Effects of climate change on methane emissions from seafloor sediments in the Arctic Ocean: A review

Abstract

Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and seaair exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments.

The global methane budget 2000–2012

“…For example, bubble plumes of CH 4 from the seabed have been observed in the water column but not detected in the Arctic atmosphere (Westbrook et al, 2009;Fisher et al, 2011). A large part of the seabed CH 4 production and emission is oxidized in the water column and does not reach the atmosphere (James et al, 2016). There are several barriers preventing methane from being expelled to the atmosphere.…”

“…There are several barriers preventing methane from being expelled to the atmosphere. From the bottom to the top, gas hydrates and permafrost serve as a barrier to fluid and gas migration towards the seafloor (James et al, 2016). First, on centennial to millennium timescales, trapped gases may be released when permafrost is perturbed and cracks or through Pingolike features.…”

“…Third, another important mechanism stopping methane from reaching the ocean surface is the dissolution of bubbles into the ocean water. Although bubbling is the most efficient way to transfer methane from the seabed to the atmosphere, the fraction of bubbles actually reaching the atmosphere is very uncertain and critically depends on emission depths (< 100-200 m, McGinnis et al, 2015) and on the size of the bubbles (> 5-8 mm; James et al, 2016). Finally, surface oceans are aerobic and contribute to the oxidation of dissolved methane (USEPA, 2010a).…”

“…Summing biogenic, geological and hydrate emissions from oceans leads to a total oceanic methane emission of 14 Tg CH 4 yr −1 (range 5-25). Refining this estimate requires performing more in situ measurements of atmospheric and surface water methane concentrations and of bubbling areas and would require the development of process-based models for oceanic methane linking sediment production and oxidation, transport and transformation in the water column and atmospheric exchange (James et al, 2016).…”

Biogeochemical Consequences of Nonvertical Methane Transport in Sediment Offshore Northwestern Svalbard

“…Calculations suggest a high probability for increasing temperatures in both the shallow and deep ocean (Stocker et al, 2013), presenting a scenario where release of methane from marine reservoirs seems possible. However, while the first projections of temperature‐related methane releases from marine hydrates were rather drastic (Archer, 2007; Buffett & Archer, 2004; Krey et al, 2009), later studies revealed that hydrate dissociation stemming from temperature rise is slow, possibly delayed by centuries or even millennia (Biastoch et al, 2011; Kretschmer et al, 2015; Ruppel, 2011), potentially providing enough time for methanotrophic microbes in sediment and in the water column to oxidize methane before catastrophic amounts enter the atmosphere (James et al, 2016; Knittel & Boetius, 2009). Still, continued discoveries of methane seeps along the gas hydrate stability zone (GHSZ) of continental margins around the world (Baumberger et al, 2018; Skarke et al, 2014; Westbrook et al, 2009) and the expectation of numerous more seeps yet to be discovered (Boetius & Wenzhöfer, 2013) place renewed importance on questions about potential releases of methane from the seafloor (Ruppel & Kessler, 2017).…”