Global and local climate change during a late Paleozoic glacial cycle revealed by conodont and sedimentary geochemistry from the Midcontinent, USA

Major changes in the Earth's climate may be tracked by glacio-eustasy, the global change in sea-level caused by the waxing and waning of continental ice volumes. Glacio-eustatic magnitudes are typically constrained by oxygen isotopes, but this proxy may be confounded by local variations in the oxygen isotopic composition of seawater. Here, we show that an influx of freshwater to the Midcontinent Sea during the late Paleozoic Ice Age lowered the oxygen isotopic composition of local seawater. Conodont oxygen and strontium isotope records from the Swope cyclothem of Kansas City, Missouri, USA reveal episodic influxes of freshwater to the Midcontinent Sea during the Pennsylvanian. Organic matter C:N ratios and carbon isotopes confirm these findings. Organic matter nitrogen isotopes suggest that these influxes of freshwater were associated with denitrification, likely caused by sub-oxic conditions. These freshwater influx events possibly resulted from increased tropical precipitation. The geochemical measurements inform an inverse model that we use to constrain the magnitude of the influx of freshwater. We use these estimates of the influx of freshwater, an approximated glacial to interglacial temperature curve and published values for the oxygen isotopic composition of freshwater to isolate the oxygen isotopic composition of seawater in the Midcontinent Sea. The resulting range suggests glacio-eustatic magnitudes of 100–108 m during the Pennsylvanian. Multi-parameter geochemical analysis and inverse modelling thus enable the separation of the influence of local precipitation and global glaciation.