Causes and consequences of oceanic anoxic events – A focus on ocean nutrient cycling
Dr Fanny Monteiro, University of Bristol, UK
Oceanic anoxic events (OAEs) reflect the most dramatic changes in ocean state of the last 250 Ma. Using an (organic geochemical) data - model comparison I provide here detailed insights into the impact of temperature and ocean nutrient inventory, and associated biogeochemical responses to two of the largest OAEs of the Mesozoic, Aptian OAE 1a (~120 Ma) and Cenomanian-Turonian OAE 2 (~93.5 Ma).
The model-data reconstructions show that the spread of anoxia that both events experienced, mainly resulted from an enhancement in ocean nutrient level (4 and 2 times for OAE 1a and OAE 2 respectively). Enhanced nutrient levels thus increased ocean production in the surface and oxygen consumption in the deep ocean, causing ~50% and at least 40% of the ocean volume to become dysoxic/anoxic during OAE 1a and OAE 2 respectively. The spread of anoxia and euxinia differ between OAEs on their regional distribution as a consequence of paleogeography.
The model shows that during OAEs the marine nitrogen (N)-cycle operated fundamentally differently to today. While the N:P ratio is closed to Redfield ratio in the modern ocean throughout the water column, OAEs N:P ratio collapses in the deep ocean despite high rates of nitrogen fixation in the surface.
Early Jurassic hyperthermals in the context of a long, continuous, integrated stratigraphy (the JET project)
Professor Stephen Hesselbo, Camborne School of Mines, University of Exeter, UK
During the Early Jurassic, the planet was subject to distinctive tectonic, magmatic, and orbital forcing, and fundamental aspects of the modern biosphere were becoming established in the aftermath of the end-Permian and end-Triassic mass extinctions. The breakup of Pangaea was accompanied by biogeochemical disturbances including the largest magnitude perturbation of the carbon-cycle in the last 200 Myr, coeval with the now well-characterised hyperthermal, the Toarcian Oceanic Anoxic Event (T-OAE). Knowledge of the Early Jurassic is, however, based on scattered and discontinuous datasets, meaning that stratigraphic correlation errors confound attempts to infer temporal trends and causal relationships, leaving us without a quantitative process-based understanding of overall Early Jurassic Earth system dynamics. The Llanbedr (Mochras Farm) borehole in west Wales, originally drilled 50 years ago, provides the basis for placing the T-OAE, and other possible Early Jurassic hyperthermals, in a long-term stratigraphic and timescale context. Here the drillcore represents 27 Myr of Early Jurassic time with sedimentation rate of approximately 5 cm/kyr. Through the Integrated Early Jurassic Timescale and Earth System project (JET), a multi-faceted, international programme of research on the functioning of the Earth system, new data from the old Mochras core will be combined with data from a new core to provide an understanding of global change and quantify the roles of tectonic, palaeoceanographic, and astronomical forcing on hyperthermal (and hypothermal) events at this key juncture in Earth history. This project is funded by the International Continental Scientific Drilling Programme (ICDP) and the UK Natural Environment Research Council (NERC).
pCO2 and temperature during Aptian OAE 1a
Dr David Naafs , University of Bristol, UK
The Oceanic Anoxic Events (OAEs) of the Cretaceous represent one of the largest climatic perturbations of the Phanerozoic and share characteristics with the Cenozoic hyperthermals. However, the response of Earth’s climate and ecosystems to OAEs is often much less well constrained. This talk will focus on Aptian Oceanic Anoxic Event (OAE) 1a, which took place about 120 million years ago. Using a range of organic geochemical proxies, combined with computer modeling, this presentation will quantify key-climatic parameters such as pCO2 and temperature across OAE 1a. It will be demonstrated that sustained volcanic outgassing was the primary source of carbon dioxide during OAE 1a and was the ultimate driver of the observed global warming with reconstructed temperatures during OAE 1a being higher than found anywhere during the Cenozoic.
Temperature change and OAEs
Dr Stuart Robinson, University of Oxford, UK
Mesozoic oceanic anoxic events (OAEs) have been mechanistically compared with the hyperthermals of the early Cenozoic, with some suggesting that they represent similar, but larger magnitude, perturbations of the Earth system. Conceptual models for explaining hyperthermals, OAEs, and other similar phenomena in Earth history, make specific predictions about the role and pattern of temperature change during such events, which can be tested through comparison with the geological record. Oceanic anoxic event 2 (OAE2) occurred approximately 94 million years ago at the Cenomanian–Turonian boundary and is often considered as the type example of an OAE, as it fulfills many of the predictions of the conceptual models. However, temperature change during OAE2 is largely constrained from Northern Hemisphere sites and, in many cases, is based on qualitative reconstructions. In order to understand the drivers of climate change during OAE2, quantitative estimates of temperature change from many different localities are required. In this presentation, the record of qualitative and quantitative temperature change during OAE2 will be reviewed, including unpublished data from the southern hemisphere. Consideration of these temperature records in the context of short-term carbon cycling, GCM modeling, and longer-term records of Cretaceous and early Cenozoic climate variability, will hopefully lead to better constraints on OAE2 and, more broadly, the understanding of carbon-cycle-driven climate change in the geological record.
Hydrological cycle and cretaceous OAEs
Professor Richard Pancost, University of Bristol, UK
Ancient warming events allow us to evaluate the impacts of global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks. There are a diverse range of biological and geochemical signatures that can be interpreted as direct or indirect indicators of hydrological change. Further complicating interpretation is the fact that changes in precipitation and its biogeochemical consequences are often conflated in interpretation of sedimentary signatures, as well as strong evidence for changes in the episodic and/or intra-annual distribution of precipitation which has not widely been considered when comparing proxy data to GCM output. This requires interpretations that integrate proxies holistically with one another and with model simulations. When done so, proxy records and climate models indicate that the response to past global warming was profound, with evidence for global reorganisation of the hydrological cycle and profound local increases and decreases in rainfall; combined with elevated temperatures and terrestrial vegetation change, this appears to often result in warming-enhanced soil organic matter oxidation, chemical weathering and nutrient cycling. All of these responses, however, are spatially and temporally complex. Key challenges, therefore, will be to increasingly: 1) interrogate extreme events in climate simulations; 2) use earth system models to disentangle the complex and multiple controls on proxies; 3) adopt multi-proxy approaches to constrain complex phenomena; and 4) increase the spatial coverage of such records, especially in arid regions, which are currently under-represented.