Frequent violent collisions of impactors from space punctuated the geological and atmospheric evolution of early Earth. It is generally accepted that the most massive collisions altered the chemistry of Earth’s earliest atmosphere, but the consequences of Archaean collisions for atmospheric oxidation are little understood. Early Archaean (4.0–3.5 billion years ago (Ga)) impact flux models are tightly constrained by lunar cratering and radiometric data. Further, a record of the late Archaean (3.5–2.5 Ga) impact flux is provided by terrestrial impact spherule layers—formed by collisions with bodies ≥10–20 km in diameter—although this record is probably incomplete and significant uncertainties remain. Here we show, on the basis of an assessment of impactor-related spherule records and modelling of the atmospheric effects of these impacts, that current bombardment models underestimate the number of late Archaean spherule layers. These findings suggest that the late Archaean impactor flux was up to a factor of ten higher than previously thought. We find that the delivered impactor mass was an important sink of oxygen, suggesting that early bombardment could have delayed Earth’s atmosphere oxidation. In addition, late Archaean large impacts (≥10 km) probably caused drastic oscillations of atmospheric oxygen, with an average time between consecutive collisions of about 15 Myr. This pattern is consistent with a known episode of atmospheric oxygen oscillation at ~2.5 Ga that is bracketed by large impacts recorded by Bee Gorge (~2.54 Ga) and Dales Gorge (~2.49 Ga) spherule layers.
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We acknowledge insightful discussions with D. Lowe, B. Black, B. Johnson and R. Fu. T.S. was funded by the German Research Foundation (DFG, SCHU 3061/1-1).
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Roxana Lupu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super.
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Gray curve is the MBA SFD from ref. 40. This SFD was able to generate a NEO SFD in agreement with observations at that time. The orange curve is an update of the MBA SFD—our nominal MBA SFD—that better fit the current NEO SFD (green). The vertical dashed lines indicate reference sizes of 33 m and 5 km (see text for details). For completeness, 1.1 km is also indicated, corresponding to an impactor size that would make a 20 km lunar crater (often used for calibrating bombardment models), for an impactor density of 2.5 g/cm3, 18 km/s impact velocity, 45 deg impact angle (see Methods).
a) Model impactor SFDs (red curves as in Fig. 2b) compared with Monte Carlo retrieval of impactor SFDs from spherule layers for the minimum number of independent layers (Extended Data Table 1). Meaning of symbols and colors as in Fig. 2b. b) Best-fit collisional model. The figure shows the model impactor SFDs (blue curves) for γbest = 10γmax. The green and yellow lines are the impactor SFDs reconstructed from spherule layers (see Fig. 2b), but now impactors sizes are reduced by a factor fbest = 1.5 (see text). The red shaded area indicates the range of impactor SFDs for γmax, as in Fig. 2b.
The calculations assume chemical equilibrium between the total impactor vapor mass and background atmosphere. Here we take a background atmosphere of 0.7 bars N2, 0.3 bars CO2, and 1 ppm O2. The initial impact conditions are taken as 100 bars and 2000 K and used to derive the composition of the impactor vapor in equilibrium with the impacting meteoritic material. We then vary the mass of impactor vapor to atmospheric mass for different impactor diameters. Atmospheric O2 abundances are strongly depleted by 1000 K, indicating that quenching of chemical reactions within the vapor plume should not strongly affect the amount of O2 consumed by the impact.
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Marchi, S., Drabon, N., Schulz, T. et al. Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth. Nat. Geosci. 14, 827–831 (2021). https://doi.org/10.1038/s41561-021-00835-9