Stable isotope ratios and CaCO3 content of sediment core MD90-963

We examined coarse fraction contents of pelagic carbonates deposited between 2000-and 3700-m water depth in the tropical Indian Ocean using Ocean Drilling Program (ODP) sites 722 (Owen Ridge, Arabian Sea) and 758 (Ninetyeast Ridge, eastern equatorial Indian Ocean), and four giant piston cores collected by the French R/V Marion Dufresne during the SEYMAMA expedition. Over the last 1500 kyr, coarse fraction records display high-amplitude oscillations with an irregular wavelength on the order of ~500 kyr. These oscillations can be correlated throughout the entire equatorial Indian Ocean, from the Seychelles area eastward to the Ninetyeast Ridge, and into the Arabian Sea. Changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation. The well-known 'mid-Bruhes dissolution cycle' represents the last part of this irregular long-term dissolution oscillation. The origin of this long-term oscillation is still poorly understood. Our observations suggest that it is not a true cycle (it has an irregular wavelength) and we propose that it may result from long-term changes in Ca++ flux to the ocean. Sites 722 and 758 d18O records provide a high-resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr. Site 722 (2030 m) lies above the present and late Pleistocene lysocline. The lysocline shoaled to the position of site 758 (2925 m) only during the interglacial intervals that occurred between about 300 and 500 ka (Peterson and Prell, 1985a). Despite these supralysoclinal positions of the two sites, short-term changes in coarse fraction contents are correctable from one site to another and probably result from regional (or global) dissolution pulses. By stacking the normalized coarse fraction records from sites 722 and 758, we constructed a Composite Coarse Fraction Index (CCFI) curve in which most of the local signals cancelled out. The last 800 kyr of this curve appear to compare extremely well with the Composite Dissolution Index curve from core V34-53 (Ninetyeast Ridge), which unambiguously records past variations of carbonate dissolution in the equatorial Indian Ocean (Peterson and Prell, 1985a). In the late Pleistocene the CCFI variations are mainly associated with glacial-interglacial changes. They show strong 100 and 41 kyr periodicities but no clear precession-related periodicities. As proposed earlier by Peterson and Prell (1985a), the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation. We cannot totally reject the possibility, however, that low temporal resolution and/or bioturbation degrade somehow the precessional signal at ODP sites 722 and 758. In contrast, spectral density of dissolution cycles in the giant (53 m long) piston core MD900963 (Maldives area) displays clear maxima centered on the precession frequencies (23 and 19 kyr**-1) as well as on the kyr**-1 frequency but shows little power at the 100- ky**-1 frequency. These high-frequency changes most probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE

本研究利用大洋钻探计划（Ocean Drilling Program, ODP）722号站位（阿拉伯海欧文海岭）、758号站位（东赤道印度洋九十度海岭），以及法国“马里安·杜弗勒”号（R/V Marion Dufresne）科考船在SEYMAMA科考航次中采集的4根巨型活塞岩芯，对热带印度洋2000~3700米水深沉积的远洋碳酸盐粗组分含量开展了分析。在过去1500千年（kyr）中，粗组分记录呈现出高振幅波动，其波长不规则，量级约为50万年。此类波动可在整个赤道印度洋范围内进行对比，从塞舌尔海域向东延伸至九十度海岭，乃至阿拉伯海。粗组分含量与基于有孔虫壳体破碎度构建的有孔虫保存指数呈正相关，这一证据表明，沉积物粒度变化主要由碳酸盐溶解作用变化所驱动。广为人知的“布鲁赫斯中期溶解旋回”即为该不规则长期溶解波动的最后阶段。该长期波动的成因目前仍未明确。本研究观测结果表明，其并非真正意义上的旋回（波长无规律性），并推测该波动可能源于输入海洋的钙离子通量长期变化。722号与758号站位的氧同位素18（δ¹⁸O）记录提供了高分辨率地层框架，使得我们能够在过去1500千年间对两处站位的粗组分记录开展详细的站位间对比。722号站位（水深2030米）位于现代及晚更新世溶跃面之上。仅在约30万~50万年之间的间冰期时段，溶跃面才会抬升至758号站位的水深（2925米）处（Peterson与Prell，1985a）。尽管两处站位均处于超溶跃面水深范围内，但粗组分含量的短期变化仍可在不同站位间进行校正，该变化大概率由区域（或全球）溶解脉冲事件所导致。通过对722号与758号站位的标准化粗组分记录进行堆叠，我们构建了复合粗组分指数（Composite Coarse Fraction Index, CCFI）曲线，该曲线可抵消绝大多数局地信号。该曲线的过去800千年时段与取自九十度海岭的V34-53岩芯的复合溶解指数曲线匹配度极高，后者清晰记录了赤道印度洋过去的碳酸盐溶解变化（Peterson与Prell，1985a）。晚更新世期间，CCFI的变化主要与冰期-间冰期旋回相关，其呈现出显著的100千年与41千年周期，但未发现明确的与岁差相关的周期信号。正如Peterson与Prell（1985a）早前提出的那样，岁差周期的缺失或许表明，区域碳酸盐溶解信号受深水环流变化所调控。但我们无法完全排除以下可能性：较低的时间分辨率以及/或者生物扰动作用，在一定程度上削弱了722号与758号站位的岁差信号。与之相反，取自马尔代夫海域的53米长巨型活塞岩芯MD900963的溶解旋回功率谱密度，呈现出以岁差频率（23千年⁻¹与19千年⁻¹）及千年⁻¹级频率为中心的显著峰值，但在100千年⁻¹频率处的功率极低。此类高频变化大概率与季风变率相关的表层生产力变化有关。该站位的溶解作用最终可能受控于有机质氧化过程——在西南季风偏弱以及东北季风更为干燥的时段，沉积物中有机质的埋藏量相对更高。