Abstract
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) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg∕Preact and Ntotal∕Preact) 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 Corg∕Preact and Ntotal∕Preact 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.
Highlights
The Cretaceous period was characterized by high atmospheric CO2 levels and temperatures and by the episodic deposition of sediments with extremely high organic carbon content, referred to as oceanic anoxic events (OAEs; e.g. Schlanger and Jenkyns, 1976; Jenkyns, 1980)
New high-resolution stable isotope and X-Ray fluorescence (XRF) scanner data were integrated with published records from Cores LB1 and LB3 in the South Provence Basin (Lorenzen et al, 2013; Moullade et al, 2015) and from Core SN◦4 in the Tarfaya Basin (Kuhnt et al, 2017; Beil et al, 2018) to contrast the temporal evolution of two of the most significant oceanic anoxic events: OAE1a and OAE2
The structure of the marine δ13C records suggests a similar evolution of the carbon cycle during both Oceanic anoxic events (OAEs), the duration of the individual phases differed substantially
Summary
The Cretaceous period was characterized by high atmospheric CO2 levels and temperatures and by the episodic deposition of sediments with extremely high organic carbon content, referred to as oceanic anoxic events (OAEs; e.g. Schlanger and Jenkyns, 1976; Jenkyns, 1980). Schlanger and Jenkyns, 1976; Jenkyns, 1980) These events represent prolonged and intense perturbations of the global carbon cycle S. Beil et al.: Cretaceous oceanic anoxic events events are characterized by positive carbon isotope (δ13C) excursions that have been related to globally enhanced rates of organic carbon burial Smaller-scale events of probably more regional extent were identified in several ocean basins (e.g. OAE1b, c, d, and OAE3). Another such event, the mid-Cenomanian Event (MCE; Coccioni and Galeotti, 2003), appears to represent a less intense precursor event of OAE2. In shelf areas of the global ocean, major sea level changes associated with the cycles Ce2.1 and Ce3 of Gale et al (2002) may have caused long-lasting hiatuses, obliterating evidence of the MCE
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