Open AccessNear‐surface hydrocarbon anomalies in shelf sediments off Spitsbergen: Evidences for past seepages
Author(s) -
Knies Jochen,
Damm Ellen,
Gutt Julian,
Mann Ute,
Pinturier Laurence
Publication year - 2004
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2003gc000687
Subject(s) - methane , sediment , geology , hydrocarbon , adsorption , seawater , environmental chemistry , geochemistry , oceanography , geomorphology , chemistry , organic chemistry
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 (∼−65‰) 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 μm) and C 2+ (ethane through pentane) (up to 1724 ng/g wet <63 μm) 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 (−38–−60‰; ∼100–∼5300 ng/g wet <63 μm) and C 2 H 6 (−20–−36‰; ∼30–∼1700 ng/g wet <63 μm) 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.