Sensor Package Data and H2 and CH4 concentrations in Pre- and Post-Drilling Water Samples: IODP Expedition 357

The sea-floor rock drills utilized on Expedition 357 were equipped with water sampling capability as well as with a CTD/water sensor package that allowed real time monitoring of temperature, dissolved oxygen, methane, pH, and oxidation-reduction potential (ORP). We typically collected near-bottom water samples at each drill site with the ship's CTD/rosette system and post-drilling samples with the automated sampling system mounted on each drill. The water samplers were flushed with a Seabird pump. Through the use of the sensor package, we usually monitored temperature, pH, ORP, and methane concentration during drilling. The sensor data exhibited considerable downhole variability. Temperature was variable (typically by a few tenths of a degree but up to 1°C at some sites) but difficult to interpret due to the potential for heating by the drill. The pH value normally increased downhole and significant downhole CH4 peaks were common at most sites. ORP minima were very common and were associated with at least two causal mechanisms: a) associated with specific horizons during continuous drilling of a specific core; b) caused by the shutting off of flushing water, which reduced the dilution of formation fluid as core barrels were changed. Here, we compare and contrast the sensor data and gas concentrations from several drill sites.