Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system Stephen Punshon a, ⁎ , Kumiko Azetsu-Scott a , Owen Sherwood b,c , Evan N. Edinger d a Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, Nova Scotia B2Y4A2, Canada b Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA c Department of Earth Sciences, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia B3H4J1, Canada d Department of Geography and Department of Biology, Memorial University of Newfoundland, St. John's, Newfoundland A1B3X9, Canada ARTICLE INFO Keywords: Methane Seepages Carbon cycle Carbon dioxide Shelf seas Carbon isotopes ABSTRACT Measurements of dissolved methane (CH 4 ) 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 δ 13 C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO 2 , 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, ara- gonite 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 re- vealed 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. 1. Introduction Methane (CH 4 ) is a powerful atmospheric greenhouse gas with the second largest contribution to radiative forcing after carbon dioxide (Forster et al., 2007). Sea floor methane seeps are widespread along continental margins, often giving rise to fluid release structures such as mud volcanoes (Paull et al., 2015), pockmarks (Hovland et al., 1984; Hovland et al., 2010) and carbonate chimneys (Bayon et al., 2013). Sedimentary methane originates from microbial and thermogenic sources; microbial methane production or methanogenesis is the anaerobic decomposition of organic matter in shallow sediments by Archaea using carbon as a terminal electron acceptor, whereas ther- mogenic methane can be produced from the breakdown of con- temporary organic matter at temperatures above about 110 °C and from the action of heat and pressure on fossil carbon at depths of several kilometers within the sediments (Archer, 2007). Under conditions of low temperature and/or high pressure both microbial and thermogenic methane seepage may result in the accumulation of gas hydrate or clathrate, a solid ice-like material where methane is incorporated within a cage of water molecules (Hester and Brewer, 2009), and it has been estimated that about 900 Gt C is stored as gas hydrate north of 60° (Biastoch et al., 2011). The concern that rising temperatures in the Arctic may destabilise hydrate deposits, either terrestrial or marine, causing a large scale release of methane to the overlying water column and atmosphere, has prompted a great deal of recent interest in high latitude methane seeps (Shakhova et al., 2010; Berndt et al., 2014). More recently, a review by Ruppel and Kessler (2017) concluded that much of the methane released from decomposing marine gas hydrates never reaches the atmosphere, but is converted through microbial oxidation to carbon dioxide. Nonetheless, this finding implies that in- creasing methane seep activity in polar seas can contribute to the effects of ocean acidification with potentially negative consequences for a https://doi.org/10.1016/j.marchem.2019.04.004 Received 14 January 2019; Received in revised form 28 March 2019; Accepted 7 April 2019 ⁎ Corresponding author. E-mail address: punshons@dfo-mpo.gc.ca (S. Punshon). Marine Chemistry xxx (xxxx) xxx–xxx 0304-4203/ Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved. Please cite this article as: Stephen Punshon, et al., Marine Chemistry, https://doi.org/10.1016/j.marchem.2019.04.004