An automated technique for measuring delta D and delta O-18 values of porewater by direct CO2 and H-2 equilibration

The stable-oxygen and -hydrogen isotopic values (deltaD, delta O-18) of por ewater in geologic media are commonly determined on water obtained by extra ction techniques such as centrifugation, mechanical squeezing, vacuum heati ng and cryogenic microdistillation, and azeotropic distillation. Each of th ese techniques may cause isotopic fractionation as part the extraction proc ess and each is laborious. Here we demonstrate a new approach to obtain aut omated, high-precision deltaD and delta O-18 measurements of porewater in g eologic sediments by direct H-2- and CO2-porewater equilibration using a mo dified commercial CO2-water equilibrator. This technique provides an import ant and cost-effective improvement over current extraction methods, because many samples can be rapidly analyzed with minimal handling, thereby reduci ng errors and potential for isotopic fractionation. The precision and accur acy of direct H-2- and CO2-porewater equilibration is comparable to or bett er than current porewater extraction methods. Finally, the direct equilibra tion technique allows investigators to obtain high-resolution (cm scale) po rewater deltaD and delta O-18 profiles using cores from individual borehole s, eliminating the need for costly piezometers or conventional porewater ex tractions.