Unravelling the sources of carbon emissions at the onset of Oceanic Anoxic Event (OAE) 1a

Markus Adloff,Sarah E Greene,Ian J Parkinson,B David A Naafs,Will Preston,Andy Ridgwell,Dan J Lunt,José Manuel Castro Jiménez,Fanny M Monteiro

doi:10.1016/j.epsl.2019.115947

Abstract

The early Aptian Oceanic Anoxic Event (OAE) 1a represents a major perturbation of the Earth's climate system and in particular the carbon cycle, as evidenced by widespread preservation of organic matter in marine settings and a characteristic negative carbon isotopic excursion (CIE) at its onset, followed by a broad positive CIE. The contemporaneous emplacement of a large igneous province (LIP) is invoked as a trigger for OAE 1a (and OAEs in general), but this link and the ultimate source of the carbon perturbation at the onset of OAEs is still debated. In this study, we simultaneously assimilate an atmospheric pCO2 reconstruction along with a δ13C record from the Spanish Cau section in an Earth system model to obtain a novel transient reconstruction of emission rates and identify the primary carbon-emitting sources across the negative CIE interval at the onset of OAE 1a. We reconstruct carbon emissions of 4,300–29,200 Pg from a mixture of carbon sources. This estimate is a lower bound, as contemporaneous organic carbon burial is not accounted for. Carbon was first released at slow rates from a 13C-depleted reservoir (e.g. thermo- and/or biogenic methane from sill intrusions). Towards the end of the negative CIE the rate of emissions increased and they became more 13C-enriched, likely from a dominantly volcanic source (e.g. LIPs). New osmium isotope (187Os/188Os) measurements, from the same section as the pCO2 reconstruction and δ13C data, reveal a shift to less radiogenic values coinciding with the change towards mantle-derived carbon emissions as indicated by our modelling results, lending further support to our interpretation. These results highlight that geologically triggered carbon emissions were likely driving the OAE onset.

Highlights

C-depleted reservoir (e.g. thermo- and/orC-enriched, likely fromThe early Aptian Oceanic Anoxic Event (OAE) 1a (∼121 Ma, Olierook et al., 2019) represents one of the largest perturbations of the climate system during the last 200 Myr (Jenkyns, 2010)
Yr−1 reconcile the negative carbon isotopic excursion (CIE) and pCO2 estimates, with the exact rate depending on the assumed duration of OAE onset
Our results suggest that an initial input of predominantly 13 C-depleted carbon was responsible for the negative CIE associated with

Summary

Introduction

C-depleted reservoir (e.g. thermo- and/orC-enriched, likely fromThe early Aptian Oceanic Anoxic Event (OAE) 1a (∼121 Ma, Olierook et al., 2019) represents one of the largest perturbations of the climate system during the last 200 Myr (Jenkyns, 2010). The resulting reduced oceanic oxygen concentrations would have led to enhanced preservation and burial of organic matter (Jenkyns, 2010; Monteiro et al, 2012). While all OAEs are associated with a positive carbon isotopic excursion (CIE), interpreted as reflecting an interval of enhanced burial of organic matter, the onset of OAE 1a is characterized by a distinct negative CIE prior to the positive CIE (Menegatti et al, 1998). Because of the broad temporal coincidence of OAE 1a with the emplacement of the Ontong-Java Plateau, the negative CIE could have resulted from carbon input from (submarine) volcanism initiating the OAE by warming the climate and releasing nutrients to the ocean

Methods

Results

Discussion

Conclusion