南海中央海盆更新世以来浊流沉积层的有孔虫记录及古环境意义

FORAMINIFERAL PALEOCEANOGRAPHY OF PLEISTOCENE TURBIDITE DEPOSITS FROM THE CENTRAL BASIN OF THE SOUTH CHINA SEA,IODP SITE U1433

本文研究IODP349航次U1433站岩芯上部0～300m更新世沉积物样品中的有孔虫,目的是寻找深水海盆钙质沉积的证据并探讨其古环境意义。该站位于南海西南次海盆残留扩张脊,水深4 379m。沉积物主要由泥质、砂质及钙质软泥层组成,具明显的深海浊流沉积特征。结合古生物、古地磁和颜色反射率的年龄框架指标,确定本段岩芯年龄为0-2.23Ma,包含氧同位素MIS 1-85期。结果显示,有孔虫丰度变化很大,个体普遍较小、分选好,持续分布差,属典型的远端浊流沉积。不同沉积层之间相对应的有孔虫特征(如丰度和保存情况)不对称,可识别17个浊积层组,标志超冰期尺度的浊流活动大事件。浊积物的源区,主要为南海西北部河流以及南部的巽他陆架和婆罗洲-巴拉望岛以及周围岛礁,而南海东北部和西部陆源区也可能有所供给。远端物源的证据是更新世浊积物含渐新世-中新世浮游有孔虫属种,这些老地层多见于南海北部和东南部陆坡。因此我们推测,U1433站的更新世浊流沉积物,主要来自周边地区,部分原产于北部陆坡,可能以与现代相似的涡流方式被搬运至西南次海盆。

Samples from the upper 300 mcores at Site U1433 Aof the Integrated Ocean Drilling Program(IODP)Expedition349were studied for the Pleistocene foraminiferal paleoceanography.The site is located in the relict spreading ridge of the Southwest Sub-basin of the South China Sea(SCS).The core is mainly composed of grayish silty clay and calcareous soft clay,indicative of turbidity deposition.Based on the shipboard biostratigraphy,postcruise paleo-magnetic and color reflectance chronology,the sampling section was assigned to the last 2.23 Ma in age,covering Marine Isotope Stages(MIS)1to85.Most foraminifers are small and well-sorted with great variations in abundance,indicating distal turbidity sedimentation.Different foraminiferal assemblages characterize different sedimentary layers,especially in species abundance and preservation states,and these foraminiferal assemblages can be used to define 17 large turbidity events or groups of events.The potential source regions for the turbidity sediments include river deltas in the northwest,and the Nansha islands and Borneo-Palawan area in the south,while the northeast SCS may also have provided some supply.The occurrence of many Oligocene-Pliocene fossil species also suggests strong mixing between the older marine facies and the Pleistocene hemipelagic clay.As the Oligocene-Miocene marine sediments better developed in the northeastern and northern SCS than elsewhere,we speculate that the Pleistocene turbidity sediment of Site U1433 was at least partially from the northeast SCS,most likely transported by the deep-sea circulation and eddies as today.