270 / 2019-07-03 17:23:45

Deciphering the Cryptic Diagentic Alteration of Paleosol and Groundwater Carbonates

paleosol,carbonates,diagenesis,stable isotope

Reliability in the use of the stable-isotopic compositions of pedogenic and groundwater carbonates in paleoclimatic and paleoenvironmental reconstructions requires a comprehensive knowledge of the degree of postdepositional alteration (diagenesis) samples have experienced. In this study, electron-beam lithography (microtextures and carbonate mineral forms) of Oligocene-Miocene pedogenic and groundwater carbonates from the Oiyug Basin in south-central Tibet are thoroughly described by using scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDX) in order to identify and interpret diagenetic degrees of studied samples. SEM-EDX analyses allowed for the identification of ten major carbonate morphotypes that resemble carbonate accumulation through pedogenic, groundwater, and diagenetic processes occurring within the Oiyug Basin strata at the time of deposition and their burial. Oiyug Basin carbonate samples with interpreted diagenetically altered or unaltered microtextures were further evaluated for clumped isotopic paleothermometry (Δ47 temperature). We found that our interpreted pristine paleosol and groundwater carbonate samples yielded Δ47 temperatures of 14.8-18.1 °C, which are reasonably higher than the inferred mean annual temperature of 8.2 ±2.8 °C derived from leaf-margin analysis, and can be interpreted as primary crystallization temperatures of the Oiyug Basin pedogenic and ground carbonates. In contrast, two interpreted diagenetically altered carbonate samples yielded Δ47 temperatures of 36.4 °C and 45.5±3.5 °C. These abnormally higher Δ47 temperatures than expected values probably reflect carbonate crystallization/recrystallization at high temperatures in burial environment. Collectively, the results of this study indicate that detailed SEM micro-scale examination on carbonate forms and microtextures can be used to justify the presence of diagenetic alteration of paleosol and groundwater carbonates in the Oiyug Basin, and overall, Oiyug Basin samples likely retain their primary isotopic compositions and thus serve as a valid proxy for meteoric water composition during late Oligocene-Miocene deposition in the basin.