Jeffrey Poort

Heat transport by groundwater flow during the Baikal rift evolution

Abstract A two-dimensional modelling study of sedimentation, fluid flow, and heat flow in the Baikal rift basin undergoing flank uplift and basin subsidence has been performed in order to understand the impact of these processes on the surface heat flow signal. Heat flow anomalies of different scales and magnitudes have been observed at the sediment surface of the lake Baikal basin, and the presence of a hydrothermal vent suggests that fluids play an important role in the regional distribution of heat flow. The BASIN-code applied for this study allows to simulate topographically and compaction-driven hydrodynamical fluid flow and coupled heat transfer. The flank uplift history provides the basis for a regional groundwater circulation towards the central basin area, with predicted Darcy velocities at present-day situation in the basement varying between 1 and 100 cm/year. Within the basin, the presence of aquifers and the pinch-out layering has a major control on the flow field, and compaction-driven flow velocities are strongly altered when combined with topography-driven flow. When velocities in the basement are larger than several centimeters per year, the regional fluid circulation is an effective mechanism of heat redistribution. Heat is brought from the flanks towards the basin area, with largest heat transported at a depth of 1–2 km at both sides. During the flank uplift, heat advection increases, with secondary variation related to the deposition of sedimentary layers. The heat flow is increased over the basin and reduced in the flanks, with a total heat output balance always positive. The extra heat output over the modelled transection is 2–10% of the initial heat output. The maximum computed heat fluxes are smaller than measured in the heat flow anomalies of the lake Baikal basin. Nevertheless, the model suggests that flow in the sedimentary basin combined with a topographically driven heat advection in the surrounding basement is a sufficient mechanism to account for the increased heat flow over the basin and the main features of the heat flow distribution.

Gas hydrate forming fluids on the NE Sakhalin slope, Sea of Okhotsk

Abstract An area of focused fluid venting off NE Sakhalin, Sea of Okhotsk, was investigated in 2003 during the 31st and 32nd international expeditions of R/V Akademik M. A. Lavrentyev within the framework of the CHAOS Project. More than 40 structures related to seafloor gas venting were discovered and gas hydrates were sampled from three of these: CHAOS, Hieroglyph and Kitami. Geochemical analyses were used to define the mechanisms of gas hydrate accumulation and the sources of fluids involved. Chemical and isotopic analyses of the interstitial and hydrate waters suggest that hydrates were formed from seawater (or in-situ pore water) and an ascending fluid enriched in salts. Hydrate formation occurs at locations of the most intensive saline water upflow, and this is probably a function of the gas solubility in water in equilibrium with hydrate. The water involved in gas hydrate formation consists of about 70% pore water derived from the host sediment and 30% from the ascending fluid. The overall isotopic composition of the ‘fluid’ taking part in hydrate formation was calculated as δ2H≈−11‰ and δ18O≈−1.5‰.

Authigenic rhodochrosite from a gas hydrate-bearing structure in Lake Baikal

Early diagenetic carbonates are rare in Lake Baikal. Siderite (Fe carbonate) concretions in the sediments were discovered only recently. Here, we discuss the first finding of rhodochrosite concretions (Mn carbonate) discovered in the near-bottom sediments of the gas hydrate-bearing seepage structure St. Petersburg-2 in the deep water environment of the Central Baikal Basin. The crystal lattice of rhodochrosite contains iron and calcium substituting to manganese. Based on pore water geochemistry and of δ13C values of rhodochrosite (− 23.3 and − 29.4‰), carbon dioxide (+ 3.8 to − 16.1‰) and methane (− 63.2 to − 67.8‰), we show that carbonate crystallization most likely occurred during microbial anaerobic oxidation of organic matter, and that part of the oxygen making up the rhodochrosite seems to be derived from the 18O-rich water released from dissociating gas hydrates.

FORMATION PROCESS OF STRUCTURE I AND II GAS HYDRATES DISCOVERED IN KUKUY, LAKE BAIKAL

Structure I and II gas hydrates were observed in the same sediment cores of a mud volcano in the Kukuy Canyon, Lake Baikal. The sII gas hydrate contained about 13-15% of ethane, whereas the sI gas hydrate contained about 1-5% of ethane and placed beneath the sII gas hydrate. We measured isotopic composition of dissociation gas from both type gas hydrates and dissolved gas in pore water. We found that ethane δD of sI gas hydrate (from -196 to -211 ‰) was larger than that of sII (from -215 to -220 ‰), whereas methane δC, methane δD and ethane δD in both hydrate structures were almost the same. δC of methane and ethane in gas hydrate seemed several permil smaller than those in pore water. These results support the following idea that the current gas in pore water is not the source of these gas hydrates of both structures. Isotopic data also provide useful information how the “double structure” gas hydrates formed.