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Nitrogen isotopic evidence for deglacial changes in nutrient supply in the eastern equatorial Pacific

机译：赤道东太平洋氮素同位素证据表明营养供应发生冰川变化

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The Eastern Equatorial Pacific (EEP) is a high nutrient–low chlorophyll region of the ocean. Downcore nitrogen isotope records from the EEP have been previously interpreted as a direct reflection of changes in nutrient consumption. However, the observed changes in sedimentary δ~(15)N since the last glacial maximum have no coherent relationship with export productivity or an inferred variation in the iron-to-nitrate ratio of the surface waters. Rather, downcore N isotope records in the EEP strongly resemble changes in the extent of water column denitrification as recorded in nearby sedimentary δ~(15)N records along the western margin of the Americas. This similarity is attributed to the overprinting of the N isotopic composition of nitrate in the EEP through the advection of nitrate westward from the margins in the subsurface. A local nitrogen isotope record of changes in the degree of nutrient consumption is extracted from the bulk sedimentary record by subtracting two different sedimentary δ~(15)N records of denitrification changes from two new EEP δ~(15)N records (TR163-22 and ODP Site 1240) The denitrification records used are from 1) the Central American margin (ODP Site 1242) and 2) the South American margin (GeoB7139-2). The degree of consumption in the surface waters declines rapidly from elevated values during the last glacial maximum to a pair of minima around 15 and 11-13 ka, and finally it increases into the Holocene. The derived EEP nitrogen isotope record indicates that the regional peak in export productivity occurred when the supply of nutrients exceeded the apparently high demand. The influx of nutrients during the deglaciation is attributed to the resumption of intense overturning in the Southern Ocean and the release of sequestered CO_2 and nutrient-rich, O_2 poor waters from the deep ocean. This has important implications for understanding the glacial-interglacial scale variation in intermediate water suboxia and water column denitrification.
机译：赤道东太平洋（EEP）是海洋中营养丰富，叶绿素含量低的地区。 EEP的下层氮同位素记录先前已被解释为营养物消耗变化的直接反映。然而，从最后一次冰期开始以来，观测到的沉积物δ〜（15）N的变化与出口生产力没有任何连贯的关系，也没有推断出地表水铁铁比的变化。相反，EEP中的下层N同位素记录与美洲西部边缘附近的沉积δ〜（15）N记录中记录的水柱反硝化程度的变化非常相似。这种相似性归因于EEP中硝酸盐的N同位素组成的叠印，这是通过从地下边缘向西向西平移硝酸盐而实现的。通过从两个新的EEPδ〜（15）N记录中减去两个不同的反硝化变化沉积物δ〜（15）N记录，从大体积沉积记录中提取局部氮同位素变化的养分消耗记录（TR163-22和ODP站点1240）使用的反硝化记录来自1）中美洲边缘（ODP站点1242）和2）南美边缘（GeoB7139-2）。地表水的消耗程度从上次冰河最大值期间的升高值迅速下降到15和11-13 ka左右的一对最小值，最后增加到全新世。得出的EEP氮同位素记录表明，当养分的供应超过明显高的需求时，出口生产力出现了区域性峰值。冰川消融期间养分的涌入归因于南大洋的剧烈翻转和从深海释放出隔离的CO_2和富含养分的O_2贫水。这对于理解中间水亚氧化层和水柱反硝化过程中的冰间变化规模具有重要意义。

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