Systematic analysis of sediments, gas seeps and sludge gas at Lake Neusiedl

Permanent gas seeps at Lake Neusiedl are indicated in literature, but not very well characterized in terms of their origin. This study aimed at analysing the source of the apparently permanent seeps and comparing their geochemical signature with sludge gas that is a common by-product of organic matter degradation in the lake sediments. The main research questions were if a thermogenic gas source exists, and if all gases of biogenic origin share a common source or can be distinguished by their organic-geochemical signature. Molecular and stable carbon and hydrogen isotope compositions were measured to analyse the origin of the gas. Methane was the only hydrocarbon that was detected, indicating the absence of a thermogenic contribution of higher hydrocarbon components. Stable hydrogen and carbon isotope signature of methane and CO¿ confirm a purely biogenic source for seep gas and sludge gas. A systematic difference in isotope composition between these two types could not be detected, so a common origin of all gases, presumably associated with organic matter degradation in the lake sediments, is indicated. It is still under question why some of the gas shows seem to occur permanently at the same spot, and if this observation holds true at all. No evidence was found that permanent shows are related to a shallow microbial reservoir associated with organic-rich strata underlying the recent lake sediments. Another point of investigation were systematic changes of mineralogy and bulk geochemical parameters across the W-E lake axis. Sediment samples along two profiles between Breitenbrunn and Podersdorf were analysed via X-ray diffraction, as well as for their total organic carbon (TOC) content and pH. The composition of the lake sediments is dominated by quartz, feldspar, clay minerals and varying carbonate minerals. Carbonate minerals present are calcite, low magnesium calcite, high magnesium calcite and protodolomite. The X-ray diffractograms showed broadened peaks for low magnesium calcite, and protodolomite. The formation of high magnesium calcite and protodolomite is currently only occurring in specific regions of the lake. The pH decreases towards the lake centre, while TOC increases accordingly, arguing for more favourable conditions for organic matter preservation.