Nitrogen cycling driven by organic matter export in the South Pacific oxygen minimum zone

Oxygen minimum zones are expanding globally, and at present account for around 20-40% of oceanic nitrogen loss. Heterotrophic denitrification and anammox-anaerobic ammonium oxidation with nitrite-are responsible for most nitrogen loss in these low-oxygen waters. Anammox is particularly significant in the eastern tropical South Pacific, one of the largest oxygen minimum zones globally. However, the factors that regulate anammox-driven nitrogen loss have remained unclear. Here, we present a comprehensive nitrogen budget for the eastern tropical South Pacific oxygen minimum zone, using measurements of nutrient concentrations, experimentally determined rates of nitrogen transformation and a numerical model of export production. Anammox was the dominant mode of nitrogen loss at the time of sampling. Rates of anammox, and related nitrogen transformations, were greatest in the productive shelf waters, and tailed off with distance from the coast. Within the shelf region, anammox activity peaked in both upper and bottom waters. Overall, rates of nitrogen transformation, including anammox, were strongly correlated with the export of organic matter. We suggest that the sinking of organic matter, and thus the release of ammonium into the water column, together with benthic ammonium release, fuel nitrogen loss from oxygen minimum zones.

Geochemistry of labile organic matter in sediments and interstitial waters of ODP Sites 107-651 and 107-653

Sediment and interstitial water from Sites 651 and 653 (ODP Leg 107) were investigated by organic geochemical methods to characterize labile organic compound classes (amino compounds and carbohydrates) and to evaluate their progressive diagenetic and thermal degradation in deep-sea sediments. Downhole distribution of dissolved organic carbon (DOC) appears related to redox zones associated with bacterial activity and of diagenetic recrystallization of biogenic tests and not so much to organic matter concentrations in ambient sediments. DOC ranges from 250 to 8300 µmol/L (3-100.1 ppm). Amino acids contribute 10%-0.3% of DOC; carbohydrates range from 78 to 5 µmol/L. Rate of degradation of amino acids by thermal effects and/or bacterial activity at both sites (significantly different in sedimentation rates: average 41 cm/1000 yr in the top 300 m at Site 651, average 3.9 cm/1000 yr in the Pliocene/Quaternary sequence at Site 653 to 220 mbsf) is more dependent on exposure time rather than on the depth within the sediment column. Variability in neutral, acidic, and basic amino acid fractions of total amino acids (with a range of 1.1-0.02 µmol/g sediment; up to 2.5% of organic carbon) varies with carbonate content and by differences in thermal stability of amino acids. Distribution patterns of monosaccharides are interpreted to result from differences in organic matter sources, sedimentation rates, and the degree of organic matter decomposition prior to and subsequent to burial. Total particulate carbohydrates range from 1.82 to 0.21 µmol/g sediment and contribute about 8% to the sedimentary organic matter. Investigation of trace metals in the interstitial waters did not show any correlation of either DOC, amino compounds, or carbohydrates.

Accumulation of particulate organic carbon at the Eurasian continental margin during late Quaternary times

Data on the amount and composition of organic carbon were determined in sediment cores from the Kara and Laptev Sea continental margin, representing oxygen isotope stages 1-6. The characterization of organic matter is based on hydrogen index (HI) values, n-alkanes and maceral composition, indicating the predominance of terrigenous organic matter through space and time. The variations in the amount and composition of organic carbon are mainly influenced by changes in fluvial sediment supply, Atlantic water inflow, and continental ice sheets. During oxygen isotope stage (OIS) 6, high organic carbon contents in sediments from the Laptev Sea and western East Siberian Sea continental margin were probably caused by the increased glacial erosion and further transport in the eastward-flowing boundary current along the continental margin. During OIS 5 and early OIS 3, some increased amounts of marine organic matter were preserved in sediments east of the Lomonosov Ridge, suggesting an influence of nutrient-rich Pacific waters. During OIS 2, terrigenous organic carbon supply was increased along the Barents and western Kara Sea continental margin caused by extended continental ice sheets in the Barents Sea (Svalbard to Franz Josef Land) area and increased glacial erosion. Along the Laptev Sea continental margin, on the other hand, the supply of terrigenous (organic) matter was significantly reduced due to the lack of major ice sheets and reduced river discharge. Towards the Holocene, the amount of total organic carbon (TOC) increased along the Kara and Laptev Sea continental margin, reaching average values of up to 0.5 g C/cm**2/ky. Between about 8 and 10 ka (9 and 11 Cal ka), i.e., during times when the inner shallow Kara and Laptev seas became largely flooded for the first time after the Last Glacial Maximum, maximum supply of terrigenous organic carbon occurred, which is related to an increase in coastal erosion and Siberian river discharge. During the last 8000 years, the increased amount of marine organic carbon preserved in the sediments from the Kara and Laptev Sea continental margin is interpreted as a result of the intensification of Atlantic water inflow along the Eurasian continental margin.