Determination of in-situ Dissolved Inorganic Carbon and Alkalinity in Marine Sedimentary Interstitial Water

Porewater inorganic carbon concentration and total alkalinity from deeply buried marine sediment reflect biological activity, mineral diagenesis, sedimentary processes and past bottom ocean water composition. Reliable interpretation of these data is often complicated and/or limited due to (i) major physical environment changes taking place during sediment core retrieval, and (ii) the resulting precipitation of calcium carbonate (CaCO3) in the course of sample collection, processing and storage. Here we describe a robust method for quantifying the in-situ porewater carbonate system chemistry in deepsea sediment cores. The method relies on the over-determination of the dissolved carbonate system by measuring three of its parameters, and explicitly assumes CaCO3 saturation in the sediment and equilibrium conditions in-situ. The principles of the method are presented. We experimentally test the proposed approach using concentration profiles of dissolved carbonate system components collected from the Integrated Ocean Drilling Program (IODP) Site U1368 in the Southern Pacific. Our results show that this method can be used to accurately reproduce the in-situ aqueous carbonate system chemistry if dissolved inorganic carbon, total alkalinity and calcium concentration are measured simultaneously. The method is well suited for use over a broad range of porewater chemistry and applicable for sediment over ca. 50% of the seafloor.