Does mountain building control long-term climate?
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This project addresses 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. It has been almost 30 years since the publication of this idea, and the original paper has been cited nearly 2000 times, but it is still unclear if this mechanism really operated (e.g. Willenbring and Von Blanckenburg, 2010). In this project the student will develop a novel method of analysis by using climate predictions from advanced General Circulation Modelling to directly estimate global chemical weathering rates as the land surface evolves. The weathering rate estimations will be used as inputs to a biogeochemical box model, which is capable of integrating them over millions of years and producing geochemical data that can be directly compared to the geological record.
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 or dolomite) requires calcium or magnesium cations, 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 is 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 (McKenzie et al., 2016). This project will produce a clear test of the idea that Himalayan uplift drove enhanced chemical weathering and climate cooling. It is expected to contribute significantly to how we view the controls on past CO2 concentration, and what methods we might use to mitigate against current anthropogenic change.
Figure 1. Mountain building, 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 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 drive a spatial chemical weathering routine (Maffre et al. 2018), and use the weathering rate information as inputs to the COPSE biogeochemical model (Mills et al., 2019). This procedure will allow the global biogeochemical model COPSE to be driven by weathering calculations from GCM climates. In this way the researcher can analyse whether the changes to topography should result in enhanced weathering at the global scale, and by using COPSE, 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.
The following key questions will guide the research:
- Did Cenozoic mountain uplift result in significant changes to chemical weathering rates?
- Are the effects of increased Ca, alkalinity and P inputs to the ocean sufficient to increase carbon burial and reduce atmospheric CO2?
- Is the modelled Cenozoic climate 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, Nature Geoscience, PNAS), as have many of their previous students.
There is scope within this project to develop a wide skill set in Earth system modelling techniques. The models used here are well known and the student 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.
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 received the ESSI ‘Star Supervisor’ award in 2018.
A good degree in the physical sciences, mathematics or computing is required, and the candidate should have a strong interest in Earth history, climate and geoscience. 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).
Raymo, M. E. & Ruddiman, W. F. Tectonic forcing of late Cenozoic climate. Nature 359, 117-122 (1992).
Willenbring, J. K. & Blanckenburg, F. v. Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature 465, 211-214 (2010).
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).
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).
Hansen, J., et al. Climate sensitivity, sea level and atmospheric carbon dioxide. Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences 371, 20120294 (2013).
Royer, D. L. Atmospheric CO2 and O2 during the Phanerozoic: Tools, Patterns, and Impacts. in Treatise on Geochemistry 251-267 (2014).
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).
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).