In Situ Temperature Measurements at the Svalbard Continental Margin: Implications for Gas Hydrate Dynamics

Abstract

During expedition MARIA S. MERIAN MSM57/2 to the Svalbard margin offshore Prins Karls Forland, the seafloor drill rig MARUM‐MeBo70 was used to assess the landward termination of the gas hydrate system in water depths between 340 and 446 m. The study region shows abundant seafloor gas vents, clustered at a water depth of ∼400 m. The sedimentary environment within the upper 100 m below seafloor (mbsf) is dominated by ice‐berg scours and glacial unconformities. Sediments cored included glacial diamictons and sheet‐sands interbedded with mud. Seismic data show a bottom simulating reflector terminating ∼30 km seaward in ∼760 m water depth before it reaches the theoretical limit of the gas hydrate stability zone (GHSZ) at the drilling transect. We present results of the first in situ temperature measurements conducted with MeBo70 down to 28 mbsf. The data yield temperature gradients between ∼38°C km−1 at the deepest site (446 m) and ∼41°C km−1 at a shallower drill site (390 m). These data constrain combined with in situ pore‐fluid data, sediment porosities, and thermal conductivities the dynamic evolution of the GHSZ during the past 70 years for which bottom water temperature records exist. Gas hydrate is not stable in the sediments at sites shallower than 390 m water depth at the time of acquisition (August 2016). Only at the drill site in 446 m water depth, favorable gas hydrate stability conditions are met (maximum vertical extent of ∼60 mbsf); however, coring did not encounter any gas hydrates.

Biogeochemical Consequences of Nonvertical Methane Transport in Sediment Offshore Northwestern Svalbard

“…Reverse polarity, high reflection amplitude, and chaotic seismic facies underneath suggest that the signal was the reflection of a gas‐charged horizon at about 20 m depth. This interpretation is consistent with drilling results (Riedel et al, 2018).…”

“…The presence of free gas within the GHSZ, as confirmed by gas bubble releases (visual observation at Peepers 5 and 6 [Figure S1] and PARASOUND reflections [Figure 4]) and the presence of a gas‐charged horizon at about 20 mbsf (geophysical reverse polarity [Figure 3]) provide evidence that sufficient free gas was present to form gas hydrate. Water temperature recording over a 2‐year period at the seafloor close to the peeper site directly prior to sampling (MASOX observatory; Berndt, Feseker, et al, 2014) showed that the site provided conditions for gas hydrate formation in the surface sediments from roughly April to mid‐June 2012, when temperatures below the gas hydrate stability limit were reached (+3.0°C at 400 m; Berndt, Feseker, et al, 2014; Riedel et al, 2018). After mid‐June, water temperatures increased above +3.0°C at the seafloor, which likely lead to a gradual reduction of the GHSZ within the top ~5 m of the surface sediment (Berndt, Feseker, et al, 2014).…”

“…The outer shelf and upper continental slope of northwestern Svalbard (80–415 m water depth) are characterized by widespread occurrence of methane gas venting from the seafloor, with strongest intensity near the 400 m isobath, which overlaps with the GHSZ (Riedel et al, 2018; Sahling et al, 2008; Westbrook et al, 2009). Waters at this depth are dominated by the northward‐flowing West Spitzbergen Current, which transports water from the North Atlantic to the Arctic Ocean.…”

“…Exact locations and water depths of sampling events of the present study are provided in Table 1. The MASOX area (394 m water depth) is at the upper limit of the GHSZ (~390 m; Riedel et al, 2018). The calculation of the limit is based on the summer month August, which coincides with the study period (late August/early September) (Riedel et al, 2018).…”

“…Over the past decade, several field studies reported methane release from the shallow seafloor of the Arctic Ocean (e.g., Mau et al, 2017; Shakhova et al, 2014, 2010; Thornton et al, 2016; Veloso‐Alarcón et al, 2019; Westbrook et al, 2009). Their sources, sinks, and potential connection to gas hydrate dissociation have been the focus of several follow‐up studies and discussions (Berndt, Dumke, et al, 2014; Berndt, Feseker, et al, 2014; Graves et al, 2017, 2015; James et al, 2016; Myhre et al, 2016; Platt et al, 2018; Riedel et al, 2018; Ruppel & Kessler, 2017; H. Sahling et al, 2014; Steinle et al, 2015; M Torres & Colwell, 2018; Wallmann et al, 2018). Different models have also been applied to investigate past, current, and future methane releases from the Arctic seafloor (Biastoch et al, 2011; Crémière et al, 2016; Marín‐Moreno et al, 2015, 2013; Reagan & Moridis, 2009; Thatcher et al, 2013).…”