GEOCHEMICAL EVIDENCE FOR OCEAN DEOXYGENATION DURING THE CAMBRIAN END-PTEROCEPHALIID EXTINCTION AT LAWSON COVE RESERVOIR, UTAH

During the early portion of the Cambrian Period (542-521 million years ago), marine animals with skeletons appeared and their diversity greatly increased during an event called the Cambrian Explosion. However, after the Cambrian explosion, their diversity plateaued for ~40 million years until the Great Ordovician Biodiversification Event or GOBE. During this diversity plateau there were a series of extinction events followed by rapid diversification events. The cause of extinctions is an open question. Here we investigate one of these extinction events: the end-Pterocephaliid extinction.

We examined the end-Pterocephaliid biomere extinction at the Lawson Cove Reservoir locality in Utah to reconstruct the environmental changes that occurred during this event. To that end, we constructed a carbon isotope record to track changes in the carbon cycle and measured the concentration of several major and trace elements (Ca, Mg, Mn, Fe, Sr, P). We document a positive carbon isotope excursion associated with the end-Pterocephaliid extinction, in line with previous work from other locations. The baseline carbon isotopes are around 1‰ and rise to around 2‰ just after the extinction horizon. Whereas previous work shows a sharp negative excursion preceding the positive excursion, we do not find this within our data and suggest that this demonstrates the local variability of the carbon cycle. There are also increased manganese concentrations in and after the extinction interval that may suggest locally more reducing depositional conditions. The positive carbon isotope excursion along with the increased manganese concentrations potentially suggests increased organic carbon burial under reducing conditions. Similarity between geochemical records of the end-Pterocephaliid extinction and those of the earlier, well-studied end-Marjumiid extinction, suggest a common driver behind these events: marine deoxygenation. More broadly, the record of the end-Pterocephaliid extinction showcases the potential links between ocean deoxygenation, changes in the carbon cycle, and marine diversity that occurred following the Cambrian Explosion and before the Great Ordovician Biodiversification Event.