冲绳海槽中全新世的浊流沉积及其控制因素

THE TURBIDITY EVENTS IN OKINAWA TROUGH DURING MIDDLE HOLOCENE AND ITS POTENTIAL DOMINATING MECHANISMS

浊流沉积是陆源物质向深海搬运的主要方式，其发生对于海底扇、峡谷以及深海油气藏的形成具有重要意义。对冲绳海槽3个柱状样岩芯的研究，均发现了发生于7300-7500cal．aB．P．的浊流层。这3个浊流层sl，s2和s3的发生伴随着粒度、粘土矿物以及log（Ti/Ca）的明显变化。浊流沉积层相比正常沉积层具有粗粒组分峰值较高以及下覆的不整合构造，粒度上呈现由粗到细的正粒序构造。伴随着粒度的变化，log（Ti/Ca）也发生明显增大。A7孔位的s2浊流层的底部以及Oki01孔位的s3浊流层的上部含有火山灰层，火山玻璃含量约为80％左右。火山灰层的粒度众数约24～32μm的单峰分布。通过对比其他古气候、古环境指标，发现7300～7500cal．aB．P．的浊流事件可能与活跃的火山地震作用以及冬季风明显增强有关。

Turbidity currents are the principal mode of transportation of clastic sediment to deep sea. The occurrence of turbidity play an important role in the formation of submarine fan, canyon and deep-sea oil. In short term, earthquake, volcano activity and internal tide are regarded as the initiation process of turbidity activity. In long term, the variation of regional climate, sea level and ocean current control the sediment supply and pore pressure, thus, initiate the turbidity currents. The Okinawa Trough is a typical curved basin behind the Ryukyu Arc of the northwestern Pacific. Turbidity is common in Okinawa Trough due to the frequent earthquake and volcano activity. During the last glacial, the global sea level was relatively low; the climate was cooler and drier than the present, which led to the exposure of East China Sea continental shelf and the progradation of coastline. However, Okinawa Trough had still been submerged and recorded the environmental changes in the area. A series of submarine canyons distributing around the west slope of middle Okinawa Trough have been found by Chinese＇s scholars by using multi beam bathymetric and high resolution seismic survey. The distribution of canyons at the west slope of Okinawa Trough provides a favorable channel for facilitating sediments from the shelf area to the Okinawa Trough. Three sediment cores Oki02（26.07°N, 125.25°E; 4.9m in length） , A7（27.82°N, 126.98°E; 4.6m in length） , Oki01 （28.33°N, 127.26°E; 3.8m in length） were retrieved from the middle Okinawa Trough at water depth of 1612m, 1264m and 1010m, respectively. These three cores are distributed around the west slope, where is under the influence of Kuroshio Current main stream. In this study, we emphasized on the Holocene interval of three cores, including upper 250cm of core Oki02, upper 200cm of Core A7 and upper 280cm of core Oki01, to reconstruct the turbidity events since the Holocene, by using grain size, clay mineral, bulk elemental ratios and scanning electron microscopy （SEM） analysis. We have identified turbidity layers basing on the theory proposed by Shanmugam, who defines the turbidity sediment as the layer with normal grading and unconformity. After detailed analysis, we found that three turbidity events S1 （ 152 - 160cm）, S2 （ 110 - 120cm）, and S3 （ 182- 202em） occurred coincidently during 7300-7500cal.a B.P. in these three cores. The occurrence of turbidity layers S1, $2, and $3 accompanied by the synchronous change of median grain size, clay mineral, and log（Ti/Ca） ratios. The median grain size and log（Ti/Ca） increased abruptly, while the clay mineral source shifted from mainly Taiwan and East China Sea continental shelf to Yellow and Yangtze River correspondingly. The scanning electron microscopy analysis shows that the turbidity layers are mainly composed of coarser grain detrital material, including feldspar, quartz and heavy mineral. Compared with normal sediment layers, the turbidity layers have coarser grain size peak, unconformity structure, and normal grading bed sequence. Furthermore, there are volcano ashes layers with around 80%-90% volcano glass above S3 turbidity layers of Core Oki01 and underlying $2 turbidity layers of A7, which have 24-32μm unimodal grain size distribution. The turbidity layers also contain 10%-20% volcano ash. The interbed of volcano ash with turbidity layers indicates that the occurrence of turbidity layers may be related to the volcano activity. Comparison between our records and other paleoceanography and paleoclimate proxies suggest that active volcano, earthquake and the enhancement of sediment delivery triggered by stronger East Asian winter monsoon may be the main reasons for the coincidently wide occurrence of turbidities in Okinawa Trough during 7300-7500cal.a B.P