Carbon flux during the last interglacial cycle in the inner continental shelf of the South China Sea off Hong Kong

Abstract

Information obtained from previous studies has been used to study carbon flux during the last interglacial cycle in the siliciclastics-dominated inner continental shelf of the South China Sea off Hong Kong. The information included grab sampling surveys, many kilometres of high-resolution reflection seismic profiles and numerous boreholes. During the high sea-level stand of the Holocene and the last interglacial period, the inner shelf of Hong Kong was a net carbon sink. This is particularly so in the present day mainly through the discharge of anthropogenic sewage. The offshore dredging of mud and the offshore mining of sand and gravel deposits have nevertheless led to the release of methane and other gases either trapped beneath Holocene deposits or originating from the bacterial breakdown of plant matter within Pleistocene deposits contributing greenhouse gases. During the last glacial period, significant quantities of carbon dioxide formed by the dissolution of carbonates through acid-sulphate development on the subaerially marine deposits of last interglacial age, and biogenic methane formed by the bacterial breakdown of fossilized plant matter in the underlying deposits was released into the atmosphere from the exposed shelf. Due mainly to the poor drainage conditions, the exposed shelf was unlikely to support a dense vegetation cover causing a lowering of carbon storage through terrestrial plants. The role of greenhouse gases generated, trapped and escaping from the present-day continental shelves during the last interglacial cycle requires consideration in the estimation of global carbon flux. Reflection seismic profiling is useful as a method for mapping the present-day shelves to permit the identification of areas affected by acoustic turbidity. For the interval between the Holocene and the last interglacial, the geochemical characterization of sediment extracts is needed for the estimation of carbon flux and for obtaining information on past vegetation history. This necessitates the study of outer shelf sediments from water depths between about 130 and 200 m below the present-day sea level. The release of greenhouse gases from the exposed shelves during the last glacial period is suggested to provide a triggering mechanism for switching the earth into the present interglacial mode. © 2002 Elsevier Science B.V. All rights reserved.