Photochemical changes in chemical markers of sedimentary organic matter source and age

Several chemical markers of organic matter source and age are shown to be susceptible to light-induced alteration. To test for the photochemical lability of markers previously employed for sediments from the Louisiana coastal zone, we subjected sediments under resuspension conditions to simulated sunlight, and monitored changes in C:N and Br:OC ratios, δ¹³C, Δ¹⁴C, and lignin composition. Markers of terrigenous origin (high C:N, lignin) decreased and δ¹³C became enriched in sediments containing primarily terrigenous organic matter, while a marker of marine organic matter (Br:OC) decreased in samples containing significant contributions from this source. Preferential loss of ¹⁴C from all sediments indicated enhanced photochemical lability of organic matter of relatively recent origin, consistent with the changes in chemical markers. Most, but not all, experimental alterations are consistent with field distributions of these markers. Relatively small experimental changes in the markers in combination with confounding processes in the environment, however, prevent these parallel trends from providing any more than a consistency test for the importance of photochemical reactions in this region.     

Settling fluxes in Saanich and Jervis Inlets, British Columbia, Canada: sources and seasonal patterns

We conducted a multi-year sediment-trap experiment in Saanich and Jervis Inlets, British Columbia, Canada. Moorings with traps positioned at three depths were placed near the mouth and toward the head of each fjord, and deployments were monthly. We present fluxes of total mass, biogenic silica (BSi), particulate organic carbon (POC) and aluminium (Al), as well as the δ¹³C signal of the POC, and we compare the sediment-trap fluxes to primary-production measurements made during the experiment.Diatomaceous silica and aluminosilicates were the primary components of the settling flux, while organic matter from marine (largely diatoms) and terrestrial sources was occasionally a significant portion of the sinking material. Fluxes of BSi and POC were highest in the spring and summer, tracing maxima in local primary production. These fluxes decreased, increased or remained constant with depth due to water-column remineralisation and variability in processes that cause fluxes to increase with depth. Al fluxes followed local precipitation and river runoff at the landward stations, and with remarkable faithfulness in Saanich Inlet. Near the mouths, there was little seasonality in Al flux, and the increases of flux with depth reveal sedimentary plumes at each fjord’s sill. Tidal and deepwater-renewal components of the plumes are evident, and the plume in Saanich Inlet was particularly intense. Fluxes of Al to deep sediment traps associated with renewal flows were also observed toward the head of each fjord.Marine δ¹³C endmembers are estimated from relationships between δ¹³C and BSi concentrations, and measures of soil δ¹³C from each fjord were available. These endmembers are used with the δ¹³C record to quantify marine and terrigenous contributions to the POC flux. Marine POC composed 54-72% of the total POC caught by shallow sediment traps in spring and summer, and 36-54% in fall and winter. Primary production and sediment-trap fluxes are used to estimate annually averaged export ratios (shallow-trap flux:autotrophic assimilation) for marine POC and Si. POC export ratios (0.092-0.14) were low for these productive waters, but they compare with other results based on sediment-trap fluxes from coastal waters where terrigenous OC has been subtracted. Export ratios of Si were calculated using an estimated Si:C assimilation ratio and, therefore, are susceptible to error, but the high results (>0.8) suggest that BSi was exported more efficiently than POC. The possibility that POC was preferentially lost after interception by sediment traps is also considered. Primary production and settling fluxes were higher in Saanich Inlet than in Jervis Inlet, while export ratios of OC and Si were similar in both fjords, away from the nepheloid layer near the sill of Saanich Inlet.     

Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen Gulf region,Canadian Arctic

To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The C_ORG:N_TOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of δ¹³C_ORG and δ¹⁵N_TOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (C_ORG:N_TOT ratio) and isotopic (δ¹³C_ORG and δ¹⁵N_TOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components.     

胶州湾沉积物中有机质的来源与组成及其对百年尺度环境变化的指示

The Jiaozhou Bay is characterized by heavy eutrophication that is associated with intensive anthropogenic activities. Four core sediments from the Jiaozhou Bay are analyzed using bulk technologies, including sedimentary total organic carbon(TOC), total nitrogen(TN), the stable carbon(δ~(13)C) and nitrogen(δ~(15) N) isotopic composition to obtain the comprehensive understanding of the source and composition of sedimentary organic matter and further shed light on the environmental changes of the Jiaozhou Bay on a centennial time scale.Results suggest that the TOC and TN concentrations increase in the upper core, having indicated a probable eutrophication process since the 1920 s in the inner bay and the 2000 s in the bay mouth. The TOC and TN concentrations outside the bay have also changed since 1916 owing to the variation of terrigenous input.Considering TOC/TN ratio, δ~(13) C and δ~(15) N, it can be concluded there is a mixture of terrigenous and marine organic matter sources in the study area. A simple two end-member(terrigenous and marine) mixing model usingδ~(13) C indicats that 45%–79% of TOC in the Jiaozhou Bay is from the marine source. The environmental changes of the Jiaozhou Bay are recorded by geochemical proxies, which are influenced by the intensive anthropogenic activities(e.g., extensive use of fertilizers, and discharge of sewage) and climate changes(e.g., rainfall).     

The behaviour of iodine and bromine in estuarine surface sediments

The distribution of iodine and bromine was examined in sediments which receive inputs of marine and terrigenous organic matter. The I and Br concentrations are directly related to the content of ‘marine’ organic matter defined using carbon/nitrogen ratios. In the Etive sediments both Br and I may be used as an indicator of ‘marine’ organic matter; Br is of general application as the $\text{Br}\text{C}{}_{\mathrm{mar}}$ ratio (180 × 10^?4) is similar to ratios in other sedimentary environments but the use of I is restricted as the $\text{I}\text{C}{}_{\mathrm{mar}}$ ratio is unlike those in other sediments. Experimental study of iodine sorption clearly shows the importance of decaying marine organic matter and oxygenated conditions in the incorporation of iodine by sediments. This suggests that the mechanism of incorporation of iodine by seston previously proposed is probably an important pathway to sediments. The similarity of Br association with marine organic matter suggests that Br sorption as opposed to residual enrichment may be important for sediment Br accumulation.     

Fluxes,source and transport of organic matter in the western Sea of Okhotsk: Stable carbon isotopic ratios of n-alkanes and total organic carbon

Settling particles and surface sediments collected from the western region of the Sea of Okhotsk were analyzed for total organic carbon (TOC), long-chain n-alkanes and their stable carbon isotope ratio (δ¹³C) to investigate sources and transport of total and terrestrial organic matter in the western region of the sea. The δ¹³C measurements of TOC in time-series sediment traps indicate lateral transport of resuspended organic matter from the northwestern continental shelf to the area off Sakhalin via the dense shelf water (DSW) flow at intermediate depth. The n-alkanes in the surface sediments showed strong odd carbon number predominance with relatively lighter δ¹³C values (from −33‰ to −30‰). They fall within the typical values of C3-angiosperms, which is the main vegetation in east Russia, including the Amur River basin. On the other hand, the molecular distributions and δ¹³C values of n-alkanes in the settling particles clearly showed two different sources: terrestrial plant and petroleum in the Sea of Okhotsk. We reconstructed seasonal change in the fluxes of terrestrial n-alkanes in settling particles using the mixing model proposed by Lichtfouse and Eglinton [1995. ¹³C and ¹⁴C evidence of a soil by fossil fuel and reconstruction of the composition of the pollutant. Organic Geochemistry 23, 969–973]. Results of the terrestrial n-alkane fluxes indicate that there are two transport pathways of terrestrial plant n-alkanes to sediments off Sakhalin, the Sea of Okhotsk. One is lateral transport of resuspended particles with lithogenic material from the northwestern continental shelf by the DSW flow. Another is the vertical transport of terrestrial plant n-alkanes, which is independent of transport of lithogenic material. The latter may include dry/wet deposition of aerosol particles derived from terrestrial higher plants possibly associated with forest fires in Siberia.     

Sources,transport, and partitioning of organic matter at a highly dynamic continental margin

Continental shelves play a major role as transition zone during transport of multiply-sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic carbon content (TOC), its stable carbon isotopic composition (δ¹³CTOC), and the organic carbon to nitrogen ratio (TOC/TN). The biomarker inventory reflected a heterogeneous mixture of organic matter from various marine and terrestrial sources. Soil- and vascular plant-derived continental organic matter, indicated by lignin phenols and plant-derived triterpenoids, was primarily associated with the silt fraction and transported by river run-off. The spatial distribution patterns of higher plant-derived waxes, long-chain n-alkanes, n-alcohols, and n-fatty acids suggested distinct different transport mechanisms and/or sources. The branched tetraether index, a molecular proxy expressing the relative abundance of branched dialkyl tetraethers vs. crenarchaeol and considered to signal soil-derived organic matter, was not as sensitive as the other molecular indicators in detecting continental organic matter. Hydrodynamic sorting processes on the shelf resulted in a separation of different types of terrestrial organic matter; grass and leaf fragments and soil organic matter were preferentially transported offshore and deposited in areas of lower hydrodynamic energy. Algal lipid biomarker distributions indicated a complex community of marine plankton contributing to organic matter. Spatial and seasonal patterns of phytoplankton growth primarily controlled the distribution of algal organic matter components. The interplay of all of these processes controls production, distribution, and deposition of organic matter and results in three distinct provinces at the Galicia–Minho shelf: (I) fresh marine organic matter dominated the inner shelf region; (II) high inputs of terrestrial organic matter and high TOC content characterized the mid-shelf deposited mudbelt; (III) lower concentrations of relatively degraded organic matter with increased proportions of refractory terrestrial components dominated the outer shelf and continental slope.     

Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments:: implications for nutrient utilization and organic matter composition

Relationships between organic carbon, total nitrogen and organic nitrogen concentrations and variations in δ¹³C_org and δ¹⁵N_org are examined in surface sediments from the eastern central Arctic Ocean and the Yermak Plateau. Removing the organic matter from samples with KOBr/KOH and determining residual as well as total N shows that there is a significant amount of bound inorganic N in the samples, which causes TOC/N_total ratios to be low (4–10 depending on the organic content). TOC/N_org ratios are significantly higher (8–16). This correction of organic TOC/N ratios for the presence of soil-derived bound ammonium is especially important in samples with high illite concentrations, the clay mineral mainly responsible for ammonium adsorption. The isotopic composition of the organic N fraction was estimated by determining the isotopic composition of the total and inorganic nitrogen fractions and assuming mass-balance. A strong correlation between δ¹⁵N_org values of the sediments and the nitrate concentration of surface waters indicates different relative nitrate utilization rates of the phytoplankton in various regions of the Arctic Ocean. On the Yermak Plateau, low δ¹⁵N_org values correspond to high nitrate concentrations, whereas in the central Arctic Ocean high δ¹⁵N_org values are found beneath low nitrate waters. Sediment δ¹³C_org values are close to −23.0‰ in the Yermak Plateau region and approximately −21.4‰ in the central Arctic Ocean. Particulate organic matter collected from meltwater ponds and ice-cores are relatively enriched in ¹³C (δ¹³C_org=−15.3 to −20.6‰) most likely due to low CO₂(aq) concentrations in these environments. A maximum terrestrial contribution of 30% of the organic matter to sediments in the central Arctic Ocean is derived, based on the carbon isotope data and various assumptions about the isotopic composition of the potential endmembers.     

Origin of sedimentary organic matter in the north-western Adriatic Sea

In order to evaluate the origin and the transformation of organic matter on the shallow shelf of the NW Adriatic Sea, organic carbon, total nitrogen and stable isotope ratios of organic carbon were analysed in riverine suspended matter and sediments as well as in marine suspended and sedimentary organic matter, in marine phytoplankton and zooplankton.The deposition of organic matter is influenced by fine sediment concentration. Surface sediments were characterised by highly variable biogeochemical conditions on the sea floor, whereas sub-surface sediments showed a more homogeneous hypoxic/anoxic environment.Low C_org/N ratio and high organic carbon and nitrogen concentrations in riverine suspended organic matter indicate an important contribution of freshwater phytoplankton within rivers, particularly during low flow regimes, which adds to the marine phyto- and zooplankton at shelf locations.In order to evaluate the importance of terrestrial, riverine and marine sources of OM in shelf sediments, a three end-member mixing model was applied to shelf surface sediments using ¹³C/¹²C values for organic matter and N/C ratios. The model showed an elevated contribution of terrestrial organic substances at intermediate depths (10–15 m), mostly corresponding to an area of coarser grain-size, whereas the riverine and marine organic fractions were mainly accumulating near the coast and offshore, respectively.     

Geochemistry and organic petrography of Cretaceous sediments of the Calabar Flank,southeastern, Nigeria

Detailed bulk geochemistry and organo-petrography of outcrop Cretaceous sediments (with no significant effects of weathering) from the Calabar Flank, southeast Nigeria were performed to understand the organic carbon source, accumulation and degradation, and paleo-climatic, paleoceanographic and paleoenvironmental conditions in West Africa during Early Cretaceous (Aptian) to Maastrichtian times. This study was based on microscopic, elemental analyses (organic carbon, nitrogen, iron and sulphur), Rock-eval pyrolysis and carbon-isotope analyses. In general, the Calabar Flank shales are characterised by highly variable total organic carbon (TOC) contents, which range between 0.1% in Aptian–Albian Mfamosing Limestone and 9.9% in the Awi Formation sediments. The organic matter (OM) is a mixture of immature to early-mature marine and terrigenous OM of types III and IV. This is indicated by low hydrogen indices (HI value (10–190 mg HC/g TOC), T_max (417–460 °C), vitrinite reflectance %R_o (0.39–0.62 %R_o), low to high C/N ratios (3.4–1158.0) and high amounts of terrigenous macerals (vitrinite + inertinite). Based on carbon isotope, C/N ratios and sulphate reduction index (SRI), OM degradation (up to 70%, SRI > 2.5) is most pronounced for shales deposited in a marine environment. The geochemical and petrographic data indicate that local factors such as low bioproductivity, down slope transport and redeposition of sediments from a fluvial–deltaic basin to nearshore facies, shallower, oxic and mildly oxygen-deficient environments, humid–arid paleogeographic conditions, specifically controlled the amount and quality of the OM during Aptian–Mastrichtian stages where marine sediments have been assumed to be deposited during the global anoxic events. Therefore, the order of the main factors controlling OM content in sediments are: input of terrigenous material transported from the land > low OM productivity by marine photoautotrophs > low preservation.     

Distribution and sources of organic carbon, nitrogen and their isotopes in sediments of the subtropical Pearl River estuary and adjacent shelf, Southern China

The isotopic composition (δ¹³C and δ¹⁵N) and organic carbon (OC) and total nitrogen (TN, organic plus inorganic) content of 37 carbonate-free surficial sediments of the subtropical Pearl River estuary and the adjacent shelf of South China Sea (SCS) was determined. The δ¹³C values indicate that the sediment organic material is a mixture from two sources, terrestrial and marine. Several of the sediments have extremely low (< 4) OC / TN ratios, which could be due to low OC contents and/or to a significant fraction of the TN present as inorganic nitrogen adsorbed on clays. In general, the spatial patterns of OC, TN, δ¹³C and δ¹⁵N are similar. Values are low at the river mouth and on the western coast, suggesting proportionally greater accumulation of terrestrial particulate organic matter relative to marine phytodetritus, which is limited by low productivity in the turbid plume of the Pearl River. Algal-derived organic carbon (al-OC) content is estimated to be low (≤ 0.06%) at the river mouth and higher (up to 0.57%) on the adjacent inner shelf based on a mixing model of end members.     

Genesis of organic matter in the Kara Sea bottom sediments

Based on simultaneous use of organic and geochemical indicators (δ¹³C, C/N, and n-alkanes), the genesis of organic matter (OM) in recent bottom sediments of the Kara Sea was characterized. Maps for percentages and absolute masses of marine and terrigenous OM were drawn. The masses of buried marine and terrigenous OM were compared to its supply to the sea and onto the sea bottom.     

Distributions and sources of bulk organic matter and aliphatic hydrocarbons in surface sediments of the Bohai Sea, China

Surface sediment samples from a matrix of fifty-five sites covering virtually the entire Bohai Sea (Bohai), China were analyzed for total organic carbon (TOC), total nitrogen (TN), n-alkanes, unresolved complex mixture (UCM), biomarkers and stable carbon isotopic composition (δ¹³C), and principal component analysis was performed for source identification of organic matter (OM). The distribution of organic carbon correlated well with sediment grain size with the finest sediments having the highest concentration, suggesting the influence of hydrodynamics on the accumulation of sedimentary organic matter (SOM). The corrected TOC/ON (organic nitrogen) ratios and δ¹³C indicated mixed marine and terrestrial sources of SOM. Results suggested that δ¹³C could be used as a potential indicator to observe the dispersion of Huanghe-derived sediments in Bohai. Total n-alkane concentrations varied over 10-fold from 0.39 to 4.94 μg g^− 1 (dry weight) with the maximum terrigenous/aquatic alkane ratio observed at the Huanghe River Estuary (HRE) due to more higher plant OM from riverine inputs. C₁₂–C₂₂ n-alkanes with even-to-odd predominance were observed in several central-eastern Bohai sites. The HRE and its adjacent area is the main sink for the Huanghe river-derived OC. The ubiquitous presence of UCM, biomarkers (hopanes and steranes) and PCA results indicated the presence of petroleum contamination in Bohai, mainly from offshore oil exploration, discharge of pollutants from rivers, shipping activities and atmospheric deposition.     

末次盛冰期以来南海北部神狐海域沉积有机质的组成特征及其古气候/环境意义

通过对两根沉积柱GHE27L和GHE24L的总有机碳（TOC）、总氮（TN）、C/N比值及稳定碳同位素（δ13C_org）的分析,本文探讨了21.1 ka BP以来南海北部陆坡神狐海域沉积有机质的组成特征及可能的古气候/环境信息。沉积柱GHE27L的TOC含量、TN含量、C/N比值及δ13C_org值分别为0.53%~1.81%,0.07%~0.18%,8.2~16.0和-23.6‰~-20.3‰。沉积柱GHE24L各参数则分布为0.45%~1.65%,0.09%~0.24%,5.3~12.2和-22.6‰~-20.4‰。沉积柱总体有机质的剖面变化显示,末次盛冰期以来南海北部沉积有机质具有海洋和陆地混合来源,但以海洋有机质来源为主。冰期陆源有机质对总有机质的相对贡献比全新世高。末次盛冰期南海北部气候相对干旱,C₄植被发育。全新世夏季风增强、降雨增多。自2.0 ka BP以来,人类活动对南海北部海洋初级生产力产生一定的影响。     

Tracing suspended organic nitrogen from the Yangtze River catchment into the East China Sea

Total suspended matter was collected along the Yangtze River (Changjiang) and in the East China Sea in April to May and in September 2003, respectively, to study origin and fate of particulate organic nitrogen. Concentrations of particulate organic carbon (POC), nitrogen (PN) and hydrolyzable particulate amino acids (PAA; d- and l-enantiomers) were higher in the Yangtze Estuary than in the river and decreased offshore towards the shelf edge. In the coastal area, higher values of PAA were observed in the surface layer than in the bottom water. Stable carbon isotope ratios (δ¹³C) of POC increased from − 24.4‰ in the river to values around − 21‰ on the East China Sea Shelf. Dominant amino acids were aspartic acid + aspartine (Asx), glutamic acid + glutamine (Glx), glycine, alanine and serine. The proportions of Asx, Glx and isoleucine were higher in the marine than in the riverine samples contrary to the distributions of glycine, alanine, threonine and arginine. The proportions of d-amino acids were highest in the riverine suspended organic matter (6% of PAA) decreasing towards the shelf edge (1.5% of PAA). d-arginine, not reported in natural aquatic samples so far, was the most abundant d-amino acid in the river. The amino acid composition of the particulate organic matter (POM) in the Yangtze River indicates an advanced stage of degradation of POM. Highly degraded organic matter from soils is probably a main source of POM in the Yangtze River, but the relatively high δ¹³C values and low C/N ratios (7.7 ± 1.6) also indicate contribution from anthropogenic sources. The degraded riverine material was a dominant organic matter source in the estuary, where aquatic primary production had only a small overall contribution. In the East China Sea, gradual settling of riverine organic matter and the addition of fresher phytoplankton impacted the amino acid composition and δ¹³C values, and on the outer shelf relatively fresh phytoplankton-derived organic matter dominated.     

The supply and preservation of ancient and modern components of organic carbon in the Canadian Beaufort Shelf of the Arctic Ocean

The provenance of organic matter in sediments from the Mackenzie River and Beaufort Shelf was investigated using the stable carbon and radiocarbon isotopic compositions of bulk organic matter and the stable carbon isotopic compositions of individual organic compounds, including lignin-derived phenols and lipid-derived fatty acids. Most river suspended sediments and shelf surface sediments contained organic carbon characterized by highly depleted Δ¹⁴C values that were consistent with average radiocarbon ages exceeding 7000 years. The stable carbon isotopic signatures of lignin phenols were uniformly depleted (−25≥δ¹³C≥−32‰), indicating the predominant contributions of C₃ vascular plant sources. The isotopic compositions of C₁₄ and C₁₆ fatty acids exhibited important contrasts between the river (−36‰ to −40‰) and shelf (−25‰ to −29‰) sediments that were consistent with contributions from freshwater algae and/or vascular plants in the former and marine phytoplankton in the latter. Using ¹⁴C isotopic mass balance, the abundances of modern and ancient organic matter were quantitatively constrained. The fate of organic matter in the Beaufort Shelf was explored by normalizing these abundances to the specific surface area of sediments. Ancient organic carbon, which may include old pre-aged soil material as well as fossil bitumen or kerogen, accounted for the majority (∼70%) of the particulate organic matter exported by the Mackenzie River and deposited in surface sediments of the Beaufort Shelf. Modern organic carbon accounted for ∼30% in both river and shelf sediments, with significant contributions from vascular plant-derived materials in both river and shelf samples and from marine algae in the shelf sediments. Respiration (and/or leaching) of particle-bound marine organic matter dominates the carbon metabolism in the Mackenzie Delta/Beaufort Shelf region. However, land-derived pools, including modern carbon derived from vascular plants as well as ancient carbon also appeared to undergo a degree of post-depositional degradation prior to burial in the shelf. These novel source apportionments are reflected in an updated carbon budget for the study area.     

Burial of organic carbon in Holocene sediments of the Zhujiang (Pearl River) and Changjiang (Yangtze River) estuaries

The East Asian marginal seas are important sinks of terrigenous materials transported by large rivers. In this study two cores from the Changjiang (Yangtze River) and Zhujiang (Pearl River) estuaries and one core from the inner shelf off of Hong Kong were investigated to examine the burial of organic matter during the postglacial period and its possible links with paleoenvironmental changes. Based on a simple two end-member mixing model, the terrestrial organic matter supplied primarily from the Zhujiang and Changjiang dominates the estuarine areas while marine organic matter contributes more to the inner-mid shelf. The competing contributions of terrestrial and marine organic matter are responsible for the downcore variations of organic elemental compositions. The overall decreasing shifts of total organic carbon concentrations and total organic carbon to total nitrogen ratios in the Zhujiang estuarine sediments since 6.5 kaBP seems to support the notion that depth profiles of organic matter compositions deposited in the Zhujiang Estuary can aid in the reconstruction of monsoon history in the Holocene. Nevertheless, organic matter compositions in the Changjiang Estuary and inner shelf off of southeastern Hong Kong respond in a different and more complex way to freshwater discharges or precipitation changes (monsoon variability) in the catchments, owing to complex controls of deposition and preservation of organic matter in these estuarine and shelf environments. Caution is therefore needed in using organic elemental and isotopic compositions to decipher paleoenvironmental changes in East Asian continental shelves where intense river–sea interactions occur and sedimentary environments change drastically.     

Dynamics and sources of suspended particulate organic matter in the Marennes-Oléron oyster farming bay: Insights from stable isotopes and microalgae ecology

The aim of this study was to distinguish between sources of the complex variety of Marennes-Oléron Bay suspended particulate organic matter (SPOM) contributing to the tropho-dynamics of the Marennes-Oléron oyster farming bay. Basic biomarkers (Chl a, C/N and POC/Chl a ratios), carbon and nitrogen stable isotopes from SPOM were analyzed and the microalgae community was characterized. The sampling strategy was bimonthly from March 2002 to December 2003; samples were taken from an intertidal mudflat. Four main sources contributed to the SPOM pool: terrigenous input from rivers, neritic phytoplankton, resuspended microphytobenthos and periodic inputs from intertidal Zostera noltii meadows. Seasonal fluctuations were observed in both years of the study period: (1) SPOM collected in the spring of 2002 (δ¹³C = −25‰ to −23‰) was mainly composed of fresh estuarine inputs; (2) SPOM from the summer and fall of 2002 and 2003 was predominantly neritic phytoplankton (δ¹³C = − 22‰ to −19‰); (3) SPOM from the winter of 2002, spring of 2003 and winter of 2003 (δ¹³C = −21 to −23‰) was composed of a mixture of decayed terrigenous river inputs and pelagic phytoplankton, which was predominantly resuspended microphytobenthos. In the summer of 2003—the warmest summer on record in southern France and Europe—SPOM was particularly enriched for ¹³C, with δ¹³C values ranging from −14‰ to −12‰. Pulses in δ¹³C values, indicative of ¹³C-enriched decaying materials, extended into the fall. These were attributed to benthic intertidal inputs, including both resuspended microphytobenthos and Z. noltii detritus. Changes in SPOM sources in Marennes-Oléron Bay may lead to differences in the quality of the trophic environment available for reared oysters.     

Late-Quaternary supply of terrigenous organic matter to the Congo deep-sea fan (ODP site 1075): implications for equatorial African paleoclimate

Late-Quaternary sections (about 1 Ma) from the Congo deep-sea fan (ODP Leg 175, site 1075) were used to reconstruct the terrigenous organic matter supply to the easternmost equatorial Atlantic Ocean. Variations in quantity and quality of the riverine organic matter reflect the interaction between the paleoclimatic development within the continental catchment area and the paleoceanographic conditions in the Congo river plume. To characterize the delivery of organic matter from terrigenous and marine sources, we used elemental and bulk carbon isotopic analyses, Rock-Eval pyrolysis, lignin chemistry, and organic petrology. High-amplitude fluctuations occurring about every 15-25 ka reveal a mainly precessional control on organic sedimentation. Results from Rock-Eval pyrolysis indicate a mixed kerogen type III/II, as would be anticipated in front of a major river. Fluctuations in T_max from Rock-Eval pyrolysis demonstrate pronounced cyclic changes in the delivery of low- and high-mature organic matter. Contribution of the low-mature organic fraction was strongest during warm climates supporting enhanced marine production offshore of the Congo. Organic petrological observations confirm the existence of abundant terrigenous plant tissues, both non-oxidized (vitrinite) and oxidized (inertinite). Charcoal-like organic matter (fusinite) is attributed to periods of increased bush fires in the continental hinterland, and implies more arid climatic conditions. Results from ratios of specific phenolic lignin components suggest that terrigenous organic matter in Late-Quaternary sections of site 1075 mainly derives from non-woody angiosperm tissue, i.e., grasses and leaves. Correlation between the amount of specific lignin phenols and the bulk '¹³C_org signature fosters the conclusion that an appreciable amount of the terrigenous organic fraction derives from C4 plant matter. This may cause an underestimation of the terrigenous proportion of bulk organic matter when assessments are based on bulk carbon isotopic signatures alone.