Deep-water carbonate dissolution in the northern South China Sea during Marine Isotope Stage 3

Na Wang; Bao-Qi Huang; He Li

摘要

Abstract The production, transportation, deposition, and dissolution of carbonate profoundly form part of the global carbon cycle and affect the amount and distribution of dissolved inorganic carbon (DIC) and alkalinity (ALK), which drive atmospheric CO₂ changes during glacial/interglacial cycles. These processes may provide significant clues for better understanding of the mechanisms that control the global climate system. In this study, we calculate and analyze the foraminiferal dissolution index (FDX) and the fragmentation ratios of planktonic foraminifera for the 60–25?ka?B.P. time-span, based on samples from Core 17924 and ODP Site 1144 in the northeastern South China Sea (SCS), so as to reconstruct the deep-water carbonate dissolution during Marine Isotope Stage 3 (MIS 3). Our analysis shows that the dissolution of carbonate increases gradually in Core 17924, whereas it remains stable at ODP Site 1144. This difference is caused by the deep-sea carbonate ion concentration ([CO₃2?]) ( [ CO 3 2 ? ] ) that affected the dissolution in Core 17924 where the depth of 3440?m is below the saturation horizon. However, the depth of ODP Site 1144 is 2037?m, which is above the lysocline where the water is always saturated with calcium carbonate; the dissolution is therefore less dependent of chemical changes of the seawater. The combined effect of the productivity and the deep-water chemical evolution may decrease deep-water [CO₃2?] [ CO 3 2 ? ] and accelerate carbonate dissolution. The fall of the sea-level increased the input of DIC and ALK to the deep ocean and deepened the carbonate saturation depth, which caused an increase of the deep-water [CO₃2?] [ CO 3 2 ? ] . The elevated [CO₃2?] [ CO 3 2 ? ] partially neutralized the reduced [CO₃2?] [ CO 3 2 ? ] contributed by remineralization of organic matter and slowdown of thermohaline. These consequently are the fundamental reasons for the difference in dissolution rate between these two sites. Keywords Carbonate dissolution ; Planktonic foraminifera ; MIS 3 ; South China Sea prs.rt("abs_end"); 1. Introduction Marine Isotope Stage 3 (MIS 3) is a warming period of the last glacial, 59–24 ka B.P. (thousand years before present), which was characterized by rapid climate transitions between cold stadials and warm interstadials at northern latitudes ( Dansgaard et?al., 1993 and Bond et?al., 1993 ). Since the identification of millennial-scale climate oscillations in ice cores from Greenland, much research has carried out on Dansgaard-Oeschger (D-O) oscillations in marine and terrestrial records ( Voelker, 2002 ). Oxygen-isotope records of five stalagmites from the Hulu Cave near Nanjing, China, show a remarkable resemblance to oxygen-isotope records from Greenland ice cores, suggesting that the intensity of East Asia monsoon changed in concert with the Greenland temperature during 11–75?ka?B.P. ( Wang et?al ., 2001 ). Ice-core records, loess successions, lacustrine sediments, stalagmite records and deep-sea sediment cores show the climate instability of MIS 3. The palaeoenvironmental changes during MIS 3 have become a main topic in the study of global changes ( Zheng et?al ., 2008 ). The world ocean is one of the main constituents of the climate system and affects climate in a multitude of ways ( Rahmstorf, 2002 ). The production, transportation, deposition and dissolution of deep-water carbonate play a significant role in global carbon cycle, because the dissolution of carbonate affects the amount and distribution of dissolved inorganic carbon (DIC) and alkalinity (ALK), and drives atmospheric pCO₂ changes in glacial/interglacial cycles ( Jansen et?al., 2002 and Yu and Elderfield, 2007 ). As the largest marginal sea of the western Pacific, the South China Sea (SCS), has favorable conditions for carbonate accumulation and preservation ( Wang and Li, 2009 ), providing valuable sedimentary records for palaeoclimate research. Previous studies of planktonic foraminifera in the SCS focused on their assemblages and distribution, or on the related palaeoenvironmental changes ( Huang and Jian, 1999 , Huang et?al., 2000 , Li and Jian, 2001 and Jin et?al., 2003 ). Investigations of carbonate dissolution concentrated on carbonate-dissolution cycles and on the way in which carbonate percentages changed in response to climate variability ( Li et?al., 2001 and Chen et?al., 2002 ). The study on glacial carbonate cycles in the western Pacific marginal seas shows that the percentage of CaCO₃ is extremely low in high-latitude seas and increases towards low-latitude basins, revealing a close relationship between this percentage and water temperature and depth ( Wang, 1998 ). Cycles of carbonate dissolution reflect the rise and lowering of carbonate compensation depth (CCD). According to opposite variable trends of CCD cycles in the Pacific and Atla

机译：摘要碳酸盐的产生，运输，沉积和溶解深刻地构成了全球碳循环的一部分，并影响了驱动大气CO _{2 在冰川/间冰期循环中的变化。这些过程可能会为更好地了解控制全球气候系统的机制提供重要线索。在这项研究中，我们计算并分析了60–25？ka？B.P。的有孔虫溶解指数（FDX）和浮游有孔虫的破碎率。时间跨度，基于南海东北部（SCS）的17924核心和ODP站点1144的样本，以重建海洋同位素第3阶段（MIS 3）期间的深水碳酸盐溶解。我们的分析表明，碳酸盐的溶出度在Core 17924中逐渐增加，而在ODP站点1144处保持稳定。这种差异是由深海碳酸盐离子浓度（[CO _{3 2 ？]）（[CO 3 2？]）影响了Core 17924中3440？m深度低于饱和层以下的溶解。但是，ODP站点1144的深度为2037？m，高于水一直充满碳酸钙的溶菌碱。因此，溶解较少依赖于海水的化学变化。生产力和深水化学演化的综合作用可能会降低深水[CO _{3 2？] [CO 3 2？并加速碳酸盐溶解。海平面的下降增加了DIC和ALK向深海的输入并加深了碳酸盐饱和深度，这导致了深水[CO _{3 2？< / sup>] [CO 3 2吗？ ]。升高的[CO _{3 2？] [CO 3 2？ ]部分中和还原的[CO _{3 2？] [CO 3 2？ ]由有机物的再矿化和热盐碱的减缓而贡献。因此，这是这两个位置之间溶出度不同的根本原因。碳酸盐溶解;浮游有孔虫; MIS 3;南中国海prs.rt（“ abs_end”）; 1.简介海洋同位素第3阶段（MIS 3）是最后一个冰期（公元前59-24 ka）的变暖期。（距今已有数千年），其特征是北部纬度在寒冷的恒星与温暖的星际之间的快速气候转换（Dansgaard等，1993; Bond等，1993）。自从格陵兰岛的冰芯中识别出千年规模的气候振荡以来，已经对海洋和陆地记录中的Dansgaard-Oeschger（D-O）振荡进行了大量研究（Voelker，2002）。来自中国南京附近的葫芦洞的5个石笋的氧同位素记录与格陵兰冰芯的氧同位素记录非常相似，这表明东亚季风的强度在11–75期间随格陵兰温度而变化？ ka？BP （Wang等人，2001）。冰芯记录，黄土演替，湖相沉积，石笋记录和深海沉积物芯显示了MIS 3的气候不稳定。MIS 3期间的古环境变化已成为全球变化研究的主要主题（Zheng等。，2008年）。世界海洋是气候系统的主要组成部分之一，并以多种方式影响气候（Rahmstorf，2002）。深水碳酸盐的生产，运输，沉积和溶解在全球碳循环中起着重要作用，因为碳酸盐的溶解会影响溶解的无机碳（DIC）和碱度（ALK）的数量和分布，并驱动大气pCO < sub> 2 在冰川/冰川间周期中的变化（Jansen等，2002; Yu和Elderfield，2007）。作为西太平洋最大的边缘海，南海（SCS）为碳酸盐岩的积累和保存提供了有利条件（Wang和Li，2009），为古气候研究提供了有价值的沉积记录。以前在南海地区的浮游有孔虫研究主要集中在它们的组成和分布上，或者集中在相关的古环境变化上（Huang和Jian，1999; Huang等，2000; Li和Jian，2001; Jin等，2003）。。碳酸盐溶解的研究主要集中在碳酸盐溶解循环以及碳酸盐百分比响应气候变化而变化的方式上（Li等，2001; Chen等，2002）。对西太平洋边缘海的冰川碳酸盐循环的研究表明，CaCO _{3 的百分比在高纬度海洋中极低，并向低纬度盆地增加，表明该百分比与水的温度和深度（王，1998）。碳酸盐溶解的周期反映了碳酸盐补偿深度（CCD）的上升和下降。根据太平洋和Atla CCD循环的相反变化趋势}}}}}}}

Deep-water carbonate dissolution in the northern South China Sea during Marine Isotope Stage 3

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