(Table 1) Age model of ODP Site 184-1144@en

Variations in nitrogen isotopic composition (d15N) and total chlorin accumulation rate (AR) are employed asproxies to reconstruct oceanic nitrate inventory, the balance between denitrification and N fixation, and paleoproductivity in a rapidly accumulating sediment drift deposit beneath the Western Pacific Warm Pool for the last 145 Kyr. Subsurface and deep waters of the northern South China Sea (SCS) are sourced from the shallow Kuroshio Current (KC) and Pacific Intermediate Water, respectively. Their relative importance in determining sedimentary d15N and paleoproductivity have been altered by changes in equatorial circulation, summer- and wintermonsoon intensity and relative sea level. The location and basin configuration of the marginal SCS renders it especially sensitive to such changes. Assuming complete annual nitrate utilization, low d15N values during glacial stages are interpreted as a reflection of reduced remote denitrification in the Eastern Tropical North Pacific (ETNP) source waters, while much of marine isotope stage (MIS) 3 and the last interglacial were characterized by high denitrification. However, intervals of anomalously low d15N values are interpreted as reflecting the contribution from regional N fixation in West Pacific surface waters, transmitted to the site by the shallow KC. Unusually, the Holocene is characterized by declining d15N values and an inverse correlation with organic matter content since ca. 8.2 Ka. Millennial-scale variations during MIS 3 indicate higher frequency variations in both ETNP denitrification and local N fixation, which may be coherent with a hemispheric response to Dansgaard-Oeschger events recorded at high latitudes. For much of the last 145 Kyr, paleoproductivity was decoupled from d15N, and instead seems to reflect the extent of the global nitrate inventory stimulated by elevated dust fertilization, and regional mixed-layer deepening associated with the relative intensity of the SE Asian winter monsoon. Despite evaluation of possible conflicting influences on the record of each proxy, we interpret our data as clear evidence of glacial/interglacial changes in marine nutrient inventory across the whole of the North Pacific, with a corresponding biogeochemical response and important implications for global CO2 drawdown via an invigorated biological pump. […]本研究以氮同位素组成（d15N，nitrogen isotopic composition）与总叶绿素积累速率（AR，total chlorin accumulation rate）为代用指标，重建了过去145千年以来西太平洋暖池（Western Pacific Warm Pool）下方快速沉积的沉积物漂积体中的海洋硝酸盐库（oceanic nitrate inventory）、反硝化作用（denitrification）与固氮作用（nitrogen fixation，N fixation）的平衡，以及古生产力（paleoproductivity）变化。南海北部（SCS，South China Sea）的次表层与深层水体分别源自浅层黑潮（KC，Kuroshio Current）与太平洋中层水（Pacific Intermediate Water）。赤道环流（equatorial circulation）、冬夏季风强度与相对海平面变化改变了这两类水体对沉积物中d15N与古生产力的相对贡献程度。作为边缘海的南海北部，其区位与盆地构型使其对这类变化尤为敏感。假设年度硝酸盐完全被利用，冰期阶段（glacial stages）较低的d15N值被解释为反映北太平洋东部热带海域（ETNP，Eastern Tropical North Pacific）源区的远程反硝化作用减弱；而海洋同位素阶段（MIS，marine isotope stage）3与末次间冰期（last interglacial）则以强反硝化作用为主要特征。不过，部分异常低的d15N值区间被认为反映了西太平洋表层海水的区域性固氮作用贡献，该信号通过浅层黑潮传递至研究站位。不同寻常的是，全新世（Holocene）的d15N值呈下降趋势，且自约8.2千年前起与有机质含量（organic matter content）呈负相关关系。海洋同位素阶段3期间的千年尺度变化显示，北太平洋东部热带海域反硝化作用与本地固氮作用均存在高频变化，这可能与高纬度地区记录的丹斯果-奥什格尔事件（Dansgaard-Oeschger events）所引发的半球响应相一致。在过去145千年的大部分时段内，古生产力与d15N呈解耦状态，反而更能反映由增强的大气粉尘补给营养所驱动的全球硝酸盐库规模，以及与东南亚冬季风（SE Asian winter monsoon）相对强度相关的区域混合层加深过程。尽管本研究评估了各代用指标记录可能受到的相互冲突的影响，但我们仍将本数据集的结果解读为北太平洋全域海洋营养盐库存在冰期-间冰期变化的明确证据，其伴随的生物地球化学响应（biogeochemical response），以及通过增强的生物泵（biological pump）作用对全球二氧化碳抽取（global CO2 drawdown）产生的重要启示。[…]