Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps

Abstract

The recent discovery of active methane venting along the US northern and mid-Atlantic margin represents a new source of global methane not previously accounted for in carbon budgets from this region. However, uncertainty remains as to the origin and history of methane seepage along this tectonically inactive passive margin. Here we present the first isotopic analyses of authigenic carbonates and methanotrophic deep-sea mussels, Bathymodiolus sp., and the first direct constraints on the timing of past methane emission, based on samples collected at the upper slope Baltimore Canyon (∼385 m water depth) and deepwater Norfolk (∼1600 m) seep fields within the area of newly-discovered venting.The authigenic carbonates at both sites were dominated by aragonite, with an average δ 13 C signature of −47h, a value consistent with microbially driven anaerobic oxidation of methane-rich fluids occurring at or near the sediment-water interface. Authigenic carbonate U and Sr isotope data further support the inference of carbonate precipitation from seawater-derived fluids rather than from formation fluids from deep aquifers. Carbonate stable and radiocarbon (δ 13 C and 13 C) isotope values from living Bathymodiolus sp. specimens are lighter than those of seawater dissolved inorganic carbon, highlighting the influence of fossil carbon from methane on carbonate precipitation. U-Th dates on authigenic carbonates suggest seepage at Baltimore Canyon between 14.7 ± 0.6 ka to 15.7 ± 1.6 ka, and at the Norfolk seep field between 1.0 ± 0.7 ka to 3.3 ± 1.3 ka, providing constraint on the longevity of methane efflux at these sites. The age of the brecciated authigenic carbonates and the occurrence of pockmarks at the Baltimore Canyon upper slope could suggest a link between sediment delivery during Pleistocene sea-level lowstand, accumulation of pore fluid overpressure from sediment compaction, and release of overpressure through subsequent venting. Calculations show that the Baltimore Canyon site probably has not been within the gas hydrate stability zone (GHSZ) in the past 20 ka, meaning that in-situ release of methane from dissociating gas hydrate cannot be sustaining the seep. We cannot rule out updip migration of methane from dissociation of gas hydrate that occurs farther down the slope as a source of the venting at Baltimore Canyon, but consider that the history of rapid sediment accumulation and overpressure may play a more important role in methane emissions at this site. Published by Elsevier B.V.

The interaction of climate change and methane hydrates

“…The large percentage of excess heat absorbed by the Atlantic Ocean over the past few decades [ Lee et al ., ; Levitus et al ., ] may imply greater dynamism for the GHSZ on upper slopes here than in other ocean basins and may also lead to more rapid downdip migration of the gas hydrate stability field [e.g., Brothers et al ., ]. Still, in situ gas hydrate dissociation has been ruled out as a gas source for one prominent seep cluster [ Prouty et al ., ] that may have been active starting after the LGM, and seepage at other upper slope locations would require updip migration of gas through permeable strata [ Brothers et al ., ; Skarke et al ., ]. In addition to seepage, certain erosional and other features have been described at the upper feather edge of gas hydrate stability on global margins [ Davies et al ., ; Pecher et al ., ].…”

Gas Hydrate Dissociation During Sea‐Level Highstand Inferred From U/Th Dating of Seep Carbonate From the South China Sea

“…gas hydrate is estimated to cover an area of about 55 km 2 today . Samples from water depths between 200 and 1,000 m represent sites with a possible link between hydrostatic pressure and hydrate dissociation (Çağatay et al, 2018;Crémière et al, 2013;Han et al, 2014;Himmler et al, 2016;Liebetrau et al, 2010Liebetrau et al, , 2014Mazzini et al, 2017;Prouty et al, 2016;Ruffine et al, 2013;Teichert et al, 2003;Tong et al, 2013;Yang, Chu, et al, 2018). 3. the average gas hydrate saturation estimated from pore water freshening analysis ranges from 45% to 55% of the total pore space we use 50% for calculation); 4.…”

“…Using a simple model, we calculate methane flux as follows: Table S4 for detail) and seep carbonates in this study (red and blue circles; Table S3). Samples from water depths greater than 1,000 m (Aharon et al, 1997;Bayon, Loncke, et al, 2009;Bayon et al, 2013Bayon et al, , 2015Çağatay et al, 2018;Crémière et al, 2013;Feng et al, 2010;Himmler et al, 2015Himmler et al, , 2019Kutterolf et al, 2008;Lalou et al, 1992;Liebetrau et al, 2010Liebetrau et al, , 2014Mazumdar et al, 2009;Prouty et al, 2016;Watanabe et al, 2008) represent sites with a weak link between hydrostatic pressure and hydrate dissociation, whereas samples from water depths less than 200 m represent sites were hydrate dissociation was not involved (Aharon et al, 1997;Crémière, Lepland, Chand, Sahy, Kirsimäe, et al, 2016;Feng et al, 2010). Samples from water depths between 200 and 1,000 m represent sites with a possible link between hydrostatic pressure and hydrate dissociation (Çağatay et al, 2018;Crémière et al, 2013;Han et al, 2014;Himmler et al, 2016;Liebetrau et al, 2010Liebetrau et al, , 2014Mazzini et al, 2017;Prouty et al, 2016;Ruffine et al, 2013;Teichert et al, 2003;Tong et al, 2013;Yang, Chu, et al, 2018).…”

Methane Seeps on the US Atlantic Margin and Their Potential Importance to Populations of the Commercially Valuable Deep-Sea Red Crab, Chaceon quinquedens

“…Methane seeps are often characterized by symbiont-bearing megafauna (e.g., tubeworms and mussels), which form biogenic habitats that support a variety of additional species (Sibuet and Olu, 1998;Tunnicliffe et al, 2003;Govenar, 2010). Until recently, only three areas of methane seepage were recognized on the US Atlantic margin: a dense mussel community near Baltimore Canyon observed in the early 1980s (B. Hecker, cited in Prouty et al, 2016), and chemosynthetic communities associated with the Blake Ridge (Paull et al, 1995;Van Dover et al, 2003) and Cape Fear salt diapirs (Brothers et al, 2013;Wagner et al, 2013). It was not until 2012 that the US Atlantic Margin was recognized for widespread methane seepage following the discovery of ∼570 gas plumes between Cape Hatteras and Georges Bank (Skarke et al, 2014).…”

“…Seeps north of Cape Hatteras are dominated by bathymodiolin mussels (Skarke et al, 2014;Quattrini et al, 2015;Ross et al, 2015;Prouty et al, 2016;Bourque et al, 2017;McVeigh et al, 2018;Coykendall et al, 2019), with Bathymodiolus childressi prevalent at Norfolk, Chincoteague, and Baltimore seeps (Coykendall et al, 2019). In addition to harboring methanotrophic endosymbionts, B. childressi is capable of filter feeding (Page et al, 1990;Pile and Young, 1999) and thus has the potential to use both photosynthetically and chemosyntheticallyderived food resources (Tyler et al, 2007).…”