Coring of sediments with minimal physical disturbance, and sampling of pore water with minimal contamination from seawater, are critical in marine geochemistry and microbiology. Yet, sediment coring generally causes smearing of sediment down along the inside of the core liner and leaves seawater and fluid mud trapped around the core. This seawater is a source of diffusive or advective exchange of solutes with the core, which affects the chemistry of retrieved pore water. The exchange depends on the time elapsed between coring and the end of pore water extraction. The degree of seawater contamination is difficult to determine and has generally been neglected in the geochemical literature. Yet, it is possible to quantify seawater contamination due to the high millimolar concentration of sulfate in seawater and low micromolar concentration in the subsurface methanic zone, where sulfate is largely depleted. Analysis of 540 sediment cores from a global database showed that sulfate concentration data beneath the sulfate-methane transition were often truncated and stated as “0” or “0,0” mM, thereby preventing an evaluation of seawater contamination. Among the 181 sediment cores for which the mM sulfate concentrations were reported with two or three decimal places, concentrations measured in methanic sediments were up to an order of magnitude higher than the estimated in situ concentrations in gravity cores and two orders of magnitude higher in cores recovered by scientific ocean drilling. A comparison between pore water extraction by Rhizon suction samplers and by high-pressure squeezers in cores from Baltic Sea Expedition 347 of the IODP (International Ocean Discovery Program) showed no systematic difference in the degree of contamination. A comparison with a perfluorocarbon (PFC) tracer injected by IODP coring into the drilling fluid showed a seawater contamination similar to that based on sulfate in the methanic zone. In IODP cores, where the true in situ sulfate concentration is generally unknown, we calculated the theoretical sulfate concentration that corresponded to equilibrium between SO, Ba and barite (BaSO4) to show that the equilibrium sulfate concentrations in the methanic zone are much lower than the sulfate concentrations measured in pore water samples, again indicating seawater contamination. We used a time-resolved diffusion model to calculate how contaminating seawater ions penetrate from the external liner fluid into sediment cores before pore water is extracted. We conclude that nearly all published pore water samples retrieved from subsurface sediments are contaminated by seawater to some degree. We also conclude that sensitive sulfate analyses in methanic sediments constitute a unique opportunity to quantify this contamination and thereby provide a better possibility to control it.

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海洋沉积物取芯和孔隙水取样造成的海水污染

在物理扰动最小的情况下对沉积物进行取芯，以及在海水污染最小的情况下对孔隙水进行取样，对于海洋地球化学和微生物学至关重要。然而，沉积物取芯通常会导致沉积物沿着岩心衬里的内部涂抹，并使海水和流体泥浆滞留在岩心周围。这种海水是与岩心进行扩散或平流交换溶质的来源，这会影响回收的孔隙水的化学性质。交换取决于取芯和孔隙水提取结束之间经过的时间。海水污染程度很难确定，并且在地球化学文献中通常被忽视。然而，由于海水中硫酸盐的毫摩尔浓度较高，而地下甲烷区的微摩尔浓度较低（硫酸盐在该区域中已大量耗尽），因此可以量化海水污染。对全球数据库中 540 个沉积物岩心的分析表明，硫酸盐-甲烷转变以下的硫酸盐浓度数据经常被截断并表示为“0”或“0,0”mM，从而妨碍了对海水污染的评估。在 181 个沉积物岩心中，mM 硫酸盐浓度报告为小数点后两位或三位，甲烷沉积物中测得的浓度比重力岩心中估计的原位浓度高出一个数量级，而岩心中的浓度则高出两个数量级通过科学的海洋钻探发现。对 IODP（国际海洋发现计划）波罗的海 347 号探险队岩心中 Rhizon 抽吸取样器和高压挤压器提取的孔隙水进行比较，结果显示污染程度没有系统性差异。与通过 IODP 取芯注入钻井液中的全氟化碳 (PFC) 示踪剂进行比较显示，海水污染与甲烷层中基于硫酸盐的污染类似。在IODP岩心中，真实的原位硫酸盐浓度通常未知，我们计算了与SO、Ba和重晶石（BaSO4）之间的平衡相对应的理论硫酸盐浓度，以表明甲烷区中的平衡硫酸盐浓度远低于甲烷区的平衡硫酸盐浓度。在孔隙水样品中测量的硫酸盐浓度，再次表明海水污染。我们使用时间分辨扩散模型来计算在提取孔隙水之前污染海水离子如何从外部衬里流体渗透到沉积岩芯中。我们的结论是，几乎所有已发表的从地下沉积物中提取的孔隙水样本都受到了一定程度的海水污染。我们还得出结论，甲烷沉积物中的敏感硫酸盐分析构成了量化这种污染的独特机会，从而提供了更好的控制它的可能性。