Major environmental events reflected by pelagic records since 3.5 Ma BP in the Ninetyeast Ridge at the equator.

Abstract: The paper presents the pelagic records of planktic foraminifera, as well as the data of lithostratigraphy, stable isotope stratigraphy and carbonate stratigraphy since 3.5 Ma BP from the core of hole ODP758 in the Ninetyeast Ridge of the Indian Ocean. Based on these data, it discusses the manifestations and related mechanisms of such major tectonic and environmental events as the rapid uplift of the Himalaya Mountains, “middle Pleistocene climatic transition” and “mid-Brunhes dissolution event” in the region. According to an analysis and comparison of various indices and the changes in the aspects of the foraminifera assemblage, paleotemperature, paleosalinity and themocline from the core of hole ODP758, the authors deduce that the paleoclimatic changes might correlate with the mid-Pleistocene transition at 1.4-0.7 Ma BP. The changes in CaCO3, mass accumulation rates (MAR) of CaCO3 and non-CaCO3 MAR indicate that the loaded terrigenous sediments increased at 1.7 Ma, which is in agreement with the uplift history of the Qinghai-Tibet Plateau as shown by the available data. The last two changes coincide with the uplift of the Qinghai-Tibet Plateau, so called “Qinghai-Tibet movement”(1.7Ma), and the “Kunlun-Yellow River movement” (1.2-0.6 Ma). The changes of the CaCO3 content, coarse fraction (>150 μm) content and planktonic foraminifera biostratigraphy show that strong dissolution of abyssal CaCO3 occurred in the study region during 0.5-0.4 Ma. The event was consistent with the “mid-Brunhes dissolution event” in the sedimentary records of the Atlantic Ocean, Pacific Ocean, Indian Ocean and Nansha sea area of the South China Sea.