南海北部滨珊瑚硼同位素气候环境指示意义再研究

REVISITING PALEOCEANOGRAPHIC APPLICATION OFδ11B IN CORAL PORITES FROM THE NORTHERN SOUTH CHINA SEA

珊瑚碳酸盐硼同位素（δ11B）是古海洋中非常重要、具有特殊环境指示意义的地球化学指标。利用δ11B重建古海水pH值，极大地推动了对海洋酸化历史以及海洋生物适应酸化潜力的认识。然而最近的研究表明，自然环境下生长的珊瑚具有非常强的调控钙化流体pH的能力，甚至有可能超过外部海水pH的影响，为δ11B—pH指标研究带来了极大的挑战。因此，需要对珊瑚钙化流体δ11B-pH气候环境指示意义重新进行分析、挖掘和再解释。本文利用前人测量的南海北部滨珊瑚（Porites）长时间尺度的钙化流体δ11B—pH记录，根据大气二氧化碳、海水温度、海水溶解无机碳含量和总碱度记录对δ11B-pH进行了重构。结果显示冬季风能够影响珊瑚礁海水pH值和珊瑚内部钙化环境，是控制南海北部珊瑚钙化流体pH值变化的主导因素。对于珊瑚礁周围的海水，冬季风通过影响珊瑚礁生态系统的钙化作用、光合作用以及水体交换来控制海水pH值；对珊瑚内部钙化流体，冬季风通过影响温度，从而影响珊瑚呼吸作用来调控钙化流体溶解无机碳含量，最终影响流体pH值。虽然目前利用珊瑚δ11B—pH精确地定量重建外部海水pH存在着挑战和不确定性，但是在南海北部这个冬季风敏感海区，珊瑚δ11B—pH可以作为冬季风强度变化的潜在新指标。

Boron isotopes of biogenic carbonate have been suggested as a key proxy to reconstruct ambient seawater pH since the 1990s. Long-term pre-instrumental records of ambient seawater based on δ11B can provide critical insights on nature variability of seawater pH and modern ocean acidification. However, recent studies suggest that under natural reef environments, some species corals exert strong physiological controls on the pH of their calcifying fluid. Thus pH of calcifying fluid derived fromδ11B is only partially dependent or independent of the ambient seawater pH. The past pH reconstruction based empirical relationships under laboratory-controlled experiments and its environmental implication may be invalid. To constrain the environmental implication of δ11B-pH in coral fluid, here we attempt a comprehensive study of ca.200 years δ11B record in a coral Porites from the northern South China Sea （SCS）. Using hydrographic data from the northern SCS and the coral-derived pH record based on the empirical relationship, we are able to differentiate pH changes due to surface sea temperature, atmosphere CO2 versus those from biogeochemical and mass exchange changes. The contribution from SST is small. We also find that the ocean pH does not simply reflect atmospheric CO2 trends but rather mainly biogeochemical/water mass exchange changes in the last two centuries, which are strongly influenced by the East Asian Winter Monsoon （EAWM）. The EAWM is one of largest circulation systems on earth during boreal winter and could strongly modulate the physical- biogeochemical conditions of the SCS. The linkage between δ11B-pH and EAWM is attributable to calcification rate of corals, the flushing efficiency of waters over the reefs and productivity. During weak EAWM periods, the build- up of CO2 due to calcification could lower pH due to poor flushing of reef waters by open seawater. In contrast, periods of relatively strong EAWM with strengthened surface currents and decreased calcification could result in higher pH values. Moreover, surface primary productivity of the SCS is also controlled by EAWM intensity. A weak （strong） EAWM would cause the nutricline to deepen （shallow） and result in lower （higher） surface productivity and decreased （increased） pH. On the other hand, enhanced （depressed） metabolic activity of zooxanthellae symbionts in a warm （cold） condition during weak （strong） EAWM periods could increase （decrease） pH in calcifying fluid. Although the exact contribution from physiological controls on the pH of coral calcifying fluid is largely unknown and may undermine the usage of this proxy on reconstructing ambient seawater pH, it seems to also be linked with the thermal condition driven by winter monsoon on the same direction of ambient seawater pH. Therefore, we suggest the δ11B of corals in the northern SCS instead to be a proxy for winter monsoon intensity which is difficult to reconstruct using traditional proxies. This is supported by a significant correlation between the δ11B-pH and Siberian High pressure, an index of EAWM intensity. Low and high δ11B-pH values respectively co-vary with weak and strong Siberian High phases on inter-decadal and seasonal timescales.