EVOLUTION OF THE MARINE CARBON CYCLE DURING THE PALEOZOIC

The Paleozoic (Middle Cambrian-Pennsylvanian) carbon isotope curve shows an amount of variation that is transitional between the highly unsettled Neoproterozoic and the increasingly stable Mesozoic to Cenozoic periods. Large positive excursions were common during cool periods (e.g., Late Ordovician-Silurian, and Late Devonian-Early Mississippian) but rare during greenhouse climates. Cool periods ventilated the oceans and switched the ultimate limiting nutrient to P. This allowed for carbon isotope excursions during episodic organic carbon burial events sustained by positive feedbacks between productivity and anoxia. In contrast, periods of stability that lasted for tens of millions of years (e.g., Late Cambrian-Early Ordovician) are interpreted to reflect negative feedbacks on productivity in a nitrogen limited (low oceanic N/P) ocean in which anoxia led to increased denitrification. Suppression of N fixation, likely due to low levels of essential trace elements, is a requirement of N limitation. All of the major mass extinctions of the Paleozoic are associated with periods of volatility in the carbon isotope curve, and may potentially be explained in the context of the oceanographic changes that accompanied ventilation of the deep ocean.