Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system

Highlights
• Evidence of sedimentary methane seepage at three sites along the high latitude eastern Canadian Shelf.

• Stable isotope measurements at one of the sites suggest methane originates from microbial methanogenesis.

• Near bottom spikes in pH seen at two sites suggest the presence of an unidentified proton acceptor in the seep fluid.

Abstract
Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

Punshon S., Azetsu-Scott K., Sherwood O. & Edinger E. N., in press. Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system. Marine Chemistry. Article (subscription required).

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