南海全新世大暖期海表水的高酸性证据

利用正热电离质谱高精度测定了南海化石珊瑚样品的δ¹¹B 比值，定量地重建了中晚全新世以来南海海表水的pH值。结果显示南海海表水pH值并不像预先设想的那样稳定。其中古海水pH值最低为6100aB.P.前的7.91；   最高为4300aB.P.前和1200aB.P.前的8.29。南海海表水pH值从全新世大暖期开始，整体上呈缓慢增加趋势，而到现代以后明显下降。珊瑚δ¹¹B记录表明南海海表水在中晚全新世有两个高酸性的时期，一个发生在全新世大暖期，一个为现代。全新世大暖期时南海出现的高海平面、偏强的东亚夏季风和偏弱的冬季风可能是导致其海表水出现高酸性的原因。现代南海海表水pH值显著偏低，背离了中晚全新世以来南海海表水pH值逐渐增加的趋势，这很可能说明人类大量排放的CO₂确实改变了南海海表水自然变化的规律，南海在变酸。

EVIDENCE FOR LOW pH VALUES OF SURFACE SEAWATER IN SOUTH CHINA SEA DURING HOLOCENE THERMAL OPTIMAL

P-TIMS isotopic technique is used in determining of the coral δ¹¹B  compositions and satisfies obtaining high analytical precision for small variability of δ¹¹B related to surface seawater pH change. The fossil and modern corals analyzed in this study are sampled from Leizhou Peninsula at the northern coast of the South China Sea(SCS)，Hainan Island，Xisha Islands in the north and middle of SCS，and Nansha Islands in the south of SCS. These samples cover a large period of time in the Mid Late Holocene spanning from the Holocene thermal optimal(around 6kaBP.)to the present and a large geographical region in SCS，which are very valuable for investigating ocean pH changes in SCS during the Holocene. All samples were run in duplicate. Our work first reveals significant variability of boron isotopic ratios recorded in fossil corals in the Mid Late Holocene that may link with synchronous climate change. The δ¹¹B  values of corals range in 22.2‰～25.5‰. Correspondingly，the reconstructed pH displays a large variability. The lowest ocean pH value is 7.91which occurred at 6.1kaB.P.while the highest pH value is 8.29 which occurred at 4.3kaB.P. and 1.2kaB.P. The amplitude of pH variability inferred from the records of our fossil corals is about 038 pH units. The most interesting feature of the coral records in SCS is that the lowest δ¹¹B(lowest pH)occurred at two warm periods：  the Holocene thermal optimal and the modern time. The global atmospheric concentrations of CO₂ increased dramatically after industrial revolution. The sharp decrease of pH of the modern seawaters is likely to be caused by large anthropogenic CO₂ emissions. However，atmospheric CO₂ concentration changed little from 7kaB.P. to pre industry. There is no correlation between atmospheric CO₂ concentration recorded in Antarctica ice cores and δ¹¹B  or reconstructed paleo pH in the Mid Late Holocene. Thus，the variation of pCO₂ in atmosphere accounts little for pH change of SCS before pre industry. We suggest that high sea level，intensified summer monsoon，and decreased winter monsoon in Middle Holocene may have caused the low pH during Holocene thermal optimal. Low ocean pH values can be expected in warm conditions when sea level is relative high. The δ¹¹Bseries in this study also shows a similar tendency with δ¹¹B of planktonic foraminifera from the western equatorial Pacific but with larger amplitude. This may be contributed by synchronous monsoon changes in SCS. In addition，interestingly，in contrast to a gradual increasing trend of pH of SCS since the Holocene thermal optimal，modern pH of SCS is very low which may indicate that the impact of increasing concentration of anthropogenic greenhouse gases may reverse the natural pH trend in SCS since the Mid Holocene. 