A Hydrothermal-Sedimentary Context for the Origin of Life

Abstract

Critical to the origin of life are the ingredients of life, of course, but also the physical and chemical conditions in which prebiotic chemical reactions can take place. These factors place constraints on the types of Hadean environment in which life could have emerged. Many locations, ranging from hydrothermal vents and pumice rafts, through volcanic-hosted splash pools to continental springs and rivers, have been proposed for the emergence of life on Earth, each with respective advantages and certain disadvantages. However, there is another, hitherto unrecognized environment that, on the Hadean Earth (4.5-4.0 Ga), would have been more important than any other in terms of spatial and temporal scale: the sedimentary layer between oceanic crust and seawater. Using as an example sediments from the 3.5-3.33 Ga Barberton Greenstone Belt, South Africa, analogous at least on a local scale to those of the Hadean eon, we document constant permeation of the porous, carbonaceous, and reactive sedimentary layer by hydrothermal fluids emanating from the crust. This partially UV-protected, subaqueous sedimentary environment, characterized by physical and chemical gradients, represented a widespread system of miniature chemical reactors in which the production and complexification of prebiotic molecules could have led to the origin of life. Key Words: Origin of life-Hadean environment-Mineral surface reactions-Hydrothermal fluids-Archean volcanic sediments. Astrobiology 18, 259-293.

FIG. 1.

(A) Location of the study area within South Africa. (B) General geological map of the Barberton Greenstone Belt showing location of the JC (white box). (C) Detailed geological map of the thin JC sediment exposures (red) within large thicknesses of mafic and ultramafic volcanics. Adapted from Westall et al. (2015a).

FIG. 2.

Early Archean, laminated shallow marine volcanogenic sediments interacting with pervasive hydrothermal effluent from the 3.33 Ga JC. (A) Field photograph of alternating ash (light) and carbonaceous (dark) layers. (B) Photomicrograph of a thin section from the same facies documenting layers disrupted, through soft sediment deformation, by infiltrating hydrothermal fluids (arrowed). Red box outlines detail in (D). (C) Raman map of (B), showing carbon (green) within the silicified matrix (orange, quartz); anatase (blue) represents altered volcanic clasts. Red box outlines details in (D). (D) Raman map (red boxes in B, C) showing carbon (green) intermixed with volcanic particles (represented by alteration phases: anatase, blue; muscovite, pink); the quartz matrix (yellow/orange) represents the silica precipitated by hydrothermal fluids. Optical image (E) and Raman map (F) are details of the red box in (D) showing carbon (green) coating volcanic particles (arrowed), which have been replaced by muscovite (pink), anatase (blue), and quartz (yellow/orange). Additional minerals: magnetite, light blue; rutile, red.

FIG. 3.

Crushed volcanic rocks (mixture of East Pacific Rise basalt and komatiite) before and after corrosion in artificial seawater (starting pH = 6.24). (A) Scanning electron microscope view of the surface of basalt glass with skeletal pyroxene crystals showing mineralogical and morphological variability on the micron scale. (B) Detail of the volcanic glass surface (with inset) documenting submicron-scale morphological heterogeneity. (C) Sample from the same crushed volcanic rocks after 15 days of corrosion (ending pH = 7.1). The pyroxene crystals show pits and pores ranging in size from submicron to a few microns. (D) Phyllosilicate-coated surface of a volcanic glass shard corroded for 15 days (ending pH = 7.1).

FIG. 4.

Corroded volcanic grains from a mixture of East Pacific Rise basalt and komatiite (as in Fig. 3). (A) Changing pH during the first 3 days of corrosion of a mixture of East Pacific Rise basalt and komatiite in artificial seawater with a starting pH of 6.24. The red curve denotes pH changes in the sediment, blue curve changes in the overlying seawater. The rapid change in pH to more alkaline conditions from an initial weak acid is ascribed to the reaction of this seawater with particularly reactive minerals in the volcanic grains; the pH gradient in the pore spaces of volcanic sediment can thus be initiated in days. (B, C) Scanning electron micrographs of an ex-gas bubble in a pyroxene grain, documenting variable surface textures including porosity and protrusions associated with compositional variability (D). (D) EDX elemental maps showing two main phases: needle-shaped crystals of pyroxene (Na, Mg, Al, Ca, trace Fe; Si not shown) co-precipitated with ilmenite (FeTiO₃).

FIG. 5.

Macroscopic to microscopic hydrothermal veins. (A) Late diagenetic, vertical hydrothermal vent (full arrow) cross-cutting hummocky-swaley storm deposits in the JC (Facies A of Westall et al., 2015a). Note also infiltrations of hydrothermal chert, emanating from the central vent, parallel to the sediment layering (outlined by dotted red lines). (B) At the thin-section scale, this optical micrograph shows a hydrothermal veinlet cross-cutting finely laminated, fine-grained sediments. (C) Raman map of carbon distribution in the sediment and in the vein shown in B); brighter color indicates higher concentration, i.e., carbon is at its highest concentration when entrained within the vein.

FIG. 6.

Silica gel-like sediment from the 3.33 Ga JC. (A) Scanned thin section slide of a deposit of hydrothermal silica containing carbonaceous clots and layering. (B) Detail of red box in (A) showing fine-scale carbonaceous layering in the lower part of the image and a mottled carbonaceous texture in the upper part. The top of the layered section shows plastic deformation (dashed white arrow) and tearing, indicative of disruption by the dynamic flow of hydrothermal fluids. Red arrows indicate the cohesive layer above the plastically deformed layer, and solid white arrows indicate detrital sedimentation below. The red box denotes the detail in (C) and the Raman scan in (D), while the yellow box denotes the area of the Raman map in (E). (C) Detail showing tearing of the cohesive surface of the finely laminated layer (black arrow). (D, E) Raman maps demonstrate that the sample consists of only quartz (yellow-orange) and carbon (green).

FIG. 7.

Hydrothermal veinlet in fine-grained carbonaceous sediments from Josefsdal. (A) Optical micrograph showing a hydrothermal veinlet cross-cutting finely laminated, fine-grained sediments. (B) Raman map of the siderite distribution in the sediment and in the veinlet; brighter color indicates higher concentration. (C, D) Optical micrograph views of a siderite (FeCO₃) and rhodochrosite (MnCO₃) co-precipitate at the edge of the veinlet shown in (A). Red box in (C) shows location of detail in (D). Arrow in (D) points to the same location as the arrow in (E). (E) PIXE elemental maps (beam size: 2 μm; map size: 500 × 500 μm; resolution: 256 × 256 pixels; 11 h acquisition time) of area denoted by red box in (C) document concentrations of other elements associated with the siderite/rhodochrosite precipitation, including Mn, Sr, Ca, Ni, Cu, Ti, Cr, and Zn scavenged from the hydrothermal fluids.

FIG. 8.

Geochemical analyses demonstrating the bulk influence of hydrothermal fluids on the JC sediments (from Hubert, 2015). (A) Shale-normalized (PAAS, McLennan, 1989) REE+Y patterns of several samples from the JC. The positive Eu anomaly is indicative of a hydrothermal influence (Danielson et al., ; Derry and Jacobsen, 1999). (B) Plot of Eu and Ce anomalies (shale-normalized; PAAS, after McLennan, 1989). Eu/Eu*: Eu/((Sm*05) + (Gd*05)) and Ce/Ce*: Ce/((La*05) + (Pr*05)). An Eu/Eu* value (related to the Eu anomaly) of >1 indicates a hydrothermal signature (Danielson et al., ; Derry and Jacobsen, 1999), while Ce/Ce* (related to a La anomaly) indicates a strong marine signal where <1. The results from two additional samples, silicified carbonates of the 2.9 Ga silicified stromatolites of the Pongola Supergroup, South Africa, and a silicified komatiitic basalt from Josefsdal are given for comparison. The Pongola stromatolite displays a marine signature (Ce/Ce* <1 plus La anomaly) but no hydrothermal signature, while the silicified basalt exhibits a hydrothermal signal (Eu/Eu* of >1 plus Eu anomaly) without marine influence. Josefsdal sediment samples show mixed signatures indicating fluids influenced by both marine waters and hydrothermal activity. (C) PIXE spectrum acquired for 8 h with a 2 μm diameter proton beam from a hydrothermal chert vug in highly silicified Facies D sediments (after Westall et al., 2015a) showing the presence of a number of hydrothermally transported elements, including Fe, Ni, Cu, Zn, As, and Ba.

FIG. 9.

Hydrothermal element scavenging by altered volcanic particles. (A) Thin section of sedimented volcanogenic particles showing dark layers comprising concentrations of volcanic particles (including spherules), as well as traces of carbon and microcrystalline pyrite (Facies D, after Westall et al., 2015a). (B) Optical micrograph of the volcanic particles in a black layer in the JC sediments. Yellow box denotes the area of the regions in (C). (C) PIXE elemental maps (beam size: 2 μm; map size: 300 × 300 μm; resolution: 256 × 256 pixels; 3.5 h acquisition) showing the concentration of Fe, As, Ni, and Cu, trace elements of hydrothermal genesis scavenged by the altered volcanic particles.

FIG. 10.

“Traffic Light— diagram comparing the potentials of the proposed environments for the origin of life. The conditions necessary for prebiotic complexification leading to the origin of life are split into origination (the ability of the environment to provide the molecular and mineral components that co-facilitate prebiotic reactions), complexification (the ability of the environment to sustain conditions conducive to both continued directional reactions and the overall diversification of the molecular complement of the system), and plausibility (the relevance of the environment to, and supposed survival on, the Hadean Earth, based upon available geological evidence). The text below the diagram outlines the rationale for the assignment of the color code. See main text for further interpretation of this diagram.

FIG. 10.

“Traffic Light— diagram comparing the potentials of the proposed environments for the origin of life. The conditions necessary for prebiotic complexification leading to the origin of life are split into origination (the ability of the environment to provide the molecular and mineral components that co-facilitate prebiotic reactions), complexification (the ability of the environment to sustain conditions conducive to both continued directional reactions and the overall diversification of the molecular complement of the system), and plausibility (the relevance of the environment to, and supposed survival on, the Hadean Earth, based upon available geological evidence). The text below the diagram outlines the rationale for the assignment of the color code. See main text for further interpretation of this diagram.

FIG. 11.

Schematic synthesis of the proposed Hadean, hydrothermal-sedimentary micro-reactor environment for complexification of prebiotic chemistry. Slightly acidic seawater entraining dissolved and particulate carbonaceous matter of diverse origins permeates through ultramafic/mafic sediments into the crust (insert 1), altering the ultramafic rocks and becoming more alkaline during these reactions. Light-weight carbon molecules and gases (e.g., H₂, CH₄) formed by Fischer-Tropsch-type processes (Shock et al., 2002), as well as molecules from ultramafic fluid inclusions (Van Kranendonk et al., 2015), were convected into reactive porous sediments at the bottom of the sea (the sediment-water interface, insert 2), where a temperature and pH disequilibrium (insert 3) with the overlying acidic seawater existed. Convection of warm, carbon-bearing hydrothermal fluids allowed prebiotic molecules to concentrate and self-assemble in pore spaces and on the surfaces of chemically reactive minerals, resulting in the formation of increasingly complex molecules.