Our primary research interests focus on the development of a habitable planet and include crustal evolution and the oxygenation history of terrestrial reservoirs (atmosphere, oceans, and mantle). We also focus on the refinement of stable and radiogenic isotope analytical techniques for these studies.
Our primary tools are radiogenic isotope systems, which we use as: 1) clocks to place rocks into the appropriate stratigraphic and Earth history context; and 2) tracers of a variety of processes, such as mantle melting or degrees of chemical weathering.
If you have any comments regarding our work or are interested in collaboration, please contact us.
Timeline of major projects in our lab, showing in-house (UW-Madison Dept. of Geoscience) science collaborators. Names in purple are current members of our lab group, as of Fall 2024.
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1. Origin and evolution of the continents
We are interested in documenting the rates and processes of continental crust creation and destruction in the Archean and Proterozoic. A further-refined crustal record will allow us to determine the initiation of plate tectonic processes on Earth.
The isotopic record of the oldest terrestrial crust, the Acasta Gneiss Complex
We have been working on the radiogenic isotope (U-Pb, Lu-Hf, Sm-Nd) and petrologic record of the Acasta Gneiss Complex of the Northwest Territories, Canada to evaluate processes documented during >1 Gyr of magmatism (2.9 to >4.0 Ga) and several contemporaneous and subsequent metamorphic episodes.
Bauer, A.M., Fisher, C.F., Vervoort, J.D., Bowring, S.A. (2017). Coupled zircon Lu-Hf and U-Pb isotopic analyses of the oldest terrestrial crust, the >4.03 Ga Acasta Gneiss Complex. Earth and Planetary Science Letters, 458, 37-48.
Reimink, J.R., Bauer, A.M., Chacko, T.S. The Acasta Gneiss Complex. In: Earth’s Oldest Rocks, 2nd edition (2018).
Reimink, J.R., Chacko, T., Carlson, R.W., Shirey, S.B., Liu, J., Stern, R.A., Bauer, A.M., Pearson, D.G., Heaman, L.M. (2018). Petrogenesis and tectonics of the Acasta Gneiss Complex derived from integrated petrology and 142Nd and 182W extinct nuclide-geochemistry. Earth and Planetary Science Letters, 494, 12-22.
Fisher, C. M., Bauer, A. M., & Vervoort, J. D. (2020). Disturbances in the Sm–Nd isotope system of the Acasta Gneiss Complex—Implications for the Nd isotope record of the early Earth. Earth and Planetary Science Letters, 530, 115900.
Fisher, C.M., Bauer, A.M., Pearson, D.G., Sakar, C., Hanchar, J.M., Vervoort, J.D., Luo, Y. (2020). Laser ablation split-stream analysis of the Sm-Nd and U-Pb isotope compositions of monazite, titanite, and apatite–Improvements, potential reference materials, and application to the Archean Saglek Block gneisses. Chemical Geology, 119493.
Reimink, J. R., Davies, J. H., Bauer, A. M., & Chacko, T. (2020). A comparison between zircons from the Acasta Gneiss Complex and the Jack Hills region. Earth and Planetary Science Letters, 531, 115975.
Bauer, A.M. and Reimink, J.R., Chacko, T.S., Foley, B.F., Shirey, S.B., Pearson, D.G. (2020). Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonics. Geochemical Perspectives Letters, 14, 1-6.
Bauer, A. M., Vervoort, J. D., & Fisher, C. M. (2020). Unraveling the complexity of zircons from the 4.0–2.9 Ga Acasta Gneiss Complex. Geochimica et Cosmochimica Acta, 283, 85-102.
Drabon, N., Byerly, B.L., Byerly, G.R., Wooden, J.L., Wiedenbeck, M., Valley, J.W., Kitajima, K., Bauer, A.M., Lowe, D.R. (2022). Destabilization of long-lived Hadean protocrust and onset of pervasive hydrous melting at 3.8 Ga. AGU Advances, 3(2), e2021AV000520.
Reimink, J.R., Crow, C., Moser, D., Jacobsen, B., Bauer, A.M., Chacko, T. (2023). Quantifying the Effect of late bombardment on terrestrial zircons. Earth and Planetary Science Letters, 604, 118007.
Droubi, O.K., Bauer, A.M., Bonamici, C., Nachlas, W., Tappa, M., Garber, J., Reimink, J.R. (2023). U-Th-Pb and trace-element evaluation of existing titanite and apatite LA-ICP-MS reference materials and determination of Th-U date discordance in Archean accessory phases. Geostandards and Geoanalytical Research, 47(2), 337-369.
Mixon, E.M., Bauer, A.M., O’Neil, J., Rizo, H., Blum, T.B., Valley, J.W., Bonamici, C.B. (2023) Mechanisms for generating elevated zircon δ18O in Archean crust: Insights from the Saglek-Hebron Complex, Canada. Earth and Planetary Science Letteers, 624, 118443.
Mixon, E. E., Bauer, A.M., Blum, T.B., Valley, J.W., Rizo, H., O’Neil, J., Kitajima, K. Zircon Geochemistry from Earliest Evolved Terranes Records Coeval Stagnant- and Mobile-lid Tectonic Regimes. In press, PNAS, August 2024.
2. The Rise of Atmospheric Oxygen
The Dynamics of the Lomagundi-Jatuli Excursion and Implications for Early Life
Temporal constraints on the rise of atmospheric oxygen have fundamental implications for the trajectory of biological evolution, yet the rate of change of atmospheric oxygen in the aftermath of the ~2.33 Ga Great Oxidation Event is uncertain. Following this rise, the Lomagundi-Jatuli positive carbon isotope excursion documents a massive and anomalous disruption to biogeochemical cycling (sustained C >10‰) for as long as ~260 Ma. The long-term enriched carbon isotopic values recorded in oceanic carbonates throughout the Lomagundi-Jatuli excursion have been interpreted to correspond with a surge in atmospheric O2, however, the mechanisms of onset and termination are poorly understood. These age constraints have different implications for the operation of global biogeochemical cycles in the Paleoproterozoic and thus it is important to discern between them in order to understand the development of complex life in response to increasing levels of atmospheric oxygen.
We are using petrography, redox-sensitive element distributions, Nd isotope stratigraphy, Os isotope stratigraphy, and the Re-Os chronometer on sedimentary sequences from this time interval to constrain the timing of chemical signatures of oxidative weathering.
Millikin, A.E.G., Uveges, B.T., Izon, G., Bauer, A.M., Summons, R., Evans, D.A., Rooney, A.D. A new Re-Os age constraint informs the dynamics of the Great oxidation Event. In press, Geology, August 2024.
3. Proterozoic Refugia for Early Eukaryotes
We are working to provide the first systematic, integrated study of the use of Sr isotope stratigraphy to evaluate syn-and post-depositional features of the Mesoproterozoic Nonesuch Formation. The integration of Sr isotope stratigraphy with detailed sedimentologic and geochemical profiles will allow us to provide fundamental constraints on the distribution, genetic relationship, and depositional setting of these Precambrian source rocks, which include perhaps the oldest record of terrestrial eukaryotes.
Stewart, E.K., Bauer, A.M., Prave, A.R. (2023). End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA. Accepted by GSA Bulletin, October 2023. https://doi.org/10.1130/B37060.1
4. Analytical Technique Development in Isotope Geochemistry
An optimized procedure for solution and laser ablation ICP-MS analysis of sub-nanogram quantities of Hf in complex zircon
The ability to run subnanogram Hf isotopic samples will enable analysis of smaller domains of zircon, which will improve our ability to resolve small-scale complexity and will also provide a basis for analyzing minerals with lower total [Hf]. We started this project in 2014 because it was too challenging to achieve meaningful precision via conventional Lu-Hf acquisition protocols for <5 ng Hf. In collaboration with Matt Horstwood at the British Geological Survey, we developed low-volume solution introduction methods without Hf-REE separation using Hf amounts as low as 0.4 ng, and laser ablation methods using successive 25um spot U-Pb and Lu-Hf ablation pits with a combined depth of 18 μm, while retaining an uncertainty level of ca. 1 ε for both methods. We investigated methods of Yb interference correction and the potential for matrix effects, with a particular focus on the accurate quantification of Lu/ Hf. We demonstrate that optimizing for low oxides, largely without compromising sensitivity, allows accurate Yb correction of neat zircon washes with the added benefit that Lu-Hf data can be gathered, too. These improvements reduce the minimum amount of material required for U-Pb and Lu-Hf isotopic analysis of zircon by about an order of magnitude.
Bauer, A.M., Horstwood, M.S.A. (2018). Small-volume Lu-Hf and U-Pb isotope determination of complex zircons by solution and laser ablation MC-ICP-MS. Chemical Geology, 476, 85-99.
