A Pleistocene paleoceanographic record from the north slope of the Spratly Islands, southern South China Sea

Abstract

A 1.4 my paleoceanographic history for offshore north central Spratly Islands, southern South China Sea, is reconstructed using stable isotope, carbonate content, coarse fraction, and planktonic foraminifera census data from gravity core SO95-17957-2. The chronological framework for SO95-17957-2 is based on stable oxygen isotope correlation with open ocean records and constrained with AMS- 14C dates, floral and faunal bioevents, and a Pleistocene microtektite event. Stable oxygen isotope composition of surface water reflects mainly changes in high latitude ice volume but also riverine water influx from surrounding land. Stable carbon isotope record suggests that surface water δ 13C in the southern South China Sea has been modulated primarily by global changes in atmospheric CO 2 since the early Pleistocene. Dissimilarities between δ 18O patterns in SO95-17957-2 and those in various cores from the rest of the South China Sea indicate that between 60 and 120 Ka surface water chemistry at offshore north central Spratly Islands may be affected by different mechanisms. Those differences likely reflect unknown hydrological conditions during stages 4 and 5. Carbonate content is lower during the interglacial periods than during the glacial–deglacial periods, which are opposite to the general trend in the shallow South China Sea areas during the same period. Faunal composition, RSP and various dissolution indices suggest that productivity is the most important factor controlling carbonate preservation. Dissolution above the lysocline may modify the productivity signal but cannot reverse the entire pattern. Faunal composition and carbonate flux suggests productivity is higher during glacial–deglacial than during interglacial. Stronger glacial winter monsoon induces stronger upwelling and deep mixing and enhances nutrient cycling in the surface ocean, which in turn increases productivity. Estimated summer sea surface temperature (SST) based on transfer function FP-12E agrees with δ 18O and is consistently between 28 and 29°C during most of the last 1.4 my Winter SST reflects geographic location and changes in surface hydrology, and fluctuates between 25 and 28°C. During the last glacial period disconnection of the South China Sea from the Indian Ocean and the influx of temperate water through the Bashi Channel allowed winter SSTs to vary by as much as 3.5°C. Comparison of modern and estimated SST and SST estimates based on different methods suggest that G. bulloides and G. menardii are probably overweighed in transfer function FP-12E, which cause discrepancies in SST estimation.