GEOCHEMICAL CHARACTERISTICS OF TRANSITION ELEMENTS IN SURFACE SEDIMENTS NORTHERN OKINAWA TROUGH
JIANG Fu-Qing, LI An-Chun, LI Tie-Gang (Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071)
The Okinawa Trough is an active back-arc basin located behind the Ryukyu Trench and Ryukyu Trench-arc system.The trough adjoins the broad expanse of the East China Sea continental shelf on the west side and faces the Ryukyu Island arc on its east side.184 surface sediments were collected in the outer continental shelf of the East China Sea, slope and the trough in northern part of the Okinawa Trough.The sediments are composed mainly of sand, silt and clay.The clay content increases from the shelf to the Okinawa Trough.The concentration of Fe 3+ and Fe 2+ , and Eh value are measured in situ.The Fe 3+ /Fe 2+ ratio in the continental shelf sediments are less than 2, some stations less than 1,which indicates a weak reduction and oxidation environment.The Fe 3+ /Fe 2+ ratios increase from the shelf to the trough, and greater than 5 in some sediments, indicating a more oxidative environment that favors for the enrichment of Mn.The contents of Al and transition elements, including Ti, Cr, Fe, Co, Ni, Cu, Zn and Zr were measured in the laboratory.The regional distributions of Cu, Zn, Co, Ni and Cr are similar to Al's.The contents of the elements are low in the continental shelf sediments that are mainly composed of sandy sediment.The lowest contents of the elements are in the north of the study area, extending in tongue-shaped form from NW to SE.The sediments in the trough are composed of mainly fine-grained sediments.The distributions of transition elements are different from those in the slope and the trough.Ti and Zr are locally rich in the slope, and increase toward the trough.Zn and Cu increase from slope to trough and their distributions change with the topography.Cr, Co and Ni increase from slope to trough generally.Mn is relatively low and shows a little change in the continental shelf and slope sediments, and increases from the slope to the trough in sediments.The correlation between Al and transition elements were analyzed.Except for Mn, the elements are positively correlated with Al.The correlation coefficient between Ti and Al is the highest (r=0.87).The average Al/Ti ratio of the sediments is 16.03, close to the value of the crust.No excess Al(non detritus Al)is available in the bulk sediments.Al and Ti are mainly derived from terrigenous matters.The Mn/Fe ratios in the continental shelf sediments bearing less than 20% of CaO and in the slope sediments, are similar to that in the Yellow Sea and East China Sea sediments, but less than those in the trough and the continental shelf sediments bearing more than 20% of CaO sediments.The high Mn/Fe ratios in the sediments from the latter two areas are caused by autogenous concentration and biological enrichment, respectively.Because of the effects of biogenic debris, grain size, volcanic activity and the autogenetic deposit, the information of the provenance of the transition elements are partly smeared in the continental shelf sediment.The enrichment factor (EF) of transition elements in the continental shelf sediments bearing more than 20% of CaO were proved to be more effective for removing biogenic debris and grain size effects.The study on transition elements enrichment factor shows that the EF values of some elements, such as Ti, Cu and Co, are similar to those of the Huanghe (Yellow) River.Some elements, such as Zn and Mn, are close to the Changjiang (Yangtze) River, and the others are close to both river sediments, which show that the sediments in the study area were derived partly from these two rivers.
【Fund】： 中国科学院知识创新工程资助项目 KXCX3-SW-220号;; 国家自然基金重点项目 90411014号;; 国家自然科学基金资助项目40506016号;; 国家重点基础研究发展规划资助项目 G2000046703号。
【CateGory Index】： P736.4
【CateGory Index】： P736.4