The Investigation Of Provenance In The Hangjiahu Plain During The Middle And Late Holocene

In this study, we collected two Holocene sediment cores DTX4 and DTX10 from the Hangjiahu Plain, and surficial sediments from adjacent sediment-source areas for clay mineral analysis. We identified changes in provenance of different periods in cores by comparing clay mineral compositions of core sediments with the surficial sediments. Combined with the results of lithofacies and AMS14C ages, the environment evolution of the Hangjiahu Plain has been rebuilt. This study aims to recognize when the Taihu Bay closed and the Dongtiaoxi-River discharged northward into the Taihu Lake.We found that since the mid-Holocene, muddy sediments prevailed and the grain sizes were distributed between tractive current and static water deposition in the C-M plot, which indicated a relatively weak hydrodynamic environment at the core sites. Besides, there are always plant debris and peaty mud in both sediment cores. Thus the sedimentary facies in both cores can be inferred to be bay phase-marsh phase-lacustrine phase.By X-ray diffraction techniques, we found clay mineral assemblage of Illite+ Kaolinite+ Chlorite+ Smectite in most surficial and cores sediments, but Smectite missed in some samples. According to the content of Illite, the provenance can be divided into two categories:one is the Yangtze Form which is characterized by higher content of Illite, another is the Qiangtang-River Form which is characterized by lower content of Illite. In the first type, the Illite content is >70% and reach 80% in sediments from the continental shelf of the East China Sea. In the second type, Illite content is 60%-70%. The differences are caused by the different weathering conditions and source rocks.Since the mid-Holocene, the changes in provenance and sedimentary environment is described as follows:Before ca.7100 cal a BP, the Hangjiahu Plain is dominated by sediments transported by tidal currents from the Yangtze River and local mountain river, which indicated an open bay environment with a wide range of sea water influence in the study area; Since 7100 cal a BP, the source changed to mixture of the Yangtze and Qiantang Rivers and remained for a long time period. During this stage, the impact of the Yangtze-River source gradually weakened, while the Taihu Bay diminished. As the chenier ridges developed on the east Taihu Plain, the bay changed to marsh environment gradually on Hangjiahu Plain. By 3500-3000 cal a BP, sediment source changed to the local mountain river, the Dongtiaoxi-River and Taihu plain sediments. The DTX4 sited at a lacustrine environment and deposited mainly the local mountain river sediments, while DTX10 sited at a flood plain environment and deposited mix-sourced sediments.In this study we suggest the Taihu Bay closed when the sediments from Qiantang-River disappeared at 3500 cal a BP. It was also deduced that the Dongtiaoxi-River migrated northward no later than 2800 cal a BP when the Dongtiaoxi-River sediments arrived at core site of DTX10. Lake expansion occurred during this period on the Hangjiahu Plain, but it remains to be further tested about influence of Dongtiaoxi-River.