南海ODP1143站上新世至更新世天文年代标尺的建立

基于底栖有孔虫δ18O的精确的年代标尺是古海洋学研究的基础, 特别是长度超过5 Ma, 分辨率小于5 ka的连续δ18O记录在全球大洋深海记录中更是凤毛麟角.在大西洋和东太平洋已经建立起了类似的代表性剖面, 如大西洋ODP659站和东太平洋ODP846站, 但在对全球气候有着重要影响的“西太平洋暖池区”还没有建立起这样的剖面.以南海大洋钻探184航次1143站底栖有孔虫的δ18O为材料, 建立了西太平洋地区跨越5 Ma、分辨率达2~ 3 ka的天文年代标尺.天文调谐的基本原理参照Imbrie et al.(1984), 并将斜率周期上8 ka的相位差和岁差周期上5 ka的相位差从晚更新世扩展到5 Ma; 调谐目标选用Laskar(1990)的斜率和岁差; 调谐方法采用了有别于ODP659站和ODP846站的自动轨道调谐方法(Yu and Ding, 1998).调谐结果显示, 1143站190.77 m、191个冰期、间冰期的深海沉积记录了5.02 Ma的南海古海洋学历史; 1143站布容/松山磁性反转事件的年龄为0.78 Ma, 与前人研究结果一致; 15个识别出的浮游有孔虫生物事件年龄部分与已经发表过的年龄相吻合, 部分为该生物事件在南沙海区的新年龄.由年代标尺推导出的南海沉积特征显示, 2.9 Ma是沉积速率的一个转折点, 在此之前, 平均线性沉积速率只有39.5 m/Ma, 冰期、间冰期平均波动幅度为50 m/Ma; 2.9 Ma以后, 平均线性沉积速率猛然上升到65.4 m/Ma, 冰期、间冰期平均波动幅度为200 m/Ma.此外, 南海的沉积速率还显示出冰期或间冰期中的亚冰阶沉积速率高, 而相邻的间冰期或亚间冰阶的沉积速率低, 这种特点在更新世尤为典型.这可能与全球冰量变化带来的冰期、间冰期差异性风化剥蚀和搬运有关. 

Establishment of the Plio-Pleistocene Astronomical Timescale of ODP Site 1143, Southern South China Sea

An accurate timescale derived from benthic foraminiferal δ18O is crucial to paleoceanographic studies. In global oceans, there are few continuous δ18O records for the benthic foraminifers which span the past 5 Ma and have time resolution better than 5 ka. Such representative profiles have been established for ODP Site 659 in the Atlantic and ODP Site 846 in the east Pacific, but still have been absent in the"Western Pacific Warm Pool", an area playing an important role in global climate changes. Based on the benthic foraminiferal δ18O of ODP Site 1143 in the southern South China Sea, we establish a 5 Ma astronomical timescale for the west Pacific Plio-Pleistocene, with a time resolution of ~ 2-3 ka. The phases are fixed at 8 ka and 5 ka for the obliquity and precession bands respectively, and have been applied throughout the whole records. Obliquity and precession are calculated and treated as the tuning targets.An automatic orbital tuning method has been applied in this study, which is more efficient than those used at ODP Site 659 and 846. The results show that the 190.77 m long deep sea sediments in the South China Sea have recorded a paleoceanographic history of ~ 5.02 Ma, corresponding to 191 glacial/interglacial cycles. The tuned Brunhes/Matuyama paleomagnetic polarity reversal agrees well with an age of 0.78 Ma dated previously. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea are also estimated. The sedimentation rates calculated from this new timescale indicate that 2.9 Ma is a threshold for the sedimentation rate in the location of Site 1143. Before this time, the average sedimentation rate is ~ 39.5 m/Ma, with a small amplitude of ~ 50 m/Ma.After this time, the average sedimentation rate jumped to ~ 65.4 m/Ma, with an amplitude as large as 200 m/Ma. In addition, the sedimentation ratesin the southern South China Sea were higher during glacials or stadials but lower during interglacials orinterstadials, especially in the late Pleistocene period. This feature might be related to the global ice volume changes, which caused different erosion, denudation and transportation during glacials and interglacials.