PP020-06
High rate of collisions on the Archean Earth leads to variable atmospheric oxidation

Wednesday, 9 December 2020: 17:50
Virtual
Simone Marchi1, Nadja Drabon2, Toni Schulz3, Laura Schaefer4, David Nesvorny5, William F Bottke Jr5, Christian Koeberl3 and Timothy Lyons6, (1)Southwest Research Institute, Boulder, CO, United States, (2)Stanford Earth Sciences, Stanford, CA, United States, (3)University of Vienna, Vienna, Austria, (4)Stanford University, Stanford, CA, United States, (5)Southwest Research Institute Boulder, Boulder, CO, United States, (6)University of California Riverside, Department of Earth and Planetary Sciences, Riverside, CA, United States
Recorded Presentation
Abstract:
Recent advances have improved our understanding of the nature and rate of collisions between the Earth and planetesimals left over from planet formation (e.g., [1,2]). Although significant uncertainties in the impact flux remain, it is clear that such energetic events may have drastically affected Earth’s earliest evolution [3], with consequences for its atmosphere [e.g., 4] and tectonics [e.g., 5].

Models for early Archean (4.0-3.5 Ga) impact fluxes are tightly constrained by lunar cratering and radiometric data [6]. Terrestrial impact spherule layers provide an additional yet less complete record of the late Archean (3.5-2.5 Ga) impact flux. Impact spherule layers form from the melting and vaporization of both target and projectile materials [e.g., 7]. There are currently 16 known impact spherule layers identified in outcrop between 3.5-2.5 Ga, some of which have only very recently been discovered [8]. Importantly, recent analyses of deep drill cores from the Fig Tree Group in South Africa (BARB5 and CT3 cores) revealed up to 19 new spherule layers [9,10].

Here we present a new bombardment model for the Hadean-Archean Earth calibrated on the number of late Archean impact-generated spherules layers. We find that the impact flux may have been up to a factor of ten higher than previously recognized. These results require the presence of a long-lived source of impactors, possibly due to leftover planetesimals. We also find that impactors 5-10 km in diameter could have delivered a significant source of reducing gases [11] to Earth’s atmosphere, thus helping to maintain low levels of atmospheric oxygen despite the onset of biological production. The models and data suggest that the impactor flux faded during the 0.5 Gyr preceding the rise of atmospheric oxygen at ~2.5 Ga [e.g., 12], suggesting that waning early bombardment could have contributed to the Great Oxidation Event.


[1] Marchi et al. Nature 511, 2014; [2] Bottke & Norman. AREPS 45, 2017; [3] Lyons et al. Decadal Science White Paper, 2020; [4] Marchi et al. EPSL 449, 2016; [5] O’Neill et al. NatGeo, 2018; [6] Morbidelli et al. Icarus 305, 2018; [7] Melosh & Vickery. Nature 350, 1991; [8] Lowe et al. Geology 42, 2014; [9] Drabon et al. Prec. Res. 325, 2019; [10] Schulz et al. GCA 211, 2017; [11] Schaefer & Fegley. Icarus 186, 2007; [12] Lyons et al. Nature 506, 2014.

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