Stromatolitic carbonates formed in ancient microbial mats and provide unique geochemical archives to reconstruct palaeo-environments of microbial life. In this study, we report C-O isotopes and trace metal concentrations of Miocene stromatolites from the Oberpullendorf Basin (Austria) that were formed during the Badenian salinity crisis at the north-western coast of the Paratethys Sea. The combined approach of trace element data with C-O isotopes of individual stromatolite laminae is used to reconstruct short-term variations of palaeo-environmental conditions in microbial habitats.
Stromatolitic carbonates in the lower units show negative δ13Ccarb values and typical seawater-like shale-normalized rare earths and yttrium (REYSN) patterns with positive LaSN, GdSN anomalies, super-chondritic Y/Ho ratios, and heavy REYSN enrichments to light REYSN, indicating an open marine depositional setting. Stratigraphic upwards, stromatolitic carbonates show suppressed seawater-like REYSN signatures and increasing δ13Ccarb values that reflect the development of a restricted environment. Seawater-like REYSN patterns and homogenously distributed negative δ13Ccarb values in the stratigraphic uppermost part resemble the transition to fully marine environmental conditions again. Enrichment factors of bio-essential elements (Fe, Mn, Co, Zn, Mo, W) reflect sufficient element availability during marine conditions but continuous limitation during the development of the (semi)closed lagoon.
We show that this approach may provide the groundwork for a better understanding of the evolution and development of microbial metabolisms under severely different atmospheric-hydrospheric conditions on planet Earth and beyond.