冲绳海槽北部15ka B.P.以来沉积物源及控制因素——稀土元素的证据

THE REE IMPRINT ON SEDIMENT PROVENANCE OF THE NORTHERN OKINAWA TROUGH SINCE THE LAST 15ka B.P.

用等离子体质谱法(ICP-MS)对冲绳海槽北部Y127孔16个沉积物样品的稀土元素(REE)进行了分析。根据AMS14C测年结果,Y127孔记录了冲绳海槽约15kaB.P.以来的沉积历史。约11.1kaB.P.以来,沉积物的稀土元素总量(ΣREE)由表层102.4μg/g向下增加到136.1μg/g,平均为123.3μg/g。15—11.1ka,ΣREE在149.1—169.2μg/g之间,平均为158.6μg/g,总体趋势由上向下逐渐增加。轻、重稀土元素比值(ΣLREE/ΣHREE)在上述两个时间段中分别为8.16和9.42,其变化趋势与∑REE的完全相同。La/Sm在两个时段分别为6.70和7.15。球粒陨石标准化的结果表明,15kaB.P.以来冲绳海槽北部沉积物具有相同的配分模式,表现为LREE富集,HREE弱亏损。δEu的值在0.69—0.76之间,中等程度的负Eu异常;δCe在0.85—0.95之间,为弱的负Ce异常。经页岩标准化(NASC)后,REE配分模式曲线平坦,弱的正Eu异常和弱的负Ce异常。通过Y127孔沉积物REE特征与长江、黄河沉积物的对比研究,在15—11.1kaB.P.沉积物的REE组成基本反映了源区的物质组成特征,沉积物主要以长江物质为主,影响沉积物REE特征的主要因素是海平面变化和源区的物质特征。11.1kaB.P.以来,由于海平面上升,可能仅有少量黄河来源物质进入冲绳海槽北部,源区沉积环境和沉积物的组织,加上火山物质和生源组分的共同作用,形成了这一时期沉积物的REE特征。

The rare earth elements （REEs） of 16 sediment samples taken from Core Y127 in the northern Okinawa Trough were measured with ICP-MS. The samples were gravity-cored at 128°18.26′E, 30°32.97′N, 422 cm long, 739 m below the sea level, by Institute of Oceanology, CAS on R/V Science-I in May 1999. The geological age of the bottom core was AMS^14C dated for 15ka B.P. The concentration of EREE was 102.4μg/g at the top and then increased to 136.1μg/g downwords, averaged at 123.3p.g/g from 11. lka B.P. to the present. From 15ka to 11. lka B.P., the EREE concentration was between 149.1 and 169.2μg/g, in average of 158.6μg/g, increasing downwards. The ratios of ΣLREE/ΣHREE are 8.16 and 9.42 respectively in the two periods, having similar variation to ΣREE. The ratios of La/Sm are 6.70 and 7.15 respectively in the two periods. The results of chondrite normalization show that the REE distribution patterri of the sediment in the northern Okinawa Trough remained the same since 15ka B.P., i.e. the enrichment of LREE and weak depletion of HREE. δEu is between 0.69 and 0.76, which is in scope of intermediate negative Eu abnormity. δCe is between 0.85 and 0.95, weak negative Ce abnormity. After normalization with NASC, REE allocation schema became even, with weak positive Eu abnormity and weak negative Ce abnormity. Comparing REE characteristics of Core Y127 with those of the Changjiang （Yangtze） River and the Yellow River, we found that REE characteristics in the period of 15 ka to 11. lka B.P. in the trough indicated the nature of provenance. The sediment mainly derived from Changjiang River, the formed REE pattern was mainly constrained by sea level change, depositional environment, and lithology of the provenance. Probably, since the last 11.1ka B.P., only a small amount of sediment was from the Yellow River into the northern Okinawa Trough because of the high sea level stand. With the additions of volcanic matters and biogenic matter, the REE pattern shows different characteristics from previous one since the last deglaciation.