Sensitivity of lacustrine stromatolites to Cenozoic tectonic and climatic forcing in the southern Junggar Basin, NW China: New insights from mineralogical, stable and clumped isotope compositions

Abstract

Lacustrine stromatolites serve as high-resolution and sensitive archives of palaeoenvironmental settings and palaeoclimatic conditions. An exceptionally well-developed stromatolite bed was discovered within nearshore shallow lacustrine sediments of the Lower Miocene Taxihe Formation on the southern margin of the Junggar Basin, north of the Tian Shan Range. We investigated the morphology, mineralogy, clumped and traditional stable isotope geochemistry of the stromatolites by using a multi-method approach with the application of X-ray diffraction analysis, advanced mineral identification and characterization (AMICSCAN), a clumped isotope palaeothermometer and modelling. The overall positive excursions and covariance of δ18O (varying from +10.61‰ to −5.82‰ V-PDB) and δ13C (ranging between −1.27‰ and +2.08‰ V-PDB) values are interpreted as the result of deposition in a closed-basin system under local progressive evaporation conditions during an episode of aridification. Clumped isotope proxy records display a significant decrease in water δ18O values and a decline of lake water temperature from ~38 to ~32 °C, suggesting a shift towards more arid and relatively cooler conditions during the course of stromatolite accretion. This interpretation agrees with lake volume and water level decreases inferred from isotope modelling results. The Late Cenozoic aridification of the southern margin of the Junggar Basin is thus interpreted from a new and unique perspective to have prevailed since the Early Miocene, driven by renewed uplift of the northern Tian Shan Range. The investigated stromatolite records reflect a snapshot of the local palaeoclimatic and palaeoenvironmental evolution, and our work provides references for potential studies of future discoveries of lacustrine microbialites in northwestern China.