MULTI-PROXY RECORD OF THE PENNSYLVANIAN-AGE SWOPE CYCLOTHEM: GLACIAL TO INTERGLACIAL CHANGES IN MIDCONTINENT SEAWATER CHEMISTRY

Cyclothem deposition during the Carboniferous-Permian resulted from eccentricity-paced changes in ice volume that fluctuated global sea level and seawater oxygen isotopic composition (δ¹⁸O). Chronicles of conodont δ¹⁸O_PO4 reconstructed at cm-scale resolution from Midcontinent USA cyclothems, however, reveal oscillations at a higher frequency than possible for glacioeustatic change. We suggest these fluctuations resulted from episodic influxes of fresh water on the basis of cm-scale records from the Missourian-age, Swope cyclothem of conodont δ¹⁸O_PO4 and strontium isotopic composition (⁸⁷Sr/⁸⁶Sr), as well as organic matter carbon:nitrogen (C/N) and nitrogen isotopic composition (δ¹⁵N). Decreases in δ¹⁸O_PO4 are coeval with increases in ⁸⁷Sr/⁸⁶Sr and C/N, indicating that contributions of δ¹⁸O-depleted freshwater lowered the δ¹⁸O of Midcontinent tropical seawater. ⁸⁷Sr/⁸⁶Sr and C/N values constrain a non-linear inverse model which estimates that 2 to 40% of the epicontinental Midcontinent Sea was fresh. The correlation of low-energy facies, high δ¹⁵N values, and fresh water maxima suggests that these influxes of terrigenous water stratified the water column, decreased oxygen, and stimulated denitrification. The influence of regional seawater δ¹⁸O variability on intra-cyclothem conodont δ¹⁸O_PO4 trends is more significant than previously thought, and challenges the use of these records for glacioeustatic estimates. When considered in combination with global climate models for the late Paleozoic that predict extremes in eccentricity-modulated precession of Earth’s rotational axis forced tropical precipitation, the geochemical trends suggest a precessional driver of tropical seawater chemistry that may eventually be used as an intra-cyclothem astrochronometer.