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Nitrogen isotope constraints on the biogeochemistry and paleoclimatology of the subarctic North Pacific.

机译：氮同位素对北极北太平洋生物地球化学和古气候的限制。

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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.
机译：氮同位素是评估现代海洋和过去生物泵强度的有力工具，因为它们记录了表层海洋中生物群对硝酸盐吸收的程度。通过测量白令海，北极西部亚太平洋北太平洋和鄂霍次克海深海沉积物核心中的大量沉积物和与硅藻结合的有机物质的氮同位素组成，可以推断出气候驱动的表层海洋养分利用变化最后两个冰期-冰间循环（通过MIS 6）。在所有检查的岩心中，较冷时期的沉积物15N / 14N升高，加上这些时期生物生产力降低的证据，表明对表层海洋的养分供应受到抑制，并且在冰河极大时期对养分池的利用几乎完成。这些结果与冰川期整个北亚大北极地区更强分层的上层水柱的发展是一致的。此外，在发生冰消融作用（I和II处）时，沉积物中15N / 14N的显着增加可能是由于地下硝酸盐的15N / 14N比值的增加，伴随着更强烈的局部和/或区域反硝化作用。然而，鉴于有证据表明在冰期间隔期间生物生产力同时增加，因此养分利用率的提高也可能有助于观测到的15N / 14N沉积物增加。来自现代白令海水柱的硝酸盐同位素数据，估计了硝酸盐同化的氮同位素效应在5至7‰之间，其中较高的估计值来自白令海南部，并且与更深的夏季混合层有关，这与同位素的培养结果一致藻类光限制的作用。相对于硝酸盐浓度而言，通过冬季剩余混合层的样品硝酸盐的15N / 14N（和18O / 16O）变化要比夏季混合层中硝酸盐同化的推断小得多。这种偏差的出现似乎是由于白垩纪北太平洋强烈的盐分分层，导致白令海盆地和陆架的硝酸盐消耗量很高，以及冬季从下方补充的硝酸盐不完全。

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