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 thermal structure and sensitive to the thermocline fluctuations and the thickness of the North Pacific subtropical mode water (NPSTMW) thermostads for the Quaternary in the South China Sea (SCS) and Okinawa Trough. During the last 1.5 Ma, the relative abundance (percentage) of G. truncatulinoides displayed long-term oscillations on a time scale longer than the glacial–interglacial 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 the 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 intermediate depths remarkably weakened and the NPSTMW strongly influenced the SCS. The predominance of left-coiling G. truncatulinoides together with the occurrence of Globoquadrina conglomerata in the western North Pacific between 115 and 55 ka BP indicates a deep thermocline and very thick NPSTMW thermostads. A decreasing trend in the relative abundance of G. truncatulinoides from marine isotopic stages 7 to 1 is evident in the SCS, indicating a progressive reduction of the NPSTMW thermostads. After the last glacial maximum, G. truncatulinoides increased in abundance, but abruptly disappeared between 8 and 4 ka BP from the SCS and Okinawa Trough, implying a mid-Holocene reduction in the NPSTMW thermostad thickness.