A geochemical investigation of seafloor methane seepage at the landward limit of the hydrate stability zone offshore Western Svalbard

Abstract

A significant proportion of the world’s organic carbon is trapped in submarine methane hydrates. When ocean bottom waters warm, these hydrates may be destabilised, releasing gaseous methane into the surrounding sediments and potentially into the overlying water column and atmosphere. Increased atmospheric methane contributes to further warming as methane is a potent greenhouse gas. Release of methane from hydrate may have accompanied some paleoclimate warming events, but observations of hydrate destabilization due to current global warming remain unconfirmed. The discovery of more than 250 seafloor methane bubble plumes close to the limit of the gas hydrate stability zone offshore Western Svalbard has recently been linked to increases in bottom water temperature in this region over the past 30 years. To assess the source and fate of this methane, this thesis presents a geochemical study of hydrate, sediments, seawater, and gas in the vicinity of the seafloor methane seepage.
Analyses of the gas molecular and isotopic compositions reveals that hydrate-bound gas, free gas in shallow sediments, and gas bubbles entering the water column at seafloor seep sites all have the same source. The gas is thermogenic gas produced offshore that has migrated laterally to the continental slope and shelf region. Transport-reaction modelling of pore water chemistry shows that active anaerobic oxidation of methane in sediments is an effective barrier to release of methane into ocean bottom waters. However, small fractures and faults allow ~90% of the methane that enters near-surface sediments to bubble into the water column at localized seafloor seeps. Analyses of the methane distribution in the water column indicate that the methane in the bubbles rapidly dissolves in seawater, and is transported northwards at depth in the West Svalbard Current. As a result, there is limited vertical exchange of methane between deep and surface waters. Surface waters are nevertheless supersaturated due to isopycnal mixing with methane-rich waters from the shallow shelf onto the upper slope. Measurements of methane mixing ratios in air indicate that the sea to air methane flux offshore Western Svalbard does not make a significant contribution to the local atmospheric methane budget.
Sedimentary records of the ?13C-CH4 signature of benthic foraminifera provide evidence for intermittent methane seepage at the current limit of hydrate stability (~400 m water depth) over the last ~20,000 years. Although this is likely due to changes in hydrate stability as a result of changes in bottom water temperature, we find no evidence for this in the current data set.

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ORCID for Rachael James: orcid.org/0000-0001-7402-2315

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