Vegetation and hydrology stratification as proxies to estimate methane emission from tidal marshes

Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. Salinity is a useful proxy for tidal marsh CH4 emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes, where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by comparing emissions from four strata differing in hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m−2 h−1 ranked as S. alterniflora (1.2 ± 0.3) ≫ High-elevation J. roemerianus (0.4 ± 0.06) > Low-elevation J. roemerianus (0.3 ± 0.07) = S. patens (0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater depletion in the S. alterniflora stratum (61 ± 4%) than in the S. patens stratum (1 ± 9%) with the J. roemerianus strata falling in between. We attribute the high CH4 emissions in the S. alterniflora stratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH4 emissions in the S. patens stratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH4 emissions from the J. roemerianus strata were likely due to plant traits that favor CH4 oxidation over CH4 production. Hydrology and plant community composition have significant potential as proxies to estimate CH4 emissions at the site scale.

Stratifying by Vegetation and Hydrology Improves Tidal Marsh Methane Emission Accounting

Methane emissions must be directly measured or estimated using methods such as proxies when managing wetlands for greenhouse gas offset activities. Salinity is a useful proxy for tidal marsh CH4 emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by identifying four strata based on hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m-2 hr-1 ranked as S. alterniflora (1.2 ± 0.3) >> High-elevation J. roemerianus (0.4 ± 0.06) > Low-elevation J. roemerianus (0.3 ± 0.07) = S. patens (0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater in the S. alterniflora stratum (61 ± 4%) than in the S. patens stratum (1 ± 9%) with the J. roemerianus strata falling in between. We attribute the high CH4 emissions in the S. alterniflora stratum to sulfate depletion likely driven by limited connectivity to tidal waters. Low CH4 emissions in the S. patens stratum are attributed to lower water levels, higher levels of ferric iron, and shallow rooting depth. Moderate CH4 emissions from the J. roemerianus strata were likely due to plant traits that favor CH4 oxidation over CH4 production. We concluded that stratification by hydrology and plant community composition can be an effective proxy to estimate CH4 emissions at the site scale.

Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 1: Fractionation of sea-salt particles

Multiple year-round records of bulk and size-segregated composition of aerosol were obtained at the inland site of Concordia located at Dome C in East Antarctica. In parallel, sampling of acidic gases on denuder tubes was carried out to quantify the concentrations of HCl and HNO3 present in the gas phase. These time series are used to examine aerosol present over central Antarctica in terms of chloride depletion relative to sodium with respect to freshly emitted sea-salt aerosol as well as depletion of sulfate relative to sodium with respect to the composition of seawater. A depletion of chloride relative to sodium is observed over most of the year, reaching a maximum of ∼ 20 ng m−3 in spring when there are still large sea-salt amounts and acidic components start to recover. The role of acidic sulfur aerosol and nitric acid in replacing chloride from sea-salt particles is here discussed. HCl is found to be around twice more abundant than the amount of chloride lost by sea-salt aerosol, suggesting that either HCl is more efficiently transported to Concordia than sea-salt aerosol or re-emission from the snow pack over the Antarctic plateau represents an additional significant HCl source. The size-segregated composition of aerosol collected in winter (from 2006 to 2011) indicates a mean sulfate to sodium ratio of sea-salt aerosol present over central Antarctica of 0.16 ± 0.05, suggesting that, on average, the sea-ice and open-ocean emissions equally contribute to sea-salt aerosol load of the inland Antarctic atmosphere. The temporal variability of the sulfate depletion relative to sodium was examined at the light of air mass backward trajectories, showing an overall decreasing trend of the ratio (i.e., a stronger sulfate depletion relative to sodium) when air masses arriving at Dome C had traveled a longer time over sea ice than over open ocean. The findings are shown to be useful to discuss sea-salt ice records extracted at deep drilling sites located inland Antarctica.

Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) Part 1: Fractionation of sea-salt particles

Multiple year-round records of bulk and size-segregated composition of aerosol were obtained at the inland site of Concordia located at Dome C in East Antarctica. In parallel, sampling of acidic gases on denuder tubes was carried out to quantify the concentrations of HCl and HNO3 present in the gas phase. These time-series are used to examine aerosol present over central Antarctica in terms of chloride depletion relative to sodium with respect to freshly emitted sea-salt aerosol as well as depletion of sulfate relative to sodium with respect to the composition of seawater. A depletion of chloride relative to sodium is observed over most of the year, reaching a maximum of ~ 20 ng m−3 in spring when there are still large sea-salt amounts and acidic components start to recover. The role of acidic sulfur aerosol and nitric acid in replacing chloride from sea-salt particles is here discussed. HCl is found to be around twice more abundant than the amount of chloride lost by sea-salt aerosol, suggesting that either HCl is more efficiently transported to Concordia than sea-salt aerosol or reemission from the snow pack over the Antarctic plateau represents an additional significant HCl source. The size-segregated composition of aerosol collected in winter (from 2006 to 2011) indicates a mean sulfate to sodium ratio of sea-salt aerosol present over central Antarctica of 0.16 ± 0.05, suggesting that, on average, the sea-ice and open ocean emissions equally contribute to sea-salt aerosol load of the inland Antarctic atmosphere. The temporal variability of the sulfate depletion relative to sodium was examined at the light of air mass backward trajectories, showing an overall decreasing trend of the ratio (i.e. a stronger sulfate depletion relative to sodium) when air masses arriving at Dome C had travelled a longer time over sea-ice than over open-ocean. The findings are shown to be useful to discuss sea-salt ice records extracted at deep drilling sites located inland Antarctica.

Biogeochemical controls on biodegradation of MC252 crude oil in coastal wetland systems

The persistence and fate of MC252 oil as well as the presence of nutrients and their effect on biodegradation were studied in both an oil-contaminated marsh and a nearby oil-contaminated mangrove site near Port Fourchon, Louisiana as well as a second marsh site in Barataria Bay adjacent to Bay Jimmy. High-resolution nutrient profiles were obtained at different locations at each site during winter and summer using dialysis samplers. Areas of sulfate depletion were observed in the top 20 cm of dialysis samplers in the Fourchon marsh and mangrove sites. When sulfate depletion was observed, it was accompanied by an increase in the ammonia and/or phosphorous concentration signifying an anoxic zone in the sediment. This anoxic zone was found more frequently in summer sampling compared to winter sampling. Flux chamber were installed at each site to capture evolved CO2 to determine if carbon from the MC252 oil was being mineralized at each site. Carbon derived from oil and carbon derived from indigenous organic matter was differentiated using the stable carbon isotope ratio. d13C signatures of the bulk soil, root matter, and soil with roots removed were measured at depth to compare contaminated sites to nearby inland sites with little to no oil contamination. A d13C signature of −24.8 ± 0.8 ‰ was observed in the bulk soil of the contaminated shoreline site in the Port Fourchon Marsh while an adjacent uncontaminated inland bulk soil sample had a d13C signature of −17.3 ± 0.8 ‰. d13C signatures were measured for sample before and after being washed with a hexane acetone solvent which removed oil. The signature of the samples became less negative after being washed with the solvent. To supplement the field findings, a laboratory microcosm study was performed to observe the effects of adding nutrients to a wetland soil/water/oil mixture. The study focuses on 5 different treatments: natural attenuation, sulfate amended, ammonia amended, sulfate and ammonia amended, and a killed control. A second set of microcosm experiments were conducted in triplicate under aerobic conditions along with a kill control sample. There was no observed degradation in the anaerobic microcosm experiments for any of the treatments during the time of the study. Aerobic microcosm results are still pending. Of particular interest are petrogenic PAHs (i.e, C1-phenanthrenes, C2-dibenzothiophenes, and C3-dibenzothiophenes) since there is little information in the peer-reviewed literature on the effectiveness of biostimulation in marsh systems.