Nitrogen isotope patterns in the oxygen-deficient waters of the Eastern Tropical North Pacific Ocean

We analyzed the stable nitrogen isotope composition of an extensive set of samples of particulate matter (PM) and seawater nitrate collected during October/November 1997 along the Mexican coastline from 24° to 11.5°N. At the northern and southern end of our study area, the δ¹⁵N of PM ranged between 5 and 7‰ in the upper 200 m of the water column with higher values at intermediate depths. These data are very similar to those reported from other parts of the open ocean. In the oxygen minimum zone (OMZ), we found significantly higher δ¹⁵N values for suspended particles. Furthermore, the δ¹⁵N of nitrate (NO₃) was elevated within the OMZ and we found a strong relationship between the oxygen concentration, nitrate deficit and the ¹⁵N content of the nitrate. The core of the OMZ between 22°N, 105°W and 15°N, 110°W coincided with higher nitrate deficits and δ¹⁵N values relative to the stations near the boundaries. The δ¹⁵N of nitrate was highest, with values up to 18.7‰, where oxygen concentrations were below 1–2 μmol/l. This pattern is consistent with an overall nitrogen isotopic enrichment factor of 22.5‰ for denitrification in the core of the OMZ using the Rayleigh equation (closed-system approach). Results from a diffusion model (open-system approach), however, gave a fractionation factor of 30±7.5‰, implying that the Rayleigh formula only gives a lower estimate of the fractionation factor ε. The vertical flux of particles collected in short-term deployments (ca. 35 h) of a drifting sediment trap was not significantly correlated with the water column nitrate deficit. The isotopic signature of the nitrate within the gradient is very similar to the δ¹⁵N value of sedimenting particles, suggesting that there might be a strong link between the production and sedimentation of particles. Upward flux of nitrate across the thermocline can account for less than half of the particle flux leaving the mixed layer. Mixing and transport of nitrate across the lower boundary of the OMZ can lead to significant enrichment in the ¹⁵N content of deep waters, and our isotopic data imply that at least 14% of the nitrate in the waters below the OMZ originates from this source.     

Sources of δ15N variability in sinking particulate nitrogen in the Cariaco Basin,Venezuela

Ten years of monthly observations of the δ¹⁵N of sinking particulate nitrogen (δ¹⁵N–PN (in ‰ versus atmospheric N₂)=[(¹⁵N/¹⁴N)_sample/(¹⁵N/¹⁴N)_standard)−1]1000) in the Cariaco Basin, Venezuela, confirm that the basin's bottom sediments store information about nitrogen dynamics related to seasonal and interannual variability in regional surface ocean processes. During the upwelling period of the southern Caribbean Sea (February–April), the δ¹⁵N–PN is similar to that of the thermocline nitrate (∼3.5‰). This nitrate is imported into the Cariaco Basin with Subtropical Underwater (SUW), which wells up near the coast. Thus, particles generated by phytoplankton photosynthesis during this productive period bear a sub-tropical North Atlantic isotopic imprint of N₂ fixation (low compared to the global average of nitrate δ¹⁵N≈5‰). During the non-upwelling period when surface waters are stratified (September–November), the δ¹⁵N–PN is also 3.5–4.0‰, and reflects a mixture of local N₂ fixation within the mixed layer, inputs of terrigenous organic matter and SUW nitrate consumption by phytoplankton below the mixed layer, which most likely exerts the strongest control on the δ¹⁵N–PN signal during this time. In the transition periods of May–July and December–January, the δ¹⁵N–PN increases to 4.5–6.5‰. This coincides with maxima of continental material fluxes (terrestrial PON δ¹⁵N is >6‰) into the Cariaco Basin. The δ¹⁵N signal in the sediments of the Cariaco Basin thus provides information about the relative strength of the local coastal upwelling, the relative input of continental material via river runoff, and local N₂ fixation. The findings contribute to interpretations of the basin's paleoclimatic nitrogen cycle variations based on observations of the sedimentary δ¹⁵N record at this location.     

Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical North Pacific waters

Isotopic analyses of nitrate by the denitrifier method, and indeed by many other analytical methods, do not discriminate between nitrate and nitrite. For samples containing both chemical species, accurate isotopic analysis of nitrate requires either removal of nitrite or independent isotopic analysis of nitrite and subtraction of its contribution to the mixed isotopic signal. This study evaluates the application of a variety of available analytical approaches to the isotopic analysis of mixed nitrate and nitrite solutions, with the goal of producing accurate coupled isotopic analyses of both nitrate and nitrite. These methods are tested on mixtures of standard solutions of nitrate and nitrite, and then applied to the coupled δ¹⁵N and δ¹⁸O analyses of nitrate and nitrite in waters of the Eastern Tropical North Pacific (ETNP). Results from standard mixtures show that even for extreme values of nitrate and nitrite δ¹⁵N and δ¹⁸O, both nitrite removal by ascorbate and nitrite isotopic analysis and subtraction from the mixed isotopic signal yield nitrate δ¹⁵N and δ¹⁸O values that are close to the expected values. Application of these analyses to samples from the ETNP yielded δ¹⁵N_NO3 and δ¹⁸O_NO3 values as high as 21‰ vs. AIR and 19‰ vs. VSMOW, respectively. Conversely, very low δ¹⁵N values were observed in nitrite, with values ranging from − 7.2 to − 18.5‰ vs. AIR. Removal of nitrite from ETNP samples thus revealed differences of up to 5‰ between NO₃^- and NO₂^- + NO₃^- for both δ¹⁵N and δ¹⁸O. Moreover, the δ¹⁵N offset between co-occurring nitrate and nitrite is greater than expected from the action of denitrification alone and may provide a unique constraint on the processes involved in the cycling of nitrite in and around oxygen deficient zones. Finally, subtraction of the nitrite δ¹⁵N and δ¹⁸O from ETNP samples allows the extension of the Δ(15,18) tracer into suboxic regions containing nitrite. The magnitude and distribution of Δ(15,18) in these samples suggests an important role for nitrite reoxidation in nitrate isotope variations.     

Seasonal variations in the nitrogen isotopic composition of settling particles at station K2 in the western subarctic North Pacific

Intensive observations using hydrographical cruises and moored sediment trap deployments during 2010 and 2012 at station K2 in the North Pacific Western Subarctic Gyre (WSG) revealed seasonal changes in δ ¹⁵N of both suspended and settling particles. Suspended particles (SUS) were collected from depths between the surface and 200 m; settling particles by drifting sediment traps (DST; 100–200 m) and moored sediment traps (MST; 200 and 500 m). All particles showed higher δ ¹⁵N values in winter and lower in summer, contrary to the expected by isotopic fractionation during phytoplankton nitrate consumption. We suggest that these observed isotopic patterns are due to ammonium consumption via light-controlled nitrification, which could induce variations in δ ¹⁵N(SUS) of 0.4–3.1 ‰ in the euphotic zone (EZ). The δ ¹⁵N(SUS) signature was reflected by δ ¹⁵N(DST) despite modifications during biogenic transformation from suspended particles in the EZ. δ ¹⁵N enrichment (average: 3.6 ‰) and the increase in C:N ratio (by 1.6) in settling particles suggests year-round contributions of metabolites from herbivorous zooplankton as well as TEPs produced by diatoms. Accordingly, seasonal δ ¹⁵N(DST) variations of 2.4–7.0 ‰ showed a significant correlation with primary productivity (PP) at K2. By applying the observed δ ¹⁵N(DST) vs. PP regression to δ ¹⁵N(MST) of 1.9–8.0 ‰, we constructed the first annual time-series of PP changes in the WSG. This new approach to estimate productivity can be a powerful tool for further understanding of the biological pump in the WSG, even though its validity needs to be examined carefully.     

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.     

酸化过程对不同碳酸钙含量的海洋沉积物中总氮及其同位素分析的影响

为节约成本和样品,一些学者同时分析海洋沉积物中的碳、氮及其同位素（TOC、TN、δ¹³C和δ¹⁵N）。分析沉积物中的δ¹³C,需要对样品进行酸化去除无机碳,但是这一酸化过程会使TN和δ¹⁵N的分析结果产生偏差,且偏差范围与沉积物中无机碳含量（CaCO₃）有关。本研究选取了低CaCO₃含量（1-16%）和高CaCO₃含量（20-40%）的海洋沉积物样品,比较了酸化过程对TN和δ¹⁵N的影响。研究结果表明,酸化过程对海洋沉积物中TN和δ¹⁵N的分析结果产生了显著影响。对于低CaCO₃含量的样品,酸化导致样品中TN流失了约0-40%,δ¹⁵N偏移了约0-2‰;而对于高CaCO₃含量的样品,酸化导致样品中TN流失了约10-60%,δ¹⁵N偏移了约1-14‰。表明酸化对TN和δ¹⁵N的影响已经超过了仪器的误差范围0.002%（TN）和0.08‰（δ¹⁵N）,将影响TN和δ¹⁵N的环境指示意义。因此,即使海洋沉积物样品中CaCO₃含量很低,也必须用原样分析TN和δ¹⁵N以避免酸化过程的影响。     

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.     

Seasonal variability of δ15N in sinking particles in the Benguela upwelling region

Temporal changes in δ¹⁵N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in δ¹⁵N (to as low as 2.5‰) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low δ¹⁵N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High δ¹⁵N values (up to 13.1‰) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower δ¹⁵N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with δ¹⁵N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on δ¹⁵N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on δ¹⁵N and possible variability in preservation of the isotope signal between periods of high and low particle flux.     

Pelagic nitrogen dynamics in the Vietnamese upwelling area according to stable nitrogen and carbon isotope data

Upwelling and nitrogen (N) fixation provide new N for primary production off southern central Vietnam. Here we evaluate the roles of both N sources for zooplankton nutrition by comparing δ¹⁵N and δ¹³C values in nitrate, particulate organic matter (POM), and six net-plankton size fractions from monsoon and intermonsoon seasons. The δ¹³C values in POM and the net-plankton size fractions differed by 2–4‰ at any time. We assume that plankton from the POM filters was dominated by nano-and picoplankton as opposed to micro- and mesoplankton in the net-samples. The implications of this are discussed in terms of size differential pathways of C and N in the planktonic food web. We used δ¹⁵N to estimate the differences in N nutrition between the actual upwelling region and the oligotrophic area further offshore. The δ¹⁵N values of the net-plankton size fractions were depleted in δ¹⁵N by ca. 2‰ outside compared to inside the upwelling area during the monsoon season. We attribute these patterns to the additional utilization of N derived from N fixation. The concomitant findings of high N fixation rates reported earlier and low subthermocline nitrate (nitrate_sub) values of 2.9–3.6‰ support this conclusion. Net-plankton δ¹⁵N values increased with size, pointing to the dominance of higher trophic levels in the larger size fractions. According to a two source mixing model N fixation may have provided up to 13% of the N demand in higher trophic levels.     

Isotopic and elemental indicators of nutrient sources and status of coastal habitats in the Caribbean Sea,Yucatan Peninsula,Mexico

Nutrient inputs associated with coastal population growth threaten the integrity of coastal ecosystems around the globe. In order to assess the threat posed by rapid growth in tourism, we analyzed the nutrient concentrations as well as the δ¹⁵N of NO₃⁻ and macrophytes to detect wastewater nitrogen (N) at 6 locations along a groundwater-dominated coastal seagrass bed on the Caribbean coast of Mexico. We predicted that locations with greater coastal development would have higher concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (P), as well as δ¹⁵N of NO₃⁻, reflecting wastewater sources of N. However, concentrations of NO₃⁻ were not significantly different between developed (3.3 ± 5.3 μM NO₃⁻) and undeveloped (1.1 ± 0.7 μM) marine embayments. The most important control on DIN concentration appeared to be mixing of fresh and salt water, with DIN concentrations negatively correlated with salinity. The δ¹⁵N of NO₃⁻ was elevated at an inland pond (7.0 ± 0.42‰) and a hydrologically-connected tide pool (7.6 ± 0.57‰) approximately 1 km downstream of the pond. The elevated δ¹⁵N of NO₃⁻ at the pond was paralleled by high δ¹⁵N values of Cladophora sp., a ubiquitous green alga (10 ± 1‰). We hypothesize that inputs of nitrogen rich (NO₃⁻ > 30 μM) groundwater, characterized by ¹⁵N enriched signatures, flow through localized submarine groundwater discharges (SGD) and contribute to the elevated δ¹⁵N signatures observed in many benthic macrophytes. However, changes in nitrogen concentrations and isotope values over the salinity gradient suggest that other processes (e.g. denitrification) could also be contributing to the ¹⁵N enrichments observed in primary producers. More measurements are needed to determine the relative importance of nitrogen transformation processes as a source of ¹⁵N to groundwaters; however, it is clear that continued inputs of anthropogenic N via SGD have the potential to severely impact ecologically and economically valuable seagrass meadows and coral reefs along the Caribbean coast of Mexico.     

Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column

We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in ¹⁵N in the eastern basin (−0.3±0.5‰) than in the western basin (+2.4±1.4‰), suggesting that nitrogen supplied by biological N₂ fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. The δ¹⁵N of nitrate in the deep Mediterranean (∼3‰ in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5‰), also suggesting an important role for N₂ fixation. The eastward decrease in the δ¹⁵N of surface PON is greater than the eastward decrease in the δ¹⁵N of the subsurface nitrate, implying that the amount of N₂ fixation in the eastern Mediterranean is great enough to cause a major divergence in the δ¹⁵N of phytoplankton biomass from the δ¹⁵N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the δ¹⁵N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. ¹⁵N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.4±0.7‰. The δ¹⁵N in sinking particles caught at 100 m depth (4.9–5.6‰) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Δδ¹⁵N[PON-chlorin]=+6.4±1.4‰) in the surface, similar to the offset reported earlier in cultures for cellular N and chl-a. This indicates that δ¹⁵N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity.     

Recovery of added 15N-labelled ammonium-N from Louisiana Gulf Coast estuarine sediment

The loss of added ¹⁵N-labelled NH₄⁺ from bottom sediment was studied in situ in a shallow saline Louisiana Gulf Coast lake. ¹⁵N-labelled NH₄⁺ was uniformly mixed with sediment at the level of 35 μg N g^?1 dry sediment for 24 h before being weighed into polyethylene containers. The ¹⁵N-enriched sediment was incubated on the lake bottom for periods up to 337 days. At intervals, triplicate samples were taken and analyzed for inorganic NH₄⁺, organic N, and denitrification rates. Concentrations of NH₄⁺ in the ¹⁵NH₄⁺ enriched sediment samples were at levels similar to those measured in vertical intact cores removed from the lake at each sampling date. The initial and final isotopic compositions of NH₄⁺ in the incubated sediments were 44.26 and 0.521 atom % ¹⁵N excess, respectively. ¹⁵N enrichment in the organic N fraction increased rapidly during the first 15 days of incubation and then increased slowly from 15 to 123 days. Denitrification, estimated by the acetylene blockage technique, ranged from 0.994 to 0.079 ng N g^?1 dry sediment per hour. The mean denitrification rate for the 337-day period was 0.28 ng N g^?1 h^?1. There was no statistical difference in the recovery of ¹⁵N between 15 and 337 days of incubation. When the added ¹⁵N became incorporated into the organic N pool, little or no further ¹⁵N was lost.     

Trophic relationships along a bathymetric gradient at the deep-sea observatory HAUSGARTEN

Deep-seafloor communities, especially those from the ice-covered Arctic, are subject to severe food limitation as the amount of particulate organic matter (POM) from the surface is attenuated with increasing depth. Here, we use naturally occurring stable isotope tracers (δ¹⁵N) to broaden our rudimentary knowledge of food web structure and the response of benthic organisms to decreasing food supplies along the bathymetric transect (～1300–5600 m water depth) of the deep-sea observatory HAUSGARTEN. Encompassing five trophic levels, the HAUSGARTEN food web is among the longest indicating continuous recycling of organic material typical of food-limited deep-sea ecosystems. The δ¹⁵N signatures ranged from 3.0‰ for Foraminifera to 21.4‰ (±0.4) for starfish (Poraniomorpha tumida). The majority of organisms occupied the second and third trophic level. Demersal fish fed at the third trophic level, consistent with results from stomach contents analysis. There were significant differences in the δ¹⁵N signatures of different functional groups with highest δ¹⁵N values in predators/scavengers (13.2±0.2‰) followed by suspension feeders (11.2±0.2‰) and deposit feeders (10.2±0.3‰). Depth (=increasing food limitation) affected functional groups in different ways. While the isotopic signatures of predators/scavengers did not change, those of suspension feeders increased with depth, and the reverse was found for deposit feeders. In contrast to the results of other studies, the δ¹⁵N signatures in POM samples obtained below 800 m did not vary significantly with depth indicating that changes in δ¹⁵N values are unlikely to be responsible for the depth-related δ¹⁵N signature changes observed for benthic consumers. However, the δ¹⁵N signatures of sediments decreased with increasing depth, which also explains the decrease found for deposit feeders. Suspension feeders may rely increasingly on particles trickling down the HAUSGARTEN slope and carrying higher δ¹⁵N signatures than the decreasing POM supplies, which elevates the δ¹⁵N value of their tissues. Our results imply that a depth-stratified approach should be taken to avoid a misinterpretation of data obtained at different depths.     

Organic carbon stable isotope ratios of continental margin sediments

Stable isotope ratios (δ¹³C) of total organic carbon were measured in surface sediments from the continental margins of the northern and western Gulf of Mexico, the north coast of Alaska and the Niger Delta. Gulf of Mexico outer-shelf isotope ratios were in the same range as has been reported for Atlantic coastal shelf sediments, ?21.5 to ?20‰. Off large rivers including the Mississippi, Niger and Atchafalaya (Louisiana), δ¹³C values increased from terrigenous-influenced (around ?24‰) to typically marine ($\text{～?20‰}$) within a few tens of kilometers from shore. This change was accompanied by a decrease in the amount of woody terrigenous plant remains in the sediment. Alaskan continental margin samples from the cold Beaufort Sea had isotopically more negative carbon (?25.5 to ?22.6‰) than did warmer-water sediments. The data indicate that the bulk of organic carbon in Recent sediments from nearshore to outer continental shelves is marine derived.     

Nitrogen isotopic discrimination by water column nitrification in a shallow coastal environment

Temporal changes in nitrogen isotopic composition (δ¹⁵N) of the NO₃ ⁻ pool in the water column below the pycnocline in Ise Bay, Japan were investigated to evaluate the effect of nitrification on the change in the δ¹⁵N in the water column. The δ¹⁵N of NO₃ ⁻ in the lower layers varied from −8.5‰ in May to +8.4‰ in July in response to the development of seasonal hypoxia and conversion from NH₄ ⁺ to NO₃ ⁻. The significantly ¹⁵N-depleted NO₃ ⁻ in May most likely arose from nitrification in the water column. The calculated apparent isotopic discrimination for water column nitrification (ɛ_nit = δ¹⁵N_substrate − δ¹⁵N_product) was 24.5‰, which lies within the range of previous laboratory-based estimates. Though prominent deficits of NO₃ ⁻ from hypoxic bottom waters due to denitrification were revealed in July, the isotopic discrimination of denitrification in the sediments was low (ɛ_denit = ∼1‰). δ¹⁵N_NO3 in the hypoxic lower layer mainly reflects the isotopic effect of water column nitrification, given that water column nitrification is not directly linked with sedimentary denitrification and the effect of sedimentary denitrification on the change in δ¹⁵N_NO3 is relatively small.     

The oxygen isotopic composition of the fraction less than 2 microns (clay fraction) in Recent sediments from western Europe

The oxygen isotopic composition of the clay fraction in Recent argillaceous sediments in western Europe shows regional differences, the δ¹⁸_SMOW values varying between + 15.8 and + 21.7‰. This phenomenon is used for sediment-transport studies. In the Rhine-Meuse estuary a landward transport of marine sediments is observed. The clay fraction of the sediments in the Wadden Sea is only for a minor part derived from the rivers Rhine and Meuse.     

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.     

South Atlantic and benthic foraminifer δ13C deviations: implications for reconstructing the Late Quaternary deep-water circulation

Benthic foraminifer species Cibicidoides wuellerstorfi and related genera are assumed to secrete calcite very close to the carbon isotope values of the ambient bottom-water ΣCO₂. Recently, attention has been focused on substantial productivity-linked δ¹³C depletions. To examine further the productivity effect on benthic δ¹³C deviations, we present data from the South Atlantic between 15 and 35°S, including water samples from 10 hydrographic stations and related surface-sediment measurements on C. wuellerstorfi. We compare open-ocean data with observations in the Namibia Upwelling area. As a result, δ¹³C_ΣCO2 values as well as phosphate concentrations in water samples of the upwelling realm differ significantly from those of the open-ocean realm at least in the upper and mid-depth water masses (SACW, AAIW, UCDW). However, deviations from the Redfield fractionation, caused by air–sea fractionation, remain constant within each water mass, which means that the carbon isotope changes toward upwelling areas are exclusively determined by biological cycling. In addition to lower δ¹³C_ΣCO2 values in upwelling areas, a depletion in the δ¹³C of epibenthic foraminifer calcite is observed, which is most likely explained by the decay of organic matter, reducing the ¹³C/¹²C ratio in the pore water and influencing the carbon isotopic composition of the C. wuellerstorfi shells of highly productive areas. The paleoceanographic implication of this effect for reconstructing the Late Quaternary deep-water circulation is discussed using carbon isotope records of several sediment cores within and outside the Namibia upwelling area.     

Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the gulf of California

Sediment cores were taken from the Gulf of California, and pore waters recovered by mechanical squeezing. The chemistry and isotopic abundance of sulfur in these pore fluids were compared with coexisting solid phases to deduce the mechanisms involved in pyrite formation. The results suggest that burrowing activities of benthonic organisms supply sulfate sulfur to sediments to depths of approximately 0.5 m from the surface. This is inferred from essentially constant pore water concentration profiles of dissolved ions in horizons where sulfate reduction is demonstrated by the presence of iron sulfides.For a core from Pescadero Basin, it is estimated that beneath the mixed zone, diffusion adds 0.4% sulfur by dry weight of sediment, whereas burial of sulfate adds less than 0.1% sulfur. It is shown that diffusion can add isotopically light sulfur to sediments, due to more rapid relative addition of ³²SO₄^2? compared to ³⁴SO₄^2? down a concentration gradient maintained by bacterial processes. The overall net isotopic value of the sulfate so added is $\text{δ}{}^{34}\text{S = ?4.5‰}$. The depth distribution of S-isotope in sulfur is controlled by the balance between a bacterial kinetic isotope effect preferentially removing ³²S relative to ³⁴S, and the supply of sulfate by diffusion. The isotopic fractionation factor, α, calculated by a mathematical formulation which takes diffusion into account, is larger (1.060±0.010) than when sulfate reduction is assumed to occur in a closed system (1.035). The larger value is supported by the sulfur isotope distribution in metastable iron sulfide. Essentially, the same open-system α was calculated for a core from Carmen Basin.     

南海神狐海域沉积物自生黄铁矿和石膏S同位素特征及对甲烷渗漏强度的示踪

The northern slope of the South China Sea is a gas-hydrate-bearing region related to a high deposition rate of organic-rich sediments co-occurring with intense methanogenesis in subseafloor environments.Anaerobic oxidation of methane(AOM) coupled with bacterial sulfate reduction results in the precipitation of solid phase minerals in seepage sediment,including pyrite and gypsum.Abundant aggregates of pyrites and gypsums are observed between the depth of 667 and 850 cm below the seafloor(cmbsf) in the entire core sediment of HS328 from the northern South China Sea.Most pyrites are tubes consisting of framboidal cores and outer crusts.Gypsum aggregates occur as rosettes and spheroids consisting of plates.Some of them grow over pyrite,indicating that gypsum precipitation postdates pyrite formation.The sulfur isotopic values(δ~(34) S) of pyrite vary greatly(from –46.6‰ to –12.3‰ V-CDT) and increase with depth.Thus,the pyrite in the shallow sediments resulted from organoclastic sulfate reduction(OSR) and is influenced by AOM with depth.The relative high abundance and δ~(34) S values of pyrite in sediments at depths from 580 to 810 cmbsf indicate that this interval is the location of a paleo-sulfate methane transition zone(SMTZ).The sulfur isotopic composition of gypsum(from–25‰ to –20.7‰) is much lower than that of the seawater sulfate,indicating the existence of a 34 S-depletion source of sulfur species that most likely are products of the oxidation of pyrites formed in OSR.Pyrite oxidation is controlled by ambient electron acceptors such as MnO_2,iron(Ⅲ) and oxygen driven by the SMTZ location shift to great depths.The δ~(34) S values of gypsum at greater depth are lower than those of the associated pyrite,revealing downward diffusion of 34 S-depleted sulfate from the mixture of oxidation of pyrite derived by OSR and the seawater sulfate.These sulfates also lead to an increase of calcium ions from the dissolution of calcium carbonate mineral,which will be favor to the formation of gypsum.Overall,the mineralogy and sulfur isotopic composition of the pyrite and gypsum suggest variable redox conditions caused by reduced seepage intensities,and the pyrite and gypsum can be a recorder of the intensity evolution of methane seepage.