Constraining the tempo of Mesozoic Oceanic Anoxic Events using extra-terrestrial He-isotopes

Key Questions

What were the rates of carbon release and sequestration associated with ‘oceanic anoxic events’? 

Background

Determining the causes and consequences of major environmental perturbations in the geological past requires a robust estimate of the rates of change. This project will use a novel proxy for sedimentation rates to reconstruct detailed chronologies for Mesozoic ‘oceanic anoxic events’ and provide new insights into the trigger mechanisms and Earth system feedbacks associated with them. 

Aims of the Project

To reconstruct detailed accumulation rate records for Mesozoic oceanic anoxic events and constrain rates of carbon cycling, organic carbon burial and other forms of environmental change. 

Project Description

Reconstructing past climatic and environmental change from the pre-Quaternary rock record relies on having a robust chronology of events that can be confidently correlated from place to place. In order to do so, a good understanding is needed of how time is recorded by sedimentation at the highest resolution possible. It is necessary to be able to identify changes in accumulation rate and gaps in sedimentation. 

The Mesozoic Era was a time-interval characterized by greenhouse climates and geologically brief intervals of carbon-cycle perturbation, often associated with widespread oceanic anoxia (the so-called “oceanic anoxic events”, OAEs). OAEs are represented in the geological record by significant excursions in stable carbon isotopes and, most dramatically, by apparently synchronous increases in the concentrations of organic-carbon burial across many localities (e.g. Jenkyns, 2010). An increasing number of organic and inorganic geochemical proxies are being generated in order to understand the processes that drove, sustained and terminated OAEs, but to achieve this aim a detailed knowledge of the rates of change is required, which, in many cases, is poorly constrained. 

Methodology 

Over relatively long time-scales, Milankovitch cycles provide a method of reconstructing sedimentation rates, but this approach can be sensitive to missing or highly condensed cycles. Furthermore, this method does not allow the identification of changes in sedimentation at sub-Milankovitch resolution (i.e. less than a precession cycle). An alternative approach, particularly applicable to fine-grained pelagic sediments, is based upon the concentration of extraterrestrial helium (3HeET; e.g. Murphy et al., 2010). Extraterrestrial helium is supplied to sediments in the form of Interplanetary Dust Particles (IDPs) that are constantly raining onto the Earth’s surface. Over fairly short-intervals of time, the supply of IDPs (and thus 3HeET) from space is constant and so any changes in the concentration of 3HeET within sediments must have been caused by changes in the amount of dilution by sediment flux (i.e. sedimentation rate). This phenomenon provides a method to identify changes in sedimentation rate at high-resolution, sub-Milankovitch scales, and detect intervals of highly condensed sedimentation. 

This project will aim to apply the 3HeET to cyclic sediments and OAEs in the Jurassic and Cretaceous building on previous work in Oxford. The project will require extensive fieldwork and sampling of cores in Europe and the UK, plus considerable geochemical work in the noble gas laboratories in Oxford 

Timeline 

Year 1: General training in sedimentary geochemistry, field work, carbon cycling and palaeoclimatology.  

Specific training in noble gas preparation and analytical methods; start of data acquisition from existing sample sets 

Year 2: Further sample collection from overseas field areas and application of methods to OAEs. 

Years 3 and 4: Data integration, comparison with climate records and models, thesis completion, and submission of papers for international journals & conferences 

References & Further Reading 

Jenkyns, H.C., 2010 Geochemistry of oceanic anoxic events. Geochemistry Geophysics Geosystems, 11, doi: 10.1029/2009GC002788 

Murphy, B.H., Farley, K.A., and Zachos , J.C., 2010, An extraterrestrial 3He-based timescale for the Paleocene–Eocene thermal maximum (PETM) from Walvis Ridge, IODP Site 1266, Geochimica et Cosmochimica Acta, 74, 5098-5108 

Methods to be used

Noble-gas and stable-isotope geochemistry, stratigraphy, sedimentology, fieldwork; potentially Earth system and carbon cycle modelling. 

Specialised skills required

The project requires a student with strong interests in geochemistry, sedimentology and stratigraphy. Additional specific training in any area will provided, including in field and laboratory techniques. 

Please contact Stuart Robinson on stuart.robinson@earth.ox.ac.uk if you are interested in this project