中布容期热带西太平洋深部[CO32-]变化：碳循环过程与气候效应

The mid-Brunhes interval is a significant period in the history of Quaternary climate. It contains the mid-Brunhes dissolution event and the onset of warmer interglacials. However, there are still major uncertainties regarding this time interval, including the extent of mid-Brunhes dissolution event, the evolution of oceanic carbon storage and its significance for the mid-Brunhes climatic transition. Application of paleo-carbonate proxies is crucial for improving our understanding of these scientific issues. Here, Mg/Ca ratios and δ18O values of planktonic foraminifera are measured to investigate the potential effect of multiple environmental parameters, such as temperature and salinity, on the “size-normalized weight” (SNW) of planktonic foraminifera. After the assessment of the proxy reliability, SNW of planktonic foraminifera are used to reconstruct deep-water carbonate ion concentrations ([CO32-]) in the western tropical Pacific during the mid-Brunhes interval. Along with foraminiferal fragmentation data, the SNW-based [CO32-] record is used to recognize the mid-Brunhes dissolution event. Then, the extent of dissolution event, and the pelagic carbonate production during the mid-Brunhes, are revealed by the deep-water [CO32-] decline. Finally, based on the comparison of deglacial variabilities in deep-water [CO32-] before and after the mid-Brunhes climatic transition, this project will provide further insights into the carbon storage in the deep tropical Pacific during the mid-Brunhes and its role in contemporaneous global climate change.

中布容期海洋环境以及全球气候发生重大转型，期间深海碳酸钙溶解作用显著加强，MIS 11及其之后的间冰期相比之前更加温暖。然而中布容期深海碳酸钙溶解加强幅度、大洋碳储量演化及其与间冰期转型间的关联悬而未决，重建过去海洋碳酸盐系统参数是研究上述问题的关键。本项目以热带西太平洋岩心为材料，应用浮游有孔虫Mg/Ca、δ18O以及粒径标准化壳体重量（SNW）等指标，在评估表层海水温度与盐度等环境因素对SNW的可能影响后，利用浮游有孔虫SNW恢复中布容期深海碳酸根离子浓度（[CO32-]）变化。基于深海[CO32-]演化，并结合有孔虫碎壳率等数据，确认中布容溶解事件在热带西太平洋的记录。根据中布容溶解事件中深海[CO32-]下降幅度，定量评估此次溶解事件强度，并揭示当时大洋碳酸钙生产状况。最后，对比中布容期间冰期转型前后冰消期深海[CO32-]特征，探讨大洋碳储量演化在中布容期间冰期气候转型中的作用。

海洋沉积物中浮游有孔虫粒径标准化壳体重量（Size-Normalized Weight，SNW）随着深海碳酸根离子饱和程度（Δ[CO32-]）下降、碳酸钙溶解作用加强而变轻，故可以作为一种古海洋代用指标来重建过去深海碳酸根离子浓度（[CO32-]）变化，为理解过去大洋碳循环演变及其气候效应提供依据。基于全球热带大洋表层沉积物SNW数据，项目评估了现代过程中浮游有孔虫Trilobatus sacculifer，Neogloboquadrina dutertrei和Pulleniatina obliquiloculata SNW的受控因素，认为T. sacculifer和N. dutertrei SNW可作为古海洋深部Δ[CO32-]代用指标，而P. obliquiloculata SNW同时受到深海Δ[CO32-]以及初始粒径标准化壳体重量（Initial Size-Normalized Weight，ISNW）的影响。随后，项目建立了适用于全球大洋的SNW-Δ[CO32-]校准公式，评估了P. obliquiloculata ISNW/钙化作用的受控机制，通过热带西太平洋MD06-3047B孔以及U1490站重建了更新世以来深海[CO32-]变化。本项目取得的主要结论有：.（1）P. obliquiloculata ISNW在现代过程、冰期旋回以及岁差周期中均响应海水温度变化，表明P. obliquiloculata钙化作用受控于海水温度变化，而非海洋酸化；.（2）冰期旋回中，热带西太平洋深部[CO32-]通过碳酸钙补偿机制响应海平面变化；.（3）中布容期，热带西太平洋深部[CO32-]从MIS 12/11的~100 μmol kg-1极大值急剧下降至MIS 11中期的~40 μmol kg-1极低值，前者表明MIS 12/11冰消期海洋碳释放加强，后者量化了中布容溶解事件在热带西太平洋的强度；.（4）中更新气候转型期，[CO32-]在~0.9-0.7 Ma阶段性下降~7-10 μmol kg-1，表明当时太平洋深海碳储量增加，这一过程受控于冰盖扩张引起的南大洋海洋垂向流通减弱。