Geochemical constraints on the Hadean environment from mineral fingerprints of prokaryotes

Abstract

The environmental conditions on the Earth before 4 billion years ago are highly uncertain, largely because of the lack of a substantial rock record from this period. During this time interval, known as the Hadean, the young planet transformed from an uninhabited world to the one capable of supporting, and inhabited by the first living cells. These cells formed in a fluid environment they could not at first control, with homeostatic mechanisms developing only later. It is therefore possible that present-day organisms retain some record of the primordial fluid in which the first cells formed. Here we present new data on the elemental compositions and mineral fingerprints of both Bacteria and Archaea, using these data to constrain the environment in which life formed. The cradle solution that produced this elemental signature was saturated in barite, sphene, chalcedony, apatite, and clay minerals. The presence of these minerals, as well as other chemical features, suggests that the cradle environment of life may have been a weathering fluid interacting with dry-land silicate rocks. The specific mineral assemblage provides evidence for a moderate Hadean climate with dry and wet seasons and a lower atmospheric abundance of CO₂ than is present today.

Figure 1

Composition of prokaryotes and dispersion of elemental abundances. (a) Observed prokaryotic compositions versus literature data. Uncertainties (1σ) comprise reported values for literature data and variations in our repeated measurements. (b) Interquartile ratios calculated for measured elemental contents in prokaryotes (n = 11, see Methods). Biologically essential chemical elements (Supplementary Discussion) are marked by green color. Also the plot illustrates the dispersion of solution species (dark green) supporting the mineral fingerprints considered in this research.

Figure 2

Mineral fingerprints in prokaryotes and minerals precipitated in altered basalts. (a) Mineral saturation indices calculated for observed and literature compositions⁴⁸ of various prokaryotes at conditions of their cultivation. For a few species, barite saturation indices (unfilled circles) were estimated using the median valuve of sulfur measured in other species. The shaded region approximates the equilibrium range. Uncertainties are 2σ. (b) An analogue for the cradle environment in the modern weathering regolith of Paraná basalts. Mineral abbreviations: Ap – apatite, Brt – barite, Chal – chalcedony, Fe-smc – Fe-smectite, Sph – sphene.

Figure 3

Si and Ti speciation in fluids saturated with respect to various minerals versus contents in prokaryotes. (a) The ΣSi and SiO₂,aq in solutions saturated with amorphous silica and chalcedony. NaHSiO₃,aq and HSiO₃⁻ trends correspond to saturation in amorphous silica. (b) The bulk Ti content and solution species correspond to saturation level of sphene and rutile at pH = 7 and cation composition of E. coli. They are propagated to other pH values assuming the same content of Ti(OH)₄,aq. In the case of rutile, ΣTi and Ti(OH)₄,aq are shown only. Uncertainties (2σ) on thermodynamically predicted total Si and Ti are shown by shaded regions. The prokaryotic abundances are marked with squares scaled by their 2σ uncertainty. The unfilled circles reveal the estimated contents of SiO₂,aq and Ti(OH)₄,aq in cells. At pH < 8 SiO₂,aq equals to the bulk Si content.

Figure 4

Constraints on the Hadean environment. (a) A pe⁻-pH diagram illustrating the redox buffers and stability fields of minerals calculated for E. coli composition at 15 °C and 1 bar. The shaded region outlines the conditions where the measured solution is stable. (b) Determination of equilibrium temperature (T_eq) for E. coli composition at pH-pe⁻ conditions marked by green star on the Fig. 4a. The grey area represents 2σ uncertainty for equilibrium. (c) The upper limit for pCO₂ in the cradle environment constrained by the precipitation of sphene. Calculations are based on temperatures and pH estimated using the mineral equilibria approach. CO₂ pressure calculated using compositions of archaea - dark green, E. coli – dark yellow, other bacteria - light green, and literature data (Supplementary Table 1) - dark cyan. Uncertainties are 1σ. Mineral abbreviations: Anl – analcime, Ap – Cl-apatite, Brt – barite, Bth – berthierine, Cer – cerianite, Chal – chalcedony, Clc – clinochlore, Fe-smc – Fe smectite, Hsm – hausmannite, Ilm – ilmenite, Lmt – laumontite, Micr – microcline, Mnt-Ca and Mnt-Mg – Ca- and Mg-montmorillonites, Py – pyrite, Pcr – pyrochroite, Qtz – quartz, Rt – rutile, SiO₂* – amorphous silica, Sp – sphalerite, Sph – sphene, Znc – zincite.

Figure 5

REE in prokaryotes versus (a) their contents in nutrient media (1-REE ratios for cells before and 2-after washing procedure) and (b) selected rocks and fluids. The C1 normalized plots are shown for Mount Roe basalt paleosol (MR#1), Paraná basalt, upper continental crust, mean trends for continental and oceanic hydrothermal fluids, Upper Paraná river water draining the Paraná basalts and seawater. The median (dark cyan) is plotted for archaea (dark green), E. coli (dark yellow) and other bacteria (light green). Uncertainties are 1σ.