Geochemistry of solid and dissolved phases of IODP EXP340 holes

We measured dithionite-extractable iron and manganese along with a variety of bulk sedimentary solid and dissolved phases to constrain diagenetic reactions occurring within the sediment package of Grenada Basin, which is within the Lesser Antilles volcanic arc region. Core material was obtained during Integrated Ocean Drilling Program Expedition 340. For this report we focus primarily on five sites; three sites are located in the northern portion of the study area off the island of Montserrat (Sites U1394-U1396), and two sites are located farther south off the island of Martinique (Sites U1399 and U1400). Sediments throughout this region include tephra-rich volcanic sands, hemipelagic mud sequences, and carbonate-rich sequences, with widely variable proportions over short (centimeter scale) depth intervals. Regardless of the main sediment type, organic carbon contents are low with average values of 0.19 ± 0.1 wt% at Site U1394, 0.13 ± 0.08 wt% at Site U1395, 0.13 ± 0.06 wt% at Site U1396, 0.28 ± 0.08 wt% at Site U1399, and 0.23 ± 0.15 wt% at Site U1400. Carbonate contents are more variable, ranging between 0 and ~80 wt%, in cores from Sites U1394-U1396 and between 1 and 40 wt% at both Sites U1399 and U1400. These variations in carbonate content likely reflect variable dilution with volcanogenic sediment. Pore fluids reflect a range of diagenetic conditions from oxidizing (sulfate rich) to sulfate-reducing conditions. Reactive major elements (Ca and Mg) as well as a number of minor elements show a range of diagenetic behaviors from reactions likely related to carbonate dissolution or precipitation to exchange reactions between pore fluids and the volcanic-rich sedimentary substrate and clay formation. However, significant site-to-site variability is seen in the diagenetic behavior of these elements. Solid-phase reactive Fe ranges from 0.18 to 0.75 wt% at the northern sites and 0.4 to 1.5 wt% at the southern sites, whereas reactive Mn ranges from 0 to 0.1 wt% in the north and 0 to 0.3 wt% in the south.本研究测定了连二亚硫酸盐可提取态铁（dithionite-extractable iron）与锰，同时同步测定了多种体相沉积固相与溶解相组分，以约束赋存于小安的列斯火山弧区域内格林纳达盆地（Grenada Basin）沉积层序中的成岩反应（diagenetic reactions）过程。岩心样品采自综合大洋钻探计划（Integrated Ocean Drilling Program）340航次。本报告主要聚焦5个钻探站位：其中3个站位位于研究区北部蒙特塞拉特岛（Montserrat）外海（U1394~U1396站位），剩余2个站位位于更南部的马提尼克岛（Martinique）外海（U1399与U1400站位）。该区域的沉积物类型涵盖富火山碎屑（tephra）火山砂、半远洋泥质序列（hemipelagic mud sequences）以及富碳酸盐序列（carbonate-rich sequences），在厘米级的短深度间隔内，各组分占比存在显著差异。无论沉积物主类型如何，各站位有机碳（organic carbon）含量均较低：U1394站位平均含量为0.19±0.1 质量百分比（wt%），U1395站位为0.13±0.08 wt%，U1396站位为0.13±0.06 wt%，U1399站位为0.28±0.08 wt%，U1400站位为0.23±0.15 wt%。U1394~U1396站位岩心的碳酸盐含量波动范围更大，介于0~约80 wt%之间；而U1399与U1400站位的碳酸盐含量则介于1~40 wt%之间。碳酸盐含量的这种差异大概率反映了火山成因沉积物对碳酸盐的稀释作用存在变化。孔隙流体（pore fluids）记录了从氧化环境（富硫酸盐，sulfate rich）到硫酸盐还原环境（sulfate-reducing conditions）的一系列成岩环境条件。活性常量元素（reactive major elements，钙与镁）以及多种微量元素表现出多样的成岩行为：从与碳酸盐溶解（dissolution）或沉淀（precipitation）相关的反应，到孔隙流体与富火山物质沉积基底之间的离子交换反应（exchange reactions），再到黏土矿物形成（clay formation）过程。但这些元素的成岩行为在不同站位间存在显著差异。固相活性铁在北部站位的含量范围为0.18~0.75 wt%，南部站位为0.4~1.5 wt%；而固相活性锰在北部站位的含量范围为0~0.1 wt%，南部站位为0~0.3 wt%。