Stable and Clumped Isotope Characterization of Authigenic Carbonates in Methane Cold Seep Environments

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ¹³C, δ¹⁸O, δ⁴⁴Ca, and Δ₄₇) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ₄₇-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ¹³C_(VPDB) values (−30 to −47‰), high δ⁴⁴Ca_(SW) values (−0.4 to −0.8‰), and Δ₄₇-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ¹³C_(VPDB) values (−18 to −58‰), lower δ⁴⁴Ca_(SW) values (−0.8 to −1.6‰) and a larger range in Δ₄₇-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ⁴⁴Ca_(SW) values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO₂ degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions

Gas seeps in Norwegian waters – distribution and mechanisms

Gas seeps and fluid-flow related seabed features are found over the entire Norwegian exclusive economic zone (EEZ). Multibeam water-column data from c. 136 000 km2 has revealed more than 5 000 gas seeps. Most of the gas seeps seem to have biogenic, thermogenic or mixed origin; some may be of abiotic origin. The spatial distribution of the gas seeps appears to correlate with: 1 – structural highs with associated faulting, exposing hydrocarbon reservoir rocks at or near the seabed; 2 – faults serving as conduits for fluid flow; 3 – settings where reservoir rocks overlain by less permeable cap rocks sub-crop at the seabed. Other mechanisms involve seepage around abandoned exploration wells, and possible abiotic gas generation from serpentinisation of ultramafic rocks near mid-oceanic ridges. The gas seeping from the Norwegian cold seeps is mostly methane and has, in many places, led to the formation of methane-derived authigenic carbonate crusts, which give evidence for either extensive gas seepage in the past or long-lived seepage. Chemosynthetic communities are commonly associated with cold seeps and may form special habitats together with the carbonate crusts. Methane seepage has been proposed to contribute significantly to the global carbon budget and may be associated with gas hydrates giving rise to potential geohazards. Gas seeps have been identified and spatially mapped as acoustic gas flares, using multibeam echosounder systems, which have the ability to record reflections from both the water column and the seabed. Water-column data have been recorded in the MAREANO seabed mapping program since 2010, covering an area of c. 262 000 km2 , with a data volume in the order of 210 Tb. The observations of extensive gas flares in the Norwegian EEZ are available to the scientific community and other users through a dedicated MAREANO data and web access system

Timescales of methane seepage on the Norwegian margin following collapse of the Scandinavian Ice Sheet

Gas hydrates stored on continental shelves are susceptible to dissociation triggered by environmental changes. Knowledge of the timescales of gas hydrate dissociation and subsequent methane release are critical in understanding the impact of marine gas hydrates on the ocean–atmosphere system. Here we report a methane efflux chronology from five sites, at depths of 220–400 m, in the southwest Barents and Norwegian seas where grounded ice sheets led to thickening of the gas hydrate stability zone during the last glaciation. The onset of methane release was coincident with deglaciation-induced pressure release and thinning of the hydrate stability zone. Methane efflux continued for 7–10 kyr, tracking hydrate stability changes controlled by relative sea-level rise, bottom water warming and fluid pathway evolution in response to changing stress fields. The protracted nature of seafloor methane emissions probably attenuated the impact of hydrate dissociation on the climate system

Strategy for Detection and High-Resolution Characterization of Authigenic Carbonate Cold Seep Habitats Using Ships and Autonomous Underwater Vehicles on Glacially Influenced Terrain

Cold seep habitats with authigenic carbonates and associated chemosynthetic communities in glacially influenced terrains constitute an important part of the benthic ecosystems, but they are difficult to detect in large-scale seabed surveys. The areas they occupy are normally small, and survey platforms and sensors allowing high-resolution spatial characterization are necessary. We have developed a cold seep habitat mapping strategy that involves both ship and autonomous underwater vehicle (AUV) as platforms for multibeam echosounder, synthetic aperture sonar (SAS) and a digital photo system. Water column data from the shipborne multibeam echosounder data are initially used to detect gas flares resulting from fluid flow from the seabed. The next phase involves mapping of flare areas by SAS, mounted on an AUV. This yields an acoustic image with a resolution up to 2 cm over a swath of c. 350 m, allowing detection of seep-related features on the seabed. The last phase involves digital photographing of the seabed, with the AUV moving close to the seabed, allowing recognition of bubble streams, seep-related features and giving a first order documentation of the fauna. The strategy was applied to a 3775 km2 large area on the continental shelf, northern Norway. This is a passive continental margin, with thick deposits of oil- and gas-bearing sedimentary rocks. Extensive faulting and tilting of layers provide potential conduits for fluid flow. The seabed is glacially influenced with a highly variable backscatter reflectivity. More than 200 gas flares have been identified, and a similar number of cold seep habitats have been characterized in high spatial detail. Two case studies are shown. In the first area, there is a close spatial relation between active gas seepage and carbonate crust fields. The second case study shows that carbonate crust fields are not necessarily spatially associated with currently active seeps, but represent dormant or formerly active gas expulsion. An important finding is that the bathymetric resolution of shipborne multibeam echosounders will often be too low to detect cold seep habitats. This means that a nested multi-resolution approach involving a multitude of platforms and sensors is required to provide the full pictur

Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea

Gas seepage through the seafloor into the water column is inferred based on acoustic mapping, video observations and geochemical analyses at multiple locations in the Viking Graben and Utsira High areas of the central North Sea. Flares in the Viking Graben occur both inside and along the periphery of a submarine melt water channel where pockmarks (up to 500 m in diameter) and methane-derived carbonate crusts are found on the seafloor, indicating focussing of fluid flow in the vicinity of the channel. The flares can be related to gas accumulations close to the seafloor as well as in Quaternary and deeper strata, observed as high-amplitude reflections on seismic data. Many palaeo-channels, which act as accumulation zones, are observed in the subsurface of both the Viking Graben and Utsira High areas. The deeper origin of gas is partially supported by results of isotope analyses of headspace gas collected from sediment samples of the Viking Graben, which show a mixed microbial/thermogenic origin whereas isotope data on free seeping gas in the Viking Graben indicate a predominantly microbial origin. Based on these lines of evidence, a structure-controlled fluid flow model is proposed whereby hydrocarbons migrate in limited amount from deep thermogenic reservoirs along faults, and these deep fluids are strongly diluted by microbial methane. Moreover, the existence of subsurface pockmarks at several stratigraphic levels indicates long-term fluid flow, interpreted to be caused by gas hydrate destabilisation and stress-related high overpressures

Image based quantitative comparisons indicate heightened megabenthos diversity and abundance at a site of weak hydrocarbon seepage in the southwestern Barents Sea

Background. High primary productivity in the midst of high toxicity defines hydrocarbon seeps; this feature usually results in significantly higher biomass, but in lower diversity communities at seeps rather than in the surrounding non-seep benthos. Qualitative estimates indicate that this dichotomy does not necessarily hold true in high latitude regions with respect to megafauna. Instead, high latitude seeps appear to function as local hotspots of both megafaunal diversity and abundance, although quantitative studies do not exist. In this study, we tested this hypothesis quantitatively by comparing georeferenced seafloor mosaics of a seep in the southwestern Barents Sea with the adjacent non-seep seafloor. Methods. Seafloor images of the Svanefjell seep site and the adjacent non seep-influenced background seabed in the southwestern Barents Sea were used to construct georeferenced mosaics. All megafauna were enumerated and mapped on these mosaics and comparisons of the communities at the seep site and the non-seep background site were compared. Sediment push cores were taken in order to assess the sediment geochemical environment. Results. Taxonomic richness and abundance were both considerably higher at the seep site than the non-seep location. However, taxa were fewer at the seep site compared to other seeps in the Barents Sea or the Arctic, which is likely due to the Svanefjell seep site exhibiting relatively low seepage rates (and correspondingly less chemosynthesis based primary production). Crusts of seep carbonates account for the higher diversity of the seep site compared to the background site, since most animals were either colonizing crust surfaces or using them for shelter or coverage. Our results indicate that seeps in northern latitudes can enhance local benthic diversity and this effect can take place even with weak seepage. Since crusts of seep carbonates account for most of the aggregating effect of sites experiencing moderate/weak seepage such as the study site, this means that the ability of seep sites to attract benthic species extends well beyond the life cycl

Image based quantitative comparisons indicate heightened megabenthos diversity and abundance at a site of weak hydrocarbon seepage in the southwestern Barents Sea

Background - High primary productivity in the midst of high toxicity defines hydrocarbon seeps; this feature usually results in significantly higher biomass, but in lower diversity communities at seeps rather than in the surrounding non-seep benthos. Qualitative estimates indicate that this dichotomy does not necessarily hold true in high latitude regions with respect to megafauna. Instead, high latitude seeps appear to function as local hotspots of both megafaunal diversity and abundance, although quantitative studies do not exist. In this study, we tested this hypothesis quantitatively by comparing georeferenced seafloor mosaics of a seep in the southwestern Barents Sea with the adjacent non-seep seafloor. Methods - Seafloor images of the Svanefjell seep site and the adjacent non seep-influenced background seabed in the southwestern Barents Sea were used to construct georeferenced mosaics. All megafauna were enumerated and mapped on these mosaics and comparisons of the communities at the seep site and the non-seep background site were compared. Sediment push cores were taken in order to assess the sediment geochemical environment. Results - Taxonomic richness and abundance were both considerably higher at the seep site than the non-seep location. However, taxa were fewer at the seep site compared to other seeps in the Barents Sea or the Arctic, which is likely due to the Svanefjell seep site exhibiting relatively low seepage rates (and correspondingly less chemosynthesis based primary production). Crusts of seep carbonates account for the higher diversity of the seep site compared to the background site, since most animals were either colonizing crust surfaces or using them for shelter or coverage. Our results indicate that seeps in northern latitudes can enhance local benthic diversity and this effect can take place even with weak seepage. Since crusts of seep carbonates account for most of the aggregating effect of sites experiencing moderate/weak seepage such as the study site, this means that the ability of seep sites to attract benthic species extends well beyond the life cycle of the seep itself, which has important implications for the larger marine ecosystem and its management policies