Table1_Formation of calcium chloride brines in volcaniclastic-rich sediments.docx

The Amami-Sankaku Basin, located in the Philippine Sea, records approximately 50 million years of sediment accumulation and diagenesis of volcanic ash derived from the Kyushu-Palau and Kyushu-Ryukyu arcs. Analyses of porewater and sediment samples from the 1461-m core recovered at IODP Expedition 351, Site U1438, included major and trace elements, strontium radiogenic isotope ratio (87Sr/86Sr), and taxonomic identification of archaeal classes and bacterial orders. Sediment X-ray diffraction and thin section analyses show that smectite, zeolites and chlorite are the main authigenic minerals. A multicomponent solute diffusion and reaction numerical model was developed to simulate the long-term diagenesis that took place in this sedimentary sequence and to account for the transition to a calcium chloride brine at about 670 mbsf. Numerical results indicate that 45% of the initial amount of volcanic ash has been dissolved at 750 mbsf. At this depth, 13% of the initial water (H2O) in the pore space is estimated to have been transferred to the solid phase by the formation of zeolites, which accounts for an increase in porewater chloride concentration. In contrast, dissolution of anorthite and volcanic ash combined with sodium uptake by zeolites accounts for the predominance of calcium in the brine. In the upper 160 m of the sedimentary column, the electrochemical migration of solutes causes chloride to move in the opposite direction of the concentration gradient, i.e., uphill diffusion, sustaining downward diffusion of seawater chloride into the sediments.

位于菲律宾海（Philippine Sea）的奄美三角盆地（Amami-Sankaku Basin）记录了约5000万年的沉积物堆积过程，以及源自九州-帕劳弧（Kyushu-Palau arcs）与九州-琉球弧（Kyushu-Ryukyu arcs）的火山灰（volcanic ash）的成岩作用演化。对国际大洋发现计划（International Ocean Discovery Program, IODP）351航次U1438站位获取的1461米岩芯中的孔隙水与沉积物样品开展的分析，涵盖了常量元素、微量元素、锶放射成因同位素比值（87Sr/86Sr），以及古菌纲与细菌目的分类学鉴定。沉积物X射线衍射与薄片分析结果显示，蒙脱石（smectite）、沸石（zeolites）与绿泥石（chlorite）为主要自生矿物（authigenic minerals）。本研究构建了多组分溶质扩散-反应数值模型，用以模拟该沉积序列经历的长期成岩作用，并解释约670海底以下米（meters below seafloor, mbsf）处向氯化钙卤水（calcium chloride brine）的转变过程。数值模拟结果表明，在海底以下750米处，初始火山灰总量已有45%发生溶解。在此深度下，据估算孔隙空间中初始水分（H₂O）的13%已通过沸石形成作用转移至固相，这也是孔隙水氯离子浓度升高的成因。与之相对，钙长石（anorthite）与火山灰的溶解，结合沸石对钠的摄取作用，共同造就了卤水中钙的主导地位。在沉积柱上部160米范围内，溶质的电化学迁移使得氯离子沿与浓度梯度相反的方向运动，即上坡扩散（uphill diffusion），维持了海水氯离子向沉积物内的向下扩散过程。