218-6 Something Calcareous this Way Comes: Role of Mineralogy and Diagenesis in the Silurian-Devonian Klonk δ13C Event

Abstract:

The Silurian-Devonian transition (419.2 Ma) coincides with an up to 5‰ positive carbon isotope (δ¹³C) change in carbonates around the world, known as the Klonk Event. Though the Klonk Event is recorded globally, peak δ¹³C values vary by location up to 2.5‰, suggesting influence of water mass variations, differences in primary minerology, or post-depositional alteration. These local processes could decouple the recorded δ¹³C signal in carbonate sections from the global carbon cycle. Small differences in δ¹³C values aimed at calculating global organic carbon burial fluxes result in large changes in CO₂ or O₂ in isotope mass balance models. We utilize calcium isotope (δ^44/40Ca) and minor-element geochemistry (Sr/Ca) paired with δ¹³C measurements in the Appalachian Basin of West Virginia (WV) and the Arbuckle Mountains of Oklahoma (OK) to investigate the impact of local variation in carbonate mineralogy and diagenesis on the records. The WV section contains open marine to peritidal carbonate facies overlain by sandstone in a rapidly subsiding foreland basin, while the OK section contains open marine to shallow subtidal limestone and shale in an aulacogen with lower sedimentation rates. The WV section’s peak δ¹³C is 1.85‰ higher. The OK section has stable δ^44/40Ca (-1.03 to -0.88‰, n = 4), while the WV section demonstrates significant variation in δ^44/40Ca (-1.57 to -1.00‰, n = 14). Since WV stratigraphic δ^44/40Ca variation is too large to attribute to seawater changes on the timescale of the Klonk Event, diagenesis regimes (fluid- vs sediment-buffered), precipitation rate, or mineralogy are likely drivers. In contrast, lack of variation in δ^44/40Ca in OK suggests little mineralogic or diagenetic changes in this section. Local process shifts may explain the higher δ¹³C peak in WV, which may have contained a greater percentage of original aragonitic skeletal grains (higher δ¹³C and lower δ^44/40Ca) or more sediment-buffered diagenesis. Alternatively, lower δ¹³C peak value in OK could be a result of increased seawater-buffered diagenesis suggested by higher δ^44/40Ca and low Sr/Ca. The Silurian-Devonian δ^44/40Ca suggest that Klonk Event records are locally influenced by carbonate mineralogy and diagenesis, but still partially represent a primary signal of global organic carbon burial which coincides with the rapid spread and diversification of early land plants and a rise of atmospheric oxygen levels.

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