Reliability of micritic carbonates in recording well-preserved isotopic composition and implications for paleoelevation estimates in central Tibet

Paleoelevation and paleoclimate reconstructions based on carbonate proxies rely on the preservation of syndepositional surface water isotopic signatures. This study investigates the reliability of fine-grained carbonates (micrites) in recording well-preserved isotopic composition. Utilizing a suite of research tools, including petrography, X-ray diffraction mineralogy, stable isotopes, clumped isotope temperatures, isotope mixing, solid-state bond reordering modeling, and trace element concentrations, we examine the diagenetic history of late Oligocene micritic carbonates from the Nima Basin in central Tibet. Our findings reveal that the lacustrine carbonates at deposition were micritic and calcite-dominated. However, they underwent partial dolomitization through cryptic recrystallization during shallow burial at temperatures of ∼34–40 °C, likely influenced by groundwaters with abundant Mg2+ and trace elements distinct from surface waters. Subsequently, during the further burial at temperatures of ∼120–130 °C, vein calcites precipitated from thermal fluids. Concurrently, the calcite component of fluid-adjacent micrites underwent open-system recrystallization, recording temperatures of 100–130 °C, while the associated dolomite component remained minimally altered due to its higher resistance to dissolution. In contrast, micritic calcites unaffected by thermal fluids experienced partial solid-state bond reordering that elevated clumped isotope temperatures to 50–60 °C. This study underscores the complex diagenetic histories that micrites can undergo, emphasizing the need for caution when interpreting carbonate stable oxygen isotopes in paleoclimate and paleoelevation studies, especially in tectonically active regions. Finally, our analysis infers that the stable oxygen isotopic values of the calcitic soil carbonates in the Nima Basin are well-preserved and reflect moderately high paleoelevations of ∼3.3 km at 26–25 Ma. This suggests a >1 km elevation gain during the Neogene, highlighting subsurface geodynamic processes as additional drivers of surface uplift in central Tibet.