Geochemical Biogenicity Criteria – BIOGEN

We combine well-established techniques (mineralogy, bulk-rock geochemical analyses, stable isotopes of Zn, radiogenic isotopes of Pb, MC-ICP-MS, determination of neutron fluence using Sm and Gd isotopic anomalies, synchrotron X-ray micro-fluorescence) with more exploratory techniques (high-precision S isotopic analyses by MC-ICP-MS, organic biomarkers in very old rocks) to analyses of rocks and fossils from the reference sites of the Paleoproterozoic Francevillian series in Gabon, fossil samples from the Cambrian Burgess Shale in Canada, and diatomites and fossils from the Miocene Andance volcanic cone in France. We further need to go back to geochronology and select samples from Gabon that have not been irradiated by the Oklo natural reactor. Additionally, we plan on expanding the project to include Zn isotope measurements of (i) carbonates through time to try to understand the Zn cycle through time and (ii) modern biomass, such as mosses and plants, in order to compare the Zn isotopic signature of fossils of putative biological origin with biological material of certain biological origin.

Project en cours.

Project en cours.

Project en cours.

At the current time of Mars missions, finding robust markers that could be used to establish the biogenicity of objects observed in ancient sediments and planetary materials is an imperative challenge. Carbon and nitrogen are particularly sensitive to degradation of organic matter and to interaction with groundwater. Most of the transition metals used as protein co-factors are redox-sensitive. Zinc, a widely distributed protein co-factor along all metabolic pathways, has been shown to be depleted in light isotopes in organic material and in sediments rich in organic components. Preliminary results on complex enigmatic structures commonly found in black shales from the ca. 2.0 Ga old Francevillian series (Gabon) and undoubtedly representing early organisms show systematic enrichment in light Zn isotopes with respect to their background rocks. These anomalies are correlated with organic carbon contents. We will expand this study with Pb-Pb isotope systematics, in order to rule out diagenetic effects. What characterizes a biomarker is not a particular threshold of a geochemical observation but the correlation between several parameters believed to signal biological material. We will explore how Zn and S isotopic anomalies associated with fossil remains correlate with organic biomarkers and geochemical anomalies of the remains with respect to the bulk-rock. This proposal will combine some well-established techniques (mineralogy, bulk-rock geochemical analyses, Zn and Pb isotopes, synchrotron X-ray micro-fluorescence) with more exploratory techniques (high-precision S isotopic analyses by MC-ICP-MS, organic biomarkers in very old rocks). In addition to the reference site of the Francevillian series, we will collect fossil samples from the Cambrian Burgess Shale and from the Miocene diatomites of the Andance volcanic cone. University de Poitiers and ENS Lyon will be partners in the project.