Microfossil Phosphatization and Its Astrobiological Implications

Abstract

One of the major tasks of astrobiology is to critically examine evidence of past or present ecosystems beyond our planet. As Earth is the only planet that is known to have hosted life, perhaps as early as 3.8–3.5 billion years ago (or Ga, Giga anna) as illustrated by biologically-meaningful carbon isotopic signatures and prokaryotic microfossils (Mojzsis et al., 1996; Schopf, 2006; but see van Zuilen et al., 2002; Brasier et al., 2006; Fedo et al., 2006), it provides the only model for us to learn how traces of life can be preserved and recognized. In this contribution, we focus on fossil preservation through phosphate mineralization and discuss its implications for the identification of possible life (particularly ancient life if it did exist) on other planets. In this contribution, we ask the question how morphological evidence (as opposed to geochemical evidence) of microbial life – if it did exist – would be best preserved in extraterrestrial environments. We approach this question by briefly reviewing the taphonomic pathways in the Proterozoic (2.5–0.54 billion years ago) fossil record. This is followed by a more detailed analysis of three-dimensional phosphatization of non-biomineralizing microorganisms in the Neoproterozoic Doushantuo Formation. We focus on the phosphatization window of the Doushantuo Formation because it represents one of the most powerful taphonomic pathways through which soft-bodied microorganisms can be preserved. We then close our chapter by discussing the astrobiological relevance of phosphatization.