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doi:10.2204/iodp.proc.311.213.2010

Lithologic controls on gas hydrate distribution

During onboard analyses of the recovered core, it became evident that most of the gas hydrate recovered occurred in the coarser grained sandy turbidite sections (see the "Expedition 311 summary" chapter). Other forms of gas hydrate were recovered within veins and fractures, especially at the cold vent Site U1328, and will be discussed later in this report. Although previously observed on other expeditions (e.g., Ginsberg et al., 2000; Weinberger et al., 2005), and thus not fully unexpected, direct observation (visually and through IR imaging) of gas hydrate in sands on the Cascadia margin was systematically documented for the first time during Expedition 311. Furthermore, the relatively high amount of sand recovered at all sites (but especially Sites U1325 and U1326) was surprising compared to results from Leg 146.

The strongest evidence for the occurrence of gas hydrate within coarser grained sediment comes from IR imaging of recovered core segments, as described by Torres et al. (2008). Typically, 10–30 cm long whole-round segments were cut from the core and taken to the geochemistry laboratory for further analyses. Although historically the entire core section would have been mechanically squeezed after cleaning to collect pore water samples, the core section was first laid out and reimaged with an IR camera (Fig. F7). Cores with IR anomalies were further sampled for individual squeezing and pore water analyses. These analyses documented the preference for gas hydrate to be present in coarser grained sandy-to-silty turbidites, with only a very minor fraction present in fine-grained sediments. At Site U1325, the gas hydrate concentration is directly correlated to the sand content of the host sediment (Torres et al., 2008). However, Site U1326 samples contained a lower average gas hydrate concentration than what would be expected based on the amount of sand present in the sediments. This was interpreted by Torres et al. (2008) as indicative of an insufficient availability of methane to fully "charge" the sand to the maximum possible gas hydrate concentration (e.g., from reduced local in situ methane production rates).

In addition to the study by Torres et al. (2008), two other studies were conducted by Hashimoto and Minamizawa and Wang (2006) to analyze grain-size distribution at the Expedition 311 drill sites with linkages to gas hydrate occurrences. Wang (2006) also used soupy and mousselike sediment textures to further infer the presence of gas hydrate. A good correlation was found between the occurrence of inferred gas hydrate and the coarser grain fraction.

An interesting general question arises from observation of the strong lithologic control on gas hydrate occurring mainly in sands and the apparent deepening of the top of gas hydrate occurrence landward from the deformation front (as modeled by Malinverno et al., 2008): What defines the shallowest occurrence of gas hydrate? Is it the combination of in situ methane production, sedimentation, and methane advection rates or simply the limit in sand occurrence at a given depth?

Recently, Malinverno (2010) showed that if gas hydrate formation is inhibited in the small pores of fine-grained marine muds, microbial methane generated in these mud layers will stay in solution. This dissolved methane will instead be transported by diffusion into the coarser grained sand or silt layers, where it forms concentrated gas hydrate. As an example, sediments recovered at Site U1325 show a large abundance of sand and silt layers across the entire 300 m cored interval. An interval with a high abundance of sand layers occurs between 57 and 67 mbsf, where ~40 individual sand layers were identified (see "Lithostratigraphy" in the "Site U1325" chapter). However, the shallowest gas hydrate at Site U1325 was inferred from the resistivity logs to be at ~73 mbsf. The first sand layer with gas hydrate was identified from IR images and pore water chlorinity freshening at ~80 mbsf. Combining these observations with the models by Malinverno (2010) and Malinverno et al. (2008) shows that the top of gas hydrate is a factor of overall in situ methane production and sedimentation rates and is not necessarily related to the amount of sand present (or lacking) in the system. Advection is not necessarily a requirement to form the observed concentrations of gas hydrate. However, if methane advection is added to the scenario, the top of gas hydrate remains at the same depth if in situ methane production is reduced proportionally.

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