The nitrogen isotope biogeochemistry of sinking particles from the margin of the eastern North Pacific

The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO₃/^-, and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is influenced seasonally by coastal upwelling. Nevertheless, sediment trap material evidently records the isotopic composition of new nitrogen sources, since average δ¹⁵N is generally indistinguishable from δ¹⁵N values for subsurface NO₃/^-. Surficial sediments are also very similar to the average δ¹⁵N value of the sediment traps, being within 1‰. This difference in δ¹⁵N between trap material and sediment is much less than the previously observed 4‰ difference for the deep sea. Better organic matter preservation at our margin sites is a likely explanation, which may be due to either low bottom O₂ concentrations or higher organic matter input to the sediments. All sites have δ¹⁵N for sub-euphotic zone NO₃/^- (8-10‰) substantially elevated from the oceanic average (4.5-5‰). This isotopic enrichment is a result of denitrification in suboxic subsurface waters (Gulf of California) or northward transport of denitrification influenced water (Monterey Bay and San Pedro Basin). Our results therefore suggest that downcore δ¹⁵N data, depending on site location, would record the intensity of denitrification and the transport of its isotopic signature along the California margin. Temporal variations in δ¹⁵N for the sediment traps do appear to respond to upwelling or convective injections of NO₃/^- to surface waters as a result of isotopic fractionation during phytoplankton uptake. Overall, though, the coupling between NO₃/^- injection, δ¹⁵N, and flux is looser than previously observed for the open-ocean, most likely the result of the smaller time/space scales of the events. In the Gulf of California, wintertime convective mixing/upwelling does produce distinct δ¹⁵N minima co-occurring with particle flux maxima. Interannual variations are apparent in this region when these wintertime δ¹⁵N minima fail to occur during El Nino conditions. There appears to be a positive relationship between the Southern Oscillation Index (SOI) anomaly and annual average δ¹⁵N. One explanation calls for hydrographic changes altering the δ¹⁵N of subeuphotic zone NO₃/^-.