Study On Biogenic Silica And ³²Si In Marine Sediment Cores From The Nansha Sea Area

32Si generated from the spallation reaction of cosmic rays and argon in atmosphere,was uptaken by siliceous organisms such as diatoms after falling into the land and sea from atmosphere.Most of the organisms deposit into the bottom of the ocean after the death and turn into biogenic silica.Most of biogenic silica will be buried and become a part of the sediments.Thus,32Si is in the biogenic silica.The cosmogenic radioactive isotope 32Si,with a half-life of approximately 150 years,represents an excellent dating tool for the age range of approximately 100-1000 years.32Si,with a single source and a constant production rate,is of importance to the ancient environments and paleoclimatology.As a proxy of paleoproductivity and paleoclimate,biogenic silica in marine sediments can carry a valuable record of paleoenvironment.The concentrations of biogenic silica and 32Si in marine sediments of the Nansha sea area were determined.The specific activity of 32Si in the rain in Xiamen,China was studied and the deposition flux of 32Si was calculated.To obtain a sediment chronology,beyondthe dating range of gamma-spectrometric methods(210Pbex,230Thex),we have applied the 32Si and 14C dating technique to sediment cores from Nansha sea area.In addition,biogenic silica,carbonate,uranium and thorium contents of marine sediments were determined.The geochemistry of biogenic silica,carbonate,uranium and thorium in sediemnts was studied and the change of sedimentary environment and paleoenvironment were discussed preliminary.The method of shimultaneous determination of biogenic silica and carbonate contents inmarine sediments was established.Leaching samples by 1 mol/L hydrochloric acid to estimate the carbonate contents and extracting biogenic silica by 1.5 mol/L sodium carbonate,with 85 ?water bath 5 h and obtaining the biogenic silica contents by spectrophotometry.The method of determining 31Si contents in larine sediments was established.Separating and purifying biogenic silica.Separating and purifying phosphorus in biogenic silica.Counting 32P by ?counter.The method of determining 32Si content in the rain was established.Adopting Fe(OH)3 coprecipitation to enrich silicon and using acid to wash brown floc to separate silicon.Using NaOH to dissolve SiO2 and HCl to precipitate and purificate SiO2 repeatedly.Finally,separation,purification and measurenent of 32P.The method of determining uranium contents in marine sediments was established:liquid laser-induced fluorescence spectrometry with sodium carbonate as the masking agent.Separating and purifying thorium in sediments by ion exchange column and determining thorium contents by spectrophotometry.Sedimentary rates of six short cores and two long cores were calculated by 210Pbex and 14C respectively.Sedimentary rates of Core NS-1 to NS-8 are 0.486,0.212,0.391,0.242,0.321,0.274 cm/a,10.5 cm/ka,13.5 cm/ka respectively.The biogenic silica(SiO2%),carbonate,uranium,thorium,organic carbon and water contents of all samples varies from 0.78-4.91%,2.9-57.0%,1.02-2.95 ?g/g,3.37-18.67 ?g/g,0.50-3.89%and 30.6-68.5%.with 2.36%,21.5%,2.10 ?g/g,11.61 ?g/g,2.13%and 54.0%as the mean value.The biogenic silica,carbonate,organic carbon and water contents of Core NS-3 is the lowest in all short cores.The horizontal distribution of biogenic silica is the same as the abundance of diatom from Lu J indicating that the biogenic silica in this area mainly comes from diatom The biogenic silica,carbonate,organic carbon and water contents from abyssal region are higher than shoal waters.The biogenic silica contents in sediments from abyssal region could indicate paleoproductivity level because of the less terrigenous material input and the more marine authigenic material.The results of Core NS-1 and NS-2 from abyssal region indicating that the region is low organic carbon and uranium contents,while high biogenic silica contents during 1978 and 1979.The vertical change of biogenic silica,carbonate,uranium,thorium and organic carbon dosen't exist obvious regularity while there are many similarities in accumulation rates.The results of Core NS-7 and NS-8 indicating that cold events corresponding to high biogenic silica,carbonate and uranium contents.The decrease of temperature results in the decrease sea level.The enrichment of uranium increased and carbonate debris piled up quickly.The more terrigenous material results in the rich nutrient in the ocean during the dry and cold climate period thus there were high productivity.Core NS-7 had environmental change period of 45.7 and 11.4 ka,and Core NS-7 had environmental change period of 14.2,7.11,4.74,and 1.02 ka.The fluctuation of biogenic silica,carbonate,organic carbon,uranium and thorium contents in the Nansha sea area are related to the paleoclimatic changes.The correlation of short cores are better than long cores because of the diagenesis and mineralization in samples from long cores.The biogenic silica,carbonate,organic carbon,water contents and sampling depth of thirty samples from six short cores have positive correlations with each other.The accumulation rates of biogenic silica,carbonate,uranium,thorum,and organic carbon of each core were analyzed and there are positive correlations between each other.The uranium and thorium contents had poor correlations with some heavy metal contents in long sediment cores which indicating that the uranium and thorium contents were related to some metal contents.Compared with biogenic silica contens obtained before,and the experimental process was monitored by silicomolybdenum blue and phosphomolybdenum blue spectrophotometry to analyze the loss of silicon and phosphorus.The specific activity of 32Si in six short cores varies from 8.39 to 33.34 mBq/kg,with 16.60 mBq/kg as the mean value.The average specific activity of 32Si is 0.715 Bq/kg SiO2 and the average species abundance of 32Si is 2.60×10-16(32Si/SiO2).Sedimentary rates of Core A and B are 0.106 and 0.191 cm/a respectively.The setting flux of 32Si is 2.14×10-6 Bq·cm-2·a-1 calculated by data of sediments,within the range of references.7783 L rain was collected in Xiamen,and the full recovery of silicon is 20.8%.The recovery of phosphorus is 96.4%.The 32Si activity of the rain in Xiamen is 2.32±0.44 mBq/m3,and the setting flux of 32Si is 3.08×10-7 Bq·cm-2·a-1 calculated by data of the rain.