末次盛冰期以来热带东北大西洋最小含氧带的演化

Due to the scarcity of geological records, mechanisms for triggering changes in the global Oxygen Minimum Zones (OMZs) under different climatic background and on different time scales have not been fully understood. We proposed here to reconstruct the millennial-scale variability of the northeastern tropical Atlantic OMZ since the Last Glacial Maximum using proxies of benthic foraminiferal faunal assemblage and concentrations of redox-sensitive trace metals. We also proposed to explore two potential reasons that might have caused past changes in the OMZ water oxygenation: thermocline ventilation and sea surface productivity. Two key scientific questions have been raised and will be addressed in this research: (1) Was the northeastern Atlantic OMZ better oxygenated during the Last Glacial Maximum relative to the Late Holocene? Did it cause by a better ventilation of glacial thermocline? (2) Over the last deglaciation, did the oxygenation level of the northeastern Atlantic OMZ elevate during the Bølling-Allerød warming interval with respect to the cold Heinrich Stadial 1? Did it cause by a decrease in the sea surface productivity? Answers to these questions would lead to a better understanding of behaviors of OMZs under different climatic background. This also would provide clues for the prediction of future OMZ changes under a global warming background.

由于地质记录的缺乏，迄今为止人类对不同气候背景和不同时间尺度下全球最小含氧带的变化机制尚未完全了解。本项目计划利用底栖有孔虫群落结构、氧化还原敏感性金属元素含量等指标重建末次盛冰期以来热带东北大西洋最小含氧带在千年时间尺度上的演化历史。同时，通过重建温跃层通风速率、表层古生产力记录来探索引起该区最小含氧带变化的原因。我们将重点探讨两个科学问题：(1)末次冰期时该区最小含氧带的含氧量是否比晚全新世时高？这一变化是否是由冰期时较高的温跃层通风速率引起的？(2) 在末次冰消期时段，从Heinrich Stadial 1冷期进入Bølling-Allerød暖期，该区最小含氧带的含氧量是否变高？这是否是由表层输出生产力降低引起的？以上问题的解决有助于我们更深入地了解最小含氧带在不同气候背景下的行为，为预测未来最小含氧带在全球变暖背景下的变化趋势提供历史背景资料。

海水溶解氧对海洋碳-氮生物地球化学循环、生物栖息环境均能产生重要影响，因此是衡量海洋环境变迁的一个重要指标。大洋溶解氧含量的地质演化历史还是一个发展中的课题。本项目利用氧化还原敏感性金属元素含量、有孔虫I/Ca等指标重建了末次盛冰期以来热带东北大西洋最小含氧带在千年时间尺度上的演化历史，并探究了背后的驱动因素。取得了以下三方面的认识：(1)相比于全新世，末次盛冰期时温跃层含氧量下降，最小含氧带在垂向上的范围扩张，但垂向梯度减弱。虽然海水温度下降和温跃层通风速率提高有助于冰期含氧量的上升，但陆地来源的有机碳降解作用消耗了次表层水的溶解氧；(2)相比于中全新世，早全新世最小含量带核部的溶解氧含量下降，推测是海水温度上升和陆源有机质输入共同造成的结果；(3)在末次冰消期，水体含氧量在Heinrich Stadial1冷期与Bølling-Allerød暖期时相当，但比冰期时高。陆源有机质输入的减少很可能是重要原因。全新世的最小含氧带演化历史对预测未来含氧带的变迁趋势具有启示意义。即在全球变暖背景下，海水温度升高会导致溶氧率下降，有可能造成最小含氧带地区的生态环境恶化。