Sediment geochemical study of hydrocarbon seeps in Isfjorden and Mohnbukta: a comparison between western and eastern Spitsbergen, Svalbard

Abstract

Methane is the most widespread volatile hydrocarbon and one of the most potent greenhouse gases. Marine sediments form the largest methane reservoir, from where large quantities of methane from gas-saturated sediments are released into the sea water and into the shallow shelf seas, through the water column almost unaltered into the atmosphere. Craters on the sea floor known as pockmarks are often related to seepage of methane-rich fluids originating either from shallow (microbial) or deep (thermogenic) sources. This paper presents the geochemical characteristics of migrating hydrocarbon gas from selected pockmarks and surrounding seabed in Isfjorden and Mohnbukta in western and eastern Spitsbergen, respectively. Collected gas samples, including wet gas fraction, were analyzed for methane concentration and methane carbon stable isotope ratio. Mixtures of microbial and thermogenic methane, together with higher homologies were found in the pockmarks as well as in the adjacent undisturbed seafloor, suggesting that the seepage activity in both areas is currently on the same level. Although the methane concentration profiles and methane stable carbon isotope ratios in Isfjorden and Mohnbukta show signs of biological oxidation and elevated concentrations of ethane/propane indicate input from deeper thermogenic sources. The gas composition and methane carbon stable isotope ratio from Isfjorden and Mohnbukta areas show similar trends, most likely due to originating in the same Mesozoic organic-rich marine mudstone.