UNDERSTANDING THE EFFECTS OF ASTEROID COLLISIONS ACROSS EARTH’S GREAT OXIDATION 3.5-2 Ga

Simone Marchi1, B. Black2, N. Drabon3, D. Ebel4, R. Fu5,

B. Johnson6, T. Schulz7,8, K. Wuennemann9,

1Southwest Research Institute, Boulder, CO ; 2City University of New York, New York, NY;

3Stanford University, Stanford, CA; 4American Museum of Natural History, New York, NY;

5Harvard University, Cambridge, MA;6Brown University, Providence, RI;

7University of Vienna, Vienna, Austria;8University of Cologne, Germany;

9Natural History Museum, Berlin, Germany.

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Collisions across the Great Oxidation Event (GOE)

We estimate that archean to paleoproterooic impacts had energies up to 500x Chicxulub.

How do we constrain the impact flux 3.5-2.0 Ga?

Knoll and Novak 2017

Lunar bombardment models 4.5-3.5 Ga

Observational constraints:

I. Radiometric ages (Apollo and Luna missions)

II. Highly siderophile elements (HSE; Os, Au, Ir, etc)

Recent bombardment models:

“Early” instability ( )

“Late” instability ( )

Marchi et al 2012, 2013Morbidelli et al 2012, 2018

I II

Neukum et al 1984; Marchi et al 2009; Robbins 2014

Terrestrial bombardment models 4.5-3.5 Ga

Observational constraints:

HSEs: 0.5%-2.5% of Earth’s mass delivered by late impactors. Walker 2009; Marchi et al 2018

Monte Carlo collisional models:

Regardless of late vs early instability, the flux at 3.5 Ga is relatively well constrained.Morbidelli et al 2018

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HSEs

Marchi et al 2014

Paraburdoo ejecta layer, 2.575 Ga

Archean and paleoproterozoic spherule layers

Archean and paleoproterozoic spherule layers

Lowe et al 2014; Byerly et al 2002; Dradon et al 2017; Fritz et al 2016; Schulz et al 2017; Drabon 2011; Ozdemir et al 2017, 2019; Glass and Simonson 2013

From spherule layers to impactors:

The impactor size can be estimated using the model by Johnson and Melosh 2012.

A layer thickness of 10 cm yields a projectile diameter 29-46 km.

The plot shows the “randomized” distribution of projectile masses.

Modeling archean and paleoproterozoic impacts

All spherule layers

Most likely spherule layers

Modeling archean and paleoproterozoic impacts

From spherule layers to impactors:

The impactor size can be estimated using the model by Johnson and Melosh 2012.

A layer thickness of 10 cm yields a projectile diameter 29-46 km.

The plot shows the “randomized” distribution of projectile masses.

The archean and paleoproterozoic impact flux

A new 3.5-2.0 Ga impact flux model:

We used lunar models to derive a new terrestrial flux.

Spherule layers and lunar craters can be used to benchmark the model:

Time(Ga) Min* Max*

2.0-2.5 1 9

2.5-3.0 2 8

3.0-3.5 13 28

2.0-3.5 (Spherules) 16 35

2.0-3.7 61 (3) 99 (5)

Lunar craters (>150 km) 4

* Number of impactors > 10 km

Environmental effects of collisions

Impacts interact withthe environment bygenerating vaporsand melts.

Hydro-codes can beused to study theproperties of impactvapors and melts.

Hydrothermal alteration of impact melts is well documented (Allen et al, 1992; Thien et al 2015; Osinski et al 2013; Kring 2003; Kring et al 1991, 2017)

Chemical alteration of impact melt

Ries crater suevite is altered by interacting with country rocks.

Alteration leads to leaching of key elements, such as Fe, P, Ni.(Staudigel and Hart 1983; Pauly et al 2011; Black et al, submitted)

An example, Nickel

Nickel in banded iron formations:

Ni is required by methanogens,the drop of Ni over time couldhave helped the rise of oxygen.

The Ni famine is ascribed toreduced rates of Ni-rich ultramafic eruptions.

Could impacts have modulated Ni?

Konhauser et al 2009

Hydro-code simulations of impacts

10 km projectile:

Projectile melt: 9 x 1014 km3

Target melt: 2.4 x 1015 km3

Could impact melts matter?

106 km3 of basaltic impact melt with 0.1 wt% Ni would contain 3 x 1015 kg Ni.

This is ~4,000x the modern ocean Ni inventory.

60 km projectile striking at 20 km/s

Conclusions

The spherule layers provide a direct constraint on the impact flux.

We find that 16-45 impacts > 10 km in diameter could have taken place in the time 3.5-2 Ga, and some of them could have been ~100 km in diameter.

This flux of impactors may have had strong effects on the environment,e.g., via release of key nutrients to the oceans and redox of the atmosphere.

back-up slides

Atmospheric redox in impact vapors

10 km projectile:

Projectile vapor: 3 x 1014 km3

Target vapor: 6 x 1014 km3