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 NO-3, 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 NO-3. 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 NO-3 (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 NO-3 to surface waters as a result of isotopic fractionation during phytoplankton uptake. Overall, though, the coupling between NO-3 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 winter-time δ15N minima fail to occur during El Niño 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 NO-3.