Oxidative conditions can lead to exceptional preservation through phosphatization

Pierre Gueriau,Didier B Dutheil,Loïc Bertrand,Sylvain Bernard,François Farges,Thierry Adatte,Sylvain Charbonnier,Dominique Thiaudière,Brahimsamba Bomou,Cristian Mocuta,Marie Godet

doi:10.1130/g45924.1

Abstract

Abstract Exceptional preservation through phosphatization is primarily controlled by a reduction in pH, favoring the precipitation of apatite over that of calcite. Laboratory experiments have suggested that phosphatization results from anoxic decay. Here we report results of the fine-scale mineralogical characterization of Cretaceous phosphatized fossils of teleost fishes and crustaceans from the Jebel oum Tkout Lagerstätte (Morocco). Data collected using complementary laboratory and synchrotron-based X-ray techniques reveal that oxidative conditions were established at a certain step of decay. Supporting these conclusions are the presence, covering and embedded in the phosphatized tissues, of Fe(III)-rich mineral phases, the precipitation of which was likely biologically induced during decay. The present study highlights that the establishment of oxidative conditions during decay can be compatible with exceptional preservation of fossils through phosphatization.

Highlights

Soft-tissue phosphatization, i.e., calcium phosphate mineralization occurring prior to the degradational collapse of cellular tissues, provides the most spectacular fossils of animals, still exhibiting subcellular details (Martill, 1990; Briggs, 2003)
The spectral decomposition of μXRF data allowed estimating the concentrations of all detected elements, including of rare earth elements (REEs) that were normalized to the post-Archean Australian shale (PAAS) reference (McLennan, 1989) to reconstruct REE patterns, ratios, and anomalies in the fossils at “local”, submillimeter scales (Gueriau et al, 2015)
Instead of a fall in pH under anoxic conditions, the fine-scale micro-geochemical characterization conducted here strongly suggests that oxic conditions prevailed during phosphatization in the OT1 Lagerstätte

Summary

INTRODUCTION

Soft-tissue phosphatization, i.e., calcium phosphate mineralization occurring prior to the degradational collapse of cellular tissues, provides the most spectacular fossils of animals, still exhibiting subcellular details (Martill, 1990; Briggs, 2003). Phosphatization requires a sufficient concentration of phosphorus (Martill, 1988; Briggs and Kear, 1993; Briggs, 2003). Closed systems, such as those built by biofilms growing around carcasses, promote apatite precipitation by trapping phosphorus (Williams and Reimers, 1983; Martill, 1988; Briggs and Kear, 1993; Wilby et al, 1996), which is released by organicmatter degradation under oxic conditions (e.g., Meunier-Christmann et al, 1989; Mort et al, 2007). We report an in-depth and in situ assessment of the redox conditions having prevailed during the phosphatization of “exceptionally preserved” Cretaceous fossils of fishes and crustaceans from the Jebel oum Tkout (OT1) Lagerstätte of Morocco

METHODS

CONCLUSION