Massive marine methane emissions from near-shore shallow coastal areas

Abstract

Methane is the second most important greenhouse gas contributing to climate warming. The open ocean is a minor source of methane to the atmosphere. We report intense methane emissions from the near-shore southern region of the North Sea characterized by the presence of extensive areas with gassy sediments. The average flux intensities (~130 μmol m−2 d−1) are one order of magnitude higher than values characteristic of continental shelves (~30 μmol m−2 d−1) and three orders of magnitude higher than values characteristic of the open ocean (~0.4 μmol m−2 d−1). The high methane concentrations (up to 1,128 nmol L−1) that sustain these fluxes are related to the shallow and well-mixed water column that allows an efficient transfer of methane from the seafloor to surface waters. This differs from deeper and stratified seep areas where there is a large decrease of methane between bottom and surface by microbial oxidation or physical transport. Shallow well-mixed continental shelves represent about 33% of the total continental shelf area, so that marine coastal methane emissions are probably under-estimated. Near-shore and shallow seep areas are hot spots of methane emission, and our data also suggest that emissions could increase in response to warming of surface waters.

Methane distribution and oxidation around the Lena Delta in summer 2013

“…The values for a stratified fjord in the Baltic Sea are comparable to those of the North Sea (Steinle et al, 2017). However, in the southern North Sea, which has a mixed water column, very high methane fluxes (> 200 µmol m 2 d −1 ) are reported, which are mainly related to organic-rich sediments (Borges et al, 2016). A summary study of European estuaries reported an average methane emission of 118 µmol m 2 d −1 (Upstill-Goddard and Barnes, 2016).…”

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…Unlike CO 2 , the most recent global estimate of estuarine CH 4 emissions is the highest because it accounts for the direct emissions of CH 4 from sediment to atmosphere (when intertidal areas are exposed) (Borges and Abril, 2011). However, published estuarine CH 4 emissions are most probably underestimated because they do not account for CH 4 ebullition and gas flaring, although emissions to the atmosphere of CH 4 originating from gassy sediments in coastal environments have been shown to be intense (Borges et al, 2016. Reported CO 2 and CH 4 emissions from rivers are also highly uncertain and the proposed values also span a considerable range.…”

Dissolved Methane Distribution in the Reloncaví Fjord and Adjacent Marine System During Austral Winter (41°–43° S)

“…In the majority of coastal systems, surface CH 4 is produced in the anoxic sediments of fjords and reaches the surface through upward transportation (Borges and Abril 2011;Borges et al 2016). However, this is not the case for the Reloncaví fjord system, as within the estuarine stations, the bottom waters and underlying sediment did not exhibit consistently higher CH 4 levels compared to surface waters (Fig.…”

“…The origin of autochthonous CH 4 is thought to be a result of in situ production via methanogenesis within anoxic sediments (Borges et al 2016), or alternatively within particles that act as anoxic micro-niches while suspended in the oxygenated water column (Ploug et al 1997). In the majority of coastal systems, surface CH 4 is produced in the anoxic sediments of fjords and reaches the surface through upward transportation (Borges and Abril 2011;Borges et al 2016).…”

“…Estuaries and continental shelves appear to be responsible for ca. 75% of the oceanic CH 4 emissions (Bange et al 1994;Borges and Abril 2011), due to the fact that these highly productive areas favor the benthic-pelagic coupling, and the underlying sediments emit large quantities of CH 4 (Borges et al 2016).…”