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 January 2025.
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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.
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 Letters, 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. Proceedings of the National Academy of Sciences, 121(39), e2405378121.
Droubi, O.K., Bauer, A.M., Bonamici, C.E., Nachlas, W.O., Garber, J.M., Tappa, M.J., Reimink, J.R. Eoarchean-Paleoproterozoic tectonothermal history of the Acasta Gneiss Complex constrained by titanite and apatite petrochronology. Accepted at Geochemistry, Geophysics, Geosystems, June 2025.
Beaudry, P., Jagoutz, O., Bauer, A.M., Rezeau, H., Reimink, J.R., Grove, T.L., Izon, G., Ono, S. (2025). Major-element, trace-element and sulfur-isotope evidence for arc magmatism in the 4.0–2.9 Ga Acasta Gneiss Complex. Contributions to Mineralogy and Petrology, 180(9), 1-23.
Valley, J.W., Blum, T.B., Kitajima, K., Shimizu, K., Spicuzza, M.J., Gonzalez, J., Kita, N.T., Bauer, A.M., Sobolev, A.V., Sobolev, S.V., Jain, C., Cavosie, A.J. Contemporaneous Subduction and Stagnant-Lid Tectonics on the Hadean Earth. In review, May 2025.
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. Geology, 52, 857–862.
Bauer, A.M., Li, W., Rybacki, K.S., Roden, E.E., Kump, L.R., Johnson, C.M. (2025). Subaerial oxidative uranium mobilization at the culmination of the Great Oxidation Event. Proceedings of the National Academy of Sciences, 122(36), e2510289122.
Eyster, A., Tappa, M.J., Santis, A., Abdul Rashid, S., Choi, A., Bauer, A.M., Lepland, A., Kirsimae, K., Prave, A.P. Geochemical evidence for a provenance shift and implications for the end of the Lomagundi-Jatuli Event ~2.0 Ga. In prep.
Hu, P., Tappa, M.J., Stevens, S., Bauer, A.M., Lepland, A., Prave, A.P., Kirsimae, K. Detrital zircons in the Jangozero Fm, Russia, and implications for sedimentary provenance in the Onega Basin. In prep.
3. Proterozoic Refugia for Early Eukaryotes
We are collaborating on multi-disciplinary studies of the Mesoproterozoic Nonesuch Formation, Midcontinent Rift, USA, and the Mesoproterozoic Belt Supergroup, Montana to better constrain the environments in which putative early eukaryotes lived. In the Midcontinent Rift, 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 Nonesuch Formation. In the Belt Supergroup, we are working to provide detrital and authigenic accessory mineral constraints on provenance and fluid flow in the Belt Basin.
Stewart, E.K., Bauer, A.M., Prave, A.R. (2024). End-Mesoproterozoic (ca. 1.08 Ga) epeiric seaway of the Nonesuch Formation, Wisconsin and Michigan, USA. Geological Society of America Bulletin, 136(7-8), 2940-2960.
Adam, Z., Slotznick, S., Bauer, A.M. Early eukaryote restriction to variably oxidized niches: A comparison of growth and excystment features of microfossils from the Belt Supergroup, Montana, USA. In review, July 2025.
Adam, Z., Slotznick, S., Bauer, A.M., Stewart, E.K., Roden, E.E., Peters, S.E., Konhauser, K., Kacar, B. Did iron suppress eukaryote emergence and early radiation? Preprint posted in bioRxiv, May 2025.
Stewart, E.K., Tappa, M.J., Bauer, A.M., Brengman, L., Prave, A., Nachlas, W. A restricted seaway in the Laurentian interior: carbonate C-O-Sr isotope geochemistry of the ca. 1081 Ma Nonesuch Formation, Midcontinent Rift. In prep.
Stewart, E.K., Tappa, M.J., Brengman, L., Prave, A., Nachlas, W., Bauer, A.M. Shale and carbonate geochemistry track redox cycling, productivity, and salinity in the late Mesoproterozoic Laurentian inland sea of the Nonesuch Formation, Wisconsin and Michigan, USA. In prep.
4. Analytical Technique Development in Isotope and Trace-Element Geochemistry
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.
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.
Zhang, M., Fukuda, K., Tappa, M.J., Siron, G., Nachlas, W.O., Kimura, M., Kitajima, K., Bauer, A.M., Kita, N.T. (2024). Forging inner-disk Al-rich chondrules by interactions of CAI-like melt and ambient gas. Geochimica et Cosmochimica Acta, 379, 89-110.
Zhang, M., Fukuda, K., Tappa, M.J.*, Nachlas, W.O., Schneider, B., Kimura, M., Kitajima, K., Bauer, A.M., Kita, N.T. Common refractory anorthite relics hint at inefficient chondrule recycling in the outer protoplanetary disk. In review, May 2025.
5. Other Geochemical/Geochronological Studies
Munroe, J.S., Santis, A.A.*, Soderstrom, E.J., Tappa, M.J.*, Bauer, A.M. (2024). Mineral Dust and Pedogenesis in the Alpine Critical Zone. SOIL, 10(1), 167-187.
Munroe, J.S., Soderstrom, E.J., Kluetmeier, C.L., Tappa, M.J., Mallia, D.V., Bauer, A.M. (2023). Regional Sources Control Dust in the Mountain Critical Zone of the Great Basin and Rocky Mountains, USA. Environmental Research Letters, 18(10), 104034.
Chang, H., Hu, P., Zhou, G., Zhang, W-X., He, Y., Zhao, Y-J., Bauer, A.M., Hu, Z., and Wu, Y-B. (2023). Development of a back-arc basin from initial rifting to seafloor spreading: Constraints from Paleozoic basic rocks in the North Qinling accretionary orogen, central China. Geological Society of America Bulletin, 136(3-4), 1511-1525.
Linol, B., Montañez, I., Lombardo, A., Kuta, D., Upadhyay, D., Arnold, A., Tripati, A., Bauer, A.M., and Musa, S. (2021). Towards disentangling co-evolving sea-level and tectonic changes in Upper Cretaceous-Cenozoic marine sequences flanking southernmost Africa through strontium isotope stratigraphy and carbonate clumped isotope thermometry. South African Journal of Geology, 124 (2), 537-564.
Hu, P., Wu, Y., Bauer, A., Zhang, W., & He, Y. (2021). Zircon U-Pb geochronology and geochemistry of plagiogranites within a Paleozoic oceanic arc, the Erlangping unit of the Qinling accretionary orogenic belt: Petrogenesis and geological implications. Lithos, 394, 106196.
Fu, R.R., Weiss, B.P., Lima, E.A., Kehayias, P., Araujo, J.F., Glenn, D.R., Gelb, J., Einsle, J.F., Bauer, A.M., Harrison, R.J., Ali, G.A., & Walsworth, R.L. (2017). Evaluating the paleomagnetic potential of single zircon crystals using the Bishop Tuff. Earth and Planetary Science Letters, 458, 1-13.
Rioux, M., Garber, J., Bauer, A., Bowring, S., Searle, M., Kelemen, P., & Hacker, B. (2016). Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U–Pb zircon geochronology. Earth and Planetary Science Letters, 451, 185-195.
Kayzar, T. M., Nelson, B. K., Bachmann, O., Bauer, A. M., & Izbekov, P. E. (2014). Deciphering petrogenetic processes using Pb isotope ratios from time-series samples at Bezymianny and Klyuchevskoy volcanoes, Central Kamchatka Depression. Contributions to Mineralogy and Petrology, 168(4), 1-28.
