Archean Environment: the habitat of early life

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Programme outline

 

|| Summary || Scientific objectives || Proposed activities ||

 

Proposed activities

The research activities of the proposed programme can be grouped into four main projects, and will involve field studies in principally three regions. The research projects are:

 

Project 1: The Composition and Temperature of Archean Atmosphere and Oceans

The question of how the composition, and particularly the redox state, of the atmosphere and oceans, changed during the first 3 billion years of Earth history has occupied geologists for the past four decades. Lively debate continues between two firmly entrenched schools, one that argues that the atmosphere was relatively reducing for the period from 4.5 Ga up to about 2.5 Ga, the other that proposes that the change to oxidizing compositions similar to those of the present atmosphere happened before 4.0 Ga. Diverse arguments are used to defend the opposing points of view: geological features that record conditions during the deposition of sediments of formation or soils are particularly important, yet in each case questions remain as to whether the inferred conditions reflect only a local environment or that of the atmosphere/hydrosphere as a whole. Other approaches use geochemical data and depend on issues such as the interpretation of mass-independent fractionation of stable isotopes.

As part of an ESF Network we would undertake the following studies:

(a) We will constrain the composition of Archean ocean water by analyzing cherts and fluid inclusions in quartz associated with pillow lavas from various areas and times. We have samples from about 3.6 Ga to 2.02 Ga, from key regions such as Isua in Greenland , Barberton in South Africa , Pilbara in Australia , Belingwe in Zimbabwe and Abitibi in Canada . Complementary information comes from stable isotope studies. The combination of O and Si isotopes in cherts and quartz from amygdales in volcanic rocks provides information on both the composition and temperature of seawater. Cl and B isotopes provide further constraints.

(b) We propose to investigate the evolution through time of the composition of the Archean atmosphere using the isotopic compositions of noble gases (in particular Ne, Ar, Kr and Xe isotopes). The 40Ar/36Ar ratio (where 40Ar is produced by the decay of 40K in the crust and mantle) appears to have changed during the Archean. If atmospheric argon was trapped in fluid inclusions containing seawater that had equilibrated with the Archean atmosphere, the Ar isotope composition of fluid inclusions should constrain the kinetics of atmospheric evolution and discriminate between two types of models, those which propose early and thorough degassing, and those that argue for more protracted degassing.

(c) The nitrogen isotopic composition is linked to the partial pressure of oxygen in the atmosphere through complex feedback mechanisms between the lithosphere, hydrosphere, atmosphere and the biosphere. In order to better understand the role of nitrogen isotopes as tracers of the oxygen fugacity shift in the Archean-Proterozoic, it is highly desirable to investigate the atmospheric isotope composition of nitrogen.

During these studies we will cooperate closely with groups working on analogous systems; on one hand we will interact with planetologists studying the terrestrial planets and satellites of the Jovian planets; on the other we will cooperate with groups working on modern oceanic sediments and hydrothermal deposits.

 

Project 2: The Nature of Archean Landmasses

The goal here will be to obtain information about one possible setting in which life may have formed or evolved. In order to establish the physical and chemical conditions on tidal flats at the margins of Archean continents, or in terrestrial thermal springs, it is essential to know the nature of the rocks and minerals that were exposed at the land surface, and the nature of the atmosphere to which they were exposed. Geological and geochemical investigations of rocks and minerals in the rare localities where subaerial Archean rocks are known, mainly in the Pilbara and Kaapvaal cratons will provide constraints on the physico-chemical conditions and the topography of the land surface.

The nature of weathering and erosion, the types of sediments that resulted from this erosion, the fluxes from land to sea may all have been radically different for the two types of land surface that might have existed in the Archean. At one extreme there may have been “continents” composed of mafic-ultramafic rock. These land surfaces would have had relatively subdued relief, which would mitigate the rate of mechanical erosion, but they would be composed of easily altered rocks. At the other extreme are granitoid continents formed at subduction zones and these would have topographies and compositions more similar to modern continents. However, because of higher temperatures in the mantle and crust itself, due to higher concentrations of heat-producing isotopes, the mountain ranges may have been lower and the topography more subdued. How would these landmasses have reacted to erosion under the more aggressive Archean atmosphere? What types of sediments were deposited around the edges of the early supracrustal masses? What is the origin of the silicification that affects such a high proportion of Archean sedimentary and volcanic rocks? Does the silicification indicate that hydrothermal circulation, both within the oceanic crust and on land, was much more common than now? How did the closer Moon influence tides and their effects on sedimentary structures? How deep was the usual wavebase?

How was the Earth affected by meteorite impacts? The lunar impact record indicates that large impacts were much more frequent in the Archean than at later times. Such large impact events may have been an important factor in the processes that determined the conditions of early life. Large impacts are thought to have had a devastating effect on life in the more recent history of Earth, such as the KT extinction event. However, in the Archean, meteorite impacts may have influenced the origin and the habitat of early life. Pre-biotic molecules, such as amino acids and polycyclic aromatic hydrocarbons, are abundant in space, and meteorites and comets could perhaps have played an important role in providing the necessary ingredients for life on Earth. The energy of the impact and brecciation of target rocks in the crater results in extensive hydrothermal systems that could provide an ideal habitat for early life.

 

Project 3: Interaction between Archean seawater and the oceanic crust.

The goal of this project is to establish the physical and chemical characteristics of oceanic hydrothermal systems in the Archean. More specifically, we would aim to establish the size, geometry, and flow rates of the hydrothermal cells and the changing temperature and chemical compositions (Eh, pH, contents of major and trace elements) of the fluids that moved through the system. Particular attention will be paid to the changes in composition that took place as the fluids flowed out of the crust and mixed with Archean seawater. As with the other projects, we will cooperate closely with groups working on modern hydrothermal systems on land and in the ocean basins.

We will obtain information on Archean systems in two ways.

(a) By studying in detail the traces of hydrothermal circulation that are recorded in the volcanic and sedimentary rocks of Archean greenstone belts. The principal field areas will be the 3.5 Ga Pilbara in Western Australia , the 3.5 Ga Barberton belt in South Africa and the 2.7 Ga Abitibi belt in Canada . In the first area (Pilbara), extensive work by Australian, American and European geologists and geochemists has demonstrated the existence of complicated and extensive zones of siliceous alteration related to hydrothermal circulation. Participants of the ESF network would continue to be involved in this research. The second region ( Barberton ) has received less attention but in some ways it is more interesting than the Pilbara. The rocks are in general better preserved and the search for traces of life is well advanced. By building on the political will to establish research ties between Europe and southern Africa , it should be possible to establish a viable research programme. The third region (Abitibi), although younger, offers the best opportunity to investigate the operation of Archean hydrothermal systems. As a result of decades of investigation by Canadian geologists and geochronologists, the stratigraphy and tectonic development of the Abitibi belt are better known than that of any other belt. Furthermore, the region contains numerous volcano-hosted massive sulfide Cu-Zn ore deposits, which are known to form at hydrothermal sources on the Archean ocean floor. Other targets will be 3.8 Ga belts in Greenland (Isua) and northern Quebec (Porpoise Cove), which contain the oldest known volcanic rocks, but our study of these highly deformed and metamorphosed regions will be postponed until we obtain a more comprehensive understanding of the operation of Archean systems through the study of better-preserved regions. When the political situation improves, we will also continue ongoing research in the Belingwe belt of Zimbabwe .

We will study the rocks in the field and in the laboratory with the aim of obtaining direct information about the nature of the circulating fluids. By mapping zones of hydrothermal alteration in well-exposed and well-preserved regions such as in the Barberton and Abitibi belts, we will establish the physical dimensions and geometry of the hydrothermal system. By determining the mineralogy and chemical compositions of altered rocks, we will evaluate the extent of fluid-rock interaction and calculate the compositions of the fluids. One approach that has yielded valuable information in studies of rocks from Greenland and the Pilbara is the geochemical analysis of quartz and other minerals in amygdales (filled gas bubbles in volcanic rocks) and investigation of the fluid inclusions in minerals within these rocks. We would extend the approach to other areas using modern in-situ methods that can establish the chemical and isotopic composition of the minerals and fluid inclusions.

(b) The second approach would be to model hydrothermal circulation through Archean oceanic crust. Parameters taken into account would include the total thickness of crust (much greater than modern crust if the volcanic rocks were derived from hotter Archean mantle), the thermal gradient through this crust (lower than across the much thinner modern crust), the distribution and intensity of heat sources (more dispersed?), the composition of the volcanic rocks (more magnesian) and the composition of seawater (see project 1). This approach will provide a model of an Archean hydrothermal system that will be used to constrain interpretation of the observations and data obtained by direct measurement and analysis of the rocks.

 

Project 4: The Search for Traces of Early Life

(a) What were the conditions that produced the emergence of life ?. A fundamental requirement for life is a steady source of energy commensurate with the requirements of metabolism. The materials and modules required for emerging life must also be delivered at the same time and in the same place. The energy must be sufficient to drive the first cells (e.g. to drive pyrophosphate formation) but not of such power to destroy them. The next fundamental question relates to the formation of RNA, DNA, other polymerised organic molecules and cross-catalytic self-replicating systems. In other words, how could sufficient and sustained concentrations of the building blocks of life be achieved in a prebiotic world? The most popular current solutions to this problem are the evaporation of terrestrial ponds or surface catalysis. Unless the emergence of life is later than generally assumed, the first solution faces the challenge that, as there may have been few if any large continents or land masses, and bodies of fresh water would have been ephemeral, the ocean surface of the rapidly rotating Earth with its closely orbiting moon would have been wracked storms and tornadoes and bombarded by numerous large meteorites. In the second solution, excess of reactants at mineral surfaces allows them to react. However, if after reaction the product diffuses into the ocean, any further reaction is highly unlikely. So, rather than merely a mineral with a reactive surface, the mineral surface, or mineral membranes, must be capable of nurturing a molecular population of the complexity of the building blocks of life. The surfaces of iron sulfides and of clay minerals, and the propensity of some of these to form inorganic membranes, may meet both these requirements.

We propose to study the surface characteristics of iron monosulfides, greigite, pyrite as well as montmorillonite and saponite in relation to the abiotic formation and polymerisation of amino acids. More specifically, the reaction of carbon dioxide and ammonium with ferrous iron ( Fe(II)) sorbed at the surface of these minerals will be studied. These minerals will need to be studied in solution of various ionic and pH, since the reactivity of sorbed Fe(II) will vary with type of mineral host and ambient conditions and that this variation changes for different reactions. Naturally, the persistence of life after emergence is equally crucial. Therefore, we will also study the metabolism at low energy yield of dissimilatory reducing bacteria in nutrient poor media. In particular, the study will aim to distinguish between electron transfers at mineral surfaces, bacterial cell walls and within the bacteria themselves. To relate this experimental work to the natural setting of the emergence of life, field analogues will be studied where low energy-yield metabolism is expected to occur; for example, at the reducing bottom of stratified lakes, at the bottom of the Black Sea, in reducing sediments, at subaqueous alkaline springs and seepages and in cold environments such near ice caps. The importance of such a study lies in the resulting knowledge of life at extreme conditions, expectedly valid for early life on Earth.

(b) Evidence for early life in terrestrial rocks. Although most opinion is that life appeared on Earth between 4.2 and 3.8 Ga ago, the evidence for this is fairly thin. There are two direct ways of searching for evidence of the existence of life during the earliest part of Earth history. The first approach involves the use of   optical, electronic (SEM, ESEM and TEM) and most recently atomic force microscopy to search for fossilized microbes. The second is geochemical and depends on the measurement, in rocks that are interpreted as metasediments, of putative organic traces in the geochemistry. These traces include clumps of carbon, which may or may not be biogenic, and biological isotopic fractionation, both of light elements such as C, N, S, O and D/H which make up the cell mass, and of the metals that constitute key housekeeping proteins (Fe, Cu, Zn). The geochemical approach also involves micro-scale in-situ analyses of isolated microfossils from shale and chert samples. Chemical compositions are measured by electron-dispersive spectroscopy coupled with a SEM, carbon structure and degree of thermal alteration by micro-Raman spectroscopy, and molecular composition by micro-Fourier transform infra-red spectroscopy. The presence of biomarkers is established by laser micro- pyrolysis/GC-MS.

Claims that ancient traces of life have been discovered are always controversial and key papers have recently been questioned. Fractionated ratios of carbon and sulfur isotopes and the occurrence of graphite had been cited as evidence that life existed during the deposition of sedimentary rocks that form part of ~ 3.8 Ga rocks in the Isua sequence in Greenland, and similar isotopic data, and observations of probable microfossils, have been made in samples from 3.5 Ga sequences of the Pilbara and Barberton regions. The graphite in samples from the Isua belt may, however, have formed by thermal disproportionation of metasomatic carbonate. Evidence for even older traces of life in a banded iron formation on Akilia Island (southern West Greenland ), has been hotly debated since the host material has been reinterpreted as an altered ultramafic rock. It has been argued that the microfossils in cherts of the Pilbara sequences may actually have formed by abiogenic processes associated with hydrothermal activity. All these debates are current, and their outcomes unresolved. Other examples of putative Archean fossils have been reinterpreted as organic material added to the rocks during recent geological processes or by contamination during sample treatment. Moreover, morphologically complex microstructures that closely resemble the microfossils in Archean metasediments have been synthesized abiotically in laboratory experiments.

The isotopic data may provide the most stringent constraints. Sedimentary organic matter strongly depleted in the carbon isotope 13C in rocks with ages from 2.8 to 2.5 Ga implies the presence of archaeal methanogens and bacterial methanotrophs. Filament tufts in 2.7 Ga lacustrine stromatolites are consistent with phototropic cyanobacteria. Fractionated sulphur isotopes in sulfate crystals suggest the existence of mesophilic bacterial sulfate-reducers in still older rocks (3.5 Ga). The oldest unambiguous fossil evidence for cyanobacteria is found in 2.15 Ga rocks, but biomarkers suggest the presence of cyanobacteria, methanotrophs or methylotrophs, and Eucarya in 2.77 Ga rocks of the Hamersley Basin, Australia. The presence of possible eukaryotic biomarkers together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least 1% of the present atmospheric level and may have been sufficient to support aerobic respiration.

To date there is no evidence for the existence of oxygenic photosynthesis in the Early Archaean but it is possible that anaerobic photosynthesisers could have formed certain filamentous mats. Samples from a 3.25 Ga massive sufide deposit in the Pilbara Craton may contain the remains of sulfide oxidizing bacteria, and filamentous structures in 3.5 Ga cherts have been interpreted as sea-floor hydrothermal-vent bacteria. Carbon and nitrogen isotopes in these cherts suggest the action of chemiosynthesisers. Given that vents are considered to be one possible location of the origin of life, a comprehensive search for microfossils in other massive sufide deposits is a necessity. In addition, silica-rich iron oxide deposits (jaspers) should be searched for iron oxide filaments of the type that are interpreted as microfossils of various iron oxidizing bacteria in Phanerozoic samples. To find identical filaments in Archean jaspers would have important implications for deducing the oxygen content of Archean seawater. In addition, experimental research on microbial mat formation under anoxic conditions could help to clarify the biogeochemical processes in the Early Archaean, as well as delineate possible products that may be incorporated into the rock record.

In view of these controversies it has become clear that the search for traces of ancient life has to be done in a rigorous, carefully controlled manner. Steps must be taken to: (1) eliminate contamination during sampling and laboratory investigations, (2) assure that the geological nature of the sampled rocks is clear and unambiguous, (3) understand alternative abiogenic mechanisms that might produce carbonaceous material or structures that resemble microfossils, (4) understand inorganic processes that fractionate the isotopic ratios used in the search for early life. Following this approach we will focus our attention on the three principal field regions described below. In each area we will sample rocks such as (a) cherts, which are (with few exceptions) of sedimentary origin and which contain the most convincing microfossils; (b) shales and other fine-grained hydrocarbon-rich sediments, which are the best candidates for isotopic analysis; and (c) sulfide deposits, which probably formed directly from circulating hydrothermal fluids. Through these studies we will not only search for indisputable evidence for the existence of life, but also investigate the nature of this life and the environment in which it evolved. For example, the investigation of samples from the Barberton region suggests the presence of microbial mats at sediment surfaces in shallow water to partially subaerial environments. An important question is whether oxygenic photosynthetic microorganisms could have formed some of these mats. Another question is when did the various groups of plankton evolve – crenarchaeota, planctomycetes, and and anoxygenic and oxygenic photosynthetic plankton. This question is critical to the interpretation of the composition of the atmosphere and oceans (see project 1).

|| Summary || Scientific objectives || Proposed activities ||