Globorotalia truncatulinoides as indicator of upper-ocean thermal structure during the Quaternary: evidence from the South China Sea and Okinawa Trough

Globorotalia truncatulinoides is shown to be indicative of upper-ocean ther mal structure and sensitive to the thermocline fluctuations and the thickne ss of the North Pacific subtropical mode water (NPSTMW) thermostads for the Quaternary in the South China Sea (SCS) and Okinawa Trough. During the las t 1.5 Ma, the relative abundance (percentage) of G. truncatulinoides displa yed long-term oscillations on a time scale longer than the glacial-intergla cial cycle, but with two major changes around 550-400 ka and 115-55 ka BP. It decreased to near zero during 550-400 ka BP and then disappeared from th e equatorial Indian and Pacific Oceans. This suggests that the thermocline depth and the scale of winter deep-mixing decreased at that time interval, and since then the linkage between the SCS and the equatorial oceans at int ermediate depths remarkably weakened and the NPSTMW strongly influenced the SCS. The predominance of left-coiling G. truncatulinoides together with th e occurrence of Globoquadrina conglomerata in the western North Pacific bet ween 115 and 55ka BP indicates a deep thermocline and very thick NPSTMW the rmostads. A decreasing trend in the relative abundance of G. truncatulinoid es from marine isotopic stages 7 to 1 is evident in the SCS, indicating a p rogressive reduction of the NPSTMW thermostads. After the last glacial maxi mum, G. truncatulinoides increased in abundance, but abruptly disappeared b etween 8 and 4 ka BP from the SCS and Okinawa Trough, implying a mid-Holoce ne reduction in the NPSTMW thermostad thickness. (C) 2000 Elsevier Science B.V. All rights reserved.