A nitrogen isoscape of phytoplankton in the western North Pacific created with a marine nitrogen isotope model

The nitrogen isotopic composition (δ15N) of phytoplankton varies substantially in the ocean reflecting biogeochemical processes such as N2 fixation, denitrification, and nitrate assimilation by phytoplankton. The δ15N values of zooplankton or fish inherit the values of the phytoplankton on which they feed. Combining δ15N values of marine organisms with a map of δ15N values (i.e., a nitrogen isoscape) of phytoplankton can reveal the habitat of marine organisms. Remarkable progress has been made in reconstructing time-series of δ15N values of migratory fish from various tissues, such as otoliths, fish scales, vertebrae, and eye lenses. However, there are no accurate nitrogen isoscapes of phytoplankton due to observational heterogeneity, preventing improvement in the accuracy of estimating migratory routes using the fish δ15N values. Here we present a nitrogen isoscape of phytoplankton in the western North Pacific created with a nitrogen isotope model. The simulated phytoplankton is relatively depleted in 15N at the subtropical site (annual average δ15N value of phytoplankton of 0.6‰), where N2 fixation occurs, and at the subarctic site (2.1‰), where nitrate assimilation by phytoplankton is low due to iron limitation. The simulated phytoplankton is enriched in 15N at the Kuroshio–Oyashio transition site (3.9‰), where nitrate utilization is high, and in the region around the Bering Strait site (6.7‰), where partial nitrification and benthic denitrification occur. The simulated δ15N distributions of nitrate, phytoplankton, and particulate organic nitrogen are consistent with δ15N observations in the western North Pacific. The seamless nitrogen isoscapes created in this study can be used to improve our understanding of the habitat of marine organisms or fish migration in the western North Pacific

A review of nitrogen isotopic alteration in marine sediments

Key Points: Use of sedimentary nitrogen isotopes is examined; On average, sediment 15N/14N increases approx. 2 per mil during early burial; Isotopic alteration scales with water depth Abstract: Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15N/14N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle

A review of nitrogen isotopic alteration in marine sediments

Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment ¹⁵N/¹⁴N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle

Application of chemical tracers to an estimate of benthic denitrification in the Okhotsk Sea

To estimate benthic denitrification in a marginal sea, we assessed the usefulness of , a new tracer to measure the excess nitrogen gas (N-2) using dissolved N-2 and argon (Ar) with N* in the intermediate layer (26.6-27.4 sigma (theta) ) of the Okhotsk Sea. The examined parameters capable of affecting are denitrification, air injection and rapid cooling. We investigated the relative proportions of these effects on using multiple linear regression analysis. The best model included two examined parameters of denitrification and air injection based on the Akaike information criterion as a measure of the model fit to data. More than 80 % of was derived from the denitrification, followed by air injection. Denitrification over the Okhotsk Sea shelf region was estimated to be 5.6 +/- A 2.4 mu mol kg(-1). The distribution of was correlated with potential temperature (theta) between 26.6 and 27.4 sigma (theta) (r = -0.55). Therefore, we concluded that and N* can act complementarily as a quasi-conservative tracer of benthic denitrification in the Okhotsk Sea. Our findings suggest that in combination with N* is a useful chemical tracer to estimate benthic denitrification in a marginal sea.Published online: 24 August 201

Decadal shift of biogenic sinking particle flux in the western North Pacific subpolar region

Time series of biogenic sinking particle flux in the western North Pacific subpolar region over two decades (1989–2008) revealed that the biogenic CaCO3 (CC) flux has shown a significant decreasing trend of 2.7% year−1 (annual average, −0.88 ± 0.13 mg m2 day−1 year−1) along with the decreasing particulate organic carbon (POC) flux of 0.7% year−1, while the biogenic opal (OP) flux had no long-term trend. Comparing these results with the decreasing rate of satellite-derived surface CC with −0.7% year−1, we concluded that three fourths of the decreasing trend of CC flux was derived from the strengthening of CaCO3 dissolution through seawater column due to the weakening of water ventilation and the rest was from the decline of CaCO3-shelled species, indicating the enhancement of the efficiency in oceanic sequestration of atmospheric CO2 in the sea surface of this region due to the increase of OP/CC ratio