Contents and stable isotopic compositions of organic carbon and total nitrogen in the surface sediment of two coastal bays in Korea

Surface sediment from the coastal bays of Gwangyang and Masan in South Korea were analyzed for their contents and isotopic values of organic carbon and total nitrogen. The sources and diagenetic alteration of organic matter were also assessed. Total organic carbon varied from 0.22% to 3.48% (average = 1.40%, n = 75), and C/N ratios varied from 2.4 to 15.2 (average = 8.79, n = 75). δ¹³C_org ranged from −19.92‰ to −25.86‰ (average = −21.21‰, n = 75), and δ¹⁵N_TN ranged from 8.57‰ to 3.93‰ (average = 6.49‰, n = 75). Total organic carbon in both areas was associated with grain-size, with higher contents in finer grained sediment. The high carbon content observed in Masan Bay sediment correlated with its higher C/N ratio. δ¹³C_org and δ¹⁵N_TN varied widely, attributable to various influences such as the input of terrestrial organic matter and diagenetic alteration. The depleted δ¹³C_org and higher δ¹⁵N_TN observed in the sediment of Gwangyang Bay reflected terrestrial supply, implying that biogeochemical processes, i.e. bacterial degradation, were more active in Masan Bay sediment, which showed less depleted δ¹³C_org and higher δ¹⁵N_TN than Gwangyang Bay sediment. δ¹⁵N_TN was the more useful indicator of biogeochemical processes in the highly anoxic sediment. These results indicate that the δ¹³C_org and δ¹⁵N_TN of sedimentary organic matter in coastal bays can indicate the source and degree of diagenetic alteration of sedimentary organic matter.     

The origin and biogeochemistry of organic matter in surface sediments of Lake Shihwa and Lake Hwaong

To understand the origin and biogeochemistry of the organic matter in surface sediments of Lake Shihwa and Lake Hwaong, organic nitrogen, inorganic nitrogen, labile organic carbon, and residual organic carbon contents as well as stable isotope ratios for carbon and nitrogen were determined by KOBr-KOH treatment. Ratios of organic carbon to organic nitrogen (C_org/N_org) (mean = 24) were much higher than ratios of organic carbon to total nitrogen (C_org/N_tot) (mean=12), indicating the presence of significant amounts of inorganic nitrogen in the surface sediments of both lakes. Stable isotope ratios for organic nitrogen were, on average, 5.2‰ heavier than ratios of inorganic nitrogen in Lake Shihwa, but those same ratios were comparable in Lake Hwaong. This might be due to differences in the origin or the degree of degradation of sedimentary organic matter between the two lakes. In addition, stable isotope ratios for labile organic carbon were, on average, 1.4‰ heavier than those for residual organic carbon, reflecting the preferential oxidation of¹³C-enriched organic matter. The present study demonstrates that KOBr-KOH treatment of sedimentary organic matter can provide valuable information for understanding the origin and degradation state of organic matter in marine and brackish sediments. This also suggests that the ratio of C_org/N_org and stable isotope ratios for organic nitrogen can be used as indexes of the degree of degradation of organic matter.     

Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors

The oft-cited general correlation between net sediment accumulation and preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and controls on storage of biogenic material at the continental-ocean boundary. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters on Earth. Available data indicate that sedimentary organic C (C_org) of both terrestrial and marine origin is efficiently decomposed in deltaic areas, with decomposition percentages reaching ≥70% and ≥90%, respectively, the latter percentage (marine) being quite comparable to deep-sea, low sedimentation environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic debris, patterns of SO₄⁼ reduction indicate that the reactivity of organic material being buried is low, and that a larger proportion of C_org is often degraded compared to other marine deposits of similar net accumulation rate. As indicated by properties of the surficial Amazon delta and downdrift coastal region of northeast South America (1600-km extent), the primary reasons for efficient remineralization are related to intense and massive physical reworking of sediment associated with estuarine fronts, upwelling, tidal oscillation, and wind-driven waves. Fluid muds and mobile surface material cause the seafloor and continental boundary to act as a massive, suboxic, fluidized bed reactor dominated in some cases by bacterial rather than macrofaunal biomass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory terrestrial components, result in an efficient decomposition system largely decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramatically expressed off the major rivers forming deltas.     

Carbonate removal by acidification causes loss of nitrogenous compounds in continental margin sediments

This study reports on measurements of organic carbon (C_org) and total nitrogen (N_tot) in surface sediments originating from 6 transects along the northwest European continental margin. After elimination of carbonates by an acidification technique using sulphurous acid, both elements were analysed in the same sediment sample using an elemental analyser. C_org and N_tot in the sample were comparatively low, ranging between 1 and 10 mg C and 0.2 to 1 mg N g⁻¹ dry sediment. In a second analysis, the samples were analysed without acid addition, resulting in N_tot concentrations of 0–50% higher compared to their acidified counterparts. As a consequence, molar C/N ratios derived from the analysis of C_org and N_tot in the acidified sample ranged between 6 to 11, while the N_tot separate analysis reduced C/N ratios to 6 to 8. It is suggested that the addition of sulphurous acid to eliminate inorganic carbon volatilises nitrogenous organic matter.     

Distribution and burial of organic carbon in sediments from the Indian Ocean upwelling region off Java and Sumatra,Indonesia

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (δ¹³C_org) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (C_org) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum δ¹⁵N values of 3.7‰ were measured in the northern Mentawai Basin, whereas they varied around 5.4‰ at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L^?1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum C_org accumulation rates (CARs) were measured in the Lombok (10.4 g C m^?2 yr^?1) and northern Mentawai basins (5.2 g C m^?2 yr^?1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1–7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.     

Oceanographic controls on the carbon isotopic compositions of sinking particles from the Cariaco Basin

This study examined the relationship between carbon isotopic composition of sinking organic matter (OM) and the biological, physical and chemical properties of the surface ocean in the Cariaco Basin. The ¹³C/¹²C ratio of OM (δ¹³C_org) in sinking particles was determined on sediment trap samples from four depths collected from 1996 to 1999 as part of the CArbon Retention In A Colored Ocean time series. Water column properties, including temperature, productivity, chlorophyll and concentration of dissolved CO₂, were concurrently measured on monthly cruises. The δ¹³C_org varied from a high of –17.7‰ to a low of –22.6‰ during the study period. The variation of the δ¹³C_org throughout seasonal cycles was directly proportional to the strength of upwelling and was negatively correlated with temperature (r²=0.64). During the 1996–1997 upwelling event, the strongest during the study period, the δ¹³C_org increased by 4.4‰ whereas during the 1998–1999 upwelling event, the weakest during the study period, the δ¹³C_org only increased by 3.3‰. Contrary to most previous studies, we observed a negative relationship (r²=0.53) between [CO_2 aq] and the estimated isotopic fractionation factor (ε_p). However, there was no correlation between ε_p and the calculated growth rates indicating that there was non-diffusive uptake of carbon into phytoplankton cells. It thus appears that [CO_2 aq] does not control the δ¹³C_org in the water column of the study site. The best explanation for the isotopic enrichment observed is a carbon concentrating mechanism (CCM) in phytoplankton. The existence of a CCM in phytoplankton has major implications for the interpretation of the δ¹³C_org in the Cariaco Basin.     

Benthic remineralization and burial of biogenic SiO2, CaCO3, organic carbon, and detrital material in the Southern Ocean along a transect at 170° West

We investigated the composition, recycling, and mass accumulation rates of sediments along a transect in the Southern Ocean located from 66°S to 57°S at 170°W. This transect also corresponds to the location of a sediment trap mooring line. The sediments at the seven sites studied range from largely terrigenous material to nearly pure (>90%) biogenic silica. CaCO₃ is a minor but persistent component at most sites. Mass accumulation rates have been determined on the basis of excess ²³⁰Th in the sediments, i.e., ²³⁰Th-normalized accumulation rates. The influence of redistribution of sediments on the sea floor has been estimated from ¹⁴C analyses. The recycling of material delivered to the sediments has been characterized on the basis of pore water studies that make extensive use of both in situ sampling and shipboard extractions. The influence of the highly variable rates of input of particulate matter that characterize much of the Southern Ocean upon pore water gradients and fluxes across the sediment water interface has been considered.We find only poor correspondence between BSiO₂ burial fraction (=burial/particulate flux), a quantifiable measure of preservation efficiency, and BSiO₂ particulate rain along the transect. However, preservation does appear to be closely linked to a combination of sedimentation rate and particulate rain.The burial fraction of BSiO₂ is small relative to benthic rain (5–19%). Despite the small fraction buried, burial flux normalized to (sedimentation rate)^1/2 appears to provide a very consistent means of predicting benthic particulate rain over a large range of rain rates, including data from a number of different studies and environments. At sites with BSiO₂ rain 250 mmol m⁻² yr⁻¹ the average difference between predicted and observed rain is 25–30%. Such rain rates occur in many marine areas, particularly the Southern Ocean, with the result that this relationship potentially provides a means of estimating BSiO₂ benthic rain over prolonged periods in the past on the basis of readily measured sediment parameters.At the southern-most deep ocean station, the particulate flux was characterized by an extremely high C_org/CaCO₃ ratio (>10), but this high ratio does not appear to have a substantial influence on CaCO₃ burial. CaCO₃ is preserved in the sediments at this site despite a particulate flux with a 10-fold excess of C_org above that required for complete dissolution in the sediments. The unexpectedly high preservation of CaCO₃ is due largely to the very steep C_org oxidation rate profile at this site. As a result, a large fraction of the organic matter oxidized in the sediments does so in close proximity to the sediment–water interface where most of the metabolic CO₂ is neutralized by CO₃²⁻ from the overlying water, rather than by the dissolution of sedimentary CaCO₃.Diagenetic modeling indicates that at several of the stations, the remineralization fluxes of carbonate species across the sediment–water interface may not have been at steady state as a result of the highly pulsed nature of particulate rain in this environment. We estimate that at the time of our sampling it is possible that near-interface fluxes could have been a factor of 1.6–2 times the annual average.At every site on the transect, the burial fluxes of detrital material are substantially greater than the detrital particulate rain measured in the sediment traps, by as much as a factor of 40. Detrital burial is bimodal, being greatest at the southern and northern extremes of the transect. We postulate that the excess of burial over particulate rain in the south reflects the contribution of ice rafted debris at these high latitudes. Increases in the supply at the northern stations must have a different source. We believe that the excess at these stations is material eroded from the sea floor to the west, possibly on the Campbell Plateau, and advected by currents to the northern portion of the transect at depths below the shallow traps.     

Organic carbon flux and remineralization in surface sediments from the northern North Atlantic derived from pore-water oxygen microprofiles

Organic carbon fluxes through the sediment/water interface in the high-latitude North Atlantic were calculated from oxygen microprofiles. A wire-operated in situ oxygen bottom profiler was deployed, and oxygen profiles were also measured onboard (ex situ). Diffusive oxygen fluxes, obtained by fitting exponential functions to the oxygen profiles, were translated into organic carbon fluxes and organic carbon degradation rates. The mean C_org input to the abyssal plain sediments of the Norwegian and Greenland Seas was found to be 1.9 mg C m⁻² d⁻¹. Typical values at the seasonally ice-covered East Greenland continental margin are between 1.3 and 10.9 mg C m⁻² d⁻¹ (mean 3.7 mg C m⁻² d⁻¹), whereas fluxes on the East Greenland shelf are considerably higher, 9.1–22.5 mg C m⁻² d⁻¹. On the Norwegian continental slope C_org fluxes of 3.3–13.9 mg C m⁻² d⁻¹ (mean 6.5 mg C m⁻² d⁻¹) were found. Fluxes are considerably higher here compared to stations on the East Greenland slope at similar water depths. By repeated occupation of three sites off southern Norway in 1997 the temporal variability of diffusive O₂ fluxes was found to be quite low. The seasonal signal of primary and export production from the upper water column appears to be strongly damped at the seafloor. Degradation rates of 0.004–1.1 mg C cm⁻³ a⁻¹ at the sediment surface were calculated from the oxygen profiles. First-order degradation constants, obtained from C_org degradation rates and sediment organic carbon content, are in the range 0.03–0.6 a⁻¹. Thus, the corresponding mean lifetime of organic carbon lies between 1.7 and 33.2 years, which also suggests that seasonal variations in C_org flux are small. The data presented here characterize the Norwegian and Greenland Seas as oligotrophic and relatively low organic carbon deep-sea environments.     

The Cenomanian/Turonian oceanic anoxic event in the Razzak Field, north Western Desert, Egypt: Source rock potential and paleoenvironmental association

The Western Desert of Egypt is one of the world’s most prolific Jurassic and Cretaceous hydrocarbon provinces. It is one of many basins that experienced organic-rich sedimentation during the late Cenomanian/early Turonian referred to as oceanic anoxic event 2 (OAE2). The Razzak #7 oil well in the Razzak Field in the northern part of the Western Desert encountered the Upper Cretaceous Abu Roash Formation. This study analyzed 23 samples from the upper “G”, “F”, and lower “E” members of the Abu Roash Formation for palynomorphs, particulate organic matter, total organic carbon (TOC) and δ¹³C_org in order to identify the OAE2, determine hydrocarbon source rock potential, and interpret the depositional environment. The studied samples are generally poor in palynomorphs, but show a marked biofacies change between the lower “E” member and the rest of the studied samples. Palynofacies analysis (kerogen quality and quantity) indicates the presence of oil- and gas-prone materials (kerogen types I and II/III, respectively), and implies reducing marine paleoenvironmental conditions. Detailed carbon stable isotopic and organic carbon analyses indicate that fluctuations in the δ¹³C_org profile across the Abu Roash upper “G”, “F”, and lower “E” members correspond well with changes in TOC values. A positive δ¹³C_org excursion (∼2.01‰) believed to mark the short-term global OAE2 was identified within the organic-rich shaly limestone in the basal part of the Abu Roash “F” member. This excursion also coincides with the peak TOC measurement (24.61 wt.%) in the samples.     

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.     

Organic geochemical characterization of Santonian to Early Campanian organic matter-rich marls (Sondage No. 1 cores) as related to OAE3 from the Tarfaya Basin,Morocco

In addition to previously analyzed sediments of Cenomanian to Santonian age in the Tarfaya Sondage No. 2 well, this study presents the results of a stratigraphically younger interval of Santonian to Early Campanian age in the adjacent well Tarfaya Sondage No. 1. This interval is part of the oceanic anoxic event 3 (OAE3), which occurred mainly in the Atlantic realm. Due to known high quality source rocks related to OAEs (i.e. Cenomanian–Turonian), the investigated sample section was tested for the quality, quantity and kind of organic matter (OM), describing also the depositional environment. The study was carried out by means of (i) elemental analysis (C_org, CaCO₃, TS), (ii) Rock–Eval pyrolysis, (iii) vitrinite reflectance measurements, (iv) gas chromatography-flame ionization detection (GC-FID) and (v) GC-mass spectrometry (GC–MS). Total content of organic carbon (C_org), values for the hydrogen index (HI) (mainly in the range 500–700 mg/g C_org) and S₂ values (10–40 mg/g rock), support the assumption of a high petroleum generation potential in these Upper Cretaceous sediments. TS/C_org ratios as well as pristane/phytane ratios indicate variable oxygen contents during sediment deposition, representing a typical depositional setting for the Late Cretaceous and are in good agreement with previously analyzed data in the Tarfaya Basin. Phyto- and zoo-plankton were identified as marine sourced. All of the investigated Early Campanian and Santonian samples are immature with some tendencies to early maturation. These results are based on vitrinite reflectance (0.3–0.4% VR_r), T_max values (409–425 °C), production indices (PI; S₁/(S₁ + S₂)< 0.1) and n-alkane ratios (i.e. carbon preference index). As the deposition of these sediments is time related to OAE3, the depositional environment was characterized by oxygen-deficiency or even anoxic bottom water conditions. This situation was favored during the Cretaceous greenhouse climate by limited oxygen solubility in the then warmer ocean water. Furthermore, local factors related to nutrient supply and primary bioproductivity led to the exceptionally thick, Upper Cretaceous organic matter-rich sedimentary sequence of the Tarfaya Basin.     

Recent sediment transport and accumulation on the NW Iberian margin

Five transects across the NW Iberian margin were studied in the framework of the EU-funded Ocean Margin EXchange II (OMEX II) project, to determine and establish recent sediment and organic carbon transport and accumulation processes and fluxes.On the Galician shelf and shelf edge, resuspension of sediments resulting in well-developed bottom nepheloid layers was observed at all stations, but transport of suspended sediment appears largely confined to the shelf. On the continental slope, only very dilute bottom nepheloid layers were present, and intermediate nepheloid layers were only occasionally seen. This suggests that cross-slope transfer of particles is limited by the prevailing northerly directed shelf and slope currents.Optical backscatter and ADCP current measurements by the BOBO lander, deployed at 2152 m depth on the Galician slope, indicated that particles in the bottom boundary layer were kept in suspension by tidal currents with highest speeds between 15–25 cm s⁻¹. Net currents during the recording period August 6th–September 10th 1998, were initially directed along-slope toward the NNW, but later turned off-slope toward the SW.The separation of the water masses on the slope from the sediment-laden shelf water by the along-slope current regime is reflected in the recent sedimentary deposits of the Galician shelf and slope. Apart from compositional differences, shelf deposits differ from those on the slope by their higher flux of excess ²¹⁰Pb (0.57–5.37 dpm cm⁻²y⁻¹ versus 0.11–3.00 dpm cm⁻²y⁻¹), a much higher sediment accumulation rate (315.6–2295.9 g m⁻²y⁻¹ versus 10.9–124.7 g m⁻²y⁻¹) and organic carbon burial rate (1.01–34.30 g m⁻²y⁻¹ versus 0.01–0.69 g m⁻²y⁻¹).In contrast to the observations on the Galician margin, pronounced nepheloid layers occurred in the Nazaré Canyon, which extended to considerably greater water depths. This indicates that significantly greater transport of fine-grained particles in both the INL and the BNL was occurring within the canyon, as reflected in the exceptionally high ²¹⁰Pb excess flux (up to 34.09 dpm cm⁻²y⁻¹), mass accumulation rates (maximum 9623.1 g m⁻²y⁻¹) and carbon burial fluxes (up to 180.91 g m⁻²y⁻¹) in the sediment. However, radioisotope fluxes in the lower canyon were only slightly higher than at comparable depths on the Galician margin. This suggests that transport and rapid accumulation is focused on the upper and middle part of the canyon, from where it is episodically released to the deep sea. Compared to the Galician margin, the Nazaré Canyon may be considered as an important organic carbon depocenter on short time-scales, and a major conduit for particulate matter transport to the deep sea on >100 y time-scales.     

Mechanisms of land–sea interactions – the distribution of metals and sedimentary organic matter in sediments of a river-dominated Mediterranean karstic estuary

This paper examines disposal of metals and the origin, characteristics, and distribution of sedimentary organic matter (SOM) in a Mediterranean karstic estuary in the north-eastern Adriatic. This environment offers a real-time, small model system for studies of geochemical processes in microtidal Mediterranean estuaries that are infilling with sediments and classified as river-dominated disequilibrium estuaries. The results have shown that the longitudinal distribution of heavy metals in sediments follows the sedimentation dynamics and deposition pattern of river-borne, clay mineral particles. The highest concentration of metals was found in the restricted upper part of the estuary, characterized by rapid deposition of clay particles and terrestrial sedimentary organic matter, and decreases toward the open sea. The vertical distribution of metals in sediment cores depends on the prevailing pH and Eh conditions. Significant increases of the concentrations of metals in the uppermost strata are the result of recent anthropogenic inputs. The share of the terrestrial component in SOM, estimated by N/C_org atomic ratios and δ¹³C values, decreases with distance from the river mouth. The small vertical variation in δ¹³C values of SOM indicates that a fast sedimentation rate overrides the diagenetically determined decomposition. The results obtained indicate that river-borne inorganic particles, natural terrigenous organic material, and anthropogenic metal loads are trapped in sediments of the estuarine system. Under the prevailing conditions, there is negligible transport towards the open sea.     

Compositions and transport of lipid biomarkers through the water column and surficial sediments of the equatorial Pacific Ocean

A systematic investigation of fluxes and compositions of lipids through the water column and into sediments was conducted along the U.S. JGOFS EgPac transect from l2°N to l5°S at 140°W. Fluxes of lipids out of the euphotic zone varied spatially and temporally, ranging from ≈0.20 – 0.6 mmol lipid-C m⁻² day⁻¹. Lipid fluxes were greatly attenuated with increasing water column depth, dropping to 0.002-0.06 mmol lipid-C m⁻² day⁻¹ in deep-water sediment traps. Sediment accumulation rates for lipids were ≈ 0.0002 – 0.00003 mmol lipid-C m⁻² day⁻¹. Lipids comprised ≈ 11–23% of C_org in net-plankton, 10–30% in particles exiting the euphotic zone, 2–4% particles in the deep EgPac, and 0.1-1 % in sediments. Lipids were, in general, selectively lost due to their greater reactivity relative to bulk organic matter toward biogeochemical degradation in the water column and sediment. Qualitative changes in lipid compositions through the water column and into sediments are consistent with the reactive nature of lipids. Fatty acids were the most labile compounds, with polyunsaturated fatty acids (PUFAs) being quickly lost from particles. Branchedchain C₁₅ and C₁₇ fatty acids increased in relative abundance as particulate matter sank and was incorporated into the sediment, indicating inputs of organic matter from bacteria. Long-chain C₃₉ alkenones of marine origin and long-chain C₂₀-C₃₀ fatty acids, alcohols and hydrocarbons derived from land plants were selectively preserved in sediments. Compositional changes over time and space demonstrate the dynamic range of reactivities among individual biomarker compounds, and hence of organic matter as a whole. A thorough understanding of biogeochemical reprocessing of organic matter in the oceanic water column and sediments is, thus, essential for using the sediment record for reconstructing past oceanic environments.     

Benthic remineralization at the land–ocean interface: A case study of the Rhône River (NW Mediterranean Sea)

Biogeochemical processes in sediments under the influence of the Rhône River plume were studied using both in situ microelectrodes and ex situ sediment core incubations. Organic carbon (OC) and total nitrogen (TN) content as well as stable carbon isotopic composition of OC (δ¹³C_OC) were analysed in 19 surface sediments to determine the distribution and sources of organic matter in the Rhône delta system. Large spatial variations were observed in both the total O₂ uptake (5.2 to 29.3 mmol m⁻² d⁻¹) and NH₄⁺ release (−0.1 to −3.5 mmol m⁻² d⁻¹) rates at the sediment–water interface. The highest fluxes were measured near the Rhône River mouth where sedimentary OC and TN contents reached 1.81% and 0.23% respectively. Values of δ¹³C_OC ranged from −26.83‰ to −23.88‰ with a significant seawards enrichment tracing the dispersal of terrestrial organic matter on the continental shelf. The amount of terrestrial-derived OC reaches 85% in sediments close to the Rhône mouth decreasing down to 25% in continental shelf sediments. On the prodelta, high terrestrial OC accumulation rates support high oxygen uptake rates and thus indicating that a significant fraction of terrestrial OC is remineralized. A particulate organic carbon (POC) mass balance indicates that only 3% of the deposited POC is remineralized in prodelta sediments while 96% is recycled on the continental shelf. It was calculated that a large proportion of the Rhône POC input is either buried (52%) or remineralized (8%), mostly on the prodelta area. The remaining fraction (40%) is either mineralized in the water or exported outside the Rhône delta system in dissolved or particulate forms.     

Meiofauna and macrofauna community structure in relation to sediment composition at the Iberian margin compared to the Goban Spur (NE Atlantic)

Meiofauna and macrofauna communities and several sediment characteristics were compared between a slope situated far from the coast (Goban Spur) and two transects across the Iberian Margin with steep slopes and close to the shore. The northern Galician transect (off La Coruña) was situated in an area subjected to wind-induced upwelling events. The western Galician transect was also subjected to upwelling, was additionally influenced by outflows of water rich in organic matter from the Rías Bajas. This transect also included the Galicia Bank. Macrofauna density decreased exponentially from the shelf edge (154 m) to the abyssal plain (4951 m) and different communities occurred on the shelf, the upper- and lower slope and on the abyssal plain. Apart from two extremely low-density stations on the Iberian Margin, there were no significant differences in the meiofauna between the Goban Spur and the Iberian Margin. Along the La Coruña-transect a station where meiofaunal densities were low occurred at a depth of 1522 m, where the sediment was characterised by having a high median-grain size, ripple structures, a low C_org and total N content. There were relatively high numbers of macrofaunal filter-feeders but low numbers of crustaceans, indicating a high current velocity regime. On top of the Galicia Bank (˜770 m) the sediment consisted mainly of shells of pelagic foraminifers, and had low contents of C_org and N. The macrofauna was dominated by filter-feeding and carnivorous taxa. At both these stations meiofauna densities were low. Meiofauna densities and community structure differed between the Goban Spur and the Iberian Margin. Meiofauna densities on the Galician shelf were more than double those on the Goban Spur shelf. The two deep stations on the La Coruña transect and the deepest station on the Galicia Bank transect all contained meiofaunal densities that were higher than found at similar depths off the Goban Spur. The meiofaunal densities were inversely correlated with %CaCO₃ content and, excluding the shelf stations, were positively correlated with both %C_org and total N at the Iberian Margin. Neither upwelling nor the enriched outflows from the rias affected the macrofauna, but meiofaunal densities were greatly enhanced.     

Sedimentary and residual gas geochemical characteristics of the Lower Cambrian organic-rich shales in Southeastern Chongqing,China

To study the sedimentary environment of the Lower Cambrian organic-rich shales and isotopic geochemical characteristics of the residual shale gas, 20 black shale samples from the Niutitang Formation were collected from the Youyang section, located in southeastern Chongqing, China. A combination of geochemical, mineralogical, and trace element studies has been performed on the shale samples from the Lower Cambrian Niutitang Formation, and the results were used to determine the paleoceanic sedimentary environment of this organic-rich shale. The relationships between total organic carbon (TOC) and total sulfur (TS) content, carbon isotope value (δ¹³C_org), trace element enrichment, and mineral composition suggest that the high-TOC Niutitang shale was deposited in an anoxic environment and that the organic matter was well preserved after burial. Stable carbon isotopes and biomarkers both indicate that the organic matter in the Niutitang black shales was mainly derived from both lower aquatic organisms and algaes and belong to type I kerogen. The oil-prone Niutitang black shales have limited residual hydrocarbons, with low values of S₂, I_H, and bitumen A. The carbon isotopic distribution of the residual gas indicate that the shale gas stored in the Niutitang black shale was mostly generated from the cracking of residual bitumen and wet gas during a stage of significantly high maturity. One of the more significant observations in this work involves the carbon isotope compositions of the residual gas (C₁, C₂, and C₃) released by rock crushing. A conventional δ¹³C₁–δ¹³C₂ trend was observed, and most δ¹³C₂ values of the residual gases are heavier than those of the organic matter (OM) in the corresponding samples, indicating the splitting of ethane bonds and the release of smaller molecules, leading to ¹³C enrichment in the residual ethane.     

Transport and fate of Danube Delta terrestrial organic matter in the Northwest Black Sea mixing zone

Within the framework of the European project EROS 21, a biogeochemical study of particles transported from the Danube Delta to the Northwestern Black Sea whose carbon cycle is dominated by riverine inputs was carried out in spring off the Sulina branch of the Danube Delta. The distribution of particulate organic carbon (POC), chlorophyll a (Chl a), C/N, and δ¹³C evidenced an omnipresent contribution of terrestrial organic matter throughout the study area together with a dilution of these inputs by freshwater and marine organisms. Four lipid series, n-alkanoic acids, n-alkanes, n-alkanols, and sterols were analyzed by gas chromatography and gas chromatography/mass spectrometry. Several signature compounds were selected to delineate dispersion of terrestrial organic carbon: (1) long-chain n-alkanoic acids in the range C₂₄–C₃₄, long-chain n-alkanes in the range C₂₅–C₃₅, long-chain n-alkanols in the range C₂₂–C₃₀, 24-ethylcholesta-5,22-dien-3β-ol (29Δ^5,22) and 24-ethylcholesterol (29Δ⁵) for vascular plant-derived material and (2) coprostanol (27Δ^0,5β) for faecal contamination associated with sewage effluents. A marked decrease was observed between the concentrations of different vascular plant markers characterizing the two end members: riverine at salinity 0.3 and marine at salinity 15.5. The decrease observed for marine/riverine end members (expressed as a function of organic carbon) varied in a large range, from 4% for n-alkanes to 18.6%, 20.4% and 24% for n-fatty acids, n-alkanols and sterols, respectively. These values reflect a combination of various processes: size-selective particle sedimentation, resuspension of different particle pools of different sizes and ages, and/or selective biological utilization. The multi-marker approach also suggested the liberation in the mixing zone of terrestrial moieties, tightly trapped in macromolecular structures of the riverine material. The greatest decrease for marine/riverine end members was observed for coprostanol (0.9%), underlining the efficiency of the mixing zone as a sink for sewage-derived carbon.     

Late Cretaceous (Late Turonian,Coniacian and Santonian) petroleum source rocks as part of an OAE,Tarfaya Basin,Morocco

Late Turonian, Coniacian and Santonian source rock samples from a recently drilled well (Tafaya Sondage No. 2; 2010) in the Tarfaya Basin were analyzed for quantity, quality, maturity and depositional environment of the organic matter (OM). To our knowledge such a thick sequence of organic matter-rich Turonian to Santonian source rocks was investigated in that great detail for the first time. Organic geochemical and organic petrological investigations were carried out on a large sample set from the 200 m thick sequence. In total 195 core samples were analyzed for total organic carbon (C_org), total inorganic carbon contents and total sulfur (TS) contents. Rock-Eval pyrolysis and vitrinite reflectance measurements were performed on 28 samples chosen on the basis of their C_org content. Non-aromatic hydrocarbons were analyzed on selected samples by way of gas chromatography–flame ionization detection (GC–FID) and GC–mass spectrometry (GC–MS). The organic matter-rich carbonates revealed a high source rock potential, representing type I kerogen and a good preservation of the organic matter, which is mainly of marine (phytoplankton) origin. HI values are high (400–900 mg/g C_org) and in a similar range as those described for more recent upwelling sediments along the continental slope of North Africa. TS/C_org ratios as well as pristane over phytane ratios indicate variable oxygen content during sediment deposition. All samples are clearly immature with respect to petroleum generation which is supported by maturity parameters such as vitrinite reflectance (0.3–0.4%), T_max values (401–423 °C), production indices (S₁/(S₁ + S₂) > 0.1) as well as maturity parameters based on ratios of specific steranes and hopanes.     

Variability in plankton community structure, metabolism, and vertical carbon fluxes along an upwelling filament (Cape Juby, NW Africa)

The variability in dissolved and particulate organic matter, plankton biomass, community structure and metabolism, and vertical carbon fluxes were studied at four stations (D₁–D₄), placed along a coastal-offshore gradient of an upwelling filament developed near Cape Juby (NW Africa). The filament was revealed as a complex and variable system in terms of its hydrological structure and distribution of biological properties. An offshore shift from large to small phytoplankton cells, as well as from higher to lower autotrophic biomass, was not paralleled by a similar gradient in particulate (POC) or dissolved (DOC) organic carbon. Rather, stations in the central part of the filament (D₂ and D₃) presented the highest organic matter concentrations. Autotrophic carbon (POC_Chl) accounted for 53% (onshore station, D₁) to 27% (offshore station, D₄) of total POC (assuming a carbon to chlorophyll ratio of 50), from which nano- and pico-phytoplankton biomasses (POC_{A < 10 μm}) represented 14% (D₁) to 79% (D₄) of POC_Chl. The biomass of small hetrotrophs (POC_{H < 10 μm}) was equivalent to POC_{A < 10 μm}, except at D₁, where small autotrophs were less abundant. Dark community respiration (R_d) in the euphotic zone was in general high, almost equivalent to gross production (P_g), but decreasing offshore (D₁–D₄, from 108 to 41 mmol C m⁻² d⁻¹). POC sedimentation rates (POC_sed) below the euphotic zone ranged from 17 to 6 mmol C m⁻² d⁻¹. Only at D₄ was a positive carbon balance observed: P_g−(R_d + POC_sed) = 42 mmol C m⁻² d⁻¹. Compared to other filament studies from the NE Atlantic coast, the Cape Juby filament presented lower sedimentation rates and higher respiration rates with respect to gross production. We suggest that this is caused by the recirculation of the filament water, induced by the presence of an associated cyclonic eddy, acting as a trapping mechanism for organic matter. The export capacity of the Cape Juby filament therefore would be constrained to the frequency of the interactions of the filament with island-induced eddies.