Sensitivity of carbonate clumped isotope thermometry to dissolution

Project description

Motivation (background):
Reconstructing robust ocean temperatures from past greenhouse climates is critical to advancing our understanding of how the Earth system may operate under increasing atmospheric CO2 levels. Carbonate microfossils, such as foraminifera, are an important archive for reconstructing past climates as they lock chemical signatures relating to past seawater chemistry. However, factors such as dissolution can impact carbonate minerals, both at the time of formation and during burial in marine sediment, and this can alter the primary signal recorded in the microfossils. We have been working on reconstructing past ocean temperatures using clumped isotope thermometry; a relatively new approach to derive robust temperature information from carbonates based on the ordering ("clumping") of stable isotopes within the molecules (Meckler et al., 2022). This new paleothermometer has the advantage of being independent of seawater chemistry and is assumed to be less sensitive to other non-thermal factors, including pH, than other more established paleothermometers (e.g., stable oxygen isotopes and Mg/Ca ratios). The effects of diagenetic alternation on clumped isotopes have been tested (Leutert et al., 2019), but the examples studied there were mostly related to additional calcite overgrowth in the sediments. In contrast, few experimental constraints exist for the influence of dissolution. Constraining the sensitivity of clumped isotopes to dissolution is of critical importance because deep-sea dissolution is a pervasive feature of extreme warming events during past warm climates (e.g., Eocene hyperthermals).

By designing and running a series of dissolution experiments on foraminifera, this masters project aims to assess the sensitivity of clumped isotope signals to dissolution. This will enable the generation of more robust reconstructions of past ocean temperatures using this paleothermometer.

The masters student will join the DOTpaleo project team who are studying Paleogene climate through a combination of clumped isotope-based temperature reconstructions and Earth System modelling. Deriving robust past ocean temperatures is central to the DOTpaleo project and this masters project will contribute to the method development and proxy validation aspects of DOTpaleo.

Research questions:

• Are foraminiferal clumped isotopes sensitive to dissolution?
• Do different carbonate microfossils exhibit differing sensitivities to dissolution?
• Can dissolution bias temperature reconstructions for the deep ocean and/or extreme past warm climate states?

References:
Meckler, A.N., Sexton, P., Piasecki, A.M., Leutert, T.J., Marquardt, J., Ziegler, M., Agterhuis, T., Lourens, L.J., Rae, J.W.B., Barnet, J., Tripati, A., Bernasconi, S.M., 2022, Cenozoic evolution of deep ocean temperature from clumped isotope thermometry, Science 377, 86-90
Leutert, T. J., Sexton, P. F., Tripati, A., Piasecki, A., Ho, S. L., Meckler, A. N., 2019, Sensitivity of clumped isotope temperatures in fossil benthic and planktic foraminifera to diagenetic alteration, Geochimica et Cosmochimica Acta, 257, 354-372

Foto/ill.:

Vicki Taylor

Proposed course plan during the master's degree (60 ECTS)

H25:
GEOV222 (10P)
free choice (10P)

V26:
GEOV302 (10P)
GEOV231 (10P)
GEOV342 (10P)
GEOV331 (5P)

H26:GEOV300 (5P)

Field-, lab- and analysis work:

Sediment samples will be wet-sieved and planktic and benthic foraminifera picked from the dried coarse fraction (some picked samples are already available). A series of dissolution experiments will be performed to subject the foraminifera to differing degrees of dissolution. Foraminifera will then be cleaned and analysed using a clumped isotope mass spectrometer, under guidance, to assess the sensitivity of stable oxygen and carbon isotopes and clumped isotopes to dissolution. Optical and scanning electron microscopy (and other microanalytical techniques) will be used to assess changes in crystalline microstructure following dissolution.

Total estimate lab/analysis time: 6 months

This project is financed through the DOTpaleo project.

NB: this project is not yet approved by the program board