STRATIGRAPHICALLY-MEDIATED DIAGENETIC OVERPRINT OF EARLY TRIASSIC CARBON CYCLE BEHAVIOR IN THE GEORGETOWN CORE, THAYNES GROUP, WESTERN UNITED STATES

The Early Triassic reflects the prolonged scars of the end-Permian extinction, captured in depauperate marine communities, unusual carbonate records, and geochemical instability on par with the most extreme variability observed in the Phanerozoic. In particular, the Smithian-Spathian boundary (SSB) interval is characterized by a positive carbon isotopic excursion in both δ¹³C_carb and δ¹³C_org, concurrent with a major marine ecosystem reorganization and the resurgence of microbialite facies. While these δ¹³C records have been traditionally interpreted as capturing global carbon cycle behavior, recent studies have suggested that at least some Early Triassic δ¹³C excursions may reflect authigenic or early diagenetic influences. To examine these potential geochemical drivers, we examine the Georgetown Core (Idaho) using a coupled δ⁴⁴Ca, δ²⁶Mg, and trace metal framework.

While the δ¹³C record in the Georgetown Core broadly reflect that in other SSB sections, it coincides with substantial shifts in δ⁴⁴Ca, δ²⁶Mg, and trace metal compositions that cannot feasibly be interpreted as primary. Furthermore, these geochemical variations correspond predictably with lithology: the δ¹³C record is strongly modulated by variations in the extent of dolomitization, and the diagenetic styles recognized here – early marine, anoxic dolomitization, and meteoric – appear to be constrained within distinct sequence stratigraphic systems tracts. Although we capture a primary shift in local seawater DIC δ¹³C from the geochemistry of the most geochemically unaltered strata, from ~3‰ in the middle Smithian to ~5‰ in the early Spathian, the timing and pathway through which this occurs cannot be readily identified nor extrapolated globally. While the Georgetown Core captures the recognized δ¹³C patterns of the SSB interval, its geochemistry reflects the differential influence of early marine diagenesis, secondary anoxic dolomitization, and meteoric diagenesis, rather than secular seawater behavior. As such, we suggest that the Georgetown Core does not directly record exogenic carbon cycle evolution, and argue that there is a widespread need for the careful reconsideration of SSB – and more broadly, Early Triassic – geochemical records to examine potential local and diagenetic influences on sedimentary geochemistry.