Chlorine isotope data of chlorides challenge the pore fluid paradigm

In order to examine the seawater-seafloor sediment interactions that influence the chemical composition of seawater through time, we examined hundreds of pore fluid geochemical analyses from 13 clay-rich sedimentary successions drilled by the ODP-IODP. Chemical trends such as monotonous increases in Ca2+, and decreases in Mg2+ and δ18O with depth are traditionally interpreted to result from water-rock interaction. In this view, the release of Ca2+ into fluids and the uptake of Mg2+ and 18O mainly results from the formation of low-temperature clays in the sediment and within underlying basalts. Chloride concentration profiles and isotopic compositions, however, suggest that different processes may influence pore water geochemistry. The data examined here show relatively constant chloride contents but with a systematic decrease in δ37Cl of chlorides with depth from 0 permil (the seawater value) down to −8.5 permil. The δ37Cl data are highly correlated with δ18O (with δ18O down to −5.7 permil).