| Nitrogen isotopes are a powerful tool for evaluating the strength of the biological pump, both in the modern ocean and the past, in that they record the extent of nitrate uptake by biota in the surface ocean. Measurements of the nitrogen isotope composition of bulk sediment and diatom-bound organic matter in deep sea sediment cores from the Bering Sea, the western open subarctic North Pacific, and the Okhotsk Sea are used to infer climatically-driven changes in surface ocean nutrient utilization over the last two glacial-interglacial cycles (through MIS 6). In all examined cores, elevated sedimentary 15N/14N during colder periods, coupled with evidence for reduced biological productivity during these times, suggests that the supply of nutrients to the surface ocean was suppressed, and utilization of that nutrient pool nearly complete during glacial maxima. These results are consistent with the development of a more strongly stratified upper water column throughout the greater subarctic North Pacific during glacial periods. Furthermore, upon deglaciations (Terminations I and II), a marked increase in sedimentary 15N/14N may be due to an enrichment in the 15N/14N ratio of subsurface nitrate associated with more intense local and/or regional denitrification; however, given evidence for a coincident increase in biological productivity during deglacial intervals, enhanced nutrient utilization may also contribute to the observed increase in sedimentary 15N/14N. Nitrate isotope data from the modern Bering Sea water column yield estimates of the nitrogen isotope effect for nitrate assimilation between 5 and 7‰, with higher estimates coming from the southern Bering Sea and correlated with deeper summer mixed layers, consistent with culture findings on the isotopic effect of algal light limitation. Samples through the remnant winter mixed layer have a much smaller change in 15N/14N (and 18O/ 16O) of nitrate relative to nitrate concentration than that inferred for nitrate assimilation in the summertime mixed layer. This deviation appears to result from the high degree of nitrate consumption in the Bering Sea basin and shelf and the incomplete wintertime resupply of nitrate from below, due to the strong salinity stratification of the subarctic North Pacific. |
