冲绳海槽中部28ka以来陆源物质输入和古环境演化的生源组分记录

沉积物中有机碳、生物碳酸盐和生物硅等生源组分的埋藏速率在很大程度上受到海洋古生产力的控制,常常被用作古生产力的替代指标。本文利用TOC、生物硅及CaCO3等生源组分探讨冲绳海槽中部DGKS9604孔(28°16.64'N,127°01.43'E;水深766m;柱长10.76m)距今28ka以来陆源物质输入和古环境演化对海洋生产力的影响。研究结果表明,在冬季风较强的末次冰期晚期(28~22kaB.P.),长江直接输运大量的陆源营养物质进入冲绳海槽,古生产力较高; 在末次盛冰期(LGM,22~18kaB.P.),虽然河口距离海槽区更近,但大陆流域降水匮乏,河流入海径流量偏低导致海槽陆源营养物质明显减少,生产力降低,表现在生物硅和有机碳堆积速率明显降低; 冰消期早期生产力与末次盛冰期相比并未明显降低,此阶段气候回暖,大陆流域降雨增加,陆源物质输入相对较高。全新世早期,古生产力快速下降,既反映海平面降低,陆源物质输入减少,还可能与冰消期寡营养盐的黑潮在海槽加强有关。DGKS9604孔CaCO3含量及堆积速率变化的总体趋势与有机碳和生物硅相反,表现为末次冰期晚期低,冰消期以来逐渐升高,一方面反映陆源物质输入的稀释作用; 另一方面与黑潮加强致使钙质生物生产力增强有关。此外,15~7kaB.P.期间CaCO3含量及其堆积速率有几次明显降低,可能对应黑潮减弱、陆源冲淡水较强的环境事件。

USING BIOGENIC COMPONENTS TO DECIPHER THE TERRIGENOUS INPUT AND PALEOENVIRONMENTAL CHANGES OVER THE LAST 28ka IN THE MIDDLE OKINAWA TROUGH

Sediment accumulation rates of total organic carbon(TOC),biogenic silica(opal)and CaCO3 in marine environments are primarily controlled by paleo-productivity,and thus,these biogenic components are treated as powerful proxies of marine productivity.In this study,we report the contents of TOC,opal and CaCO3 in sediments recovered from Core DGKS9604(28°16.64'N,127°01.43'E),with aim to understand the changing influx of terrigenous materials and paleo-environmental evolution of the middle Okinawa Trough during the last 28ka.This core was taken from the west slope of the middle Okinawa Trough at a water depth of 766m which is under control of modern Kuroshio Current. The core is 10.76m long and mainly composed of clayey silt without apparent ash layers. We selected 93 samples to measure the opal by colorimetric measurement of phosphorus as molybedenum blue and 188 samples to analyze total carbon, total organic carbon and carbonate. The chronostratigraphy of core was established on the basis of AMS 14 C dates and oxygen isotope of representative foraminifera.The results indicate that the paleo-productivity in the middle Okinawa Trough was predominantly controlled by changing sea level and monsoon climate as well as current variation since the last glaciation.The paleo-productivity reached the highest during the late last glaciations(28~22kaB.P.)due to the large influx of nutrients derived from paleo-Changjiang and/or paleo-Huanghe Rivers.Though the Changjiang Estuary might be closer to the Okinawa Trough during the Last Glaciation Maximum(22~18kaB.P.),the paleo-productivity decreased obviously primarily because of the decrease of terrigenous nutrient inputs in response to the decline of precipitation in the drainage basins.In the Early deglaciation,paleo-productivity did not vary considerably relative to LGM,which may be related to the enhancing precipitation in the catchments during that period and increasing influx of terrigenous materials.In the Early Holocene,paleo-productivity declined rapidly,reflecting either the decrease of terrigenous input of nutrients or the strengthening of the Kuroshio Current with low nutrition.The changes of CaCO3 content and accumulation rate were contrary to the downcore variations of opal and TOC,showing lower values during the late last glaciations and gradual increase during the last deglaciation,which suggests the dilution effect induced by the terrigenous materials.On the other hand,it may be related to the strengthening of Kuroshio Current in the Holocene,which might increase the calcium productivity.Furthermore,the drops of CaCO3 content and accumulation rate from 15kaB.P.to 7kaB.P.might be due to the weakening events of the Kuroshio Current and/or the strengthening of freshwater.