins: Case studies from the central Baltic Sea and the Kyllaren fjord (Norway)

Recent investigations have revealed significant fractionation of 238U/235U between organic-rich sediments of anoxic marginal seas and seawater, indicating redox-dependent U isotope fractionation. This study explores the conditions controlling U isotope fractionation in selected modern anoxic basins (Baltic Sea: Landsort and Gotland Deeps and the Kyllaren fjord in Norway) and compares U with Mo isotope fractionation. Therefore, the concentrations and isotope compositions of dissolved U and Mo from the water column and organic-rich sediments from three currently anoxic basins have been analysed.The water column samples from the Kyllaren fjord display a moderate depletion of U and a strong depletion of Mo with increasing depth. These variations are associated with a decrease in δ238U and an increase in δ98Mo with depth, from −0.35 ‰ to −0.70 ‰ and from 2.4 ‰ to 2.6 ‰, respectively. From the U isotope composition of the deep euxinic water column of the Kyllaren fjord, a minimum value of Δ238Ured≈0.7 ‰ for the U isotope fractionation during reduction is inferred. Due to the high sedimentation rate in the Kyllaren fjord, surface sediment samples are only moderately enriched in U and Mo (~4μg/g and 6μg/g to 37μg/g, respectively) and display δ238U and δ98Mo averages of −0.26 ‰±0.10 ‰ and 2.18 ‰±0.21 ‰, respectively.Water column samples from the Baltic Sea display a minor decrease in the U and Mo concentrations and only negligible U and Mo isotope fractionation compared to open seawater. The sediment layers from the Baltic Sea (Landsort Deep), which were deposited under mostly anoxic conditions, are moderately enriched in U (up to 8μg/g) and highly enriched in Mo (up to 222μg/g). However, U and Mo isotopic compositions are significantly lighter (with δ238U of −0.36 ‰±0.08 ‰ and δ98Mo of −0.04 ‰±0.11 ‰) than those of typical organic-rich sediments from anoxic basins.These findings reveal that compared to Mo, the U isotopic composition of sediments in restricted and strongly euxinic basins like the Kyllaren fjord is significantly more dependent on (1) the extent of U removal from the water column, which is lower than that of Mo, and on (2) the sedimentation rate (i.e., the fraction of authigenic U relative to detrital U in the sediment), which is also lower compared to that of Mo. In more open and only temporarily euxinic basins such as the investigated basins of the Baltic Sea, strong Mo is coupled with weak U isotope fractionation between water and sediment. These signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light Mo during frequently occurring flushing events with oxygen-rich water from the open sea. This implies that Mo and U isotope signatures of sediments only record paleo-water column redox conditions of restricted basins if the water column was permanently stratified.

Lead in ancient Rome's city waters.

It is now universally accepted that utilization of lead for domestic purposes and water distribution presents a major health hazard. The ancient Roman world was unaware of these risks. How far the gigantic network of lead pipes used in ancient Rome compromised public health in the city is unknown. Lead isotopes in sediments from the harbor of Imperial Rome register the presence of a strong anthropogenic component during the beginning of the Common Era and the Early Middle Ages. They demonstrate that the lead pipes of the water distribution system increased Pb contents in drinking water of the capital city by up to two orders of magnitude over the natural background. The Pb isotope record shows that the discontinuities in the pollution of the Tiber by lead are intimately entwined with the major issues affecting Late Antique Rome and its water distribution system.

Characterization of n-alkanes and their carbon isotopic composition in sediments from a small catchment of the Dianchi watershed

The biomarker composition and stable carbon isotope values of organic matter (OM) in sediment cores from Shuanglong catchment of the Dianchi watershed show an unimodal n-alkane distribution ranging from C15–C33 with a strong predominance of odd-numbered n-alkanes, maximizing at n-C27, n-C29 and n-C31. Organic carbon to nitrogen (OC/N) ratio indicates a strong terrestrial influence on the OM. The values of δ13C27, δ13C29 and δ13C31 of n-alkanes range from −36.1‰ to −26.1‰, −34.1‰ to −30.1‰ and −33.8‰ to −28.7‰, respectively, suggesting a mainly C3 land plants origin. The carbon preference index (CPI25–31), odd-even preference (OEP27–31), average chain length (ACL25–33), pristine/phytane (pr/ph), Paq, (C27+C29)/2C31, nC16–23/nC24–33 and 3C17/(C21+C23+C25) values are also consistent with the predominance of C3 land plant-derived OM. Different sources of OM are reflected by the peak of n-C15, ascribed to a contribution by aquatic algae and photosynthetic bacteria. Eutrophication seems to be enhanced by both autochthonous (weak) and allochthonous (strong) contributions of OM. A major factor affecting the OM accumulation in the catchment and OM transportation to the Dianchi Lake may be increased by human activities from 1871 to 2011.

Characteristics of Heavy Metals and Pb Isotopic Composition in Sediments Collected from the Tributaries in Three Gorges Reservoir, China

The concentrations, distribution, accumulation, and potential ecological risk of heavy metals (Cr, Cu, Zn, Ni, As, Pb, Cd, and Hg) in sediments from the Three Gorges Reservoir (TGR) tributaries were determined and studied. Pb isotopic compositions in sediments were also measured to effectively identify the potential Pb sources. The results showed that the average concentrations of heavy metals in sediment of TGR tributaries were higher than the local background values of soils and sediments in China. The assessment by Geoaccumulation Index indicated that Cu, Ni, and Hg were at the “slightly polluted” level and Cd was ranked as the “moderately polluted” level in tributary sediments of TGR. The assessment by Potential Ecological Risk Index showed that Hg and Cd were the predominant elements in tributary sediments in TGR. The Pb isotopic ratios in sediments varied from 1.171 to 1.202 for 206Pb/207Pb and from 2.459 to 2.482 for 208Pb/207Pb in TGR. All Pb isotopic ratios in sediments were similar to those from coal combustion, lead ores (the mining activities and smelting process), and cement material, indicating that these anthropogenic inputs may be the main sources for Pb pollution in sediments of TGR tributaries.

Heavy metals pollution and pb isotopic signatures in surface sediments collected from Bohai Bay, North China.

To investigate the characteristics and potential sources of heavy metals pollution, surface sediments collected from Bohai Bay, North China, were analyzed for the selected metals (Cd, Cr, Cu, Ni, Pb, and Zn). The Geoaccumulation Index was used to assess the level of heavy metal pollution. Pb isotopic compositions in sediments were also measured to effectively identify the potential Pb sources. The results showed that the average concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were 0.15, 79.73, 28.70, 36.56, 25.63, and 72.83 mg/kg, respectively. The mean concentrations of the studied metals were slightly higher than the background values. However, the heavy metals concentrations in surface sediments in Bohai Bay were below the other important bays or estuaries in China. The assessment by Geoaccumulation Index indicated that Cr, Zn, and Cd were classified as “the unpolluted” level, while Ni, Cu, and Pb were ranked as “unpolluted to moderately polluted” level. The order of pollution level of heavy metals was: Pb > Ni > Cu > Cr > Zn > Cd. The Pb isotopic ratios in surface sediments varied from 1.159 to 1.185 for 206Pb/207Pb and from 2.456 to 2.482 for 208Pb/207Pb. Compared with Pb isotopic radios in other sources, Pb contaminations in the surface sediments of Bohai Bay may be controlled by the mix process of coal combustion, aerosol particles deposition, and natural sources.

Biocoenosis response to hydrological variability in Southern Africa during the last 84 ka BP: A study of lipid biomarkers and compound-specific stable carbon and hydrogen isotopes from the hypersaline Lake Tswaing

Sea surface temperature (SST) changes in the SW Indian and SE South Atlantic Ocean are proposed to have controlled environmental changes at the southern tip of the African continent during the past. For a better understanding of the hydrological variations and their environmental impact during the past 84kabeforepresent (BP), we analyzed lipid biomarkers and their stable carbon (δ13C) and hydrogen (δD) isotopic compositions in sediments from Lake Tswaing (25°24′30″ S, 28°04′59″ E). δD values of long-chain n-alkanes from higher plants varied between −155 and −82‰ and matched well with SST gradient changes between the tropical and subtropical SW Indian Ocean. Increased SSTs correlate with deuterium-depleted δD values of n-alkanes and indicate a higher convection transporting more moisture across South Africa in the time from 84 to 80kaBP, during the marine isotope stage (MIS) 4 and from 50 to 39kaBP. During these humid periods high amounts of organic matter (up to 9.8% total organic carbon (TOC)) were deposited in Lake Tswaing. Periods with deuterium-enriched δD values of n-alkanes (76kaBP, 73 to 69kaBP, 55 to 51kaBP, and 31.4 to 12.6kaBP), indicating dry climate and increased lake water salinity, were associated with lower TOC contents and a higher degradation state of the terrestrial organic matter. We occasionally detected algae-derived C37- and C38-alkenones in the core, particularly in times when lake salinity and alkalinity were elevated. The disappearance of alkenones around 25kaBP was accompanied by highly depleted δD values of n-alkanes from aquatic organisms (−180‰ for n-C23) indicating a substantial freshening of the lake water. The vegetation surrounding Lake Tswaing during the last 84kaBP consisted mainly of savanna-type plants, such as trees and shrubs with varying contributions from C4 grasses. The latter predominated particularly at the end of MIS 5 and during arid climatic conditions in MIS 3. We show that the changing vegetation controls the n-alcohol to n-alkane ratio of the leaf waxes; this is reflected by the higher plant alkane (HPA) index which has potential as a new climate-controlled vegetation proxy; in semi-arid periods HPA index values were elevated reflecting savanna vegetation which is adapted to drier climate, whereas HPA index values were lower during more humid conditions.

Multiple sulfur and carbon isotope composition of sediments from the Belingwe Greenstone Belt (Zimbabwe): A biogenic methane regulation on mass independent fractionation of sulfur during the Neoarchean?

To explore the linkage between mass-independent sulfur isotope fractionation (MIF-S) and δ13Corg excursions during the Neoarchean, as well as the contemporary redox state and biogeochemical cycling of carbon and sulfur, we report the results of a detailed carbon and multiple sulfur (δ34S, δ33S, δ36S) isotopic study of the ∼2.7Ga Manjeri and ∼2.65Ga Cheshire formations of the Ngezi Group (Belingwe Greenstone Belt, Zimbabwe).Multiple sulfur isotope data show non-zero Δ33S and Δ36S values for sediments older than 2.4Ga (i.e. prior to the Great Oxidation Event, GOE), indicating MIF-S thought to be associated with low atmospheric oxygen concentration. However, in several 2.7–2.5Ga Neoarchean localities, small-scale variations in MIF-S signal (magnitude) seem to correlate with negative excursion in δ13Corg, possibly reflecting a global connection between the relative reaction rate of different MIF-S source reaction and sulfur exit channels and the biogenic flux of methane into the atmosphere during periods of localized, microbiologically mediated, shallow surface-water oxygenation.The Manjeri Formation black shales studied here display a wide range of δ13Corg between −35.4‰ and −16.2‰ (average of −30.3±6.0‰, 1σ), while the Cheshire Formation shales have δ13Corg between −47.7‰ and −35.1‰ (average −41.3±3‰, 1σ). The δ34S values of sedimentary sulfides from Manjeri Formation vary between −15.15‰ and +2.37‰ (average −1.71±4.76‰, 1σ), showing very small and mostly negative Δ33S values varying from −0.58‰ to 0.87‰ (average 0.02±0.43‰, 1σ). Cheshire Formation black shale sulfide samples measured in this study have δ34S values ranging from −2.11‰ to 2.39‰ (average 0.25±1.08‰, 1σ) and near zero and solely positive Δ33S anomalies between 0.14‰ and 1.17‰ (average 0.56±0.29‰, 1σ). Moreover, Δ36S/Δ33S in the two formations are comparable with a slope of −1.38 (Manjeri Formation) and −1.67 (Cheshire Formation), respectively. This differs from the Archean reference line (i.e. −0.9).The sulfur and carbon isotopic signatures recorded here likely reflect different environmental conditions and/or ecosystems within the sampled Cheshire and Manjeri formations paleofacies. The differences in carbon isotopes in different sedimentological facies are interpreted as recording different metabolic pathways, including photosynthesis, methanogenesis and methanotrophy. Low δ13Corg values in the Cheshire Formation black shales suggest a strong influence of methanotrophy (likely preceded by bacterial methanogenesis), while the large range of Manjeri Formation δ34S may record bacterial sulfate reduction. The C and multiple S isotopic variations recorded here may imply different patterns of C and S fractionation, perhaps dependent on variations in the biogenic egress of methane to the atmosphere.

Cd isotopes as a potential source tracer of metal pollution in river sediments

Tracing the sources of heavy metals in water environment is key important for our understanding of their pollution behavior. In this present study, Cd concentrations and Cd isotopic compositions in sediments were determined to effectively identify possible Cd sources. Results showed that elevated concentrations and high enrichment factor for Cd were found in all sediments, suggesting anthropogenic Cd origin. Cd isotopic compositions in sediments yielded relative variations ranged from −0.35‰ to 0.07‰ in term of δ114/110Cd (the mean: −0.08‰). Large fractionated Cd was found in sediments collected from a smelter and an E-waste town. Cd isotopic compositions and Cd concentrations measured in sediments allowed the identification of three main origins (dust from metal refining (δ114/110Cd < 0), slag of metal refining (δ114/110Cd > 0), and those δ114/110Cd = 0, such as background and mining activity). According to the actual precision obtained, Cd isotopes could be a potential tool for tracing metal pollution sources in water environment.

Environmental drivers of megafaunal assemblage composition and biomass distribution over mainland and insular slopes of the Balearic Basin (Western Mediterranean)

The influence of mesoscale physical and trophic variables on deep-sea megafauna, a scale of variation often neglected in deep-sea studies, is crucial for understanding their role in the ecosystem. Drivers of megafaunal assemblage composition and biomass distribution have been investigated in two contrasting areas of the Balearic basin in the NW Mediterranean: on the mainland slope (Catalonian coasts) and on the insular slope (North of Mallorca, Balearic Islands). An experimental bottom trawl survey was carried out during summer 2010, at stations in both sub-areas located between 450 and 2200m water depth. Environmental data were collected simultaneously: near-bottom physical parameters, and the elemental and isotopic composition of sediments. Initially, data were analysed along the whole depth gradient, and then assemblages from the two areas were compared. Analysis of the trawls showed the existence of one group associated with the upper slope (US=450–690m), another with the middle slope (MS=1000–1300m) and a third with the lower slope (LS=1400–2200m). Also, significant differences in the assemblage composition were found between mainland and insular slopes at MS. Dominance by different species was evident when the two areas were compared by SIMPER analysis. The greatest fish biomass was recorded in both areas at 1000–1300m, a zone linked to minimum temperature and maximum O2 concentration on the bottom. Near the mainland, fish assemblages were best explained (43% of total variance, DISTLM analysis) by prey availability (gelatinous zooplankton biomass). On the insular slope, trophic webs seemed less complex and were based on vertical input of surface primary production. Decapods, which reached their highest biomass values on the upper slope, were correlated with salinity and temperature in both the areas. However, while hydrographic conditions (temperature and salinity) seemed to be the most important variables over the insular slope, resource availability (gelatinous zooplankton and Calocaris macandreae) predominated and explained 59% of decapod assemblage variation over the mainland slope. Both fish and decapods were linked to net primary production recorded over the mainland 3 months before sampling, while the delay between the input of food from the surface and fish abundance was only 1 month on the insular slope. Our results suggest that trophic relationships over insular slopes probably involve a shorter food chain than over mainland slopes and one that is likely more efficient in terms of energy transfer.

Asian dust input in the western Philippine Sea: Evidence from radiogenic Sr and Nd isotopes

AbstractThe radiogenic strontium (Sr) and neodymium (Nd) isotope compositions of the detrital fraction of surface and subsurface sediments have been determined to trace sediment provenance and contributions from Asian dust off the east coast of Luzon Islands in the western Philippine Sea. The Sr and Nd isotope compositions have been very homogenous near the east coast of the Luzon Islands during the latest Quaternary yielding relatively least radiogenic Sr (87Sr/86Sr = 0.70453 to 0.70491) and more radiogenic Nd isotope compositions (εNd(0) = +5.3 to +5.5). These isotope compositions are similar to Luzon rocks and show that these sediments were mainly derived from the Luzon Islands. In contrast, the Sr and Nd isotope compositions of sediments on the Benham Rise and in the Philippine Basin are markedly different in that they are characterized by overall more variable and more radiogenic Sr isotope compositions (87Sr/86Sr = 0.70452 to 0.70723) and less radiogenic Nd isotope compositions (εNd(0) = −5.3 to +2.4). The Sr isotope composition in the Huatung Basin is intermediate between those of the east coast of Luzon and Benham Rise, but shows the least radiogenic Nd isotope compositions. The data are consistent with a two end‐member mixing relationship between Luzon volcanic rocks and eolian dust from the Asian continent, which is characterized by highly radiogenic Sr and unradiogenic Nd isotope compositions. The results show that Asian continental dust contributes about 10–50% of the detrital fraction of the sediments on Benham Rise in the western Philippine Sea, which offers the potentials to reconstruct the climatic evolution of eastern Asia from these sediments and compare this information to the records from the central and northern Pacific.