| The climate in Pearl River Delta(southern China, Guangdong Province) is subject to the impact of the East Asian monsoon, the vast majority of its precipitation belongs to the East Asian monsoon rainfall; it is also a region subject to frequent typhoons and storm surges. In this paper, a gravity core ZJK03-Z collected from the Wanshan Islands in Ling Ding Yang, Pearl River Delta was studied, discussed and speculated the summer monsoon rainfall changes and deposit events during the late Holocene, explored the possible relationship between the Environmental Change in Pearl River Delta and ITCZ, as well as ENSO.Water salinity affects the reproductive mode of foraminifera, which expressed in the way of external morphology, such as mean proloculus size MPS and the ratio of microspheric/meglospheric forms; salinity also affects the δ18O value of foraminiferal shells and abundance of porcellaneous tests in assemblage. Reduced East Asian monsoon rainfall lead to the reduce of freshwater runoff input, resulting in the high salinity in Pearl River estuary, then the shallow water benthic foraminifera respond to the water environment changes for their survival:The reproduction of A.annectens favor of sexual way, the MPS value decreases, the ratio of microspheric/meglospheric forms becomes larger, the δ18O value is positively biased, the abundance of porcellaneous tests increases.MPS, the ratio of microspheric/meglospheric forms, the δ18O value of the shallow water benthic foraminifera A.annectens and the abundance of porcellaneous tests in assemblage from the upper0-80cm of the drilling core ZJK03-Z were analysed, obtained a total of14summer monsoon rainfall reduction in the Pearl River Delta during1260～3114a BP in Late Holocene, which were3120～3100,3020～2980,2840～2760,2630～2590,2550～2520,2470～2410,2370～2320,2150～2120,2070～2020,1805～1760,1675～1650,1500～1480,1400～1390,1335～1320a BP. Comparision with the Ti content curve from Cariaco basin revealed that, these East Asian summer monsoon rainfall records are closely related to the north-south movement of ITCZ, the southward movement of ITCZ is in line with the reduced monsoon rainfall in this region.By comprehensive analysis of the composition of sand, silt and clay percentage, mean grain size(Φ), median grain size(Φ), sort, skew, kurt data, and grain-size frequency curve, as well as the abundance of benthic foraminifera and diameter of planktonic foraminiferal shells from the core ZJK03-Z, the results consistently showed:Between the layers from80cm (3114a BP) to190cm (～4376a BP), the volatility of the grain size parameters was very large, particles were coarse, poor sorting, positively skewed, very leptokurtic, the frequency curves had more multi-peak. The abundance of benthic foraminifera and diameter of planktonic foraminifera shell were relatively smaller. The hydrodynamic conditions of this period fluctuated frequently, sediments from nearby islands might be involved and rapidly accumulated at the station by strong hydrodynamic force. The sediments were coarsest above about110cm of this part, while in the section below110cm, skewness was relatively negatively skewed, some clay-peeks were appeared, just as the ones appeared in the bottom190-236cm part. The granularity indices of top0-80cm (1260～3114a BP) and bottom190-236cm (4376～4657a BP) parts were relatively stable, showing a relatively stable hydrodynamic conditions.The high-energy hydrodynamic events happened during3114～3927a BP led to the reversal of the core ZJK03-Z formation, and also brought the sediments elsewhere to deposit to approximately110to190cm of the drilling core. Comprehensive analysis of the storm surge record in the South China Sea as well as the El Nino indices from central and eastern tropical pacific suggested that, these deposition events might be strong storm surge events, but there was no evidence to prove it, also we could not rule out the possibility of other sedimentary events. |
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