Response of planktonic foraminifera to glacial cycles: Mid-Pleistocene change in the southern South China Sea

Abstract

A high-resolution study (∼2 ka) of planktonic foraminifers in 997 samples from ODP Site 1143 (9°21.72′N, 113°17.11′E, water depth 2772 m) in the southern South China Sea (SCS) reveals faunal response to glacial cycles in the last 2.1 Ma. Most distinctive is the contrasting variations in the abundance of two tropical–subtropical species that exhibit similar resistance to dissolution in modern oceans. The relative and absolute abundances of Globorotalia menardii are high in interglacials and low in glacials and covary with δ 18O fluctuations over the studied interval. Pulleniatina obliquiloculata, however, follow oxygen isotopes only before the Mid-Pleistocene Revolution (MPR), showing high abundances during glacials and low during interglacials after ∼850 ka. Three abundance maxima of P. obliquiloculata are observed, corresponding to three major glacial episodes, Marine Isotope Stages (MIS) 2, 12, and 16, respectively. This change in abundance patterns of P. obliquiloculata appears to be unique to the southern SCS, as in late Pleistocene records from other western Pacific regions, including the Okinawa Trough and the northern SCS, all show co-varying patterns with δ 18O as G. menardii does. Cross-spectral analyses performed between percentages of Globorotalia menardii and δ 18O records show high coherency at 41 ka obliquity and then at 100 ka eccentricity bands. We suggest that the high abundances of G. menardii during interglacials in the past 2.1 Ma reflect warmer temperatures. Percentages of Pulleniatina obliquiloculata vs. oxygen isotope records show quite large phase difference at 41 ka obliquity band before the MPR. As a contrast, there are negligible phase differences in the vicinity of 100 and 41 ka after the MPR, which means reverse trend between percentages of P. obliquiloculata and δ 18O fluctuations. The differences of P. obliquiloculata from G. menardii in abundance and orbital forcings are interpreted as resulting from a profound change in the regional upper ocean structure during the MPR time, probably relating to a final glacial cut-off of the southern SCS from the Indian water at ∼850 ka and to higher salinity during glacial times after 850 ka. We speculate that this change is related to the decreased connection between the Pacific and Indian Ocean.