Stromatolitic carbonates from the Middle Miocene of the western Pannonian Basin reflect trace metal availability in microbial habitats during the Badenian Salinity Crisis

Stromatolitic carbonates of the Middle Miocene Oberpullendorf Basin (Austria) provide a great opportunity to study the evolution of microbial habitats under extreme environmental changes during the Badenian (Langhian and early Serravallian) Salinity Crisis. We here present the first geochemical data for Badenian stromatolites and show in a combined approach using major, trace element, and C - O isotope compositions obtained in individual stromatolitic carbonate laminae that short-term variations of palaeo-environmental conditions within the Oberpullendorf Basin coincide with individual microbialite morphologies. The studied carbonates were affected by both detrital contamination and post-depositional alteration pro-cesses to different degrees. While fluid-mobile elements show clear evidence for post-depositional alteration processes, the rare earth element and yttrium (REY) as well as bio-essential element (Fe, Mn, Co, Zn, Mo, W) compositions of the carbonates remained unaffected. Stromatolitic carbonates that are devoid of detrital contamination (< 300 ppm Al) show typical shale-normalized seawater-like rare earth element and yttrium (REYSN) patterns with positive LaSN, GdSN anomalies, super-chondritic Y/Ho ratios, and heavy over light REYSN enrichments in the lower stromatolite units. These features suggest an open ocean seawater influenced deposi-tional setting at the north-western margin of the Paratethys Sea. Stratigraphically upwards, pure stromatolitic carbonates show suppressed seawater-like REYSN signatures that argue for the development of a (semi)closed lagoon with restricted access to the open sea. Seawater-like REYSN patterns in the uppermost part resemble a reappearance of open marine environmental conditions. Interestingly, geochemical data of the upper part of the section contradict the ambient fossil record, showing the urge for future interdisciplinary approaches targeting the understanding and interplay of geochemistry, palaeontology, and geomicrobiology in modern and ancient microbial habitats. Enrichment factors of bio-essential trace elements that are either used as co-factors in met-alloenzymes or metal-activated enzymes in biochemical reactions can be directly linked to the reconstructed environmental conditions: Sufficient element availability is ensured during marine conditions in the lower and uppermost stratigraphic sections; in contrast, continuous decreasing element availability of these elements is directly related to the temporary development of a (semi)closed lagoon.

Automated summary

Stromatolitic carbonates from the Oberpullendorf Basin in Austria provide a valuable opportunity to study the evolution of microbial habitats during the Badenian Salinity Crisis. Geochemical data of the carbonates show that variations in environmental conditions within the basin coincide with microbialite morphologies. Post-depositional alteration processes affected the carbonate samples to different degrees.