Composition and diagenesis of Pleistocene aeolianites at Shidao, Xisha Islands: Implications for palaeoceanography and palaeoclimate during the last glacial period

Abstract

Pleistocene aeolianites on Shidao, Xisha Islands are one of the rare aeolianite successions found in Asia. There, the aeolianites, which unconformably overlie Pleistocene coral reefs (Marine Isotope Stage 5e), are 21.8m thick and formed largely of grains derived from heterozoan organisms, including foraminifera (predominantly Amphistegina), geniculate red algae, echinoderms, and molluscs. The aeolian sands were probably derived from a subtropical shallow subtidal environment. The variation in the distribution of metastable aragonite and high-Mg calcite (HMC) divides the aeolianite succession in core XK#1 into three intervals, which are unit 1 (11.5–21.8m), unit 2 (3–11.5m), and unit 3 (upper 3m of core). The entire aeolianite succession underwent diagenetic alterations in relatively closed diagenetic environments with low water/rock ratios. Compared to the aeolianites in units 1 and 3, however, the aeolianites in unit 2 display lower amount of HMC, lower Sr and Mg concentrations, and more negative δ18O values of bulk samples. This evidence demonstrates that unit 2 underwent more extensive diagenetic alteration, probably due to the invasion of larger volumes of meteoric water.Correlation of the aeolianites in XK#1 to the aeolianites in surface exposures and subsurface core indicates that most Shidao aeolianites were deposited in MIS 3. The dry-cold climate combined with strong onshore winds blew the exposed marine sediments onshore where they formed the aeolian dunes. The palaeosols present in the Shidao aeolianite succession indicate multiple episodes of subaerial exposure with relatively more humid climate. Collectively, the results imply that aeolianites in ancient rock record can be used to examine the variations in palaeoceanography and the palaeoclimate.