Short-lived trace gases (DMS, isoprene, acetaldehyde and acetone) in the surface waters of the western Pacific and eastern AtlanticOceans
The exchange of trace gases between the surface ocean and the lower atmosphere has received increasing attention during the last years especially in view of ongoing global environmental changes such as eutrophication, warming of the ocean, ocean acidification etc.. The ocean has been identified as a huge reservoir of various climate relevant trace gases. However, the distributions and the pathways of the trace gases such as isoprene, acetone and acetaldehyde, in the surface seawater is poorly understood. Even consensus regarding the intensively studied dimethylsulphide (DMS) pathways continues to elude researchers to date. This thesis compiles different studies which contribute new insight into the fates of a group of short-lived, climate-relevant traces gases including DMS, isoprene, acetaldehyde and acetone in the surface layers of the Pacific and Atlantic Oceans: • The distribution of and the interactions between DMS, dimethylsulphoniopropionate (DMSP) and dimethylsulphoxide (DMSO) in surface seawater were examined in conjunction with the phytoplankton composition and methane along a northsouth transit in the western Pacific Ocean on board the R/V Sonne (TransBrom) from Tomakomai (Japan) to Townsville (Australia) from 9th to 24th 2009. A close link between DMSP and DMSO was found based on correlations between the two compounds and similar phytoplankton pigments, which were identified as presumably sources of both DMSP and DMSO. The detected DMSPp:DMSOp seemed to be typical for an oligotrophic tropical ocean and might indicate stress conditions for phytoplankton due to intensive solar radiation and nutrient limitation which in turn may have led to an accumulation of DMSO in the surface water of the western Pacific Ocean. It seems that DMSP and DMSO were more closely related to each other than to DMS. It was evident that different factors influence the DMS distribution, as underlined by the failure to identify phytoplankton groups as sources for DMS. Moreover, DMSP and DMSO were identified as possible substrates for methane production along the entire north-south transit, emphasizing the potential role of both compounds as precursors of a climate relevant trace gas. • The atmospheric distribution of DMS was also examined along the same northsouth transit in the western Pacific Ocean. DMS emitted from the western Pacific Ocean appeared to be influenced by storms events which were encountered along the same north-south transit. The distribution of the computed DMS flux differed from the distribution of measured atmospheric DMS concentrations, indicating that atmospheric transport processes governed the pattern of atmospheric DMS concentrations. The Langrangian FLEXPART model was used to examine the influence of transport processes on the distribution of DMS in the troposphere. Although the concentration of DMS in the surface ocean was not elevated above background levels, there were certain instances of elevated flux relating to the high winds from storm events. In regions of the cruise track influenced by convective processes, the amount of DMS (and likely its oxidation products) transported to the tropical tropopause layer, as computed by the model, was regionally significant. The modeled DMS concentrations with altitude were in general agreement with aircraft measurements. However, because DMS has never been considered as a source of sulphur to the upper troposphere/lower stratosphere, there are hardly measurements above 12 km. The actual amount of DMS crossing the tropopause could not be determined, but it is possible that one regionally and seasonally important source of sulphur to the persistent stratospheric sulphur layer is marine derived DMS. • Because DMS and isoprene have both been identified as potential cloud condensation nuclei precursors over the remote oceans, the distribution and the relationship between the two gases were studied in surface seawater along a north-south transit in the eastern Atlantic Ocean on board of R/V Polarstern (ANTXXV-1) from Bremerhaven (Germany) to Cape Town (South Africa) in November 2008. Positive and negative correlations between DMS and isoprene were found in different regions which extended over two-thirds of the transit. Additionally, DMS and isoprene showed a similar distribution pattern together with phytoplankton groups like dinoflagellates, haptophytes and chrysophytes when clustered by N:P, indicating a biological source of both compounds. However, DMS is known to be produced by bacteria rather than by phytoplankton. Thus, isoprene and DMSP were most likely produced by phytoplankton, which was reflected in the correlation between the two compounds dependent on the N:P ratio. The relationship between DMS and isoprene observed in oligotrophic to eutrophic regions might be based instead on microbial activities. It might be possible that bacteria exist which can produce both DMS and isoprene concurrently. However, this is highly speculative and needs to be further investigated. In addition, DMS:isoprene corresponded to the hydrographic regimes in the eastern Atlantic Ocean. Enhanced DMS concentrations occurred in upwelling regions, while isoprene showed elevated concentrations in oligotrophic regions. However, both compounds showed the highest concentrations in a spring bloom near South Africa, likely pointing to biological activities as the main source. The possibility that transport could be an important control on the isoprene distribution in the surface ocean was also determined. • In order to identify marine sources and sinks of acetaldehyde and acetone, incubation experiments were conducted with water samples from the Baltic Sea taken at the GEOMAR institute pier (Kiel Fjord) and samples from the eastern equatorial Atlantic Ocean taken during the research cruise MSM 18/3 on board of the R/V Maria S. Merian from Mindelo (Cape Verde Islands) to Libreville (Gabon) from 21st of June to 19th of July 2011. The effect of biological and chemical processes on the production and consumption of the two compounds was investigated under light and dark conditions. Acetaldehyde and acetone were consumed rather than produced in both oceanic regions. The effect of solar radiation was of minor importance for the production and degradation of acetone and acetaldehyde. Although, the consumption of acetone in biology treatments was slightly higher compared to the chemical treatments, no overarching differences could be determined between biology and chemistry samples, in general. However, there were hints that the biota was important for the production and consumption of both acetaldehyde and acetone, which needs to be further examined in detail. The measured production and degradation rates were variable among the individual experiments, pointing to a complex interaction between different sources and sink processes under varying environmental controls. That will be elucidated in future work with a detailed examination of the individual experiments.
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