Distribution Characteristics Of Sporopollen In The Southern South China Sea And Its Implications For Regional Climate And Environmental Evolution Since 40 Ka

Through the study of the 150 samples of surface sediments in the southern South China Sea by analyzing pollen assemblage,pollen concentration and pollen percentage,it can be inferred the pollen source and the transport mechanisms.The vegetation evolution history,hydrological and climatic characteristics of the southern South China Sea were reconstructed by analyzing pollen data of sediment core CG2 and core CB19and comparing with other paleo-climatic records.1.Pollen and spores distribution of surface sediments in the southern South China SeaIn the study of 150 surface sediment samples in the southern South China Sea,it is found that the pollen and spores in the sediment samples was dominated by pteridophyte spores,followed by trees pollen.The characteristics of pollen and spores distribution were as follows:1)the distribution characteristics of trees pollen concentration and percentage showed that the high value area was located near the coastline,and the farther away from the coastline,the lower the value.Among them,pine pollen may come from the mountains areas around the northern South China Sea,while the source area of the tropical high mountain rainforest pollen is the southern Borneo?mainly in the northeastern mountainous area?,which is transported by surface runoff.The distribution of low mountain rainforest pollen is generally less in the study area,which is related to the small and dense pollen grains,which can only be transported by means of water flow.The pollen distribution characteristics of the tropical lowland rainforest indicate that the pollen source area is located in the southern islands of the South China Sea,such as Borneo and Natuna Islands,and the main carriers are surface runoff and rivers.The main source of mangrove pollen is the coastal zone of the Natuna Islands and Borneo,and is transported by surface ocean currents.2)Herb pollen are scarce in surface sediments of the Southern South China Sea,and mainly derived from Natuna and Borneo.Poaceae pollen mostly distributed in the Nansha Trough and Sunda slope,and probably related to the widespread distribution of farmland in Borneo and the Natuna Islands.3)The high concentration of fern spores is located in the Nansha trough on the west coast of Palawan and the northern coast of Borneo,where the concentration of spores is greater than 800 grains/g.The Sunda shelf and continental slope concentrations are also relatively high,typically greater than 400grains/g.This distribution pattern shows that the main source of fern spores is Palawan,Borneo and the spores carry by river and ocean current.The distribution of percentage of fern spores showed the opposite pattern to it concentration.This pattern reflects the dominance of ferns in the Nansha Islands vegetation and the spores can be long distance transport by ocean current.The concentration and percentage of Dicranopteris spore have the same pattern of distribution and display a decreasing trend from the North of Borneo to Nansha Islands which reveal the deforestation in the Borneo.4)Based on the study of two pollen profiles in the study area,it is found that pollen and spore are more likely to be deposited in the Nansha trough with higher surrounding terrain.The higher surrounding terrain makes it difficult to transfer pollen,which indicate the seafloor topography has a strong influence on the spatial distribution of pollen.2.Vegetation evolution and hydrological climate records in the southwestern South China Sea since LGMPrecipitation data(fern spore percentage,?¹⁸O_seawater,smectite/?illite+chlorite?ratio)inferred form sediment core CG2 well reflect hydroclimate changes in the Southern South China Sea and consistent with the trend of tropical convective intensity derived from Borneo stalagmite.Hydroclimate in study area during Heinrich stadial 1change significantly,and low fern spore percentage and high herb pollen content accompany with heavier?¹⁸O_seawater and lower smectite/?illite+chlorite?ratio,indicate precipitation reduce drastically.According to the distribution of modern Pinus vegetation and Pinus pollen transport,the Pinus pollen concentration of core CG2 was used as a indicator for EAWM intensity.The Pinus pollen concentration of core CG2was positively correlated with the EAWM.During the last deglaciation,when the AMOC was shutdown,the strong EAWM could even affectted the ASM across the equator.Comparing the hydrological and climatic data of the CG2 with the north and south hemisphere summer monsoon intensity,it is found that the rainfall in the north and south hemispheres is in the form of“seesaw”in the last glacial.During the Heinrich stadial 1,the AMOC is closed which make northern hemisphere cool and strengthens the East Asian winter monsoon.Enhanced East Asian winter monsoon can flow cross-equatorial and increase the Australian summer monsoon.This process drives ITCZ move southward and reduces rainfall in the study area.Although the change of modern ENSO has a significant effect on the precipitation change in the southern South China Sea,the precipitation change since the late glacial period in the study area is not consistent with the long-term change of the ENSO state.The destruction of vegetation can reduce the amount of land evaporation,thereby reducing the convection precipitation in the study area.In the last deglacial,sea level"played"the role of deforestation,affecting the distribution of vegetation while also reducing land precipitation.The destruction of vegetation during the rise of the sea level in the deglacial may also be a factor in the reduction of precipitation of H1 events.3.Vegetation evolution in the southeastern South China Sea since 40,000 yearsThe pollen sequence of core CB19 showd the evolution of vegetation in the southern the South China Sea during the late last glacial period.The results showd that,in the MIS 3,the hydrothermal conditions in northern Borneo were relatively suitable.The vertical distribution structure of vegetation on Borneo was relatively stable,and it was distributed in high mountain rainforest,tropical lowland rainforest,tropical low mountain rainforest and mangrove forest.During MIS 2,although the precipitation remained stable,the temperature was greatly reduced,and the expansion of the tropical high mountain rainforest vegetation compressed the living space of the tropical low mountain rainforest and lowland rainforest.Although the sea level was reduced,the shorter shelf is not enough to provide sufficient expansion space for the tropical low mountain rainforest and low land rainforest,and the steep slope area also affects the growth of mangrove.Repeated high-level high mountain rainforest pollen in core CB19corresponds to the Heinrich event.By comparing it with the temperature records in the tropics,the relative content of tropical high mountain rainforest components?mainly Podocarpus?can be used as an effective indicator of millennium-scale climate change in the tropics,and revealed the response patterns of vegetation on the millennium-scale rapid cold event in the high latitudes of the Northern Hemisphere.In the CB19 pollen sequence,two peak concentrations of the Pinus pollen appeared at 25 ka and 31 ka,which corresponded with the Heinrich event 2 and 3,and consistent with the change of EAWM intensity recorded by loess.It is further verified that the Pinus pollen concentration in the southern South China Sea can be used as an indicator of EAWM intensity.