Benthic-pelagic coupling in the East Siberian Sea from nitrate isotopes

Over the East Siberian Sea and common to the western Arctic Ocean, a subsurface nutrient maximum is reported in the halocline, and generally attributed to both nutrient-rich Pacific inflow and intensive remineralization in shelf bottom waters being advected into the central Arctic basin. We report nitrogen and oxygen isotopic measurement of nitrate in the water column. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic T°C maximum into the halocline, respectively. Nitrate δ18O follows the decrease in water δ18O, harboring low-δ18O from large Arctic river discharge. This imprint is transmitted with nitrification to the ambient nitrate δ18O,suggesting that most of the nitrate being supplied into the Arctic Ocean has been reprocessed at least once within the Arctic. The associated increase in nitrate δ15N, correlated with the fixed N deficit, indicates that a significant share of benthic denitrification is supplied from nitrate produced by partial nitrification in the reactive sediment layer. Following an imbalance between remineralization and nitrification, the residual high-δ15N ammonium is accumulated in pore waters and diffuses into shelf bottom waters to be ultimately nitrified. Following the advection into the central basin, this processes could explain both the nutrient maximum, with high-δ15N and low-δ18O nitrate, and the accentuated fixed Ndeficit in the western Arctic halocline in comparison to the Pacific inflow. A sedimentary isotope effect is reported for benthic denitrification, 2.3-3.1‰, in the middle range given in the literature.info:eu-repo/semantics/publishe

Iron-controlled oxidative sulfur cycling recorded in the distribution and isotopic composition of sulfur species in glacially influenced fjord sediments of west Svalbard

This study investigates how glacially delivered reactive iron (oxyhydr) oxide and manganese oxide phases influence the biogeochemical cycling of sulfur in sediments of three Arctic fjords and how the biogeochemical signatures of these processes are preserved. Results reveal differences in the concentrations of dissolved iron and manganese in pore-waters and the concentration of solid-phase sulfur species within individual fjords and amongst the three fjords, likely controlled by the varying input of reactive iron (oxyhydr) oxides to the sediment. Broadly, the stations can be divided into three categories based on their biogeochemical signals. Stations in the first category, located in Smeerenburgfjorden, are characterized by relatively low concentrations of (easily) reducible particulate iron phases, increasing concentrations of iron monosulfides, pyrite, and elemental sulfur with depth, and low pore-water dissolved iron and manganese concentrations. Biogeochemical processes at these stations are primarily driven by organoclastic sulfate reduction, sulfur disproportionation and the subsequent reaction and sequestration of sulfide in the sediment as iron monosulfide and pyrite. Sulfur and oxygen isotope values of sulfate display progressive enrichment in heavy isotopes with depth at these stations. In contrast, concentrations of (easily) reducible particulate iron phases and pore-water dissolved iron (up to 850 mu M) and manganese (up to 650 mu M) are very high at stations of the second and third category, located in Kongsfjorden and Van Mijenfjorden, while iron monosulfide and pyrite contents are extremely low. The amount of pyrite and its isotope values in conjunction with organic sulfur compounds provide evidence for a detrital origin of a fraction of these sulfur compounds. At the Kongsfjorden and Van Mijenfjorden stations, oxidative pathways of the sedimentary sulfur cycle, controlled by the high availability of reducible particulate iron phases, play an important role, leading to the effective recycling of sulfide to sulfate through sulfur intermediates and concomitant resupply of the sulfate reservoir with S-32. In both fjords, elemental sulfur was only detected at the outer fjord stations grouped into the third category. Our study provides a framework for interpreting the Fe-S-C geochemistry of similar continental shelf areas in modern settings and ultimately for identifying these environments in the rock record

Influence of the bordering shelves on nutrient distribution in the Arctic halocline inferred from water column nitrate isotopes

The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ18O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ15N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the 15N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual 15N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters

Sulfur stable isotopes indicate the source of sinking materials in a coastal bay: Otsuchi Bay, Sanriku, Japan

Through 2004 and 2005, δ 34S of sinking material from Otsuchi Bay was measured at the center and rocky shore of the bay. At the center of the bay δ 34S was high (18∼21‰) in the material collected from April to November. However, δ 34S was low (9∼14‰) in the material collected from December to March. The increase in δ 34S in April was attributed to an increase in phytoplankton biomass because marine phytoplanktonic δ 34S is high. When δ 34S of sinking material was low, input of riverine material or bottom sediment resuspension were considered as the probable causes, because their δ 34S is low. Marine sulfur was always high (more than 70%) at both stations. The difference between the δ 34S of sinking material collected from the different sampling stations indicates that marine macroalgae contribute to sinking material near the shore when phytoplankton is scarce. In conclusion, the relative influence of different material sources to sinking materials could be successfully estimated using δ 34S