200 ka以来澳大利亚西北岸外沉积物源区风化的Mg同位素记录及其对澳洲古季风的响应  

A 200 ka record of continental weathering for northwestern Australian margin based on magnesium isotopes and its response to Australian paleo-monsoon

作　　者：可菲 徐建[1] 张鹏[1] 包志安[1] 马龙 宗春蕾[1] KE Fei;XU Jian;ZHANG Peng;BAO Zhi'an;MA Long;ZONG Chunlei(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi'an,Shaanxi 710069,China)

机构地区：[1]西北大学地质学系大陆动力学国家重点实验室,陕西西安710069

出　　处：《地质学报》2023年第2期565-582,共18页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41776060、41806066、41830539、41576045)资助的成果。

摘　　要：本文通过对IODP 363航次于澳大利亚西北岸外陆架上钻取的U1483站岩芯进行沉积物粒度、镁同位素以及主量元素成分等分析,重建200 ka以来帝汶海西南部沉积物输入演化及其指示的源区风化和侵蚀历史,探讨海洋沉积物中细粒硅质碎屑组分镁同位素的古环境指示意义。U1483站陆源碎屑组分主要由黏土质粉砂组成,其δ^(26)Mg变化范围超过0.25‰。通过计算化学蚀变指数(chemical index of alteration,CIA)和两种物理侵蚀指标(包括陆源物质堆积速率和Ti/Ca比值),结果显示200 ka以来δ^(26)Mg指标与CIA具有非常一致的曲线形态。本文推测陆源碎屑组分的Mg同位素组成主要反映物源区大陆硅酸盐风化强度的变化,而受矿物学分选、岩性和早期成岩作用的影响较小。结合区域古气候记录,我们发现本研究中的风化和侵蚀记录与同一个站位已发表的澳洲夏季风降水和表层海水温度变化具有较好的耦合关系,而与赤道太平洋海平面变化的相关程度较低,反映了轨道时间尺度上沉积物所记录的物源区风化和侵蚀演化主要受控于澳洲古季风系统,表现为气候驱动型风化模式。本研究δ^(26)Mg指标的应用为我们理解区域大陆风化及其对气候演化的响应和反馈机制提供了全新的同位素视角。We present a new multiproxy dataset of grain size,Mg isotopes and major elemental compositions of sediments from the International Ocean Discovery Program Site U1483,situated in the northwestern Australian margin,in order to reconstruct the terrestrial sediment supply to the southwestern Timor Sea,as well as the continental weathering and erosion history in the source region since 200 ka.A second objective is to explore the potential ofδ26Mg values of fine-grained siliciclastic marine sediments in tracing paleoclimate changes.The results demonstrate that terrestrial siliciclastic fraction of sediments at IODP Site U1483 are mainly composed of clay silt,withδ26Mg values ranging larger than 0.25‰.The Chemical index of alteration(CIA)and two physical erosion parameters including mass accumulation rates(MAR)and Ti/Ca ratios are reported.Over the last 200 ka,theδ26Mg index shows a shape of curve in a great similarity with that of the CIA,suggesting that Mg isotope compositions are mostly indicative of weathering intensity changes in the source region,and are little influenced by mineralogical sorting,lithology and early diagenesis.Combined with regional paleoclimate records,we notice that our new weathering and erosion data show nearly synchronous co-variations with published Australian summer monsoonal precipitation and the sea surface temperature records from the same site since 200 ka.However,there is relatively low coherence between our data and the sea level in the equatorial Pacific.By compiling the data,this study suggests that the weathering and erosion evolution of the source region on the orbital timescale were mainly controlled by the Australian paleo-monsoon system during the last 200 ka,pointing to the“climate-driving weathering”pattern.The application ofδ26Mg index provides a new isotopic perspective on understanding regional continental weathering and its response as well as feedback mechanisms to the climate evolution.

关 键 词：粒度 镁同位素 陆源硅酸盐碎屑 帝汶海 澳洲古季风 

分 类 号：P736.22[天文地球—海洋地质]