Quantitative constraints on carbon fluxes and volcanic activity during Oceanic Anoxic Event 1a

Abstract

The geologic record provides opportunities to study the long-term Earth-System response to exogenic Carbon (C) emissions. In this thesis, I extend and apply the Earth-System model cGENIE to reconstruct and evaluate the uncertainty of C emissions and volcanism during the Oceanic Anoxic Event 1a(OAE 1a) based on sedimentary records of C, Os, Sr, Li and Ca isotopes. OAE1a(120Ma) was a major C cycle perturbation featuring the formation of a transient marine organic C sink. Combining C isotope records with reconstructed CO2 concentration changes, I determine the size and isotopic composition of net C fluxes at the event onset. This inverse C flux reconstruction indicates that 4,300-29,200Pg C were emitted, predominantly from volcanism. Radiogenic Os and Sr isotopes have previously been used to reconstruct volcanic activity during the main event, when the effect of enhanced organic C burial prevents C flux reconstructions from the C isotope record. The weathering response has been quantified using Li and Ca isotopes. However, interactions between the C and these metal cycles during C injections are poorly understood. After implementing Os cycling into cGENIE and validating existing parameterisations of Sr, Li and Ca cycling, I show that concurrent changes in volcanism and weathering noticeably affect Os, Sr, Li and Ca isotope excursions in cGENIE simulations, and that further constraints on the isotopic effect of weathering flux changes are required to use trace metals as C flux proxies during long-lasting volcanic episodes. Further analysis of metal cycling prior to OAE1a reveals that substantial amounts of volcanic C must have been emitted throughout the event to cause the observed isotope excursions and support organic C burial and enhanced weathering for millions of years. C flux reconstructions remain too uncertain to determine the relative importance of these negative C cycle feedbacks.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Fanny M Monteiro (Supervisor), Dan Lunt (Supervisor), B D A Naafs (Supervisor) & Sarah E. Greene (Supervisor)