Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

Oceanic anoxic events (OAEs) document major perturbations of the global carbon cycle with repercussions for the Earth's climate and ocean circulation that are relevant to understanding future climate trends. Here, we compare the onset and development of Cretaceous OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their carbon isotope (delta C-13) records, with long-lasting negative excursions preceding the onset of the main positive excursions, supporting the view that both OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. However, there are substantial differences, notably in the durations of individual phases within the delta C-13 positive excursions of both OAEs. Based on analysis of cyclic sediment variations, we estimate the duration of individual phases within OAE1a and OAE2. We identify (1) a precursor phase (negative excursion) lasting similar to 430 kyr for OAE1a and similar to 130 kyr for OAE2, (2) an onset phase of similar to 390 and similar to 70 kyr, (3) a peak phase of similar to 600 and similar to 90 kyr, (4) a plateau phase of similar to 1340 and similar to 200 kyr, and (5) a recovery phase of similar to 380 and similar to 440 kyr. The total duration of the positive delta C-13 excursion is estimated at 2700 kyr for OAE1a and 790 kyr for OAE2, and that of the main carbon accumulation phase is estimated at 980 and 180 kyr. The long-lasting peak, plateau and recovery phases imply fundamental changes in global nutrient cycles either (1) by submarine basalt-seawater interactions, (2) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge, or (3) through nutrient recycling from the marine sediment reservoir. We investigated the role of phosphorus in the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (C-org/P-react and N-total/P-react) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in C-org/P-react and N-total/P-react above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs.