Extraterrestrial organic matter preserved in 3.33 Ga sediments from Barberton, South Africa

It is widely believed that most of the water and organic matter on the Hadean Eon (4.54–4 Ga) was of extraterrestrial material (Morbidelli et al., 2012, Marty et al., 2013 and references therein). While recent models suggest that the majority of extraterrestrial flux occurred before 4.4 Ga (Boehnke and Harrison, 2016, Genda et al., 2017), there is nonetheless evidence for continued input of extraterrestrial material throughout the Early Archaean (∼4–3.2 Ga). For example, Schoenberg et al. (2002) document evidence for meteoritic input through tungsten isotope analysis of 3.7 Ga metamorphosed sediments from Isua, Greenland, while a number of layers of impact spherules have been observed in Early-Mid Archean (∼3.5–3.2 Ga) sediments from the Barberton Greenstone Belt (South Africa) and the Pilbara Craton (Western Australia) (Glikson et al., 2004, Gomes et al., 2005, Krull-Davatzes et al., 2010, Bottke et al., 2012, Lowe et al., 2014). This flux still continues today, albeit to a lesser extent (e.g., Love and Brownlee, 1991, Yada et al., 2004).

Extraterrestrial material may have provided substantial sources of complex organic molecules for the emergence of life (e.g. Cooper et al., 2001, Strasdeit, 2005, Pasek and Lauretta, 2008). Organic matter in carbonaceous chondrites can reach up to 4% (Sephton, 2002) and up to 85% in ultracarbonaceous micrometeorites (Duprat et al., 2010, Dartois et al., 2018). About 75% of organic matter in carbonaceous chondrites is refractory, consisting mostly of small aromatic moieties linked by short and branched aliphatic chains (Derenne and Robert, 2010). This insoluble component will hereafter be referred to as insoluble organic matter (IOM). Although representing only a small fraction, soluble components exhibit a vast range of composition, with more than 14,000 different molecules containing C, H, O, N and S having been identified to date (Schmitt-Kopplin et al., 2010, Remusat, 2014). Meteoritic debris and micrometeorites falling into the ocean of the early Earth would have been rapidly broken up and partially dissolved in the globally acidic and warm ocean (Pinti, 2005, Tartèse et al., 2017), thus liberating both the soluble and the refractory fractions (Westall et al., 2018). The soluble molecules would be rapidly dissolved into seawater whereas the refractory materials (the IOM) would remain in the detrital fraction of the volcanic sediments coating the floor of the ocean and the numerous platform-covering (submerged continents) shallow seas. Of particular interest is the fate of this latter fraction, its potential involvement in prebiotic reactions leading to the emergence of life, and its distinction from organic biosignatures of ancient traces of life. In this study, we describe evidence of the oldest traces of extraterrestrial organic matter yet identified through a detailed, sedimentological, geochemical, and Electron Paramagnetic Resonance (EPR) study of Early Archaean (3.33 Ga) sediments in the Barberton Greenstone Belt (BGB). These results will certainly have implications for the search for organic biosignatures of primitive life on Earth and on Mars

The josefsdal chert and its sedimentological context

The 3.33 Ga Josefsdal Chert (the stratigraphic equivalent of unit K3c in the Kromberg Formation), located in the southern part of the Barberton Greenstone Belt (Fig. 1A) (Lowe et al., 2012) is an extensive deposit of mostly basaltic volcanic sediments deposited on top of hydrothermally silicified basalts extruded onto a shallow water platform (Westall et al., 2006, Westall et al., 2011, Westall et al., 2015). The 7 m to up to 20 m thick deposit (variations in thickness depending upon

Samples

Samples were obtained during numerous field campaigns. Dedicated field work to elucidate the geological context was undertaken in 2012 and 2014. Two samples, collected in upper shoreface facies C and D of unit 3 (Fig. 1) were selected for study by EPR. The first sample, labelled α (size 5.7 cm × 2 cm) was collected from the tuffaceous layer of Facies D marked by an arrow in Fig. 1C. This sample, shown in Fig. 4, Fig. 6, contains four of the eight black horizons of the 7th bundle, labelled E, F,

General EPR features of Josefsdal chert

The EPR spectra of the 40 slices of the laminated chert (sample α) vary significantly according to their stratigraphic (vertical) position. Fig. 3 shows two examples of EPR spectra corresponding to slice 33 (located above black horizon E) and slice 25 which contains lamina F′ of horizon F. The locations of these slices in the sample are shown in Fig. 4. Slice 33 (Fig. 3A and B) is lithologically representative of the bulk sample, except for the relative intensity of the EPR signals, which

Discussion

Conclusions

We have shown that a 1 mm-thick lamina of carbonaceous matter in rhythmically-graded tuffaceous sediments deposited in a littoral (shoreface to upper shoreface) environment exhibits EPR characteristics of IOM signature (cw-EPR spectrum lineshape and linewidth; pulse-EPR (HYSCORE) spectrum of ¹H and ¹³C) that are similar to those observed in the IOM of carbonaceous chondrites. These extraterrestrial IOM signatures are co-located with the EPR signature of submicrometric-sized particles of

Acknowledgments