Research

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.

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1. The Great Oxygenation Event

The Dynamics of the Lomagundi-Jatuli Excursion and Implications for Early Life

Temporal constraintson 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.

Temporal variability in carbonate δ13C across the Lomagundi-Jatuli isotope excursion. The shaded blue region documents the median ages and δ13C signatures ±1σ (modified after Martin and Condon, 2013). Syndepositional age constraints are indicated with green symbols and are sparse; the gray symbols are ages bracketed by upper and lower constraints obtained using U-Pb. We are working on providing syndepositional Re-Os age constraints for sequences from this time interval.

Coupled S-MIF deviations and Fe isotope signatures recorded in pyrite of the Griqualand-West Basin, Kaapvaal Craton

The evidence for a shift from an oxygen-poor to a slightly more oxygenated atmosphere includes the termination of sulfur mass independent fractionation (S-MIF) signals in the sedimentary rock record after ~2.32 Ga. These S-MIF signals, which are ultimately recorded in early diagenetic pyrite, are attributed to photolysis of gaseous sulfur species in the atmosphere and require low oxygen concentrations for their production, transfer, and preservation.

In collaboration with Shuhei Ono at MIT and researchers at DTM and ASU, we have determined sulfur, iron, and carbon isotopic ratios of pyrite from a 2.50- 2.65 Ga interval of the Agouron GKP-01 drill core of the Kaapvaal craton in order to evaluate fluctuations in S-MIF at a finer stratigraphic scale than previous work (see Ono et al., 2009 Precambrian Research). Our results indicate that pyrite of differing morphologies carry different S-MIF isotopic signatures and we are using Fe isotopes to understand the formation of these different types of pyrite.

Drilling sulfides from carbonaceous shale in the Archean Klein Naute Formation, Campbellrand platform, South Africa.

2. 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 zircon Lu-Hf isotopic record of the oldest terrestrial crust, the Acasta Gneiss Complex

The Lu-Hf isotopic system in zircon is used to track the chemical evolution of the mantle and crust during the Archean. The interpretation of radiogenic isotopic compositions of complex Archean rocks and minerals that have been multiply deformed and metamorphosed requires a targeted approach integrating accurate age determinations with radiogenic isotopic compositions in order to provide meaningful primary isotopic signatures. We have been working on the radiogenic isotope (U-Pb, Lu-Hf, Sm-Nd) 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.

An island in the middle of the Acasta River, Northwest Territories, Canada.

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 Letters530, 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). Zircons document the gradual onset of mobile-lid tectonics. Geochemical Perspective 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 Acta283, 85-102.

3. 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.

Fig. 2 of Bauer and Horstwood (2018). SEM images of ablation spots in small-volume Hf laser ablation study; scale bars are 20 μm and analyses were 10 seconds in duration.