The challenges of quantifying the carbon stored in Arctic marine gas hydrate

Abstract

Highlights• The amount of carbon stored in hydrate below the Arctic Ocean remains uncertain• A function for the fluid flow that gives observed hydrate saturations is proposed• Arctic marine gas hydrates likely form by upwards-advection of carbon-rich fluids• Equivalent fluid flows of 0.02-0.04 cm yr -1 result in hydrate saturations of 5-10%The quantification of the carbon stored in gas hydrate (GH) bearing marine sediments still remains a challenge. Despite recent efforts to develop approaches to better estimate the GH inventory globally, these estimates are still highly unconstrained due to insufficient field data and poor understanding of the mechanisms fuelling the GH stability zone (GHSZ). Here we use geophysically-derived GH saturations to constraint estimates of model-derived Arctic marine GH inventory at present. We also estimate the potential carbon released from GH dissociation under a seabed warming of 2°C over 100 yr. We estimate an inventory ranging between 0.28-541 Gt of C, which upper bound results in average GH saturations of 0.25%. Our upper bound is mainly controlled by our imposed upwards carbon-rich fluid flow of 0.01 cm yr -1 and it is five times greater than the most recent estimate that only considers in-situ degradation of particulate organic carbon (POC). To obtain the seismically-inferred GH saturations of 5-10% offshore west of Svalbard and in the Beaufort Sea, an upwards advection of carbon-rich fluids equivalent to 0.02 to 0.04 cm yr -1 is required. This mechanism may be the most important source of carbon reaching the GHSZ in Arctic marine sediments. A 2°C seabed temperature increase over 100 yr may reduce the GH inventory by about 88.44% (0.7 Gt C) if POC is the only source, and by about 5.4% (29.7 Gt C) if the main source of carbon is the upwards advection of carbon-rich fluids.

Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

“…This difference is in turn about 10 times the upper boundary in the Arctic carbon inventory estimated by Marín‐Moreno et al . [] (i.e., ~541 Gt).…”

“…Estimates of the global and Arctic carbon inventory based on different approaches remain highly divergent [Biastoch et al, 2011;Kretschmer et al, 2015;Marín-Moreno et al, 2016;Moridis et al, 2013]. Observations from Vestnesa Ridge highlight the importance of geochemically and geothermally constrained field predictions of the GHSZ thickness.…”

Modeling fracture propagation and seafloor gas release during seafloor warming‐induced hydrate dissociation

“…These overpressures often exceed the minimum (horizontal) and maximum (vertical) confining pressures and would result in hydraulic fracturing [ Fracturing would enhance the permeability of the sediments and, by allowing the rapid release of fluids, dissipates overpressure. A common method to circumvent this problem is to artificially increase the sediment permeability [e.g., Thatcher et al, 2013;Marín-Moreno et al, 2016]. The argument is that in natural systems fluid will preferentially escape along fractures, so it makes sense to prescribe fracture permeability into the model from the start.…”

Gas Hydrate and Free Gas Concentrations in Two Sites inside the Chilean Margin (Itata and Valdivia Offshores)

“…The estimation of the global methane reservoir has decreased, while the knowledge about hydrate reservoirs have increased. Actually, few authors estimated 11,000 Gt of carbon in hydrate [5], while the last estimations are significantly lower [2,[6][7][8].…”