Sea surface water isotope changes recorded by alkenone δ²H from the northern South China Sea over the last 260 kyrs

The stable hydrogen isotope compositions of long-chain unsaturated C₃₇ alkenones (δ²H_C37) in sediments were measured to estimate palaeohydrological changes over the last 260 kyrs in the northern South China Sea (SCS). Values of δ²H_C37 ranged from −212‰ to −166‰, with a value of −212‰ in the surface sediment. The residual δ²H value was estimated by subtracting the global ice volume effect from the hydrogen isotope composition of seawater (δ²H_sw), which was calculated with empirical relationships based on the δ²H_C37. δ²H_residual showed clear glacial-interglacial cycles with lower values during glacials than interglacials, a pattern that is not apparent in the analogous δ¹⁸O_residual record from the same core. The latter was estimated from δ¹⁸O of the planktonic foraminifer Globigerinoides ruber by correcting for temperature and ice volume effects. We argue that δ²H_residual may be better than δ¹⁸O_residual as a record of surface seawater isotope changes in the SCS. The lower δ²H_residual values during the glacials in the northern SCS were likely due to the La Niña-like conditions in the Western Tropical Pacific and the SCS, which resulted in increased precipitation and suppressed Kuroshio intrusion. Conversely, the higher δ²H_residual values during the interglacials were likely due to El Niño-like conditions. Alternatively, the “extra” isotopic depletion of precipitation resulting from the reorganization of the regional atmospheric circulation under glacial conditions could also contribute to the lower δ²H_residual values during the glacials.