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To help constrain models involving the chemical stability and lifetime of gas clathrate hydrates exposed at the seafloor, dissolution rates of pure methane and carbon-dioxide hydrates were measured directly on the seafloor within the nominal pressure-temperature (P/T) range of the gas hydrate stability zone. Other natural boundary conditions included variable flow velocity and undersaturation of seawater with respect to the hydrate-forming species. Four cylindrical test specimens of pure, polycrystalline CH4 and CO2 hydrate were grown and fully compacted in the laboratory, then transferred by pressure vessel to the seafloor (1028 m depth), exposed to the deep ocean environment, and monitored for 27 hours using time-lapse and HDTV cameras. Video analysis showed diameter reductions at rates between 0.94 and 1.20 μm/s and between 9.0 and 10.6 · 10−2 μm/s for the CO2 and CH4 hydrates, respectively, corresponding to dissolution rates of 4.15 ± 0.5 mmol CO2/m2s and 0.37 ± 0.03 mmol CH4/m2s. The ratio of the dissolution rates fits a diffusive boundary layer model that incorporates relative gas solubilities appropriate to the field site, which implies that the kinetics of the dissolution of both hydrates is diffusion-controlled. The observed dissolution of several mm (CH4) or tens of mm (CO2) of hydrate from the sample surfaces per day has major implications for estimating the longevity of natural gas hydrate outcrops as well as for the possible roles of CO2 hydrates in marine carbon sequestration strategies. [Copyright &y& Elsevier]

This study examined the CO 2 system in the estuary of the Oder River, one of the largest rivers entering the Baltic Sea. Three measurable parameters describing the CO 2 system, namely total alkalinity (A T), total CO 2 (C T), and the partial pressure of CO 2 (pCO 2), were investigated together with dissolved oxygen, salinity (S), and temperature during two RV Oceania cruises, in May and November of 2016. Large spatial variabilities of A T (1771–2940 μmol kg−1) and C T (1676–2972 μmol kg−1) were determined along the S gradient between the open Baltic Sea and river mouth. In November, the relationships of A T –S and C T -S indicated conservative mixing whereas in May both were strongly affected by biomass production and calcium carbonate formation. The waters of the Oder were oversaturated with CO 2 compared to the atmosphere , irrespective of the season, with pCO 2 values of 1351 ± 42 μatm in May and 1120 ± 32 μatm in November. In the Szczecin Lagoon, however, pCO 2 levels dropped significantly, to 63 μatm, in May, accompanied by an O 2 saturation of up to 134% during the same period. The inverse correlation of pCO 2 and O 2 saturation indicated that the distribution of CO 2 and O 2 in the estuary at the time of sampling was controlled mostly by biological activity. The very large drop in the pCO 2 of the Szczecin Lagoon induced an extreme oversaturation of CaCO 3 that triggered mineral calcite precipitation. The mineral precipitation of carbonates in the lagoon may have accounted for as much as 40% of the C T depletion determined in May, with the remaining 60% attributed to the joint effect of net ecosystem production and CO 2 air/water gas exchange. • The CO 2 system in the Oder River Estuary was investigated for the first time. • OM production and remineralization affect the CO 2 system in the Oder River Estuary. • Extreme primary production may initiate mineral precipitation of calcite in high A T. • Estuarine processes may modify the riverine loads of A T and C T to the Baltic Sea. [ABSTRACT FROM AUTHOR]

Extremely intense bubble plumes, like the North Sea 22/4b blowout megaplume (defined as more than 10 6 L day −1 ), create very strong upwelling flows (>1 m s −1 ), which lead to detrainment of methane-enriched water, but leave direct bubble-mediated transport unaffected. Dissolved CH 4 depth profiles and atmospheric measurements during a fall 2011 survey of the 22/4b site suggest strong constraint of seabed CH 4 below the thermocline. Seabed bubbles were nearly pure CH 4 . The effect of the upwelling flow on the fate of bubble plume CH 4 was investigated with a numerical bubble-propagation model. The model considered different representative bubble plume size distributions, ϕ , and a global (total) megaplume bubble size distribution, Φ, synthesized from video survey data and ϕ from the literature. Simulations showed that none of the literature plumes or variations in the upwelling flow could constrain CH 4 sufficiently below the thermocline. Two new bubble megaplume processes were simulated, vortical bubble trapping (slow rise) and a hypothesized, enhanced bubble gas exchange, k BE , an enhancement factor applied to the normal bubble gas exchange rate, k B . The latter could arise from plume turbulence increasing bubble boundary-layer turbulence and thus its gas exchange. Observations could not be reproduced solely by slow rise, however, simulations with k BE ∼6 reproduced observational constraints, as could weaker k BE in conjunction with slow rise. Field validation of k BE is needed given its implications for the fate of megaplume CH 4 emissions (anthropogenic or natural) for stratified and unstratified conditions. k BE suggests marine CH 4 geologic contributions to the atmosphere from all but shallow waters primarily arises from bubble plumes that are less than megaplume size. [ABSTRACT FROM AUTHOR]

A fully automated measurement system for recording of the surface water CO 2 partial pressure, pCO 2 , was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH 4 . The ship commutes regularly at 2–3 day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea. The pCO 2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO 2 consumption in spring/summer caused pCO 2 that were by up to 300 μatm below the atmospheric level. In contrast, the pCO 2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO 2 -enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO 2 , an air–sea CO 2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO 2 with uptake rates ranging between 0.60 and 0.89 mol m − 2 yr − 1 . The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO 2 uptake. The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events. Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130 nmol L − 1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air–sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage. [ABSTRACT FROM AUTHOR]

Abstract: The surface water CO2 partial pressure, pCO2, was measured during nine cruises in the Gulf of Bothnia at different seasons between 1999 and 2010, with continuous recording during most of the cruises using a bubble-type equilibrator and infrared CO2 detection. The data were used to construct the mean seasonality of the pCO2 in characteristic sub-areas. The low pCO2 measured in spring/summer and the high pCO2 measured in fall/winter indicated that biological production, stratification, mineralization and deep mixing, controlled the seasonal pCO2 cycle in both the Bothnian Bay and the Bothnian Sea. To calculate CO2 exchange with the atmosphere, the gas exchange transfer velocity was computed as a function of wind speed according to . Based on the mean seasonal pCO2 distribution, CO2 fluxes were calculated, taking into account the differing wind conditions and the varying ice coverage. The resulting mean annual flux of +140mmolm−2 yr−1 in the Bothnian Bay indicated that this area is a weak source for atmospheric CO2. In contrast, the Bothnian Sea was found to act as a sink for atmospheric CO2, with a mean flux of −730mmolm−2 yr−1. [Copyright &y& Elsevier]

Air–sea gas fluxes are commonly estimated using wind-based parametrizations of the gas transfer velocity. However, neglecting gas exchange forcing mechanisms – other than wind speed – may lead to large uncertainties in the flux estimates and the carbon budgets, in particular, in heterogeneous environments such as marginal seas and coastal areas. In this study we investigated the impact of including relevant processes to the air–sea CO 2 flux parametrization for the Baltic Sea. We used six parametrizations of the gas transfer velocity to evaluate the effect of precipitation, water-side convection, and surfactants on the net CO 2 flux at regional and sub-regional scale. The differences both in the mean CO 2 fluxes and the integrated net fluxes were small between the different cases. However, the implications on the seasonal variability were shown to be significant. The inter-annual and spatial variability were also found to be associated with the forcing mechanisms evaluated in the study. In addition to wind, water-side convection was the most relevant parameter controlling the air–sea gas exchange at seasonal and inter-annual scales. The effect of precipitation and surfactants seemed negligible in terms of the inter-annual variability. The effect of water-side convection and surfactants resulted in a reduction of the downward fluxes, while precipitation was the only parameter that resulted in an enhancement of the net uptake in the Baltic Sea. • Variability of air–sea CO 2 fluxes is modulated by mechanisms other than wind speed. • Gas transfer velocity parameterizations are not adequate for coastal/marginal seas. • Water-side convection enhances air–sea fluxes during winter in the Baltic Sea. • Surfactants suppress CO 2 downward fluxes during summer. • Precipitation represents a net increase of the carbon uptake. [ABSTRACT FROM AUTHOR]

Both the frequency and intensity of drought events are expected to increase, with unresolved alterations to peatland methane cycling and the involved microbial communities. While existing studies have assessed drought effects via experimental approaches under controlled conditions, to our knowledge, no studies have examined the in-situ effects of natural drought in restored temperate fens. In this study, we used quantitative polymerase chain reaction (qPCR) and high throughput 16S rRNA gene amplicon sequencing of DNA and complementary DNA (cDNA) to determine the abundances and community structure of total and putatively active microbial communities following the 2018 European summer drought. Together with geochemical and methane flux data, we compared these results to a non-drought reference dataset. During drought, water level and methane flux rates decreased to a new recent minimum in both fens. This corresponded with pronounced shifts in porewater geochemistry. Microbial community composition in the drought year differed markedly, and was characterized by a greater relative and total abundance of aerobic methanotrophs, and, in one of the two sites, by a decrease in total methanogen abundance. In contrast to the non-drought reference years, type I methanotrophs were clearly more dominant than type II methanotrophs in both fens. cDNA sequencing confirmed the activity of type I methanotrophs during drought, with Methylomonaceae having the highest average relative abundance of bacterial cDNA transcripts. We show that changes in microbial community dynamics, porewater geochemistry, and ecosystem methane fluxes can be substantial following natural drought in restored fens, and provide the first in-situ evidence from a natural drought which suggests type I methanotroph populations are more active than type II methanotrophs in response to drought effects. Type I methanotrophs may represent a key microbial control over methane emissions in restored temperate fens subject to natural drought. • Drought decreased water level and ecosystem methane fluxes substantially. • Microbial community dynamics reflected drought conditions at the sites. • Methanogen gene abundances decreased in one of the two fens. • Proportion of type II methanotrophs was low and similar to a non-drought year. • Type I methanotrophs dominated the active bacterial community profile at both sites. [ABSTRACT FROM AUTHOR]