(Table 1) Amount of calcium carbonate precipitation researched in IODP Hole 329-U1368C

Dissolved inorganic carbon (DIC) concentration and total alkalinity in marine sediment vary with biological activity, mineral diagenesis and past bottom ocean water composition. Reliable interpretation of this data is often compromised due to precipitation of calcium carbonate (CaCO3) during sediment recovery, processing and sample storage. Here we present and test a method that corrects for this precipitation and consequently allows quantification of in situ carbonate system chemistry. Our method relies on the over-determination of the dissolved carbonate system by (i) measuring DIC, alkalinity and calcium, and (ii) explicitly assuming CaCO3 saturation in the sediment. We experimentally tested this method using data from Integrated Ocean Drilling Program (IODP) Site U1368 in the South Pacific Gyre. Our results show that we can accurately reproduce in situ aqueous carbonate system chemistry if DIC, alkalinity and calcium concentration are measured simultaneously. At Site U1368, the correction for sampling associated precipitation is equivalent to 4.5 and 8.9% of the measured DIC and alkalinity, respectively. The method is well suited for any sediment porewater that is saturated with respect to calcium carbonate; consequently, it is applicable for approximately 50% of the global oceanic seafloor.海洋沉积物中的溶解无机碳（Dissolved inorganic carbon, DIC）浓度与总碱度（total alkalinity），会随生物活动、矿物成岩作用以及古底层海水组分发生变化。可靠解读这类数据时常因沉积物采样、处理及样品储存过程中碳酸钙（calcium carbonate, CaCO₃）的析出而受到干扰。 本文提出并验证了一种可校正此类析出效应的方法，借此可定量还原原位碳酸盐体系化学特征。该方法通过两点实现溶解碳酸盐体系的超定约束：其一，测定溶解无机碳、总碱度与钙离子浓度；其二，明确假设沉积物内碳酸钙处于饱和状态。我们利用南太平洋环流区综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）U1368站位的数据对该方法开展了实验验证。结果表明，若同步测定溶解无机碳、总碱度与钙离子浓度，即可准确还原原位水相碳酸盐体系化学特征。在U1368站位，采样相关析出效应的校正量分别相当于实测溶解无机碳与总碱度的4.5%与8.9%。该方法适用于所有碳酸钙饱和的沉积物孔隙水，因此可应用于全球约50%的海洋海底区域。