Modelling the physical and biogeochemical causes of OAE2

Abstract

Oceanic anoxic events (OAEs) are periods in Earth’s history during
which a large portion of the ocean became dysoxic. As large perturbations
of the earth’s carbon cycle, the mechanics of what caused
OAEs are particularly interesting, especially given contemporary concerns
about decreasing oxygen levels in a warming ocean. OAE2, one
of the more globally widespread OAEs, occurred at 103Ma and marks
the boundary between the Cenomanian and the Turonian stage.
OAE2 has been extensively studied, both using data and modelling.
However, most modelling relies on box models or low-resolution
Earth-System models, with few studies using full GCMs due to the
prohibitive modelling spin-up times. This thesis explores the physical
and biogeochemical causes of OAE2 using the computationally
efficient HadCM3L GCM. It covers three aspects:
(1) A model comparison of the Cenomanian and Maastrichtian
paleogeographies shows that a restricted circulation is necessary but
insufficient for OAE2 to develop. This restricted paleogeography is
particularly important in the proto-North Atlantic, the area in which
anoxia first develops.
(2) An ensemble of model runs for the Cenomanian with varying
orbital parameters shows that orbital variation can alter the spread
and magnitude of ocean anoxia without obliterating it. Orbital variation
is unlikely the driving cause behind OAE2 but could have caused
anoxic variations within the OAE, resulting in changes in distribution
or sediment layering.
(3) Comparing HadCM3L with previous OAE2 modelling highlights
the importance of a detailed representation of the ocean ecosystem.
HadCM3L, which includes an NPZD ocean ecosystem, does not
achieve global anoxia when the nutrient inventory is doubled in the
ocean, a difference caused by the top-down control of phytoplankton
and detritus.
Overall, the results show the importance of a more comprehensive
representation of the physics and biology of the ocean. Future
modelling OAEs work needs to make more use of these more comprehensive
models to progress our understanding of OAEs.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Paul J Valdes (Supervisor) & Fanny M Monteiro (Supervisor)