末次冰期以来南海北部沉积有机碳记录及其古植被环境信息

摘要

末次冰期以来南海北部深海沉积物中陆源植被碎屑来源及其指示的植被类型变化等一直存在争议.正构烷烃比值、正构烷烃碳同位素(δ<'13>C<,alk>)及总有机碳同位素(δ<'13><,TOC>)等有机碳指标的应用可能为之提供新的证据.南海北部MD05-2905岩芯中正构烷烃比值C31/C27,C31/(C31+C27+C29)和ACL(长链烷烃平均链长)等呈现末次冰期高,全新世低的变化,指示末次冰期草本植物相对增多,全新世木本植物较多.δC<,TOC>及δC<,alk>均呈现末次冰期相对偏正,全新世偏负的变化,指示C<,4>植物在冰期增多,间接印证草本植物的增多.末次冰期,由于相对冷干的气候、大气CO2浓度的降低,以及低海平面导致陆架的广阔出露,均有利于更多草本植物的生长.末次冰消期及全新世随着温度的升高及降水的增多,森林植被发育,C<,3>植物相对较多.该岩芯碳同位素结果与南海北部孢粉结果的对比表明,南海北部深海沉积中正构烷烃指标指示南海北部周边较大区域范围的植被变化信息.%The pollen-based vegetation type and the provenance of terrestrial plant remnants since the last glacial period retrieved from the Northern South China Sea ( SCS ) have been controversial. Organic carbon records such as n-alkanes ratios,stable carbon isotopic compositions of n-alkanes( δ13C.alk )and total organic carbon( δ13CToc )in the sediments from core MD05-2905 (20°08. 17＇N, 117°21.61′E, 1647m water depth)which located on the continental　slope of the Northern SCS, may supply new evidence for changes in terrestrial vegetations. This core has a total　length of 11.94m, and spanned the time period of ca. 28kaB. P. to 2kaB. P. according to ten AMS 14C dates.n-Alkanes ratios such as C31/C27, C31 / ( C31 + C27 + C29 ), ACL ( average chain length of long-chain n-alkanes )from core MD05- 2905, showing similar variation trend, revealed a pattern of glacial to interglacial variability.n-Alkanes ratios of C31/C27 presents fluctuation variations with higher average value of 2.2 during the last glacial　period and lower average value of 1.6 during the Holocene, while decreasing gradually with a average value of 2.0during the deglaciation of 16 ～ l0kaB. P. This indicates that there was increasing grassy vegetation in the last glacial　period and more woody vegetation in the Holocene. δl3Calk and δ13CToc values were more positive in the glacial period than in the Holocene, which indicates that C4 plants increased at the LGM, and that more woody plants in the Holocene. At the LGM, increasing grassy vegetations developed in land and in the enlarged exposed continental shelf due to relatively cold-arid climate and lower atmospheric CO2 concentration. During the deglaciation and Holocene,more closed forest (C3 plants)flourished due to higher temperature and precipitation. Comparison between carbon isotopic compositions and pollen data from adjacent cores in Northern SCS at LGM,indicated that plant remains in the core were not only from the continental shelf dominated by C3 grass Artemisia, but also from southeast mainland and Taiwan island that were characterized by abundant C4 flora. Alkanes records from core MD05-2905 can recover the regional vegetation changes around the Northern South China Sea.