Pore-water chemistry of ODP Leg 115 samples@en

Analyses of the Sr2+ concentrations of interstitial fluids obtained from sediments squeezed during Leg 115 were used to estimate the rates and total amount of recrystallization of biogenic carbonates. The total amount of recrystallization calculated using this method varies from less than 1 % in sediments at Site 706 to more than 40% at Site 709 in sediments of 47 Ma. Five of the sites drilled during Leg 115 (Sites 707 through 711) were drilled in a depth transect within a restricted geographic area so that theoretically they received similar amounts of sediment input. Of these, the maximum rate of recrystallization occurred in the upper 50 m of Site 710 (3812 m). The amount of recrystallization decreased with increasing water depth at Sites 708 (4096 m) and 711 (4428 m), presumably as a result of the fact that most of the reactive calcium carbonate was dissolved before burial. We also observed significant alkalinity deficits at many of these sites, a condition which most likely resulted from the precipitation of calcium carbonate either in the sedimentary column, or during retrieval of the core. Precipitation of CaCO3 as a result of pressure changes during core retrieval was confirmed by the comparison of Ca2+ and alkalinity from water samples obtained using the in-situ sampler and squeezed from the sediments. At Sites 707 and 716, the shallowest sites, no calcium or alkalinity deficits were present. In spite of our estimations of as much as 45% recrystallization at Site 709, all the carbonate sites exhibited what would be previously considered conservative Ca2+/Mg2+ profiles, which varied from -1 to -0.5. By virtue of the position of these sites relative to known basaltic basement or through the actual penetration of basalt (i.e., Sites 706, 707 and 712), these sites are all known to be underlain by basalt. Our results suggest, therefore, that more positive Ca2 + /Mg2+ gradients cannot necessarily be used as indicators of the nature of basement material.本研究以第115航次（Leg 115）中挤压沉积物获取的孔隙流体（interstitial fluids）的锶离子（Sr²+）浓度分析数据为基础，估算了生物碳酸盐（biogenic carbonates）的重结晶（recrystallization）速率与总重结晶量。采用该方法计算得到的总重结晶量差异显著：706站位（Site 706）的沉积物中重结晶占比不足1%，而在年代为47百万年（Ma）的709站位（Site 709）沉积物中则超过40%。第115航次共钻探5个站位（707至711站位，对应Site 707至Site 711），这些站位位于同一受限地理区域内的深度断面（depth transect）上，理论上接收的沉积物输入（sediment input）量相近。其中，710站位（Site 710，水深3812 m）的上部50米范围内重结晶速率最高。708站位（Site 708，水深4096 m）与711站位（Site 711，水深4428 m）的重结晶量随水深（water depth）增加而降低，这一现象大概率源于多数活性碳酸钙在埋藏前已被溶解。本研究还在多数站位观测到显著的碱度亏损（alkalinity deficits）现象，该情况极有可能是在沉积柱内或岩心提取（core retrieval）过程中发生碳酸钙沉淀所导致。通过对比原位采样器（in-situ sampler）获取的原位水样与挤压沉积物得到的水样中的钙离子（Ca²+）浓度与碱度，证实了岩心提取过程中因压力变化引发的碳酸钙沉淀。在最浅的707站位（Site 707）与716站位（Site 716），未观测到钙离子或碱度亏损。尽管本研究估算709站位的重结晶占比最高可达45%，但所有碳酸盐站位均呈现出此前被认定为保守型的钙离子/镁离子（Ca²+/Mg²+）剖面，其比值介于-1至-0.5之间。鉴于这些站位相对于已知玄武岩基底（basaltic basement）的位置，或是实际钻穿了玄武岩（如706、707与712站位，对应Site 706、Site 707及Site 712），可以确定所有站位下方均发育玄武岩基底。因此，本研究结果表明，不能单纯将更正向的钙离子/镁离子梯度（gradients）作为基底物质类型的指示指标。