Abstract:

 The lacustrine stromatolites from the Oligocene Yaxicuo Formation of the northern Tibetan Plateau display regular growth rhythms of dark (organicrich layer) and light (sedimentrich layer) lamination alternations with millimeter scale. The lamination morphologies exhibit dome and columnar shapes in a vertical section. In order to understand the origin and time length scale of a couplet of laminae in stromatolites, the stable oxygen and carbon isotopic compositions of individual lamina were analyzed by using laser microsampling technique. The analysis results showed that the dark lamina in stromatolites commonly has relatively higher δ18O and δ13C values than the light lamina. The δ18O and δ13C values display positive covariance, i.e., simultaneous enrichment in dark laminae and coeval depletion in light laminae. We believe that the cyclic fluctuations of isotopic signals among couplets of laminae were caused by seasonal climatic variations. We suggest that the couplets of laminae in lacustrine stromatolites indicate the origin of annual lamination. This study demonstrates the lamination in lacustrine stromatolites can provide us a kind of geologic material with highresolution to reconstruct terrestrial palaeoclimate and palaeoenvironment changes during the geologic history.

Key words: Key words: stromatolite; oxygen and carbon isotope; lamina origin; seasonal variation; lacustrine environment; Tibetan Plateau