Paleo-ecological changes and sedimentary evolution of the Hainan Delta, NW South China Sea

Abstract

The Hainan Delta, a paleo-delta was penetrated by a 101-meter-long core (ZBW) off the southwestern shore of Hainan Island, in the Beibu Gulf, northwestern South China Sea. Grain size, dinoflagellate cysts, diatoms, and bulk carbon stable isotopes and C/N ratios of the Hainan Delta clearly support four stages of the delta development. Those stages, along the core profile of ca. 74.50–27.0 m below seafloor (mbsf), were reconciled with the deltaic facies; their depth ranges and dates are as follows: 74.50–56.80 mbsf (ca. 65.07–62.01 kyr. BP), 56.80–42.10 mbsf (ca. 62.01–58.61 kyr. BP), 42.10–30.95 mbsf (ca. 58.61–57.02 kyr. BP), and 30.95–27.00 mbsf (ca. 57.02–55.60 kyr. BP). Core ZBW is composed mainly of fine to medium sand, with admixture of coarse sand and shell debris throughout the deltaic facies. The C/N ratio in the sediment indicates mixed terrigenous and marine origin of the organic matter. The bulk carbon isotope record clearly indicates domination of terrigenous sources. The dinoflagellate cyst ('dinocyst') assemblages provided information on in-situ produced marine organic matter delivered to the deltaic sediments. The abundant presence of dinocysts indicates a high nutrient supply and favorable trophic conditions of the waters the donor species dwelled in. A constant supply of organic matter from terrigenous sources and its rapid mineralization is interpreter as factors conducive to a higher abundance of dinocysts. The low abundance of fossil diatoms is attributed to high dilution and the lateral transport of strongly silicified coastal diatoms down the Continental slope.