THE ONSET OF GLACIATION ~8 MYRS PRIOR TO THE HIRNANTIAN

The end Ordovician mass extinction was coincident with the Ordovician glacial maximum and terminated one of the most significant periods of biological radiation in Earth history. Constraining the duration and pacing of glacial intervals prior to the Hirnantian glacial maximum is key to understanding the nuanced relationship between glaciation and the mass extinction. Indirect evidence has been accumulating which suggests that glaciation may have occurred >5 Myrs prior to the Hirnantian Stage. Here, we present stratigraphic and isotopic evidence that suggests the onset of glaciation began around the Sandbian-Katian Stage boundary (~453 Ma). A platform-to-basin transect in what is today West Virginia and Pennsylvania shows a shift in the sedimentation locus during the Guttenberg carbon isotope excursion (GICE). Prior to the GICE, the sedimentation locus is intermediate between the most basinal and landward sections. During the rising limb of the excursion, the locus shifts to the most basinal sections of the Taconic Foreland Basin while the rest of the excursion shows a landward shift in sedimentation. The duration of the excursion (~400 kyrs) and bidirectional shift in sedimentation suggests that sea level is best explained as glaciogenic rather than tectonic in origin. The isotopic compositions (δ⁷Li) of lithium in carbonates, a proxy for weathering intensity, also show a positive excursion, but are out of phase with the δ¹³C_carb whereby the positive shift in δ⁷Li predates the δ¹³C_carb. The initial rise in δ⁷Li is thought to represent a decrease in weathering congruency due to pre-glacial cooling, while the subsequent drop in δ⁷Li represents an increase in weathering intensity during deglaciation. The temporal relationship between δ⁷Li and δ¹³C_carb during the GICE is very similar to that observed during the Hirnantian. This suggests a common source of sea level change between the two intervals, namely glaciation.