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Mid-term Conference of the RNP

Archean Environment: the habitat of early life

 

 

Scientific Summary

 

The key physical and chemical conditions at the surface of the Archean Earth when life emerged remain poorly constrained and the timing and mechanisms of formation and development of the earliest continents are unclear. Extrapolating our extensive knowledge of the modern Earth, including plate tectonics, (bio)geochemical cycles and microbiological ecology back to the first two billion years of Earth history is fraught with difficulty. Life emerged in conditions that may or may not have a modern analogue. Research in these fields is truly multidisciplinary and has led to a wide range of theories that benefit from communication between scientists from diverse fields. This meeting is intended to bring together predominantly earth and planetary scientists but also biologists, chemists, physicists who have an interest in observing, modelling or experimenting to solve problems related to the early Earth. Research into the Archaean environment is currently highly active in Europe and internationally. The proposed meeting will highlight the work supported by the ESF Archaean environment: the habitat of early life network and bring together other prominent contributors to the field.

During its early stages, the Earth was much different from more recent Proterozoic and Phanerozoic times (i.e. <2.5 Ga). Generally accepted boundary conditions that characterized the early Earth include a predominance of oceanic over continental crust, a higher heat flow and possibly stronger mantle convection, surface temperatures possibly substantially higher than today, and an anoxic ocean-atmosphere-system. Crustal rocks, that formed the basis of the substrate on which an early biosphere may have developed were radically different. Archaean ultramafic lavas of have unusual compositions that have been used to define the so-called Al-depleted or Barberton-type komatiite. These rocks formed through melting under unusual conditions in the mantle. Melts have been variably attributed to formation in an unusually hot mantle plume or under cooler conditions in an Archean subduction zone. Resolution of the issue has important implications for our understanding of Archean geodynamics.

The timing and order of the emergence of key metabolic processes amongst prokaryotic microorganisms on the Archaean Earth has not been fully established. The nature and activity of the young biosphere was both strongly influenced by, and in turn modified the chemical and physical development of the oceans, atmosphere and lithosphere. For example the appearance of an oxygenated atmosphere was a product of an active biosphere which went on to have a major impact on biogeochemical and geological processes (e.g. weathering) as well as leading to the dominance of new metabolisms such as aerobic respiration. The timing of atmospheric oxygenation is underpinned by studies of Archaean and Proterozoic sedimentary rocks and is highly debated with estimates ranging from pre 3.5 to 2 Ga. Important evidence for palaeoenvironments is provided by rocks such as cherts and banded iron formations (BIFs) that were extensive on the early Earth whilst having relatively few analogue depositional environments on the modern Earth. Measurements of isotopic and chemical biomarkers in the rock record often provides one of the most robust methods for gaining an insight into ancient redox cycles and possible metabaolic processes. However biomarker evidence is equivocal and often relies on a large amount of supporting geological data. Kinetic and thermodynamic modelling of the most critical dissimilatory respiratory pathways of (heterotrophic or lithotrophic) organic carbon decomposition, and laboratory experimentation with pure microbial cultures and natural microbial consortia can provide further constraints to support data obtained from the geological record. Each of these approaches outlined above will be represented by contributors to this science meeting.

Some of the most important problems to be discussed will include:

geodynamics and thermal structure of the Archaean crust,
style and nature of early volcanism,
hydrothermal circulation through the Archaean oceanic crust
the temperature and geodynamic activity in the Archaean mantle,
emergence of the first continents
sedimentation rates and sedimentary basin formation
the temperature and composition of the Archaean oceans,
timing of oxygenation of the atmosphere
microbial habitats and metabolic activity of microorganisms in volcanic and sedimentary settings,
development of more robust biomarkers for use with Archaean rocks