Chlorine isotope data of chlorides challenge the pore fluid paradigm

In order to examine the seawater-seafloor sediment interactions that influence chemical composition of seawater through time, we examined hundreds pore fluid geochemical analyses from 13 clay-rich sedimentary successions drilled by ODP-IODP. Chemical trends such as monotonous increases in Ca2+, and decreases Mg2+ δ18O with depth are traditionally interpreted result water-rock interaction. this view, release Ca2+ into fluids uptake 18O mainly results formation low-temperature clays within underlying basalts. Chloride concentration profiles isotopic compositions, however, suggest different processes may water geochemistry. The data here show relatively constant chloride contents but a systematic decrease δ37Cl chlorides 0 permil (the value) down −8.5 permil. highly correlated (with −5.7 permil). δ37Cl-depletions found all studied piles regardless tectonic or history. These cannot be explained water–rock exchange reactions because minerals formed at low temperature have Cl too compensate for depletions observed fluids. Accordingly, hypothesize fluid-specific responsible δ37Cl-enriched were expelled out sediments ocean. After reviewing known change chlorine isotope ratios chlorides, rule diffusion gravitational fractionations could generate pattern. flow -depleted basaltic basement explain data. But mechanism produces depletion remains unknown. It exchanges between rocks. Here compaction-induced ion filtration membranes can produce δ37Cl-depletions, fractionation factors ranging 1.000 1.008 permeate) those residual retentate). We find smectite-rich associated higher factors, while illite/chlorite-rich intermediate values clay-poor lower values. This suggests might controlled surface charge specific clay minerals. Our calculations has capacity 18O-depletion oxygen retentate 1.005. 18O-enrichment is agreement experimental Haydon Graf (1986). Overall, although further work on both isotopes certainly needed, study indicate ion-filtration should considered potential fractionating species waters, particularly whose variations often commonly attributed exchange.