| Beibu Gulf is located in the northwestern part of the South China Sea.Due to its geographical location,the climate is sensitive to the east asian monsoon and the southwest monsoon and has a high deposition rate.Therefore,it is an ideal climate change research site.Looking back at the previous studies on the paleoenvironment and paleoclimatic changes in the Beibu Gulf,most of them were in the Holocene period,and the resolution was not high.Therefore,this study determined two sediment cores in the Beibu Gulf,a total of 387 samples of biological substance(biosilicon,carbonate and organic carbon)and elements(cobalt,zinc,barium,plumbum,thorium and uranium).The content and the geochemistry of the biological substance and elements in the region is studied.Then combined with the vertical distribution of each indicator and the chronological framework of the two cores,the paleoclimatic climate change since the Holocene and the late Pleistocene in the Beibu Gulf was studied.Which provided a basis for reconstructing the paleoenvironmental and paleoclimate change in the Beibu Gulf.The research content and results mainly include the following four aspects:(1)Geochemistry of biological substanceThe average biosilicon content of the two cores is 2.22%,and the range is 0.97%-4.17%.The average carbonate content was 16.22%and the range is between 2.23 and 36.24%.The average organic carbon content is 1.64%,and the range is 0.43%-3.29%.The results were all within the range given in other literature.The vertical distribution of the biological substance in the two sediment cores are quite different.The comprehensive analysis suggests that the distribution of core BW1 biological substance is due to changes in terrigenous material input and the effects of climate on siliceous and calcareous productivities in seawater.The biological substance content distribution of the core BW2 is caused by changes in sedimentary units and climate change.Correlation analysis shows that biosilicon,carbonate and organic carbon in the core BW1 are mainly the accumulation of marine organisms.The carbonate content in the core BW2 sample is greatly affected by the terrigenous components.(2)Geochemistry of elementsThe average contents of the elements cobalt,zinc,barium,plumbum,thorium and uranium in the two sediments cores of Beibu Gulf were 10.40,75.72,315.02,18.99,12.46 and 2.19 ?g/g,and range values of 5.50-17.02,29.45-118.94,172.53-468.81,13.99-29.87,6.50-16.25 and 1.20-3.44 ?g/g.Elemental analysis indicates that there are three different sedimentary units in the core BW1.It is speculated that changes in elemental content are related to changes in material sources and sea level.The sediments in the core BW2 are subject to a certain compaction.It is believed that there are seven sedimentary units,which correspond well with the seismic section of the core BW2.The correlation analysis between uranium,thorium and biological substance indicates that uranium in core BW1 sediment is mainly from the input of terrestrial materials,and some is the adsorption and deposition of organic matter.And the content of other substances in the core BW2 is diluted by terrigenous carbonate.(3)Environmental changes in the Beibu Gulf during the HoloceneIn this study,the environmental changes during the Holocene in the Beibu Gulf were studied based on the apparent traits the content,the distribution of the biological substance and elements and the accumulation rate of the biological substance of the core BW1 sediments.The study shows that during the Holocene,the Beibu Gulf experienced sea levers changes that first rise and then fall.The paleoproductivity has high and low cycles,which are at about 384,1050,2207,3184,4627 and 10700 cal.aBP have relatively high values,corresponding to the occurrence of a series of cold events;During the Holocene,the study area experienced six climate change stages:cold and dry period(12480-11360 ca1.aBP),cold and humid period(11360-10195 cal.aBP),warm and humid period(10195-5140 cal.aBP),cool,dry and humid alternating period(5140-3447 cal.aBP),cold and dry and hot and humid alternating period(3447-1376 cal.aBP),cool and humid period(1376-0 cal.aBP).And there are climate change cycles of 6400,3200,2133,1600,1280,914 and 711 a.(4)Environmental changes in the Beibu Gulf during the late PleistoceneStudies have shown that the Beibu Gulf has experienced six climate change stages during the late Pleistocene,the penultimate glacial period(165.0-125.0 kaBP),the last interglacial period(125.0-68.0 kaBP),the last ice age early ice level and weak warm period(68.0-30.0 kaBP),the last glacial late ice level(30.0-12.0 kaBP),holocene(12.0-0 kaBP).There are also climate change cycles of 128.0,42.7,32.0,21.0,18.3,16.0,11.6,9.1 and 7.5 ka. |
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