Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

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.

Carbon isotope and sequence stratigraphy of the upper Isachsen Formation on Axel Heiberg Island (Nunavut, Canada): High Arctic expression of oceanic anoxic event 1a in a deltaic environment

“…5A), corresponding to durations of ~100-420 k.y. and ~3.0-3.2 m.y., respectively (Malinverno et al, 2010;Beil et al, 2020). Estimated durations of CISs Ap4-Ap9 agree well among the different orbital chronologies (Table 1), resulting in overall consistent duration and sedimentation rate estimates for the Walker Island Member (Fig.…”

“…The shape of the δ 13 C org curve at Glacier Fiord shows a close similarity to those of biostratigraphically well-dated early Aptian-early late Aptian reference sections from low latitudes, which record isotopic changes in the reduced carbon reservoir (Figs. 5B, 5C; Herrle et al, 2015;Beil et al, 2020). Given this precise reproducibility of δ 13 C patterns between hemipelagic records from low latitudes and the Isachsen Formation, and the exceptionally low level of noise in the δ 13 C org record at Glacier Fiord, we consider ecological and taphonomic effects on the bulk δ 13 C org composition to be negligible, as discussed by Herrle et al (2015), Jarvis et al (2015), and Wagner et al (2018).…”

“…This stratigraphic framework provides the base for correlating our new δ 13 C org profile with other high-resolution Aptian reference sections from low latitudes. For correlation, we chose a δ 13 C carb record from the lower Aptian stratotype at Roquefort-La Bédoule, southern France (Flögel et al, 2010;Lorenzen et al, 2013;Moullade et al, 2015;Beil et al, 2020), and a δ 13 C carb composite curve compiled by Herrle et al (2015). This composite curve consists of δ 13 C carb records from the Apticore drilled at Cismon, northern Italy (Erba et al, 1999), and the Serre Chaitieu section in the Vocontian Basin, southern France (Herrle et al, 2004), which have been spliced based on common biostratigraphic datums (Herrle et al, 2015).…”

“…We develop floating chronologies for both δ 13 C carb reference curves (i.e., time relative to the onset of OAE 1a) based on available orbital chronologies (Huang et al, 2010;Malinverno et al, 2010;Ghirardi et al, 2014;Beil et al, 2020), which constrain the duration of individual Aptian CISs (Table 1). Assuming similar durations in the High Arctic, we calculate the depositional durations of the Rondon and Walker Island Members as well as sedimentation rates for the Glacier Fiord section.…”

“…These outcrops were revisited during a field campaign in 2014, and densely spaced samples were collected from the upper part of the Isachsen Formation (the uppermost Paterson Island, Rondon, and Walker Island Members). In this study, we present new highresolution δ 13 C org profiles, which provide a robust chemostratigraphic framework for OAE 1a in the sedimentary record of the High Arctic Sverdrup Basin and allow us to precisely correlate its different CISs with biostratigraphically well-dated reference curves of lower latitudes (Herrle et al, 2015;Beil et al, 2020). To assess potential impacts of local environmental factors (e.g., changes in the source and preservation state of OC) on bulk δ 13 C org composition, lipid biomarker data are presented.…”