Geochemistry and strontium isotopic composition of interstitial waters of marine carbonates

The concentrations and isotopic compositions of strontium in interstitial waters from several DSDP sites, where sediments consist chiefly of carbonate oozes and chalks, are used as indicators of carbonate diagenesis by reference to a recently-produced curve showing detailed variations in the 87Sr/86Sr ratio of seawater with time. Carbonate sediments of the Walvis Ridge show increases in interstitial Sr[2+] concentrations in the upper carbonate-ooze sections with the highest concentrations near the ooze-chalk boundary where maximum rates of carbonate recrystallization occur. Below this, in situ production of Sr[2+] diminishes and there is a diffusive flux of Sr to an underlying sink, presumably volcanogenic sediments or basalts, leading to Sr isotopic disequilibrium between carbonates and interstitial waters. In some other sites, however, there is no apparent Sr sink at depth and isotopic equilibrium is retained. Overall, diffusive smoothing of profiles exerts an important control on the 87Sr/86Sr ratios, although lower ratios than contemporaneous seawater values in the carbonate oozes often correlate with zones of Mg[2+] loss and reflect a combination of a flux of Sr[2+] from the zone of maximum recrystallization rates together with a contribution from the in situ alteration of volcanic matter.

针对数个深海钻探计划（Deep Sea Drilling Project, DSDP）站位的孔隙水锶浓度与同位素组成展开分析，这些站位的沉积物以碳酸盐软泥和白垩为主。研究以新近建立的海水87Sr/86Sr比值随时间的详细变化曲线为参照，将其作为碳酸盐成岩作用的指示指标。沃尔维斯海岭（Walvis Ridge）的碳酸盐沉积物显示，上部碳酸盐软泥层的孔隙Sr²+浓度呈升高趋势，在软泥-白垩分界处浓度达到峰值，该位置恰好是碳酸盐重结晶作用速率最高的区域。在此分界之下，Sr²+的原位生成量减少，同时存在Sr向下方储层的扩散通量，该储层推测为火山成因沉积物或玄武岩，由此引发碳酸盐与孔隙水之间的Sr同位素不平衡。但在其他部分站位中，深部并未出现明显的Sr储层，因此维持了同位素平衡。总体而言，剖面的扩散平滑效应对87Sr/86Sr比值具有重要调控作用，尽管碳酸盐软泥中低于同期海水比值的Sr同位素值，通常与Mg²+流失带相关，其反映了来自重结晶速率峰值带的Sr²+通量与火山物质原位蚀变带来的Sr贡献共同作用的结果。