Characterization of microbial activity in pockmark fields of the SW-Barents Sea

Abstract

15 16Multibeam bathymetry revealed the occurrence of numerous craterlike depressions, 17 so-called pockmarks, on the sea floor of the Hammerfest Basin and the Loppa High, 18 south-western Barents Sea. To investigate whether these pockmarks are related to 19 ongoing gas seepage, microbial processes associated with methane metabolism 20 were analyzed using geochemical, biogeochemical and microbiological techniques. 21Gravity cores were collected along transects crossing individual pockmarks, allowing 22 a direct comparison between different locations inside (assumed activity center), on 23 the rim, and outside of a pockmark (reference sites). Concentrations of hydrocarbons 24 in the sediment, particularly methane, were measured as headspace (free) gas, and 25 in the occluded and adsorbed gas fraction. Down to a depth of 2.6 m below sea floor 26 2 (mbsf) sulfate reduction rates were quantified by radiotracer incubations. 27Concentrations of dissolved sulfate in the porewater were determined as well. Neither 28 the sulfate profiles nor the gas measurements show any evidence of microbial activity 29 or active fluid venting. Methane concentrations and sulfate reduction rates were 30 extremely low or even below the detection limit. The results show that the observed 31 sediment structures are most likely paleo-pockmarks, their formation probably 32 occurred during the last deglaciation. 33 34 Introduction 35Pockmarks are craterlike depressions in the seabed that form due to expulsion of 36

Role of Faults in Hydrocarbon Leakage in the Hammerfest Basin, SW Barents Sea: Insights from Seismic Data and Numerical Modelling

“…Implicitly, fluctuating gas leakage in combination with variable gas hydrate stability conditions controlled by glacial loading, unloading and erosion point towards systematic hydrate growth and decay events each time a glaciation-deglaciation cycle takes place ( Figure 4). As shown by our model (Figure 7C,D), these dynamics explain the multiple fluid escape events, reported by the occurrence of pockmarks and mega pockmarks on the URU (Figures 4 and 5) and the present-day seabed ( Figure 5) in the study area [16] and elsewhere in the Barents Sea [15,84,85].…”

“…In addition, evidence of large seabed pockmarks in the study area ( Figure 5) and elsewhere in the Barents Sea [16,88], as well as offshore Norway [89], suggests that gas hydrates could have existed in the past elsewhere on the Barents Shelf beneath the grounded ice sheet. Such conditions could have prevailed in the Hammerfest Basin during the time of glaciations beneath the Barents Sea Ice Sheet [15,16,89,90] and could have resulted in rapid destabilization of gas hydrates following the ice retreat, resulting in numerous pockmarks on the seabed [16,85,91].…”

Sedimentary environments in the south-western Barents Sea during the last deglaciation and the Holocene: a case study outside the Ingøydjupet trough

“…Glacial unloading and rising seawater temperatures during the early deglaciation destabilized methane hydrate deposits, leading to free gas release through the seafloor and the formation of the pockmarks visible in the study area ( Fig. 2; Chand et al 2012;Nickel et al 2012;Pau et al 2014). During the deposition of Subunit IIc, Nonionellina labradorica reached peak abundance in all cores investigated, marking the base of Assemblage Zone B (Fig.…”

Deep-seated faults and hydrocarbon leakage in the Snøhvit Gas Field, Hammerfest Basin, Southwestern Barents Sea

“…In the special case of the Snøhvit Field, uplift and erosion has been invoked as the major factor for fluid leakage at the detriment of tectonics and other mechanisms (Cavanagh et al, 2006;Chand et al, 2014Chand et al, , 2008Laberg et al, 1998;Ostanin et al, 2012b Gabrielsen et al,1990;Nickel et al, 2012). N.B: The red line is the location of Figure 2.2…”