南海北部珊瑚高分辨率硼同位素组成及其对珊瑚礁海水pH变化的指示意义

摘要

造礁珊瑚是研究热带海洋高分辨率气候环境演变的重要载体。对采自海南岛南部三亚湾的活体滨珊瑚SY10进行了约为月分辨率的碳、氧、硼同位素组成分析,并利用珊瑚δ11B重建了海水pH。结果显示,所测量样品的δ13C变化范围为–3.32‰～–1.76‰,δ18O为–6.13‰～–4.78‰,δ11B为23.51‰～26.23‰,且这些珊瑚样品的碳、氧、硼同位素组成均存在明显的季节性周期波动。其中 pH 与δ18O 之间存在明显的正相关关系,高的 pH 值更倾向于在低温的季节出现,这意味着短时间尺度该处珊瑚礁海水 pH 可能主要不是受海水CO2溶解度控制,而是与生物活动有密切的关系。利用SY10珊瑚样品硼同位素组成重建的海水pH值变化范围为7.77～8.37,并呈季节性周期波动,这种大幅度的周期波动与我们对三亚珊瑚礁海水 pH 进行现场观测所得到的结果以及前人的研究成果相符,说明了利用珊瑚δ11B重建海水pH记录是可靠的。%Coral is a good medium for the study of high resolution climate changes in tropical oceans. In this paper, we analyzed the C, O, B isotopic compositions of 49 samples from a living Porites lutea coral, SY10, from Sanya Bay, south of Hainan Island. The result shows that, theδ13C,δ18O andδ11B values of the SY10 coral range from–3.32‰ to –1.76‰, from –6.13‰ to –4.78‰ and from 23.51‰ to 26.23‰, respectively. They all exhibited obvious seasonal fluctuations. A significant positive correlation occurs between the reconstructed seawater pH andδ18O, which is converse to the temperature effect for the solubility of CO2 in seawater that low temperature favors the dissolution of CO2 and hence decreases seawater pH. Our observation that higher pH values tend to appear in low temperature seasons suggests that seawater pH on coral reefs should be largely controlled by biological activities rather than changes in CO2 solubility. Meanwhile, the reconstructed seawater pH values range from 7.77 to 8.37, with seasonal fluctuations similar to that of theδ11B. Furthermore, the substantial periodic fluctuations of reconstructed pH are in consistent with the variability of observed seawater pH records on the same coral reef, and agree with the seawater pH changes in Xisha Islands mentioned by Dai et al. (2009). We therefore conclude that it is feasible to reconstruct high resolution seawater pH by using boron isotopic composition of corals.