Near‐surface hydrocarbon anomalies in shelf sediments off Spitsbergen: Evidences for past seepages

Abstract

Laurence PinturierTotalFinaElf Petroleum Norge AS, N-4000 Stavanger, Norway (laurence.pinturier@ep.total.no) [1] As global warming occurs, the dissociation of bound methane on Arctic shelves due to ocean current temperature changes may become a major contributor to the global methane budget, and thus contribute to strong positive climate feedback mechanisms. However, little is known about the magnitude and fate of methane emissions from shallow submarine sediments to the atmosphere in the peculiar area. In this paper, we present one of the first direct evidences for seepage on the northwestern Barents Sea shelf. By studying the molecular and isotopic signatures of low-molecular-weight hydrocarbons in seawater, near-surface sediment pore space and the sediment matrix at 26 locations, we provide a detailed view on the partitioning of gaseous hydrocarbons in the sediment-water interface off Spitsbergen. In the free gas phase, low concentration of methane ($28 ng/g wet Sediment) paired with constantly high isotopic values ($À65%) is consistent with high impact of methane oxidation on the isotopic composition. In contrast, high concentrations of adsorbed CH 4 (up to 5292 ng/g wet <63 mm) and C 2+ (ethane through pentane) (up to 1724 ng/g wet <63 mm) in the sediment matrix suggest that the adsorbed gas is reasonably well protected against microbial degradation. Moreover, the isotopic and molecular signatures of adsorbed CH 4 (À38-À60%; $100-$5300 ng/g wet <63 mm) and C 2 H 6 (À20-À36%; $30-$1700 ng/g wet <63 mm) indicate an integrated, but strongly varying signal of historic thermally derived hydrocarbon plumes plus in situ adsorbed gas of biological origin. This suggests a minimal exchange and unlike histories of free and adsorbed gas. The majority of gaseous hydrocarbons in the adsorbed pool may be the result of past seepages. Past upward flow of thermogenic gas and impregnation with mature hydrocarbons was particularly strong along tectonic lineaments. Active petroleum source rocks along the continental margin and migration of gaseous hydrocarbons at re-activated fault systems might explain the anomalies along these conduits.

Gas emissions at the continental margin west of Svalbard: mapping, sampling, and quantification

“…During a cruise in 2011 with the R/V James Clarke Ross, gas emissions were found at the Forlandet moraine complex (Wright, 2012), an area that, for simplicity, we call Area 1 in the following. Additional evidence for hydrocarbon seepage at the shelf was presented by Knies et al (2004), who discovered seep-typical sulfur-oxidizing bacterial mats using an ROV.…”

“…Gas emissions occur not only at the 396 m flares on the upper slope but also at the outer shelf at water depths up to 150 m (Westbrook et al, 2009). Typical hydrocarbon seep-related bacterial mats were observed at the shelf (Knies et al, 2004). Elevated bottom-water methane concentrations and the stable carbon isotope composition of methane in the water column indicate seepage at the shelf (Damm et al, 2005;Gentz et al, 2014).…”

Norwegian margin outer shelf cracking: a consequence of climate-induced gas hydrate dissociation?

“…More recently, echo sounder surveys provided evidence for gas venting on the outer shelf of the northern Norwegian ) and the W-Svalbard margin (Knies et al 2004) where hundreds of methane plumes were discovered (Westbrook et al 2009). The gas flares occur in water depth at \400 m that is close to the required depths for methane hydrate stability on the continental shelf.…”

Biological formation of ethane and propane in the deep marine subsurface

“…Earlier reports describe sediments offshore Peru (3) and Spitsbergen (4), from which similar mixtures of hydrocarbons could be released by treatment with hot solutions of phosphoric acid. In each case, the carbon-isotopic compositions and abundance ratios (C 1 ͞C 2ϩ ) led to reluctant suggestions that the gases must be of thermogenic origin and thus have migrated into the unconsolidated seafloor sediments: ''the [postulated] migration of C 2ϩ hydrocarbons.…”

“…. it remains speculative whether the detected hydrocarbon anomalies are related to reservoirs and͞or active source rocks'' (4). No mechanism for sorbing the putatively migrated hydrocarbons more strongly than indigenous microbial products has been offered.…”