Combined textural and stable isotopic data as proxies for the mid-Cretaceous paleoclimate: A case study of lacustrine stromatolites in the Gyeongsang Basin, SE Korea

Abstract

Microbial interference of the prevailing sedimentary dynamics during stromatolite growth, records vital biogeochemical information, which reflects the prevailing paleoclimatic conditions. This study documents the simultaneous geochemical and textural changes in mid-Cretaceous lacustrine stromatolites (Gyeongsang Basin, SE Korea), deposited in a semi arid setting. Fibrous calcite/micrite couplets in the stromatolites occurring in the Sinyangdong, Hwasan and Banyawol formations constitute the basic growth cycles and can be divided into first-, second and third-order cycles. Second-order cycles show couplets with gradual textural changes from fibrous calcite to micrite laminae, and three different types of micrite are present. Sometimes, one type of micrite evolves into another, within the second-order cycle in the Sinyangdong stromatolites. Consistent enrichments in δ 18O and depletion in δ 13C towards micrites within the Sinyangdong and Hwasan stromatolite couplets and a concomitant depletion of both isotopes from fibrous calcite to micrite in the Banyawol sample can be observed. These shifts reflect paleohydrological changes in the lake water, related to paleoproductivity and paleosalinity fluctuations. Relatively higher paleosalinity values within the Sinyangdong lake water system is registered by higher δ 18O values of fibrous calcite and micrite in the SY-3, than in the SY-1 and SY-2 stromatolites. The consistent shifts in δ 18O and δ 13C isotopes in different textural growth fabrics indicate that paleohydrological factors could play a significant role in the formation of these primary lacustrine carbonates. Textural changes shown by the frequency of the growth cycles also corresponds with oxygen and carbon isotopic values. These results appear to have been influenced by climate-related paleohydrological changes.