ORBITAL FORCING, GLACIOEUSTACY, AND BIOTIC IMMIGRATION: EXPLORING LATE ORDOVICIAN CLIMATIC AND PALEOBIOGEOGRAPHIC PATTERNS IN EASTERN LAURENTIA

The Late Ordovician Period is renowned for ending with one of the “Big 5” mass extinctions (LOME), which is believed to be induced by extensive continental glaciation over the southern hemisphere. However, over the last several decades evidence has accumulated which indicates the presence of continental glaciation extending well into the Katian Stage, and potentially as early as the Darriwillian Stage. The rhythmic and laterally traceable alternation (nearly ~200km in some cases) between siliciclastics and carbonate packages in the 3rd order C5 depositional sequence (Late Katian) suggests an allocyclic influence on its deposition. Spectral analysis of geochemical data collected from a well preserved drill core from near Dayton, Ohio, indicates that these patterns may have been driven by orbitally induced climate cycles and their resulting impacts on glacial ice volume.

The C5 sequence also records a significant biotic immigration event, known as the Clarksville Phase of the Richmondian Invasion, during which a variety of extrabasinal taxa entered the Cincinnati Sea. Previous studies have shown that these taxa arrived within the transgressive systems tract of the 4th order C5C sequence; likely within the first thousand years of the transgression. This incursion occurred approximately synchronously over a broad area from the Cincinnati arch (OH, IN, KY) as well as the Nashville Dome in Tennessee to Alabama. We propose that the mechanism for this invader introduction was glacioeustatic transgression driven by orbitally induced climate change. Additionally, we propose that Milankovitch effects played a crucial role in regulating Late Ordovician glacial ice volume. This regulation, in turn, exerted strong controls on biotic immigration patterns by initiating and terminating dispersal routes, regulating propagule pressure, and producing climatic shifts that enabled organisms from bordering provinces to expand their ranges.