IAP2-22-348

Tracking Deep time Glacial Intervals

The impacts of future sea-level rise on the coastal zone in response to ongoing climate change are of increasing concern. One way to more accurately predict future changes in sea level is to improve our understanding of past sea-level changes. Past sea-level reconstructions provide crucial input data with which to test and further refine models of future change. Associated with this, understanding past environmental geochemical changes in ancient (deep time) seawater through the study of sedimentary units is key. Marginal marine environments are particularly sensitive to changes in climate influencing both sea level and ocean circulation. This project aims to improve our ability to reconstruct past sea-level and environmental changes by furthering the growing application of osmium isotope geochemistry to oceanography.
Osmium isotope (187Os/188Os) values of organic-rich marine sediments have been used to reconstruct changes in seawater 187Os/188Os during climatic/glacial events, e.g., during the Pleistocene, Silurian, Ordovican, and Neproterozoic. During interglacial periods, seawater 187Os/188Os ratios are shown to be more radiogenic than during the glacial period. This has been interpreted to reflect the reduction in weathering of radiogenic continental crust in response to burial beneath ice sheets and decreased rates of chemical weathering. However, following deglaciation increased chemical weathering combined with exposure of easily leached glacial deposits releases more radiogenic 187Os/188Os into the oceans thus causing a rise in seawater 187Os/188Os.
Given that seawater typically has a distinct osmium-isotope composition to that of river (fresh) water (although dependent on local geology), this project will apply the osmium isotope technique to reconstruct past sea-level changes and environmental/climate changes during the Late Carboniferous / Early Permian.
Overarching aim: Investigate the osmium isotope signature from marine, marginal marine and freshwater deposits preserved in sedimentary units from the Late Carboniferous to Early Permian glacigenic deposits in the Falkland Islands. Analysis of this novel dataset will assess whether the different sediment provenances (freshwater vs marine source) can be distinguished based on osmium isotope geochemistry to more accurately constrain sea-level and environmental change, which will ultimately be applied to sedimentary sequences that lack glacigenic deposits (e.g., diamictite as in previous Ordovician and Silurian studies).

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Methodology

To achieve the above aim a series of sediment cores and outcrop samples will be examined (archived by the British Geological Survey – BGS). The cores were obtained as part of an onshore exploration programme between 2005 and 2007 and sample the syn-, and post-glacial succession which comprise part of the Falklands outcrop. The core includes the top of the Fitzroy Tillite Formation and the succeeding strata at the base of the Port Sussex Formation, within which three members are recognized, in upward succession: the Hells Kitchen (sequence of mudrocks, rhythmites, and diamictite), Black Rock (mudstone) and Shepherds Brook members (mudstone, siltstone, fine-grained sandstone, with sporadic bentonite). The lower two of these three members record the deglaciation process, the ‘icehouse to greenhouse’ transition. A single core also penetrates preglacial sedimentary units of the Bluff Cover Formation, but also outcrops comprising a range of mudstones and sandstones, with dropstones increasing in abundance upwards towards the Fitzroy Tillite.
Sedimentological and geochemical analyses will also be carried out on the available cores. The osmium isotope data from the cores will be used to reconstruct environmental changes and, hence, relative sea-level and climate change. The project will provide the necessary understanding of the suitability of osmium isotopes as a technique to independently reconstruct deep time glacial episodes and changes in sediment provenance linked to climate changes.

Project Timeline

Year 1

Review existing literature on deep time glaciations, and more recent glaciations utilizing osmium isotope techniques. Engage in Faculty training programme and complete first year Progression Paper to detail project overview, research questions, and planned methodology. Assess core archive at the BGS. Initial lab work collecting osmium isotope data.

Year 2

Main laboratory analysis phase – collection of osmium stable isotope data from all cores. Assessment of available proxy data from archived sites and identification of additional data to be collected. Collection of additional proxy data – e.g., TOC, XRF scanning, x-rays and MSCL. Preparation of preliminary review chapters. Presentation of initial results at national postgraduate conference.

Year 3

Completion of laboratory work. Data analysis and interpretation – comparison of osmium isotope data with traditional proxies to assess potential of osmium isotopes for environmental and relative sea-level/climate reconstruction. Preparation of major data and interpretation chapters for thesis. Preparation of first research paper. Presentation of research at international conference (target EGU, AGU, GSA).

Year 3.5

Preparation and completion of final chapters for thesis. Submission of papers for publication.

Training
& Skills

The student will receive training/advice in appropriate handling and sampling of the archived core material (BGS).
Training in state-of-the-art osmium isotope geochemical analyses will be provided in a world leading laboratory in the Department of Earth Sciences, Durham. Training in additional proxy techniques (sedimentological and geochemical teachniques) will be provided in state-of-the-art laboratories in the Department of Geography, Durham.
The student will be a member of the Geochemistry Research Group in Earth Sciences: (https://www.dur.ac.uk/earth.sciences/research/res_groups/geochemistry/) and the Ice Sheets and Sea Level Research Cluster in Geography (https://www.dur.ac.uk/geography/icesheetsandsealevel/).

The student will also have the opportunity to benefit from a broad range of skills training provided by the graduate training programme at Durham and through the IAPETUS2 Doctoral Training Partnership framework (e.g., thesis and paper writing, presentation skills etc.). The student will also attend and contribute to the programme of regular departmental seminars and discussion groups to support their general development as a scientist.

References & further reading

Finlay, A. J., Selby, D., Grocke, D. R., 2010. Tracking the Hirnantian glaciation using Os isotopes. EPSL, 293, 339-348.
Pogge von Strandmann, P. Desrochers, A., Murphy, M.J., Finlay, A.J., Selby, D., Lenton, T.M., 2017. Global climate stabilisation by chemical weathering during the Hirnantian glaciation. Geophysical Letters. 3, 230–237. doi: 10.7185/geochemlet.1726
Qie, W., Algeo, T.J., Luo, G., Herrmann, A., 2019.Global events of the Late Paleozoic (Early Devonian to Middle Permian): A review. Palaeogeography, Paeoclimatology, Palaeoecology, 531, 109259, doi: 10.1016/j.palaeo.2019.109259
Rooney, A.D., Selby, D., Lloyd, J.M., Roberts, D.H., Luckge, A., Sageman, B.B., Prouty, N.G., 2015. Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet. Quaternary Science Reviews 138, 49-61.
Sproson, A.D., Pogge von Strandmann, P.A.E., Selby, D., E. Jarochowska, E., Fryda, J., Jindřich H., Loydell D.K., Slavik, L., Calner, M., Maier, G., Munnecke, A., Lenton, T.M., 2022. Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations. EPSL. 577 (2022) 117260 doi: 10.1016/j.epsl.2021.117260.
Stone, P., Horan, K., 2016. Early Permian climate change in the Falkland Islands, Geology Today, 32, 107-114.
Horan, K., Stone, P. Crowhurst, S.J. 2018. Sedimentary record of Early Permian deglaciation in southern Gondwana from the Falkland Islands, In: Le Heron, D.P. et al. (eds) Glaciated Margins: The sedimentary and geophysical archive. Geological Society, London, Special Publications, 475, 131-147, doi: 10.1144/SP475.1.

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