Diatom preservation in an inner continental shelf borehole from the South China Sea

Diatom distribution in a 60-m long borehole from a siliciclastics-dominated 'stable' inner continental shelf in the South China Sea off Hong Kong was investigated. The Quaternary sediments contained evidence for five interglacial marine transgressions dating back to oxygen-isotope Stage 12, and yielded 53 genera and 186 taxa of diatoms. Intertidal species and nearshore shallow water species, including both planktonic and benthonic tropical and subtropical varieties, were dominant. Four diatom zones reflecting the shallow water environment have been recognized in the top 13.8 m of the borehole, Diatoms are missing in Stage 11 while Stage 7 and Stage 9 are very low in diversity and abundance compared to Stage 1 and Stage 5. The distribution pattern found is accounted for by the duration of time available for frustule dissolution through acid-sulphate soil development during Stages 2, 6, 8 and 10 when the shelf was subaerially exposed. Preservation of diatoms is best in Stage 1 because the deposits have never been subaerially exposed followed closely by Stage 5. Aging mainly through post-depositional dissolution is identified as the important controlling factor in determining diatom distribution in pre-Holocene deposits. (C) 2000 Elsevier Science Ltd. All rights reserved.