The cycles of carbon and oxygen are intimately linked, and have both fluctuated over Earth’s history. These fluctuations are in-turn thought to have driven major shifts in Earth’s environment and the life it supports. However until recently methods of reconstructing past oxygen and CO2 levels in the ocean and the atmosphere were largely qualitative. Thanks to recent developments in geochemistry, rapid progress is now being made on more quantitative tracers of planetary redox and CO2 conditions, and these methods are now primed to provide new insights into major events in the geological record of environmental change.
This project will use novel trace element ratios and isotope systems in marine carbonates to reconstruct changes in ocean/atmosphere CO2 and ocean redox conditions in key intervals of the last 100 Million years.
CO2 reconstructions will be based on the boron isotope composition (d11B) of foraminifera (Foster & Rae 2016; Rae 2018), which reflects water pH – and thus CO2 chemistry. This method has provided several high profile reconstructions during this time period (e.g. Anagnostou 2016, Gutjahr 2017), but records remain limited in temporal and spatial resolution.
To reconstruct redox conditions we will measure a suite of novel trace elements in foraminifera and bulk sediments, including iodine, uranium, manganese, cerium, molybdenum, and rare earth elements (e.g. Zhou et al., 2016). This will be paired with measurements of carbon isotope gradients between different species of benthic foraminifera to reconstruct bottom water oxygen concentrations (e.g. Hoogakker et al., 2015), and may be extended in key intervals with measurements of novel isotope systems (e.g. sulphur, molybdenum, uranium).
To aid interpretation, we will also use a quantitative modelling approach, pairing sedimentary redox modelling with Earth system model output.
The results of this work will improve understanding of environmental change during major transitions of the last 100 Myr, and will also help inform the use of these methods deeper in the geological record.
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Marine carbonate samples are made available from the IODP and may also be collected during dedicated fieldwork. These will be analysed for boron isotopes and trace elements in the St Andrews Isotope Geochemistry (STAiG) labs, following techniques established by Foster (2008) and Rae et al. (2011), and recently developed to improve precision on small samples. Trace elements will be measured on a new state-of-the-art triple quadrupole ICPMS, allowing removal of interferences from several key elements (e.g. REEs).
We will also take advantage of – and continue to develop – new protocols for sample preparation (including automation) that will allow faster throughput of samples and thus higher resolution records to be generated.
Controls on redox proxies will be further explored using sedimentary redox modelling, in collaboration with Dr Sandra Arndt at the Universite Libre de Bruxelles. These may also be paired with output from the GENIE Earth system model.
The project is designed to be flexible, with the opportunity to focus on approaches, time intervals, and techniques of particular interest to the student.
Sediment core sampling, fieldwork, training in clean laboratory methods and mass spectrometry, initial measurements and training, literature review
Generate long-term records. Sediment redox modelling. First manuscript.
Finalize data sets including higher resolution intervals, apply numerical techniques
Finalise manuscripts and write thesis
The student will gain specific training in mass spectrometry, fieldwork, clean lab chemistry, and geochemical modelling, as well as broader education in geochemistry, oceanography, and climate science. Over the course of the PhD the student will gain transferable skills such as scientific writing, statistics and data analysis, and problem-solving, as well as time management and working towards a long-term goal.
References & further reading
Anagnostou et al. (2016), Nature, 533(7603), 380
Foster (2008), EPSL, 271, 254-266.
Foster & Rae (2016), AnnRev, 44, (207-237)
Gutjahr et al. (2017), Nature 548.7669 (2017): 573
Hoogakker et al. (2015), Nature Geoscience 8.1 40
Rae et al. (2011), EPSL, 302, 403-413.
Rae (2018), Boron Isotopes, Springer, 107-143
Zachos et al. (2008), Nature, 451, 279-283.
Zhou et al. (2016), Paleoceanography and Paleoclimatology, 31(12), 1532-1546