Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard

Abstract

Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

“…Methane concentrations were determined using a headspace technique as described previously [Berndt et al, 2014;Steinle et al, 2015]. Briefly, 20 mL of N 2 was added to each sample bag and mixed vigorously.…”

“…Seawater sampling sites are shown in Figure 1 and listed in supporting information Tables S1 and S2, and air sampling sites are shown in supporting information Figure S1. Bubble plumes were identified by shipboard sonar: a Simrad EK60 ''fishfinder '' sonar (38,120,and 200 kHz) and a 12 kHz Simrad EM122 sonar were used on the RRS James Clark Ross [Westbrook et al, 2009], and a PARASOUND sonar (18 kHz parametric echo sounder) was used on the RV Maria S. Marian [Berndt et al, 2014]. In 2012, an equilibrator system [G€ ulzow et al, 2011] was used to continuously measure surface water methane concentrations along the cruise track of RV Heincke during cruise He-387 (20 August 2012to 6 September 2012; the area covered by the cruise track is shown in Figure 1a [Sahling et al, 2014]).…”

“…Samples for methane analysis were collected immediately upon recovery of the rosette frame into 1 and 0.5 L triple-layer Evarex Barrier Bags (Oxford Nutrition, UK), which had been flushed twice with nitrogen [Berndt et al, 2014;Steinle et al, 2015]. Samples for methane isotopes were taken into 125 mL glass vials with crimp seal tops, and poisoned with 2 mL saturated mercuric chloride solution (MSM21/4, MASOX site only).…”

“…However, new sites of seafloor methane seepage continue to be discovered [e.g., R€ omer et al, 2014;Skarke et al, 2014], and recent studies suggest that sea to air methane fluxes at some locations may be far higher than previously thought [e.g., Shakhova et al, 2010a]. Seepage of methane from seafloor sediments in the Arctic Ocean has been linked to release from temperature-sensitive shallow marine sediment reservoirs, including permafrost and methane hydrate [Berndt et al, 2014;Ferre et al, 2012;Sahling et al, 2014;Shakhova et al, 2014;Westbrook et al, 2009]. As Earth's climate continues to warm [Intergovernmental Panel on Climate Change, 2013], there is a need to constrain the potential for feedback between seafloor and atmospheric methane systems.…”

“…The presence of hydrate reduces sediment permeability such that subsurface gas flow can be directed along the base of the hydrate layer, with seepage at the seafloor in waters shallower than the landward limit of the hydrate stability zone [Naudts et al, 2006;Schmale et al, 2011]. Destabilization of methane hydrate in seafloor sediments is proposed to have contributed to previous episodes of major climate change, including the Paleocene-Eocene thermal maximum [e.g., Dickens, 2011], and recently discovered methane emissions from the seafloor offshore western Svalbard may, at least in part, be related to hydrate dissociation linked to warming of bottom waters [Berndt et al, 2014;Biastoch et al, 2011;Reagan and Moridis, 2009;Thatcher et al, 2013;Westbrook et al, 2009].…”