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.
Phosphorus-cycle disturbances during the Late Devonian anoxic events