Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

Abstract. 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 (δ¹³C) 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 δ¹³C 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 ～430？kyr for OAE1a and ～130？kyr for OAE2, (2)？an onset phase of ～390 and ～70？kyr, (3)？a peak phase of ～600 and ～90？kyr, (4)？a plateau phase of ～1340 and ～200？kyr, and (5)？a recovery phase of ～380 and ～440？kyr. The total duration of the positive δ¹³C 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)