Mountains, volcanoes or plants? Why did climate cool during the Cenozoic Era?

Mountains, volcanoes or plants? Why did climate cool during the Cenozoic Era?

Dr Benjamin Mills (SEE), Professor Alan Haywood (SEE), Professor Yves Goddéris (CNRS, France), Professor Chris Scotese (PALEOMAP project, USA)

Contact email: b.mills@leeds.ac.uk

 

Summary

The Earth’s climate is colder now than it has been for much of its history. Looking back over many millions of years, permanent ice caps are a relatively rare feature of the Earth, and much of its history seems to have been spent in a ‘greenhouse’ state, driven by high concentrations of atmospheric CO2. This project will explore what caused our planet to cool over the last 65 or so million years (the Cenozoic). Central to this investigation is the famous uplift-weathering hypothesis. Raymo and Ruddiman (1992) proposed that the uplift of the Tibetan Plateau over the last 50 million years has been the principal driver of the substantial global cooling over this interval. It is suggested that mountain-building can drive global cooling by amplifying the rate of chemical weathering of silicate minerals, ultimately leading to carbon sequestration on the seafloor and a reduction in atmospheric CO2 concentration. But it remains unclear whether mountain building was the only important change happening during this time. Large changes to tectonic CO2 degassing rates were also occurring (McKenzie et al., 2016) and Earth’s biosphere was being revolutionised by the appearance of grasses and new nitrogen-supplying species (Epihov et al., 2017). In this project the researcher will develop a method to test these competing ideas by using a new type of linked climate-chemical model developed by the supervisory team.

 

Background

The Earth cooled significantly during the last 50 Million years, and this cooling was most likely driven by a long-term reduction in atmospheric CO2 concentration (Figure 1). The reasons for CO2 decline are uncertain, but one prominent hypothesis is that carbon was sequestered as the result of the collisional uplift of the Himalayas and Tibetan Plateau (Raymo and Ruddiman, 1992). Over multimillion-year timescales, the amount of carbon contained in the atmosphere-ocean system is controlled by the rate of volcanic degassing, and the rate of removal of carbon to the sediments (‘carbon burial’). This burial of carbon can be achieved in organic or inorganic form. Inorganic carbon burial (e.g. as limestone) requires silicate cations (e.g. calcium), and organic carbon burial requires a supply of the key limiting nutrient phosphorus to the biosphere. All of these prerequisite elements are derived from the weathering of Earth’s crust (primarily silicate rocks like granite and basalt). Thus, it is expected that the uplift of mountains led to increased liberation of these elements through weathering processes (due to higher rainfall and erosion rates in mountains), and eventual burial of carbon in sediments, which ultimately led to a reduction in atmospheric CO2 and global cooling. This ‘uplift-weathering’ hypothesis is quite famous, and as well as being a key cornerstone of research into the ancient Earth system, it is also driving huge investment in present day geoengineering efforts which seek to replicate this weathering-burial effect by spreading finely-ground basaltic rocks on the Earth’s surface to mitigate against anthropogenic warming (Taylor et al., 2016). However, the uplift-weathering hypothesis has not been directly tested at the global scale, because this requires a combined approach utilizing both spatial climate models and long-term biogeochemical models, and such approaches are only just being developed. It is certainly possible that the uplift-weathering hypothesis may be incorrect: weathering is a complex process and is not always enhanced in high mountains (Joshi et al., 2019) and CO2 and temperature decline may potentially be explained by changes to carbon input rates through volcanic degassing (McKenzie et al., 2016).

In addition to this, the terrestrial biosphere was changing substantially over the Cenozoic Era. The advent of nitrogen-fixing legumes, and the evolution of grasslands with their C4 photosynthesis pathway, may have both altered the productivity (and therefore carbon sequestration potential) of the biosphere (Epihov et al., 2017). This project will use a new type of chemical-climate model to simulate this linked climate and biological evolution forwards in time and compare the results to the geological record, in order to understand which processes are the most important.

 

Figure 1. Our changing planet, CO2 and temperature. A. Creation of the Himalayas: India moves northwards and collides with Asia, resulting in the highest continental mountains on Earth. B. Estimates of global average surface temperature over this time show a substantial decrease (Hansen, 2013). C. The decrease in temperature broadly coincides with a reduction in atmospheric CO2 concentrations (Royer et al., 2014; Mills et al., 2019).

 

Aims and approach

To test the uplift-weathering hypothesis the researcher will use the SCION climate-chemical model (Mills et al., 2021). SCION combines a set of previously-computed climate model runs with a dynamic land surface model for vegetation dynamics and weathering. It is uniquely capable of integrating spatial representations of climate, surface processes and biogeochemistry over many millions of years. The researcher will run the Hadley Centre general circulation climate model (Gordon et al., 2000) for a selection of paleogeographies representing the Cenozoic Era and at a variety of different CO2 levels. They will then utilize the model outputs to inform the SCION model, which includes a spatial chemical weathering routine (Maffre et al. 2018), and global biogeochemistry (Mills et al., 2019). In this way the researcher can analyse whether the changes to topography, CO2 degassing, and the composition of the land biosphere, should result in enhanced weathering at the global scale, and can decide whether or not the degree of weathering enhancement can cause the observed changes in CO2 and in other geochemical proxies such as the isotope ratios of strontium, carbon and sulfur.

 

Figure 2. SCION Earth Evolution model. The model consists of a single-box ocean, 2D land surface scheme and steady state 2D atmosphere generated from a 3D atmosphere-ocean general circulation model.

 

 

The following key questions will guide the research:

  • Did Cenozoic changes to mountain uplift and CO­2 degassing result in significant changes to the atmospheric CO2 level?
  • Did the changing composition of the terrestrial biosphere allow more carbon to be sequestered?
  • Is the modelled Cenozoic climate and chemistry consistent with the geochemical record of stable isotopes of carbon, sulfur and strontium?

 

Impact of the research

The question of the regulation of global climate is a top priority in the Earth sciences, and papers on this subject appear regularly in top geoscience journals, as well as leading interdisciplinary publications. The combined field of paleoclimate-biogeochemistry is only just emerging, and many topics remain unaddressed. This project will directly address some of the key questions in the field that are also of interest to the general public, and to climate change policymakers. We therefore expect the impact of this project to be highly significant within the scientific community and beyond. The project supervision team encompasses paleoclimate, chemical weathering and biogeochemical modelling expertise from leaders in these fields, who have published in the top journals (Nature, Science, PNAS etc.), as have many of their previous students.

 

Training

There is scope within this project to develop a wide skill set in Earth system modelling techniques. The researcher will work with key figures in the respective fields of climate and biogeochemical modelling. The combined model to be developed here will be very powerful and should lead to many opportunities to expand the work, e.g. for different time periods, or to explore other chemical cycles such as oxygen or sulfur. The researcher will also be trained in the scientific method, concise writing and presentation, along with a broad range of additional courses offered by Faculty Graduate School.

 

Working environment

The research will be based in the Earth Surface Science Institute (ESSI), within the School of Earth and Environment at the University of Leeds, with occasional visits to see Professor Goddéris in Toulouse. ESSI is a medium sized and friendly research institute that includes analytical geochemists, biogeochemists, sedimentologists, palaeontologists and modellers of climate and biogeochemistry. The group holds an annual science day, a BBQ, a pub quiz and weekly informal get-togethers, as well as monthly science meetings for each of the ‘paleo’ and ‘geochemistry’ subdivisions. Project supervisor Mills has received multiple ESSI ‘Star Supervisor’ awards.

 

Entry requirements

A good degree in the natural sciences, mathematics or computing is required, and the candidate should have a strong interest in climate-biosphere-geosphere links and in Earth history. Formal training in numerical techniques is not essential, but some experience in (or at least, enthusiasm for) computing is advisable. All necessary training will be provided as part of the project.

 

References and further reading

Gordon, C., Cooper, C,. Senior, C,. Banks, H,. Gregory, J., Johns, T., Mitchell, J. & Wood, R. The simulation of SST, sea ice extents and ocean heat transports in a coupled model without flux adjustments. Climate Dynamics 16, 147-168 (2000).

Epihov, D. Z., et al. (2017). “N2-fixing tropical legume evolution: a contributor to enhanced weathering through the Cenozoic?” Proc Biol Sci 284(1860).

Hansen, J., et al. Climate sensitivity, sea level and atmospheric carbon dioxide. Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences 371, 20120294 (2013).

Joshi, M. M., Mills, B. J. W., & Johnson, M. A capacitor‐discharge mechanism to explain the timing of orogeny‐related global glaciations. Geophysical Research Letters, 46, 8347–8354 (2019).

McKenzie, N, R., Horton, B. K., Loomis, S. E., Stockli, D. F., Planavsky, N. J. & Lee, C-T. A. Continental arc volcanism as the principal driver of icehouse-greenhouse variability. Science 352, 444-447 (2016).

Maffre, P., J.-B. Ladant, J.-S. Moquet, S. Carretier, D. Labat and Y. Goddéris. Mountain ranges, climate and weathering. Do orogens strengthen or weaken the silicate weathering carbon sink? Earth and Planetary Science Letters 493, 174-185 (2018).

Mills, B. J. W., A. J. Krause, C. R. Scotese, D. J. Hill, G. A. Shields and T. M. Lenton. Modelling the long-term carbon cycle, atmospheric CO2, and Earth surface temperature from late Neoproterozoic to present day. Gondwana Research 67, 172-186 (2019).

Mills, B. J. W., et al. (2021). “Spatial continuous integration of Phanerozoic global biogeochemistry and climate.” Gondwana Research.

Raymo, M. E. & Ruddiman, W. F. Tectonic forcing of late Cenozoic climate. Nature 359, 117-122 (1992).

Royer, D. L. Atmospheric CO2 and O2 during the Phanerozoic: Tools, Patterns, and Impacts. in Treatise on Geochemistry 251-267 (2014).

Taylor, L. L., J. Quirk, R. M. S. Thorley, P. A. Kharecha, J. Hansen, A. Ridgwell, M. R. Lomas, S. A. Banwart and D. J. Beerling. Enhanced weathering strategies for stabilizing climate and averting ocean acidification. Nature Climate Change 6, 402-406 (2015).

Willenbring, J. K. & Blanckenburg, F. v. Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature 465, 211-214 (2010).