A new high-resolution Paleozoic δ13Ccarb curve from the Great Basin shows an amount of variation that appears 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. Some periods of stability in δ13Ccarb lasted for >107 yr and 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. In contrast, cool periods that ventilated the oceans switched the ultimate limiting nutrient to P and allowed for δ13C excursions, which signal episodic organic carbon burial that could be sustained by positive feedbacks between productivity and anoxia.

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