Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane
Edited by Jonathan J. Cole, Cary Institute of Ecosystem Studies, Avon, NC, and approved March 8, 2017 (received for review November 15, 2016)
Significance
Methane released from the seafloor and transported to the atmosphere has the potential to amplify global warming. At an arctic site characterized by high methane flux from the seafloor, we measured methane and carbon dioxide (CO2) exchange across the sea−air interface. We found that CO2 uptake in an area of elevated methane efflux was enhanced relative to surrounding waters, such that the negative radiative forcing effect (cooling) resulting from CO2 uptake overwhelmed the positive radiative forcing effect (warming) supported by methane output. Our work suggests physical mechanisms (e.g., upwelling) that transport methane to the surface may also transport nutrient-enriched water that supports enhanced primary production and CO2 drawdown. These areas of methane seepage may be net greenhouse gas sinks.
Abstract
Continued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 106 tons) of methane from thawing subsea permafrost on shallow continental shelves and dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO2) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous sea−air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (−33,300 ± 7,900 μmol m−2⋅d−1) twice that of surrounding waters and ∼1,900 times greater than the diffusive sea−air methane efflux (17.3 ± 4.8 μmol m−2⋅d−1). The negative radiative forcing expected from this CO2 uptake is up to 231 times greater than the positive radiative forcing from the methane emissions. Surface water characteristics (e.g., high dissolved oxygen, high pH, and enrichment of 13C in CO2) indicate that upwelling of cold, nutrient-rich water from near the seafloor accompanies methane emissions and stimulates CO2 consumption by photosynthesizing phytoplankton. These findings challenge the widely held perception that areas characterized by shallow-water methane seeps and/or strongly elevated sea−air methane flux always increase the global atmospheric greenhouse gas burden.
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Contemporary methane emissions from continental shelves to overlying oceans are estimated at 8 Tg to 65 Tg (1 Tg = 1012 g) annually (1), and will likely increase with future global warming. Inundation of high-latitude continental shelves accompanying sea level rise for the past ∼20 ka and superposed, shorter-lived warming events may be the triggers for ongoing methane release from thawing subsea permafrost, dissociating gas hydrate, and/or recent accumulations of microbial methane in newly warmed, organic-rich sediment (2–4). In deeper continental slope waters (200 m to 600 m) at high latitudes and midlatitudes, widespread methane seepage has been observed where warming intermediate ocean waters impinge on the shallowest extent of the gas hydrate stability zone (3, 5–9).
Methane emitted from seafloor seeps may exacerbate global warming if this potent greenhouse gas crosses the sea−air interface. However, most methane released at the seafloor may never reach the uppermost oceanic mixed layer due to a combination of gas exchange during the ascent of bubbles (10, 11), methane dissolution (12), and microbial oxidation of dissolved methane to carbon dioxide (13–15) (CO2). The exception is methane emitted from the seabed in shallow-water marine settings (<100 m water depth), of which a substantial fraction may enter the atmosphere (2, 16, 17). Studies at these shallow-water locations have taken on urgency given their wide geographic distribution and potential for large emissions (18). On the other hand, little attention has been given to CO2 exchange as a component of the net greenhouse gas flux balance at gas seepage sites. If CO2 efflux is enhanced at seepage sites, it will have a positive effect on radiative forcing. On the other hand, if CO2 is absorbed, it could mitigate the positive radiative effects from methane efflux.
In this study, we quantify methane and CO2 sea−air fluxes along the western Svalbard margin (WSM) (Fig. 1A) to determine the net global warming potential for these two important greenhouse gases. The study was conducted in summer when light availability was unlimited, thereby maximizing the potential for CO2 uptake by primary production (i.e., photosynthesis). The areas surveyed include a stable deep-water gas hydrate system (1,700 m to 2,600 m depth), a gas seepage area (240 m depth) suggested to be geologically linked to deeper climate-sensitive gas hydrate occurrences (5, 7, 19), a shallow-water (80 m to 90 m depth) active gas seep field along the ridge of a glacial moraine (19), and a nearshore (<110 m depth) coastal zone. These sites represent the full depth range of potential methane-emitting sites on high-latitude continental margins (3), including the potential for contributions from thawing subsea permafrost (4, 19).
Fig. 1.
Materials and Methods
In June 2014, we obtained a 1,600-km-long near-continuous record of methane and CO2 concentrations and carbon isotopes from surface water of the WSM aboard the R/V Helmer Hanssen (Fig. 1A). These data, in addition to methane and CO2 concentrations from the atmospheric marine boundary layer, were acquired with the US Geological Survey-Gas Analysis System (USGS-GAS; SI Appendix, Fig. S1). The USGS-GAS is a dual cavity ring-down spectrometer (CRDS) analytical system that constantly circulates gas from the headspace of a Weiss-type equilibrator through a Picarro G-2201i CRDS and sequentially measures air concentrations received from intakes set at three to four different elevations on the ship exterior with a Picarro G-2301f CRDS. Stable carbon isotope values measured with the G-2201i were calibrated against standard gases to obtain accurate δ13C values of surface water methane (±4‰ at 2 ppm; ±1.5‰ at 5 ppm) and CO2 (±1.5‰). Gradients in methane and CO2 concentrations were not detected in air samples collected at different elevations in the atmospheric marine boundary layer, so data from only one elevation (∼22 m above the sea surface) are reported here. The gas concentration data were combined with meteorological (wind speed, air temperature) and sea surface water environmental parameters (salinity, water temperature; SI Appendix, Fig. S2) and averaged at 30-s intervals to determine the flux of methane and CO2 across the sea−air interface (20) in shallow-water (Fig. 2) and deep-water (SI Appendix, Fig. S3) settings (21). To constrain biological activity in surface water, additional environmental parameters [dissolved oxygen (DO), pH, fluorescent dissolved organic matter (fDOM)] were measured in seawater pumped aboard the ship (SI Appendix, Table S4).
Fig. 2.
Surface water methane concentrations from 191 discrete water samples analyzed using the traditional gas chromatograph (GC) method and the USGS-GAS instrumentation were positively correlated (r2 = 0.86, P < 0.001) with slope of 0.99 (Fig. 3), which indicates excellent agreement between the analytical methods. The SD of the difference between the methods was 2.1 nM, with a small, but significant, 0.48 nM (P < 0.001) bias toward lower values measured by the USGS-GAS system (SI Appendix, Fig. S7A). To provide constraints on the subsurface methane distribution and its environmental controls, we also acquired dissolved methane concentration (Fig. 1B) and conductivity-temperature-depth (CTD) vertical profiles (Fig. 4).
Fig. 3.
Fig. 4.
Greenhouse Gas Dynamics
Methane Concentration and Fluxes on the WSM.
For the deep-water and shelf-edge systems (Fig. 1A and SI Appendix, Fig. S3), surface water methane concentrations ranged from 3.2 nM to 4.3 nM, corresponding to saturation anomalies of −1.5 to 36%. Sea−air fluxes ranged from 0.0 μmol⋅m−2⋅d−1 to 2.8 μmol⋅m−2⋅d−1 (Table 1 and SI Appendix, Fig. S3). The low saturation anomalies and fluxes for the 240-m water depth region are comparable to those in the open ocean (23) and are similar to those previously reported for this site (24), confirming that this setting is not a significant source of methane to the atmosphere.
Table 1.
| Setting | Survey area (km2) | CH4 flux (μmol·m–2·d–1) | CO2 flux (μmol·m–2·d–1) |
|---|---|---|---|
| Shallow shelf seep field | |||
| All | 150.5 | 3.8 ± 5.5 | –18,037 ± 8,464 |
| High flux* | 17.6 | 17.3 ± 4.8 | –33,317 ± 7,927 |
| Background | 132.9 | 2.0 ± 1.9 | –16,017 ± 6,152 |
| Nearshore coastal zone | 38.7 | 5.5 ± 6.5 | –24,944 ± 17,818 |
| Deep shelf seeps | 11.5 | 0.30 ± 0.26 | –2,166 ± 1,117 |
| Deep-water gas hydrate | 112 | 1.05 ± 0.61 | –42,001 ± 24,528 |
See SI Appendix, Table S1 for additional site summary details.
*
High flux defined as areas with CH4 flux > 10 μmol·m–2·d–1.
The highest surface water methane concentrations (Fig. 2A) and dissolved-phase fluxes (Fig. 2B) were detected at the shallow continental shelf site (“Shallow shelf” in Fig. 1A), where gas bubbles emanate from a seep field on a glacial moraine at 80 m to 90 m water depth (Fig. 1C) and dissolved methane is released from the adjacent nearshore coastal zone seafloor (<110 m water depth). The median flux from more than 7,000 averaged intervals (30 s) for the shallow shelf was 3.9 μmol⋅m−2⋅d−1, which is similar to a median value of 3.5 μmol⋅m−2⋅d−1 calculated from hydrocast samples collected from 10 m water depth (25). In the seep field, methane concentrations ranged from 3.4 nM to 10.8 nM (Fig. 2A), representing an 8 to 235% saturation anomaly and supporting a sea−air flux of 0.1 μmol⋅m−2⋅d−1 to 31.8 μmol⋅m−2⋅d−1 (Fig. 2B and Table 1). Where the diffusive sea−air methane flux exceeded 10 μmol⋅m−2⋅d−1 in the seep field (“high-flux” region in Table 1), values average 17.3 ± 4.8 μmol⋅m−2⋅d−1, almost 9 times greater than the background flux of 2.0 ± 1.9 μmol⋅m−2⋅d−1 (Table 1). Methane concentrations in the nearshore coastal zone ranged from 3.2 nM to 11.0 nM (Fig. 2A), with corresponding sea−air fluxes of 0.1 μmol⋅m−2⋅d−1 to 28.7 μmol⋅m−2⋅d−1 (Fig. 2B and Table 1).
Gridded and normalized to an area of 100 km2, the daily sea−air methane flux from each area ranged from 0.5 kg to 8.8 kg per 100 km2, with highest values in the nearshore (SI Appendix, Table S1). In the context of a well-constrained global atmospheric methane source (e.g., ruminants), the flux from the shallow-water continental shelf seep field [6.1 kg CH4 (100 km−2)⋅d−1, SI Appendix, Table S1] is equivalent to that from ∼320 sheep, each emitting 18.9 g CH4 d−1 (26). To match the methane output of the 3 × 107 sheep in New Zealand alone would require more than 90,000 multiseep clusters of the type investigated here. Although tens of thousands of discrete seeps likely remain undiscovered on global margins (8), there is no evidence that such a large number of multiseep clusters exists. Even if there were, the annual cumulative atmospheric methane flux would be ∼0.15 Tg CH4 y−1, a negligible (0.03%) quantity relative to the 580 Tg of methane emitted to the atmosphere annually (18).
WSM Fluxes Compared with Siberian Shelf Seas.
Our data show that shallow arctic methane seeps like those we investigated on the WSM emit negligible methane to the atmosphere. However, in comparison with the seeps we investigated, methane fluxes from the shallow East Siberian Arctic Shelf (ESAS), which may be underlain by thawing subsea permafrost, are ∼100 times greater. Shakhova et al. (2) report average fluxes of 229 μmol·m−2·d−1 for “background” areas and 738 μmol·m−2·d−1 from “hotspots” during the ice-free summertime. These values are comparable to an average ice-free flux for the middle and outer East Siberian Arctic shelf of 238 μmol·m−2·d−1, as measured by Thornton et al. (17). Given that (i) gas flares (evidence of seafloor gas ebullition) are prominent features on the WSM (ref. 15 and Fig. 1C) and ESAS (2), (ii) maximum bottom-water methane concentrations at the WSM seeps (Fig. 1B; ∼300 nM) are comparable to summertime bottom-water concentrations in ESAS hotspots (2), and (iii) WSM gas could also have a component derived from thawing subsea permafrost (4, 19), it is difficult to reconcile why the diffusive fluxes we report from the WSM differ so greatly from those of the ESAS. Methane may be more rapidly oxidized from the WSM water column (13, 24); however, a more likely explanation is that lateral transport of methane from the relatively small and narrow WSM shelf dilutes and disperses methane into the deeper ocean. Similar dispersion and dilution on the shallow, expansive ESAS is not possible, which could permit a greater fraction of methane released from the seafloor to transfer to the atmosphere.
CO2 Flux and Net Global Warming Potential Flux on the WSM.
Within the shallow-water gas seep field, pCO2 in the surface water was substantially less than in the surrounding area (Figs. 2 C and D and 5A) and correlates negatively with methane concentration (r2 = 0.61; SI Appendix, Fig. S4). These undersaturated pCO2 values support a CO2 influx rate of −33,300 ± 7,900 μmol⋅m−2⋅d−1 (Table 1), which is about twice that of the surrounding background area (−16,000 ± 6,000 μmol⋅m−2⋅d−1) and more than 1,900 times greater than the efflux of methane (17.3 ± 4.8 μmol⋅m−2⋅d−1). Taking into account the 25 times greater global warming potential of methane relative to CO2 for a 100-y timescale on a per unit mass basis (18), the strongly negative CO2 flux at the seep offsets the positive effect of methane expelled by a factor of 231 despite methane’s greater global warming potential. Even on a 25-y timescale, for which methane has stronger GWP of 84 (18), the cooling effect of CO2 uptake is 69 times greater than methane’s warming effect. Our comparisons consider only the dissolved phase gas fluxes. However, hydroacoustic imaging (Fig. 1C) and bubble modeling (SI Appendix, Fig. S8A) suggest minimal direct bubble transport to the atmosphere. Furthermore, a recent study from the ESAS suggesting that turbulence-driven diffusive methane flux (not ebullition) is the primary transport mechanism for sea−air methane flux (17) supports our assessment that bubble transport of methane to the atmosphere is not important at this setting.
Fig. 5.
Stimulation of CO2 Uptake over Shallow-Water Methane Seeps.
At least two processes could be responsible for the reduced concentrations of CO2 observed over the shallow-water methane seeps: (i) Methane bubbles ascending from the seafloor dissolve methane, strip CO2 from the water column, and transport this CO2 to the sea−air interface and release it to the atmosphere (12), or (ii) a physical and/or biological mechanism stimulates photosynthesis, and thus CO2 drawdown, above the seep area. To test the first hypothesis, we applied a numerical bubble-stripping model (12). Reproducing the low CO2 concentrations requires (i) bubble diameters of 14 mm, which is much larger than the most frequent diameter of ∼6 mm (range 2 mm to 16 mm) observed in the area (27), and (ii) a volumetric gas flux of 34 L⋅m−2⋅min−1 from the seabed at 90 m (∼13.6 mol/min, at 4 °C), compared with reported values of 3 mL⋅min−1 to 41 mL⋅min−1 per seep at 385 m (5.4 mmol/min to 74.5 mmol/min, at 4 °C) (19). Bubble stripping is therefore not a plausible mechanism for removing CO2.
The alternate hypothesis for lower surface-water pCO2 is that upwelling of cold, nutrient-rich water stimulated CO2 assimilation by phytoplankton, a phenomenon also observed in areas of strong upwelling associated with eastern boundary currents of major ocean basins (28). Surface water within the high-methane, low-CO2 seep area was 0.65 °C colder than the surrounding surface water (Figs. 4A and 5C and SI Appendix, Table S1), and the estimated δ13C of the seabed-sourced methane measured at the sea surface (−54.6‰; SI Appendix, Methane Isotopic Mass Balance for Determination of Seabed) was similar to that reported at the seafloor (29) and emanating from seeps downslope (19). We are therefore confident the cold and methane-rich surface water originated from near the seafloor close to the seep area. Furthermore, CO2 uptake rates we measured (2,200 μmol⋅m−2⋅d−1 to 42,000 μmol⋅m−2⋅d−1; Table 1) are comparable to primary production rates reported from nearby Kongsfjorden (30) (600 μmol⋅m−2⋅d−1 to 184,000 μmol⋅m−2⋅d−1), confirming the plausibility that phytoplankton-related processes altered the surface water CO2 budget. A possible subsurface manifestation of high surface productivity is that benthic chlorophyll and phaeopigment concentrations at this seep were the highest among nine stations investigated in the western Svalbard−Barents Sea region (31).
Upwelling on the WSM shelf is driven by Ekman transport during northerly or onshore wind events that can occur during any season (32). On a smaller scale, the topographically steered Spitsbergen Polar Current encountering the high-relief glacial moraine may upwell locally along steeply tilted isopycnals (Fig. 4D). Bubble-driven buoyancy and entrainment of bottom waters may also transport bottom water to the photic zone from depths as great as 1,000 m (33), a mechanism invoked to explain elevated surface-water chlorophyll above a Gulf of Mexico hydrocarbon seep (34). The relatively low seafloor methane flux at the WSM seep sites between 240 m and 385 m water depth (19) renders it unlikely that bubble-associated buoyancy caused the upwelling, supporting the assumption that physical oceanographic processes alone are responsible for upwelling, independent of the presence of gas seepage.
Regardless of the upwelling mechanism, multiple lines of evidence support the interpretation that primary production and consequent CO2 drawdown are enhanced where methane-charged bottom water emerges: (i) Chlorophyll-fluorescence, a proxy for photosynthesis, is elevated (Fig. 4E); (ii) DO, a product of photosynthesis, is ∼1 mg/L higher in surface waters with high methane and low CO2 concentrations (Fig. 5D); (iii) pH, which increases when CO2 is removed from solution by photosynthesis, is elevated by as much as 0.6 units compared with background (Fig. 5D); and (iv) δ13C−CO2, a metric that becomes more positive when algae preferentially remove 12CO2 during photosynthesis, is 13C-enriched (more positive) by as much as 2‰ within the upwelling area of methane-charged bottom water (Fig. 5B).
Similar, yet more pronounced, patterns of high methane, low CO2, and changes in water chemistry indicative of upwelling-induced photosynthesis were observed in the nearshore coastal zone (Fig. 5 and SI Appendix, Fig. S4). However, the coastal zone lacks pervasive discrete bubble-releasing methane seeps (Fig. 2A). Most methane in that region (up to 150 nM in bottom waters; Fig. 1B) likely originates from in situ production in organic-rich, anoxic sediment. Elevated methane in marine surface waters can also be a product of dimethylsulfoniopropionate demethylation (35), but the high bottom-water methane content and δ13C signature of the nearshore methane are most consistent with a sediment source.
Despite the spatiotemporal coincidence between high concentrations of methane and enhanced CO2 uptake at seeps on the WSM continental shelf (Fig. 5A) and in some other settings such as the Santa Barbara Basin seep field (36), we suggest that high methane concentrations are an indicator of, but not a necessary condition for, enhanced CO2 drawdown. Instead, the surface-water methane observed on the WSM is a chemical tracer for cold, nutrient-rich upwelled water that supports enhanced photosynthesis within the euphotic zone. A relationship of higher methane efflux and CO2 influx that correlated with colder surface waters was also observed near the >2,000-m deep-water gas hydrate site (SI Appendix, Fig. S5). This observation suggests enhanced CO2 drawdown is likely to occur whenever deep nutrient-rich (and perhaps methane-charged) waters are upwelled to the surface, and conditions for photosynthesis are suitable.
Methane seepage from high-latitude shallow continental margins is an atmospheric methane source (2) that could become more substantial as the climate continues to warm. Evidence that the cooling potential from CO2 influx at this shallow-water arctic methane seep overwhelms the greenhouse warming potential from the emitted methane suggests that methane seeps can nevertheless be net sinks for climate-forcing gases. If the sedimentary efflux of nutrients that support photosynthesis is related to methane discharge intensity from the seafloor, a positive feedback between accelerated methane release from the seafloor and amplified atmospheric warming may be offset by atmospheric CO2 drawdown. Further investigation of sea−air greenhouse gas fluxes at methane seep sites where upwelling-driven outputs are counteracted by photosynthetic CO2 drawdown (including light-limited wintertime conditions) would provide data to constrain which processes are responsible for enhanced CO2 uptake, quantify net greenhouse gas fluxes globally for shallow-water methane seepage areas, and determine if accelerated seafloor methane release will be offset by enhanced CO2 uptake at the sea−air interface in the future.
Data Availability
Data deposition: Archival data are available through the USGS ScienceBase-Catalog at https://doi.org/10.5066/F7M906V0.
Acknowledgments
E. Bergeron, E. Moore, and P. Bernard at the USGS contributed engineering and logistical expertise that led to the successful completion of this project. We thank Andrea Bodenbinder and Mario Veloso from GEOMAR for providing technical assistance. J.W.P., C.R., and M.C. were supported by the USGS and interagency agreements DE-FE0002911 and DE-FE0005806 with the US Department of Energy. A.S., J.M., and S.B., as well as R/V Helmer Hanssen cruise costs, were supported by University of Tromsø − The Arctic University of Norway and the Research Council of Norway. The research is part of the Centre for Arctic Gas Hydrate, Environment and Climate and was supported by the Research Council of Norway through its Centres of Excellence funding scheme Grant 223259. Any use of trade names is for descriptive purposes and does not imply endorsement by the US government.
Supporting Information
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Data Availability
Data deposition: Archival data are available through the USGS ScienceBase-Catalog at https://doi.org/10.5066/F7M906V0.
Submission history
Published online: May 8, 2017
Published in issue: May 23, 2017
Keywords
Acknowledgments
E. Bergeron, E. Moore, and P. Bernard at the USGS contributed engineering and logistical expertise that led to the successful completion of this project. We thank Andrea Bodenbinder and Mario Veloso from GEOMAR for providing technical assistance. J.W.P., C.R., and M.C. were supported by the USGS and interagency agreements DE-FE0002911 and DE-FE0005806 with the US Department of Energy. A.S., J.M., and S.B., as well as R/V Helmer Hanssen cruise costs, were supported by University of Tromsø − The Arctic University of Norway and the Research Council of Norway. The research is part of the Centre for Arctic Gas Hydrate, Environment and Climate and was supported by the Research Council of Norway through its Centres of Excellence funding scheme Grant 223259. Any use of trade names is for descriptive purposes and does not imply endorsement by the US government.
Notes
This article is a PNAS Direct Submission.
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The authors declare no conflict of interest.
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