One year of continuous measurements constraining methane emissions from the Baltic Sea to the atmosphere using a ship of opportunity

Abstract

Methane and carbon dioxide were measured with an autonomous and continuous running system on a ferry line crossing the Baltic Sea on a 2–3 day interval from the Mecklenburg Bight to the Gulf of Finland in 2010. Surface methane saturations show great seasonal differences in shallow regions like the Mecklenburg Bight (103–507%) compared to deeper regions like the Gotland Basin (96–161%). The influence of controlling parameters like temperature, wind, mixing depth and processes like upwelling, mixing of the water column and sedimentary methane emissions on methane oversaturation and emission to the atmosphere are investigated. Upwelling was found to influence methane surface concentrations in the area of Gotland significantly during the summer period. In February 2010, an event of elevated methane concentrations in the surface water and water column of the Arkona Basin was observed, which could be linked to a wind-derived water level change as a potential triggering mechanism. The Baltic Sea is a source of methane to the atmosphere throughout the year, with highest fluxes during the winter season. Stratification was found to intensify the formation of a methane reservoir in deeper regions like Gulf of Finland or Bornholm Basin, which leads to long lasting elevated methane concentrations and enhanced methane fluxes, when mixed to the surface during mixed layer deepening in autumn and winter. Methane concentrations and fluxes from shallow regions like the Mecklenburg Bight are rather controlled by sedimentary production and consumption of methane, wind events and the change in temperature-dependent solubility of methane in the surface water. Methane fluxes vary significantly in shallow regions (e.g. Mecklenburg Bight) and regions with a temporal stratification (e.g. Bornholm Basin, Gulf of Finland). On the contrary, areas with a permanent stratification like the Gotland Basin show only small seasonal fluctuations in methane fluxes

Enhanced methane emissions from oil and gas exploration areas to the atmosphere – The central Bohai Sea

“…Around those selected oilfield-neighboring CH 4 hotspots, SSS was always higher than that in the background area (surveyed areas excluding those hotspots indicated in Section 3.2), while SST was higher than that in the background area in the colder months (November and May) and lower in the warmer months (July and August) ( Table 1). The oilfield-neighboring high CH 4 associated with abnormal SSS and SST indicated upwelling or vertical mixing of the water column, presumably driven by gas seepages and/or oil spills at the seafloor, similar to the upwelling-associated high CH 4 signals in surface water off Oregon (Rehder et al, 2002) and in the Baltic Sea (Gülzow et al, 2013). Since the average depth of the Bohai Sea is only 18 m, the water column provides a short conduit for the seafloor-released CH 4 to rise to the surface layer.…”

“…A secondary equilibrator was used to pre-equilibrate headspace gas as well as compensate for gas loss in the main equilibrator. Due to the slowness of CH 4 to equilibrate between the gaseous and the water phases, the analysis results are smoothed and delayed by about half an hour (Gülzow et al, 2011(Gülzow et al, , 2013. Hence, with the relatively slow ship speed of 6-10 knots in this study, the worst spatial uncertainty of the underway methane signals was 5 nautical miles.…”

“…In waters overlying continental shelves, however, the organic-rich sediment serves as a major source of CH 4 in the water column compared to net production in the mixed layer (Martens and Berner, 1974;Martens and Klump, 1980;Reeburgh, 2007). The shallow-water [CH 4 ] can also be elevated due to gas seepages and/or oil spills (Bernard et al, 1976;Reed and Kaplan, 1977;Rehder et al, 1998Rehder et al, , 2002Kessler et al, 2011;Gülzow et al, 2013) and advection of CH 4 -rich freshwaters (Sackett and Brooks, 1975;Cline et al, 1986;Zhang et al, 2008). Since CH 4 is oxidized to dissolved inorganic carbon via microbial oxidation processes (Sieburth et al, 1987;Jones, 1991;Reeburgh, 2007) and/or lost via sea-air gas exchange (Crutzen, 1991;Bange et al, 1994), determining [CH 4 ] in surface water with high spatiotemporal data coverage is critical in adding to our understanding of the marine CH 4 cycle and furthermore of the marine carbon cycle.…”

“…So far the impacts of marine oil and gas exploration on the sea-air outgassing of CH 4 are still poorly understood, due to the fact that determination of CH 4 concentration has long been limited to the gas chromatography (GC) method equipped with flame ionization detector (FID), based on which it is difficult for researchers to obtain sufficient spatiotemporal resolution of [CH 4 precise, and on-the-spot monitoring techniques for [CH 4 ] (e.g., Gülzow et al, 2011Gülzow et al, , 2013Yvon-Lewis et al, 2011).…”

“…The concentration of the gas sample is proportional to the area under the measured spectral features. So far WS-CRDS has been successfully used for the continuous measurement of CH 4 concentrations in the atmosphere (Crosson, 2008;Chen et al, 2010;Winderlich et al, 2010;Zang et al, 2013) and in surface seawater (Gülzow et al, 2011(Gülzow et al, , 2013Yvon-Lewis et al, 2011).…”