Methane seepage in the Triassic Calcareous Alps in Austria

Monospecific brachiopod mass occurrences are known from the Triassic Hallstatt facies of the Calcareous Alps in Austria for nearly two centuries, but their origin has never been satisfyingly explained. Our aim is to test the recent hypothesis that such deposits, especially when found in neptunian dikes, represent ancient methane-seep deposits. This hypothesis is based on recent findings of closely related brachiopods at ancient methane-seep deposits in Oregon, USA. Furthermore, present-day methane seeps in the Gulf of Mexico are typically associated with salt diapirism. Neptunian dikes in the Calcareous Alps are related to diapirism of underlying Permian salts and, therefore, we will focus on mass occurrences of the brachiopods Halorella, Halorelloidea and Sulcirostra that are found in such dikes. Our approach to test this hypothesis includes: 1) field work to assess the general geologic context and to collect samples, 2) petrographic, stable isotope, and biomarker techniques to detect a possible influence of methane seepage in the formation of the respective deposits, and 3) a new approach to test for a possible influence of methane seepage based on trace metal abundances. In a second line of research, we will investigate the adaptations of these brachiopods to their environment. Here we will focus on potential adaptations to cope with different fluid flow regimes and predation pressure, which will be assessed by comparing petrographic and geochemical proxies for seepage intensity with size and predation scar pattern among the brachiopods. The anticipated results will provide new insights into habitat diversity and rock-forming processes in the Austrian Calcareous Alps and the Tethys ocean in general, in particular in relation to salt diapirism. Furthermore, we will establish a new method to identify hydrocarbon seeps in carbonate settings where the classical carbon isotope approach is likely to fall short, which has the potential to augment the current scarcity of seep communities of Paleozoic age. From an evolutionary and paleoecologic point of new, the project will (1) provide new insight into the long-term evolutionary history of the deep-sea vent and seep fauna, (2) result in a better understanding of the adaptations of extinct taxa to these ecosystems and (3) help identifying the overall factors that favored fossil brachiopod mass occurrences.

The mode of life and feeding strategies of extinct animals become increasingly difficult to reconstruct the further back in time they have lived. Brachiopods at seafloor methane seeps are a good example for this they were abundant at these sites in the geologic past but are virtually absent from them in the present-day ocean. In the present project we have identified several new cases that are between 180 and 230 million years old. We have found them together with various bivalves (clams and mussels), some of which are related to bivalves known from geologically younger methane-seeps. At present, our findings seem to support our earlier results that the brachiopods preferred sites with little sulfide but much methane, whereas the bivalves preferred the opposite. In a long-term perspective, it appears that the bivalves have increased in diversity at these sites throughout Earths history, whereas the brachiopods have never been diverse. This is in marked contrast to the general pattern in the evolution of life in the oceans, where brachiopods were very abundant and diverse during the Paleozoic era but played only a subordinate role afterward. Thus it appears that the evolutionary history of brachiopods at methane-seeps is very closely tied to the peculiarities of this environment, rather than just mirroring the overall trends in the oceans.