A new system is presented that allows the continuous measurement of methane and carbon dioxide concentrations in surface waters autonomously using ships of opportunity. The analytical setup consists of a methane carbon dioxide-Analyzer (MCA, Los Gatos Research) joined to an established equilibrator setup. The analyzer uses off-axis integrated cavity output spectroscopy (ICOS) and combines a highly specific infrared band laser with a set of strongly reflective mirrors to obtain an effective laser path length of several kilometers. This allows detecting methane and carbon dioxide in the equilibrated gas phase with high precision (less than 0.1%) and frequency. The system was installed on the cargo ship Finnmaid (Finnlines) in November 2009, which commutes regularly in the Baltic Sea between Travemünde (Germany), Gdynia (Poland), and Helsinki (Finland). Methane concentrations of the equilibrated gas phase measured by gas chromatography and by the MCA during lab tests are in excellent agreement. The comparison of carbon dioxide data measured by the MCA system to CO 2 values gathered from the same type of equilibrator in combination with a LICOR CO 2 detector (Schneider et al. 1992) during onboard operation show concordant results. The time constant for the system in freshwater at room temperature was determined to be 676 s for CH 4 and 226 s for CO 2 . Additional performance tests are presented. First field results show large regional differences with remarkable features, especially in shallow regions, demonstrating the need for the high spatiotemporal data coverage provided by the instrument.
[1] The distribution of dissolved methane in the water column of the Baltic Sea was extensively investigated. A strong correlation between the vertical density stratification, the distribution of oxygen, hydrogen sulfide, and methane has been identified. A widespread release of methane from the seafloor is indicated by increasing methane concentrations with water depth. The deep basins in the central Baltic Sea show the strongest methane enrichments in stagnant anoxic water bodies (max. 1086 nM and 504 nM, respectively), with a pronounced decrease towards the pelagic redoxcline and slightly elevated surface water concentrations (saturation values of 206% and 120%, respectively). In general the more limnic basins in the northern part of the Baltic are characterized by lower water column methane concentrations and surface water saturation values close to the atmospheric equilibrium (between 106% and 116%). In contrast, the shallow Western Baltic Sea is characterized by high saturation values up to 746%.
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 occurring during the winter season. Stratification was found to promote 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 predominantly 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.
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
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