Stable isotope records and biogenic barium fluxes of sediment core SO202-07-06@en

Reduced nitrate supply to the subarctic North Pacific (SNP) surface during the last ice age has been inferred from coupled changes in diatom-bound d15N (DB-d15N), bulk sedimentary d15N, and biogenic fluxes. However, the reliability of bulk sedimentary and DB-d15N has been questioned, and a previously reported d15N minimum during Heinrich Stadial 1 (HS1) has proven difficult to explain. In a core from the western SNP, we report the foraminifera-bound d15N (FB-d15N) in Neogloboquadrina pachyderma and Globigerina bulloides, comparing them with DB-d15N in the same core over the past 25 kyr. The d15N of all recorders is higher during the Last Glacial Maximum (LGM) than in the Holocene, indicating more complete nitrate consumption. N. pachyderma FB-d15N is similar to DB-d15N in the Holocene but 2.2 per mil higher during the LGM. This difference suggests a greater sensitivity of FB-d15N to changes in summertime nitrate drawdown and d15N rise, consistent with a lag of the foraminifera relative to diatoms in reaching their summertime production peak in this highly seasonal environment. Unlike DB-d15N, FB-d15N does not decrease from the LGM into HS1, which supports a previous suggestion that the HS1 DB-d15N minimum is due to contamination by sponge spicules. FB-d15N drops in the latter half of the Bølling/Allerød warm period and rises briefly in the Younger Dryas cold period, followed by a decline into the mid-Holocene. The FB-d15N records suggest that the coupling among cold climate, reduced nitrate supply, and more complete nitrate consumption that characterized the LGM also applied to the deglacial cold events.通过硅藻结合态氮同位素δ¹⁵N（diatom-bound d15N, DB-d15N）、全样沉积物δ¹⁵N（bulk sedimentary d15N）与生源通量的协同变化，学界已推断末次冰期期间亚北极北太平洋（Subarctic North Pacific, SNP）表层海水硝酸盐供给量有所降低。然而，全样沉积物δ¹⁵N与DB-d15N的可靠性受到学界质疑，且此前报道的海因里希冰阶1（Heinrich Stadial 1, HS1）期间的δ¹⁵N低值现象始终难以得到合理解释。我们对西SNP的一根沉积物岩芯开展研究，报道了其中厚壳新抱球虫（Neogloboquadrina pachyderma）与泡抱球虫（Globigerina bulloides）的有孔虫结合态氮同位素δ¹⁵N（FB-d15N）记录，并将其与同一岩芯过去25 kyr的DB-d15N记录进行对比。所有记录载体的δ¹⁵N值在末次盛冰期（Last Glacial Maximum, LGM）均高于全新世（Holocene），表明彼时硝酸盐消耗更为彻底。厚壳新抱球虫的FB-d15N值在全新世时期与DB-d15N值相近，但在末次盛冰期时较后者高出2.2‰。这一差异表明，FB-d15N对夏季硝酸盐浓度降低与δ¹⁵N升高的变化更为敏感，这与该高季节性环境下有孔虫相较于硅藻达到夏季生产峰值的时间滞后性相符。与DB-d15N不同，FB-d15N值并未从末次盛冰期向HS1阶段出现下降，这一结果支持了此前的假说：HS1阶段的DB-d15N低值是由海绵骨针污染所致。FB-d15N值在波令-阿勒罗德暖期的后半段出现下降，并在新仙女木冷期短暂升高，随后持续下降至全新世中期。FB-d15N记录表明，末次盛冰期所特有的寒冷气候、硝酸盐供给减少与硝酸盐消耗更彻底三者之间的耦合关系，同样适用于冰消期的冷事件。