Gas, water and volume characteristics during controlled degassing experiments on ODP Leg 164 PCS cores

A pressurized core with CH4 hydrate or dissolved CH4 should evolve gas volumes in a predictable manner as pressure is released over time at isothermal conditions. Incremental gas volumes were collected as pressure was released over time from 29 pressure core sampler (PCS) cores from Sites 994, 995, 996, and 997 on the Blake Ridge. Most of these cores were kept at or near 0ºC with an ice bath, and many of these cores yielded substantial quantities of CH4. Volume-pressure plots were constructed for 20 of these cores. Only five plots conform to expected volume and pressure changes for sediment cores with CH4 hydrate under initial pressure and temperature conditions. However, other evidence suggests that sediment in these five and at least five other PCS cores contained CH4 hydrate before core recovery and gas release. Detection of CH4 hydrate in a pressurized sediment core through volume-pressure relationships is complicated by two factors. First, significant quantities of CH4-poor borehole water fill the PCS and come into contact with the core. This leads to dilution of CH4 concentration in interstitial water and, in many cases, decomposition of CH4 hydrate before a degassing experiment begins. Second, degassing experiments were conducted after the PCS had equilibrated in an ice-water bath (0ºC). This temperature is significantly lower than in situ values in the sediment formation before core recovery. Our results and interpretations for PCS cores collected on Leg 164 imply that pressurized containers formerly used by the Deep Sea Drilling Project (DSDP) and currently used by ODP are not appropriately designed for direct detection of gas hydrate in sediment at in situ conditions through volume-pressure relationships.

Temperature measurements, gas chemistry, and gas hydrates of the Dvurechenskii mud volcano

The sediment temperature distribution at mud volcanoes provides insights into their activity and into the occurrence of gas hydrates. If ambient pressure and temperature conditions are close to the limits of the gas hydrate stability field, the sediment temperature distribution not only limits the occurrence of gas hydrates, but is itself influenced by heat production and consumption related to the formation and dissociation of gas hydrates. Located in the Sorokin Trough in the northern Black Sea, the Dvurechenskii mud volcano (DMV) was in the focus of detailed investigations during the M72/2 and M73/3a cruises of the German R/V Meteor and the ROV Quest 4000 m in February and March 2007. A large number of in-situ sediment temperature measurements were conducted from the ROV and with a sensor-equipped gravity corer. Gas hydrates were sampled in pressurized cores using a dynamic autoclave piston corer (DAPC). The thermal structure of the DMV suggests a regime of fluid flow at rates decreasing from the summit towards the edges of the mud volcano, accompanied by intermittent mud expulsion at the summit. Modeled gas hydrate dissociation temperatures reveal that the gas hydrates at the DMV are very close to the stability limits. Changes in heat flow due to variable seepage rates probably do not result in changes in sediment temperature but are compensated by gas hydrate dissociation and formation.