The environmental record of an early Jurassic carbonate platform

The early Jurassic (201-174 Myrs ago) represents a time of marked environmental changes and evolution of the Earth system: Climate oscillated between cold and warm modes (Korte and Hesselbo, 2011), seawater chemistry changed substantially (Weldeghebriel et al., 2022) and, along with the evolution of calcifiers, drove changes in the location and mineralogy of carbonate deposition (Ridgwell, 2005; Sandberg, 1983). The carbon cycle underwent several perturbations thought to be driven by volcanism and often associated with the reduced availability of oxygen in the oceans. The interval is bracketed by two well studied events: The end-Triassic extinction and the Toarcian anoxic event, but the intervening period contains several events that have received less attention: The recovery from the end-Triassic extinction in the Hettangian, the Sinemurian-Pliensbachian boundary (SPB) and the end Pliensbachian. Records have mostly focussed on the SPB (e.g. Franceschi et al., 2019) and little work has been done on the long term geochemical record to provide context for these events (with the exception of Han et al., 2022).

The Dinarides of the Adriatic coast contain a continuous record of this time period preserved in platform carbonate deposits. The platform was attached to Gondwana until the mid-Triassic when it started drifting northwards becoming the huge isolated Southern Tethyan Mega-platform. During the Toarcian this broke up to become a complex of smaller platforms separated by deep basins (Vlahović et al., 2005). The aim of this project is to investigate the environmental record of platform carbonate sections in Croatia and their deeper water expression in Montenegro, requiring the integration of field geology, sedimentology, geochemistry and stratigraphy.

Figure 1. A) Unstudied Pliensbachian and Sinemurian platform carbonates of the Velebit-B section of Sabatino et al (2013). B) Characteristic Pliensbachian lithiotid bivalves. C) Toarcian ‘spotted limestone’ facies. D) Carbonate-carbon isotope profile through the Toarcian showing the well-known global negative excursion linked to volcanism and anoxia, although this site remains oxygenated.

Objectives

1. Conduct fieldwork in Croatia and Montenegro to log and sample sections from shallow platform carbonates and their deeper water counterparts.
2. Create records of carbonate sedimentology through optical and SEM microscopy.
3. Analyse carbon isotopes of carbonates or organic matter, and/or the strontium isotopes of carbonates as appropriate to set up a chemostratigraphy.
4. Characterise the sediment composition and generate major and trace element records of carbonate chemistry to explore the evolution of seawater and its response to environmental perturbations. Possible approaches include trace element concentrations (e.g. Mn) and iodine/calcium ratios as measure of oxygenation (e.g. He et al., 2022), phosphate/calcium ratios as a measure of water column phosphorus (Dodd et al., 2021), and mercury or tellurium/thorium ratios as a record of volcanism (e.g. Regelous et al., 2020)

Potential for high profile outcome

The project will explore a novel archive of environmental information for an understudied but important time period. The project therefore has the potential to generate a number of significant papers with one or more being suitable for high-impact journals

Training & skills

The successful candidate will be fully trained in all the necessary techniques. Completing a PhD develops a broad array of transferable skills such as written communication, public speaking, project management, leadership, collaboration and perhaps most importantly, critical thinking. All of the analytical techniques are available in the School’s excellent laboratory facilities. The student will benefit from being part of the Earth Surface Science Institute, and the Cohen Geochemistry and Palaeo@Leeds research groups. This organisational framework provides a broader supportive environment which allows the cross fertilisation of ideas and expertise. In addition to the bespoke training for the PhD, the student will have access to a wide range of other general training and support. Examples would include useful scientific and transferrable skills such as statistics, time management, writing and giving presentations, and skills specific to a PhD programme such as managing your degree and preparing for your viva.

Bibliography

Dodd, M.S., Zhang, Z., Li, C., Algeo, T.J., Lyons, T.W., Hardisty, D.S., Loyd, S.J., Meyer, D.L., Gill, B.C., Shi, W., Wang, W., 2021. Development of carbonate-associated phosphate (CAP) as a proxy for reconstructing ancient ocean phosphate levels. Geochim. Cosmochim. Acta 301, 48–69. https://doi.org/10.1016/j.gca.2021.02.038

Franceschi, M., Corso, J.D., Cobianchi, M., Roghi, G., Penasa, L., Picotti, V., Preto, N., 2019. Tethyan carbonate platform transformations during the Early Jurassic (Sinemurian–Pliensbachian, Southern Alps): Comparison with the Late Triassic Carnian Pluvial Episode. GSA Bull. 131, 1255–1275. https://doi.org/10.1130/B31765.1

Han, Z., Hu, X., He, T., Newton, R.J., Jenkyns, H.C., Jamieson, R.A., Franceschi, M., 2022. Early Jurassic long-term oceanic sulfur-cycle perturbations in the Tibetan Himalaya. Earth Planet. Sci. Lett. 578, 117261. https://doi.org/10.1016/j.epsl.2021.117261

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Sabatino, N., Vlahović, I., Jenkyns, H., Scopelliti, G., Neri, R., Prtoljan, B., Velic, I., 2013. Carbon-isotope record and palaeoenvironmental changes during the early Toarcian oceanic anoxic event in shallow-marine carbonates of the Adriatic Carbonate Platform in Croatia. Geol. Mag. 150, 1085–1102. https://doi.org/10.1017/S0016756813000083

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Weldeghebriel, M.F., Lowenstein, T.K., García-Veigas, J., Cendón, D.I., 2022. [Ca²⁺] and [SO₄^2-] in Phanerozoic and terminal Proterozoic seawater from fluid inclusions in halite: The significance of Ca-SO₄ crossover points. Earth Planet. Sci. Lett. 594, 117712. https://doi.org/10.1016/j.epsl.2022.117712