A depositional history of particulate organic carbon in a floodplain lake from the lower Ob’ River, Siberia

Northern, high latitude soils have stored vast amounts of organic carbon (OC) in permafrost and peats for many millennia, however, climate change may mobilize and release this particulate OC (POC) to arctic rivers. Deltaic and floodplain lakes that receive fluvial sediments, primarily during the spring freshet, may provide records of such changes in riverine POC. Here, we examine properties of OC in a sediment core from a lake in the lower floodplain of the Ob’ River, west Siberia, to determine how the properties of OC deposited in this lake varied over many decades and to evaluate use of this sedimentary OC as a recorder of riverine POC load and properties. The core predates the most recent, dramatic changes in arctic climate and hence may serve as a benchmark against which to contrast future variations in fluvial POC discharge. Elemental, stable carbon and radiocarbon isotopic analyses, along with nuclear magnetic resonance (NMR) spectroscopy and molecular-level information (lignin phenol composition), indicate two major sources of OC to most of the sediments in this lake: plant-derived OC and algal-derived OC. However, a mixing model indicates that the nature and ¹⁴C content of these two sources change with depth in the sediment, resulting in three distinct layers: surface horizons, a “high-OC” layer and “mixing” horizons found above and below the high-OC layer. The plant-derived component is significantly aged throughout the core (¹⁴C ages of 1300-3900 years) and appears to derive from primarily local, tundra sources, whereas the algal component is modern. Our analysis suggests that the usual mode of OC deposition, as exemplified by the “mixing” and surface horizons, involved mixing of varying amounts of new algal production (35-65%) with aged permafrost- or peat-derived OC. This deposition was interrupted by an event, such as the collapse of a riverbank, which laid down the compositionally distinct “high-OC” layer in which plant-derived OC mixes with aged mineral-soil-derived OC without clear input from algae. The relative amounts of the plant and algal components in the lake sediments appear to be controlled primarily by local hydrological conditions rather than by river-wide processes, suggesting that comparison of sediment records from multiple lakes within a floodplain will be important to assess changes in POC export by arctic rivers. However, the flux and nature of the higher plant-derived OC may carry important information on the sources and dynamics of OC stored within the drainage basin.     

A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).     

Variability of organic carbon isotope and C/N in the Hemudu Area,Hangzhou Bay and its environmental implications in the Holocene

With the purpose of tracing the variations of the organic matter sources of sediments, a sample column (25.96 m in size) from the Hemudu Area of Hangzhou Bay was put through AMS¹⁴C dating and biogeochemical analysis. TOC and TN presented similar variation trends, suggesting the same and stable sources of organic matter; the bad correlation between the grain size and TOC content indicated that the organic matter occurrence was neither controlled by the grain size nor the surface absorption of the fine particles, but it may has something to do with the complicated sedimentary hydrodynamic force, the input of organic matter and microbial action. Judging on the basis of C/N ratio and δ¹³C_TOC, the organic carbon in sediments was a mixture of terrigenous and marine organic carbon, testifying to the land-sea interaction characteristic of the study area. The indexes experienced abrupt changes at about 6.5 ka BP, when the lighter terrigenous organic carbon made an increased proportion to the heavier marine organic carbon. The phenomenon reflected the enclosure of the lagoon and the reduced exchange interactions with the seawater of open seas.     

Natural-abundance radiocarbon as a tracer of assimilation of petroleum carbon by bacteria in salt marsh sediments

The natural abundance of radiocarbon (¹⁴C) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA)—one heavily contaminated by petroleum residues—was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C₁₅ and C₁₇, 10-methyl-C₁₆) were depleted in ¹⁴C in the contaminated sediment (mean Δ¹⁴C value of +25 ± 19‰ for bacterial PLFAs) relative to PLFAs in uncontaminated “control” sediment (Δ¹⁴C = +101 ± 12‰). We suggest that the ¹⁴C-depletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms. Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates.     

18.5kaB. P.以来东北四海龙湾玛珥湖全岩有机碳同位素记录及其古气候环境意义

结合含水量、TOC含量和TOC/TN比值变化曲线,18·5kaB·P·以来的四海龙湾玛珥湖沉积物全岩有机碳同位素组成（^δ13C_TOC）记录可划分为3个阶段:1)末次冰期晚期（18·5～14·7kaB·P·）,^δ13C_TOC值偏正,变化范围为-29·50‰～-26·18‰,平均值约为-28·10‰;2)末次冰消期（14·7～11·7kaB·P·）,^δ13C_TOC值显著偏负,变化范围为-33·92‰～-28·40‰,平均值约为-31·75‰,在^δ13C_TOC值变化曲线上表现为一个低谷,但在类似YoungerDryas的冷干事件期间（12·7～11·7kaB·P·）,^δ13C_TOC值再次显著偏正,最高可达-28·4‰;3)全新世以来（11·7kaB·P.至今）,^δ13C_TOC值变化幅度不大（-30·85‰～-27·37‰）,基本上都在平均值-29·1‰左右。研究表明,大气CO₂浓度变化是影响18·5kaB·P·以来四海龙湾玛珥湖^δ13C_TOC值变化的主导因素。     

Carbon Geochemistry of Gas Hydrate‐associated Sediments in the Southwestern Taiwan Basin

Marine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate-bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973-3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon(DIC) and high total organic carbon(TOC)(mean = 0.46%) originated from terrigenous organic matter(mean δ~(13)C_(TOC) value of-23.6‰) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon(IC), and the depleted δ~(13)C_(IC), coupled with the increase of TOC and the decrease of δ~(13)C_(TOC) values owing to the methanotroph ‘Methanosarcinales/ANME' in 430–840 cm. Environmental factors and archaeal communities in core 973-3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate-associated sediments.     

The carbon isotopic response of algae, (cyano)bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367)

The stable carbon isotopic compositions of free and sulfur (S)-bound biomarkers derived from algae, (cyano)bacteria, archaea and higher plants and total organic carbon (TOC) during the first phase of the late Cenomanian/Turonian oceanic anoxic event (OAE) were measured in black shales deposited in the southern proto-Atlantic Ocean in the Cape Verde basin (DSDP Site 367) to determine the response of these organisms to this major perturbation of the global carbon cycle resulting from widespread burial of marine organic matter. The average positive isotope excursions of TOC and biomarkers varied from 5.1‰ to 8.3‰. The δ¹³C values were cross correlated to infer potential common sources of biomarkers. This revealed common sources for C₃₁ and C₃₂ hopanes but no 1:1 relationship for pristane and phytane. The correlation of δ¹³C_TOC with the δ¹³C value of sulfur (S)-bound phytane is the strongest. This is because S-bound phytane is derived from phytol that originates from all marine primary producers (algae and cyanobacteria) and thus represents a weighted average of their carbon isotopic compositions. The δ¹³C values of S-bound phytane and C₃₅ hopane were also used to estimate pCO₂ levels. Before the OAE burial event, pCO₂ levels are estimated to be ca. 1300 ppmv using both biomarkers and the independent maximum Rubisco fractionation factors. At times of maximum organic carbon burial rates during the OAE, reconstructed pCO₂ levels are estimated to be ca. 700 ppmv. However, compared to other C/T OAE sections the positive isotope excursion of S-bound phytane is also affected by an increased production during the OAE. When we compensate for this, we arrive at pCO₂ levels around 1000 ppmv, a reduction of ca. 25%. This indicates that burial of organic matter can have a large effect on atmospheric CO₂ levels.     

Recycling of Organic Matter in the Sediments of Santa Monica Basin,California Borderland

Geochemical and isotopic data for the uppermost 1.2 m of the sediments of the central Santa Monica Basin plain were examined to better understand organic matter deposition and recycling at this site. Isotopic signatures (Δ¹⁴C and δ¹³C) of methane (CH₄) and dissolved inorganic carbon (DIC) indicate the occurrence of anaerobic oxidation of CH₄ that is fueled by CH₄ supplied from a relict reservoir that is decoupled from local organic carbon (C_org) degradation and methanogenesis. This finding was corroborated by a flux budget of pore-water solutes across the basal horizon of the profile. Together these results provide a plausible explanation for the anomalously low ratio between alkalinity production and sulfate consumption reported for these sediments over two decades ago. Shifts in Δ¹⁴C and δ¹³C signatures of C_org have previously been reported across the 20-cm depth horizon for this site and attributed to a transition from oxic to anoxic bottom water that occurred ~350 years BP. However, we show that this horizon also coincides with a boundary between the base of a hemipelagic mud section and the top of a turbidite interval, complicating the interpretation of organic geochemical data across this boundary. Radiocarbon signatures of DIC diffusing upward into surface sediments indicate that remineralization at depth is supported by relatively ¹⁴C-enriched C_org within the sedimentary matrix. While the exact nature of this C_org is unclear, possible sources are hemipelagic mud sections that were buried rapidly under thick turbidites, and ¹⁴C-rich moieties dispersed within C_org-poor turbidite sections.     

Environmental changes and carbon cycle perturbations at the Triassic–Jurassic boundary in northern Switzerland

The Triassic–Jurassic boundary is characterized by strong perturbations of the global carbon cycle, triggered by massive volcanic eruptions related to the onset of the Central Atlantic Magmatic Province. These perturbations are recorded by negative carbon isotope excursions (CIEs) which have been reported worldwide. In this study, Triassic–Jurassic boundary sections from the southern margin of the Central European Basin (CEB) located in northern Switzerland are analyzed for organic carbon and nitrogen isotopes in combination with particulate organic matter (POM) analyses. We reconstruct the evolution of the depositional environment from Late Triassic to Early Jurassic in northern Switzerland and show that observed negative shifts in δ¹³C of the total organic carbon (δ¹³C_TOC) in the sediment are only subordinately influenced by varying organic matter (OM) composition and primarily reflect global changes in the carbon cycle. Based on palynology and the stratigraphic positions of isotopic shifts, the δ¹³C_TOC record of the studied sections is correlated with the GSSP section at Kuhjoch (Tethyan realm) in Austria and with the St. Audrie’s Bay section (CEB realm) in southwest England. We also show that in contrast to POM analyses the applicability of organic carbon/total nitrogen (OC/TN) atomic ratios and stable isotopes of total nitrogen (δ¹⁵N_TN) for detecting changes in source of OM is limited in marginal depositional environments with frequent changes in lithology and OM contents.     

Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland)

We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ¹³C_TOC and δ¹⁵N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ¹³C_DIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH₄ and CO₂ in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO₃.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes.     

Stable C and N isotope record of short term changes in water level in lakes of different morphometry: Lake Anastazewo and Lake Skulskie,central Poland

We present the stable C isotope record of the changes within the past 20 yr in water level of two morphologically different lakes in central Poland. The aim was to explain the relationship between lake water level and the δ¹³C signature of bulk sedimentary organic matter (δ¹³C_TOC) and to assess the potential of δ¹³C_TOC as a paleolimnological proxy of lake level change. This was done by comparison of the fossil δ¹³C_TOC record with instrumental data for lake level change in a shallow and small lake as well as in one large and deep basin. In both lakes the water table varied greatly between 1980 and 2000 AD. The δ¹³C_TOC data were supplemented with δ¹⁵N and bulk geochemistry data, as well as paleoecological data. We show that δ¹³C_TOC reacted to short term and low amplitude fluctuation in water level, but the response was highly dependent on the morphometry of the lake. In the shallow and small basin, δ¹³C_TOC decreased along with lake level drop due to oxidation and greater input of organic matter from macrophytes colonizing the lake bottom. On the contrary, in the deep/large lake δ¹³C_TOC decreased with increasing water level due to enhanced delivery of soil-derived OM to the lake during highstands. Our results have broad paleolimnological implications as they show that δ¹³C_TOC cannot act as a universal paleohydrological proxy. Its interpretation for a particular lake can be ambiguous and must be supported with additional geochemical and paleocological information.     

Thermal maturity and hydrocarbon potential of the sedimentary succession from the Hecla field in Sverdrup Basin, Arctic Canada

The thermal maturity and source-rock potential of the Upper Palaeozoic and Mesozoic sediments in the Hecla field, Melville Island, Arctic Canada, have been studied using reflected-light microscopy and Rock-Eval pyrolysis. Approximately 250 polished whole-rock samples were examined and their reflectance (% R₀, random) measured. In addition, approximately 100 samples were subjected to Rock-Eval/TOC analyses.Hydrogen-rich organic matter in the Schei Point Group sediments is dominated by alginite (Tasmanales), dinoflagellate cysts with minor amounts of sporinite, cutinite, resinite and liptodetrinite in an amorphous fluorescing matrix. Vitrinite reflectance in Cretaceous sediments ranges from 0.41 to 0.54%; in Jurassic sediments it ranges from 0.43 to 0.64% and in Triassic sediments from 0.50 to 0.65%. The Triassic Schei Point Group calcareous shales and marlstones contain organic matter mainly of marine origin, whereas the predominantly terrestially-derived organic matter present in the Jameson Bay (Lower Jurassic) and in the Upper Jurassic to Lower Cretaceous Deer Bay formations have ower TOC. Only the Ringnes Formation has a TOC content of equivalent to or greater than Schei Point source rocks. Within the Schei Point Group, the Cape Richards and Eden Bay members of the Hoyle Bay Formation are slightly richer in TOC than the Murray Harbour Formation (Cape Caledonia Member). Higher average TOC contents (>3.0%) have been reported in the Cape Richards and Eden Bay members in almost all Hecla drillholes.Variations in the level of thermal maturity of Mesozoic sediments in the Hecla field are a function of burial depth. The stratigraphic succession thickens towards the main Sverdrup Basin depocentre located in a N-NE direction. The pattern of the isoreflectance contours at the top of the Triassic (Barrow Formation) is similar to that of formation boundary lines of the same formations, an indication that present-day maturation levels are largely controlled by basin subsidence.     

Aerobic respiration in pelagic marine sediments

Analyses for dissolved oxygen, nitrate and total CO₂ in the interstitial water have been combined with solid phase sediment analyses of carbon and nitrogen to calculate the rates of reaction and stoichiometry of decomposing organic matter in central Equatorial Pacific pelagic sediments. The diagenesis is dominated by aerobic respiration and nitrification.Organic carbon and total nitrogen decrease exponentially with depth in both red clay and carbonate ooze sediments. In addition, there is a correlation between surface organic carbon and total nitrogen with distance from the equator. Fixed NH₄ is relatively constant with depth and constitutes 12 to 64% of the total nitrogen. The remainder is considered to be organic nitrogen.The $\text{C}\text{N}$ ratio of the decomposing organic matter was obtained using three approaches. Using the correlations of organic carbon with total nitrogen or organic nitrogen the molar ratios varied from 3.4 to 18.1. The average of all stations was 12.6 using total nitrogen and 13.7 using organic nitrogen. The Redfield ratio is 6.6. Approaches using interstitial water chemistry gave lower ratios. The average value using correlations between dissolved oxygen and nitrate was 8.1. The same approach using total CO₂ and nitrate gave an average of 9.1. Due to difficulties in unambiguously interpreting the solid phase data we favor the ratios obtained from the pore water analyses.The rate of organic matter decomposition can be obtained from model calculations using the dissolved oxygen and solid organic carbon data. Most gradients occur in the upper 10 to 20 cm of the sediments. Assuming that bioturbation is more important than sedimentation we have calculated first order rate constants. The average values using organic carbon and dissolved oxygen was 3.9 kyr^? and 4.2 kyr^? respectively using a biological mixing coefficient of 100 cm² kyr^?1. These rate constants decrease in direct proportions to the mixing coefficient.     

Marine biota, nearshore sediments, and the global carbon balance

Nearshore waters to a depth of 200 m constitute only 0.2% of the volume of the world's oceans, but they receive and process 25% of the oceans' input of organic carbon, between 6 and 7 × 10¹⁵ g C per year. Man's activities during the past century have significantly increased both the supply of nutrients from land to coastal waters and the total sediment load carried by rivers and deposited in nearshore waters. The combination of increased primary productivity, due to increased supply of nutrients, and increased burial of organic matter, due to increased sedimentation, could sequester a significant fraction of the carbon released into the atmosphere by man as organic carbon in recent shallow-water sediments.     

Uranium diagenesis in sediments underlying bottom waters with high oxygen content

We measured U in sediments (both pore waters and solid phase) from three locations on the middle Atlantic Bight (MAB) from the eastern margin of the United States: a northern location on the continental shelf off Massachusetts (OC426, 75 m water depth), and two southern locations off North Carolina (EN433-1, 647 m water depth and EN433-2, 2648 m water depth). These sediments underlie high oxygen bottom waters (250-270 μM), but become reducing below the sediment-water interface due to the relatively high organic carbon oxidation rates in sediments (EN433-1: 212 μmol C/cm²/y; OC426: 120 ± 10 μmol C/cm²/y; EN433-2: 33 μmol C/cm²/y). Pore water oxygen goes to zero by 1.4-1.5 cm at EN433-1 and OC426 and slightly deeper oxygen penetration depths were measured at EN433-2 (∼4 cm).All of the pore water profiles show removal of U from pore waters. Calculated pore water fluxes are greatest at EN433-1 (0.66 ± 0.08 nmol/cm²/y) and less at EN433-2 and OC426 (0.24 ± 0.05 and 0.13 ± 0.05 nmol/cm²/y, respectively). Solid phase profiles show authigenic U enrichment in sediments from all three locations. The average authigenic U concentrations are greater at EN433-1 and OC426 (5.8 ± 0.7 nmol/g and 5.4 ± 0.2 nmol/g, respectively) relative to EN433-2 (4.1 ± 0.8 nmol/g). This progression is consistent with their relative ordering of ‘reduction intensity’, with greatest reducing conditions in sediments from EN433-1, less at OC426 and least at EN433-2. The authigenic U accumulation rate is largest at EN433-1 (0.47 ± 0.05 nmol/cm²/y), but the average among the three sites on the MAB is ∼0.2 nmol/cm²/y. Pore water profiles suggest diffusive fluxes across the sediment-water interface that are 1.4-1.7 times greater than authigenic accumulation rates at EN433-1 and EN433-2. These differences are consistent with oxidation and loss of U from the solid phase via irrigation and/or bioturbation, which may compromise the sequestration of U in continental margin sediments that underlie bottom waters with high oxygen concentrations.Previous literature compilations that include data exclusively from locations where [O₂]_bw < 150 μM suggest compelling correlations between authigenic U accumulation and organic carbon flux to sediments or organic carbon burial rate. Sediments that underlie waters with high [O₂]_bw have lower authigenic U accumulation rates than would be predicted from relationships developed from results that include locations where [O₂]_bw < 150 μM.     

Holocene permafrost history and cryostratigraphy in the High‐Arctic Adventdalen Valley,central Svalbard

This paper presents the history and cryostratigraphy of the upper permafrost in the High‐Arctic Adventdalen Valley, central Svalbard. Nineteen frozen sediment cores, up to 10.7 m long, obtained at five periglacial landforms, were analysed for cryostructures, ice, carbon and solute contents, and grain‐size distribution, and were ¹⁴C‐ and OSL‐dated. Spatial variability in ice and carbon contents is closely related to the sedimentary history and mode of permafrost aggradation. In the valley bottom, saline epigenetic permafrost with pore ice down to depths of 10.7 m depth formed in deltaic sediments since the mid‐Holocene; cryopegs were encountered below 6 m. In the top 1 to 5 m, syngenetic and quasi‐syngenetic permafrost with microlenticular, lenticular, suspended and organic‐matrix cryostructures developed due to loess and alluvial sedimentation since the colder late Holocene, which resulted in the burial of organic material. At the transition between deltaic sediments and loess, massive ice bodies occurred. A pingo developed where the deltaic sediments reached the surface. On hillslopes, suspended cryostructure on solifluction sheets indicates quasi‐syngenetic permafrost aggradation; lobes, in contrast, were ice‐poor. Suspended cryostructure in eluvial deposits reflects epigenetic or quasi‐syngenetic permafrost formation on a weathered bedrock plateau. Landform‐scale spatial variations in ground ice and carbon relate to variations in slope, sedimentation rate, moisture conditions and stratigraphy. Although the study reveals close links between Holocene landscape evolution and permafrost history, our results emphasize a large uncertainty in using terrain surface indicators to infer ground‐ice contents and upscale from core to landform scale in mountainous permafrost landscapes.     

Role of a strong oxygen-deficient zone in the preservation and degradation of organic matter: a carbon budget for the continental margins of northwest Mexico and Washington State

Rates of organic carbon oxidation in marine sediments were determined for the continental margins of northwest Mexico and Washington State, with the goal of assessing the role of oxygen in the preservation of organic matter on a margin with a strong oxygen-deficient zone and on a typical western continental margin. Total carbon oxidation rates (including rates for individual electron acceptors: O₂, NO₃⁻, and SO₄⁼) were determined at depths ranging from 100 to 3000 m on both margins. Carbon oxidation rates were generally higher on the Washington margin than on the Mexican margin. The relative importance of the different electron acceptors varied across the two margins and was related primarily to the availability of O₂ and NO₃⁻ from the overlying water. The relative contribution of O₂ consumption increased in deeper sediments (>2000 m) as aerobic processes began to dominate the total carbon oxidation rate. Denitrification rates were highest in Washington sediments; however, denitrification represented a larger fraction of the total carbon oxidation rate in the Mexican sediments (∼40% for Mexico vs. ∼30% for Washington). Sulfate reduction accounted for as much as 79% of the total carbon oxidation rate in shallow sediments and less than 20% in deep sediments on both margins. The offshore trends in carbon oxidation rate appeared to be related to the organic carbon input rate. Pore-water O₂ and NO₃⁻ penetration depths were shallowest in nearshore stations and increased offshore. Regeneration ratios of C:N:P reveal “non-Redfield” behavior on both margins. Carbon budgets for the two margins demonstrate that off Mexico, a much greater percentage of the organic matter produced in the surface ocean reached the sediments (>15% vs. <8% for Mexico and Washington, respectively). On the Mexican margin, ∼8% of the primary production escaped oxidation in the surface sediments to be permanently buried, as compared with only ∼1.2% of the primary production on the Washington margin. This suggests that oxygen-deficient conditions on Mexican margin are linked to enhanced carbon preservation.     

Disentangling marine, soil and plant organic carbon contributions to continental margin sediments: A multi-proxy approach in a 20,000 year sediment record from the Congo deep-sea fan

A 20 kyr long sediment sequence from the Congo deep sea fan (core GeoB 6518-1), one of the world’s largest deep sea river fans, has been analysed for bulk and molecular proxies in order to reconstruct the marine, soil and plant organic carbon (OC) contributions to these sediments since the last glacial maximum. The bulk proxies applied, C/N ratio and δ¹³C_org, ranged from 10 to 12.5 and from −24.5 to −21‰ VPDB, respectively. As molecular proxies, concentrations of marine derived alkenones and terrestrial derived odd-numbered n-alkanes were used, which varied between 0.2 and 4 μg/g dry weight sediment. In addition, the branched vs. isoprenoid tetraether (BIT) index, a proxy for soil organic matter input, was used, which varied from 0.3 to 0.5 in this core.Application of binary mixing models, based on the different individual proxies, showed estimates for terrestrial OC input varying by up to 50% due to the heterogeneous nature of the OC. Application of a three end-member mixing model using the δ¹³C_org content, the C/N ratio and the BIT index, enabled the distinction of soil and plant organic matter as separate contributors to the sedimentary OC pool. The results show that marine OC accounts for 20% to 40% of the total OC present in the deep sea fan sediments over the last 20 kyr and that soil OC accounts for about half (∼45% on average) of the OC present. This suggests that soil OC represents the majority of the terrestrial OC delivered to the fan sediments.Accumulation rates of the plant and soil OC fractions over the last 20 kyr varied by a factor of up to 5, and are strongly related to sediment accumulation rates. They showed an increase starting at ca. 17 kyr BP, a decline during the Younger Dryas, peak values during the early Holocene and lower values in the late Holocene. This pattern matches with reconstructions of past central African humidity and Congo River discharge from the same core and revealed that central African precipitation patterns exert a dominant control on terrestrial OC deposition in the Congo deep sea fan. Marine OC accumulation rates are only weakly related to sediment accumulation rates and vary only little over time compared to the terrigenous fractions. These variations are likely a result of enhanced preservation during times of higher sedimentation rates and of relative small fluctuations in primary production due to wind-driven upwelling.     

Sedimentary record of black carbon in the Pearl River estuary and adjacent northern South China Sea

The concentrations of black carbon (BC) and δ¹³C_BC were determined in sediments of three dated cores from the Pearl River estuary (core PR-3) and adjacent northern South China Sea (cores SS-30, E2). For comparison, the total organic C (TOC) contents and δ¹³C_TOC in the sediments were also measured. Relatively higher concentrations and fluxes of BC were found in sedimentary core PR-3, taken in the Pearl River estuary. The BC concentration profiles or fluxes correlated well with fossil-fuel usage in the Pearl River Delta. Maximum BC fluxes occurred in the late 1970s to early 1980s as recorded in core PR-3, and in the 1950s (core SS-30), reflecting the maximum BC emission in the Pearl River Delta and Hong Kong region, respectively. After the 1980s, a rapid decrease of BC fluxes and a light δ¹³C_BC excursion were presumably due to improvements in combustion and pollution-control technologies and a shift of energy structure from biomass and coal to a mixture of coal, gas, oil and biomass. The fossil BC that contributed to total BC in core PR-3 increased from 20–30% to 70–80% during the last five decades. The study also shows that BC correlates well with terrestrial organic matter and that the ratio of BC to TOC is a good pollution indicator in relation to anthropogenic activities.