在阿拉伯海氧氣最小區域內1.2個百萬年的有孔蟲氮同位素紀錄

Abstract

在現今的海洋中，阿拉伯海是三個主要氧氣最小區域（OMZ）之一，其中反硝化作用發生在150-1200m的深度，並從海洋中還原生物固氮作用的氮（N）。阿拉伯海的含氧量低歸因於由於當地生物生產力較高而導致的氧氣消耗或是海洋環流流動遲緩所致。前人研究主要是使用沉積物中的總氮同位素（δ15N），且研究顯示在溫暖時期（包括間冰期和間冰段），由於有較高的反硝化作用導致δ15N升高。在此研究中，我們是第一個使用來自阿拉伯東部的IODP Site U1456中的浮游有孔蟲Trilobatus sacculifer建立在過去的1.2 Myr中有孔蟲δ15N（FB-δ15N）記錄，研究結果顯示在冰期間FB-δ15N值約為5‰，而間冰期則增加到12‰以上，此結果與前人發表在這跟岩心中的沉積物δ15N一致，但是在間冰期FB-δ15N值比沉積物的δ15N略大1-2‰。在間冰期時較高FB-δ15N值通常與較高的總氮含量（TN）有關，因此我們的研究結果支持了之前的論點，即在間冰期/間冰段時較高的生物生產力可能會促使淺層的溫躍層的氧氣消耗量增加，從而推動阿拉伯海OMZ的反硝化速率提高。

The north Arabian Sea is one of the three major oxygen minimum zones (OMZ) where denitrification occurs in the intermediate depths and removes biologically fixed nitrogen (N) from the ocean. The low oxygen content in the Arabian Sea has been attributed primarily to sluggish ocean circulation or oxygen consumption due to high levels of local productivity. Previous studies using bulk sedimentary nitrogen isotopes (δ15N) show elevated δ15N caused by higher denitrification during warm periods including interglacials and interstadials. Here, we evaluate these findings by generating the first foraminifera-bound δ15N (FB-δ15N) record using planktonic foraminifera Trilobatus sacculifer from IODP Site U1456 in the Eastern Arabian Sea over the last 1.2 Myr. The FB-δ15N value is around 5‰ in the glacials and increased to above 12‰ during the interglacials. The results are consistent with previously published bulk sediment δ15N at this site, but interglacial FB-δ15N values are 1-2‰ greater than bulk sediments. The high FB-δ15N value in the warm interglacials is generally associated with higher total nitrogen content (TN). Our findings support previous arguments that higher productivity during interglacials/interstadials may prompt greater oxygen consumption in the shallow thermocline, driving higher denitrification rates in the Arabian Sea OMZ.