Dry climate near the Western Pacific Warm Pool: Pleistocene caliches of the Nansha Islands, South China Sea

Abstract

The Nansha (Spratly) Islands are located in the middle of the South China Sea (at about 10°N) near the northwestern margin of the Western Pacific Warm Pool (WPWP). A borehole was drilled at an atoll at Nansha, and cores were taken. The upper 165.4 m of the borehole consists entirely of limestone of reef facies and lagoon facies, which had been influenced by extensive meteoric diagenesis. Petrographic and geochemical studies have identified at least four subaerial exposure surfaces (SES) in the Pleistocene carbonate sequence. These SES are thought to have resulted from global sea-level changes and characterized by caliche formation. When developed in limestone, caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm, compared with 1800–2200 mm of annual rainfall of the present-day Nansha Islands. The Nansha caliche therefore indicates the existence of several dry climate episodes during Pleistocene sea-level lowstands. Lower rainfall and higher evaporation during such dry conditions may explain the higher sea-surface salinities reported elsewhere in the South China Sea. The Nansha caliche may also indicate reduced extent of the WPWP and southerly shift of the Intertropical Convergence Zone during major sea-level lowstands.