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Abstract Dissolved organic nitrogen (DON) is the dominant form of fixed nitrogen in most low and middle latitude ocean surface waters. Here, we report measurements of DON isotopic composition (δ15N) from the west South China Sea (SCS), with the goal of providing new insight into DON cycling. The concentration of DON in the surface ocean is correlated (r = 0.70,p < 0.0001) with chlorophyllaconcentration, indicating DON production in these surface waters. The concentration and δ15N of DON fall in a relatively narrow range in the surface ocean (4.6 ± 0.1 μM and 4.3 ± 0.2‰ vs. air, respectively; ±SD), similar to other ocean regions. The mean DON δ15N above 50 m (4.5 ± 0.3‰) is similar to the δ15N of nitrate in the “shallow subsurface” (i.e., immediately below the euphotic zone; 4.6 ± 0.2‰) but is higher than the δ15N of suspended particles in the surface ocean (~2.3‰). This set of isotopic relationships has been observed previously (e.g., in the oligotrophic North Atlantic and North Pacific) and can be explained by the cycling of N between particulate organic nitrogen (PON), DON, and ammonium, in which an isotope effect associated with DON degradation preferentially concentrates15N in DON. Consistent with this view, a negative correlation (r = 0.70) between the concentration and the δ15N of DON is observed in the upper 75 m, suggesting an isotope effect of ~4.9 ± 0.4‰ for DON degradation. Comparing the DON δ15N data from the SCS with other regions, we find that the δ15N difference between euphotic zone DON and shallow subsurface nitrate δ15N (Δδ15N(DON‐NO3)) rises from ocean regions of inferred net DON production to regions of net DON consumption, with the SCS representing an intermediate case.
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A Test of the Diatom‐Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient‐Nitrogen Into Southern Ocean Particles and Sediments
Abstract Sedimentary nitrogen isotope (as δ15N) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmosphericpCO2. We present a field assessment of how surface nitrate consumption is reflected in δ15N values of total nitrogen and diatom‐bound nitrogen pools of particles and sediments across the Southern Ocean along 170°W during late austral summer. Mixed layer nitrate δ15N values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom‐bound δ15N values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between δ15N values of the total nitrogen and diatom‐bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the δ15N value(s) of the assimilated nutrient(s). Surface sediment δ15N values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in δ15N values of the diatom‐bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained.
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- Award ID(s):
- 1757572
- PAR ID:
- 10449605
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Global Biogeochemical Cycles
- Volume:
- 34
- Issue:
- 10
- ISSN:
- 0886-6236
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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