Recovery on the rocks: Can we use carbon isotope variability to constrain the Early-Middle Triassic Boundary?
Date
2025
Authors
Lovett, Kascia
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Abstract
Following Earth’s largest mass extinction, the Early Triassic represents a time of continued environmental disruption that has been recorded in the rock record. Specifically, marine carbonate rocks of this time indicate large perturbations to the global carbon cycle. These perturbations can be utilized as a proxy to help identify the Early Triassic Stage in the sedimentary rock record. As the boundary between the Early and Middle Triassic has had issues with its biostratigraphy, this study aims to use chemostratigraphy to further constrain its dating as the North American fossil record is limited. The Union Wash Formation in the Inyo Mountains of eastern California is composed of marine sedimentary rocks of Early Triassic age deposited along the eastern Panthalassa Ocean coastline. It consists of deep water, low energy mixed carbonate and siliciclastic lithologies with three major lithologic units based on carbonate or siliciclastic dominance. The stratigraphically lowest unit features an initially high organic matter content with no fossils while the two upper units are each host to ammonoids from the Neopopanoceras haugi Zone indicating a late Spathian age. Stable isotope geochemistry indicates high variability in the δ¹³C values of carbonate sediments throughout the formation. This trend is like that found globally throughout the late Early Triassic but shows evidence of diagenetic overprinting of δ¹³Ccarb and δ18Ocarb. There is also a possible facies dependence resulting from local variations in the carbon cycle, with low δ¹³Ccarb values within siliciclastic-dominant lithologies and higher δ¹³Ccarb values within carbonate-dominant lithologies. The similarities between global isotopic data and the Union Wash Formation are compelling but the evidence of local control on δ¹³Ccarb values (e.g. facies dependence) and possible diagenetic overprinting needs further investigation.
Supervisor: Jon Husson
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Keywords
Early Triassic, age constraints, Union Wash Formation, Inyo Mountains, carbon cycle recovery, carbonate geochemistry, carbon isotope chemostratigraphy