A CA. 0.5 MA SILICICLASTICS-DOMINATED INNER SHELF SEQUENCE FROM THE SOUTH CHINA SEA OFF HONG KONG

Studies of offshore boreholes from the ‘stable’ South China Sea inner shelf off Hong Kong have revealed a ca. 0.5 Ma siliciclastics-dominated sequence. Up to 5 marine units and 5 terrestrial units corresponding to oxygen-isotope (OI) stages 1, 2-4, 5, 6, 7, 8, 9, 10, 11 and 12 can be recognized. Four authigenic minerals - pyrite, glauconite, limonite and siderite are associated with the sequence. Pyrite, formed under anoxic conditions on the seabed, is preserved entirely only in Holocene marine deposits because they have never been sub-aerially exposed. During OI stages 2-4, 6, 8 and 10 when sea level fell up to ca. 130 m, pyrite in marine deposits of OI stages 5, 7, 9 and 11 respectively were oxidized to different degrees through acid-sulphate soil development. Glauconite occurs in greater abundance in the deeper (> 15 m) and more exposed parts of the inner shelf. Both limonite and siderite are products of palaeosols referred to as ‘palaeo-desiccated crusts’ formed during periods of low sea levels. Limonite was produced by the oxidation of pyrite while siderite was produced by the in situ replacement of plant rootlets with iron deriving from pyrite and carbonate deriving from the dissolution of shells and shell fragments. The chronology of the sequence is supported by dating results and correlation with the Vostok ice core. The maximum radiocarbon age of OI stage 1 is found to be ca. 8.1 ka. Uranium-thorium dating of mollusks yielded ages of OI stage 5, 7, 9 and 11 while thermo-luminescence dating yielded ages of up to OI stage 8 for terrestrial deposits. The agreement with the five interglacial-glacial cycles identified in the Vostok ice core is confirmative of a eustatic sea-level signature. A study of acoustic turbidity in high-resolution boomer siesmic profiles indicates a possible role for carbon dioxide and methane released from the sub-aerially exposed inner shelf in switching the earth from a glacial mode into an interglacial mode.