海洋Nd同位素演化及古洋流循环示踪研究

Nd isotope evolution of ocean waters and implications for paleo-ocean circulation

海洋Nd同位素演化已经成为示踪陆源风化输入和洋流循环改变的最重要的手段之一,得到了越来越多的应用,并取得了许多重要的成果。海水的Nd同位素组成主要受陆源输入物质控制,热液输入几乎可以忽略。由于Nd在海洋中的停留时间(约500～1000a)略小于海水的平均混合时间(约1500a),且各洋盆有不同的Nd同位素风化输入,因此现代各大洋海水具有不同的Nd同位素组成。在陆源输入稳定的情况下,可以利用海水的Nd同位素组成和演化来示踪水体的混合或洋流循环的改变。目前主要依靠对海洋中水成铁锰结壳、海洋钙质有孔虫壳体、磷酸质鱼骨头或鱼牙齿化石以及沉积物中铁锰氧化物组分等的研究来恢复和反演古海水的Nd同位素组成和演化。4种分析材料各有其优缺点。其中,通过对水成铁锰结壳的Nd同位素分析,基本建立了各大洋新生代以来的主要洋流的Nd同位素组成的长尺度演化。通过有孔虫壳体、鱼化石碎片和沉积物中Fe-Mn氧化物组分可以进行高时间分辨率的古海水Nd同位素演化示踪。利用海水Nd同位素演化可以示踪古洋流通道的开启或闭合,以及获得水体交换的直接信息,为研究构造运动与气候变化之间的关系提供指示。同时,将海水Nd同位素演化与气候变化的指标结合起来,可以用于示踪各种气候条件下洋流循环的改变,将洋流循环的改变与气候变化联系起来,研究两者之间的成因关系。对表层水体的Nd同位素组成的研究则可以示踪不同气候条件下大陆陆源风化输入的改变。

Nd isotope of ocean waters has become an important tracer for continental weathering and oceanic circulation.Dissolved oceanic Nd is controlled by input from continental weathering,while hydrothermal input can be ignored.The Nd isotope composition of seawater is not uniform in the present-day oceans because the oceanic residence time（500-1000 a） of Nd is shorter than the mixing time（～1500 a） of the ocean and the relative contributions of Nd from ancient continental versus young volcanogenic materials differ in various basins.Therefore,the Nd isotope system has been used as a tracer for water exchange and paleo-ocean circulation.There are four major marine materials which can preserve Nd isotope compositions of paleo-ocean waters：hydrogenetic ferromanganese crusts,planktonic or benthic foraminifera,fossil fish teeth or bones,authigenic Fe-Mn oxide fraction in marine sediments.Each material has its own advantages and disadvantages.Cenozoic Nd isotopic evolutions of major oceans on timescales of millions have been acquired by analyzing Fe-Mn crusts.High resolution Nd isotopic evolution can be acquired through studying foraminifera,fossil fish debris and Fe-Mn oxides.According to Nd isotopic evolution we may reconstitute the water exchange to trace open and closure of paleo-seaways,which can be used to study the relationship between tectonic activities and climate changes.The combination of Nd isotope and the proxies for climate change can be used to study the relationship between ocean circulation and climate shifts.Nd isotope of surface water can provide information about continental weathering input.