Archean Environment: the habitat of early life

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Programme outline

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

Scientific objectives

The starting point of the programme is the notion that conditions at the surface and in the interior of the Archean Earth may have been very different from those at present, but in a manner that is as yet poorly understood.

We can be fairly certain that the sun was less luminous, the mantle was hotter and the atmosphere was less rich in oxygen, but we do not know how such differences influenced the way the Earth functioned. Our lack of a complete understanding of the interplay between solar radiation and atmospheric composition means that we are unsure whether the atmosphere and oceans were hotter or colder that those of today. Rates of heat production were far higher in the Archean mantle, but because our understanding of mantle convection at that time is rudimentary, we do not know if this difference resulted in internal temperatures that were extremely high, or close to those of the present-day mantle.

We know very little about the environments in which life may have appeared and later evolved. Two commonly cited settings for the cradle of life are hydrothermal springs on the ocean floor and tidal flats on the early continents. Our views on the first are based on studies of black or white smokers on the modern ocean floor, but there is every reason to believe that their Archean counterparts were very different. Archean oceanic crust probably was much thicker than modern crust and the thermal gradient across this crust would have been low: temperatures in the shallow interior of Archean oceanic crust may have been lower than those in modern crust, not higher as is commonly assumed. Archean volcanic rocks were more magnesian than their modern counterparts and Archean ocean water more reducing. How did these differences influence conditions in Archean hydrothermal springs and what was their influence on the appearance and early evolution of life?

Our knowledge of the second setting is even more rudimentary. The survival of 4.2-4.4 billion year old zircons for about a billion years on the turbulent surface of the Archean Earth tells us that felsic continents formed very early on and that these continents stayed on the surface throughout the early Archean. 3.8 Ga sediments in Isua, Greenland , and in the Porpoise Cove region of Canada tell us that this crust was exposed to weathering and erosion: some of the emergent land in the Archean was very similar to that of today. But there may also have been vast “ melano” (dark-coloured) continents – emergent volcanic plateaus composed of mafic to ultramafic volcanic rocks. The best modern analogue of an Archean tidal flat may be the shores of a tropical Iceland .

It is probable that the volume of ocean water was greater than today’s. Was the Archean a waterworld in which only a few small landmasses (mafic or felsic) breached the surface of a global ocean; or did higher mantle temperatures and more vigorous convection produce an extensive network of emergent mountain ranges along the mid-ocean ridges? The scientific objectives of the programme will be to provide answers to these questions and many others, all crucial to our understanding of the origin and evolution of life.

A list of important scientific questions includes:

  • What was the temperature and composition of the ocean and atmosphere?
  • How did these parameters change during the first 2-3 billion years?
  • What was the volume of the oceans?
  • What were the composition, structure and relative proportions of the oceanic and continental crust?
  • How much land was there in the Archean and what was its composition?
  • What were conditions on the land surface and at the ocean floor?
  • How did seawater circulate through oceanic crust? What were the temperature and the composition of hydrothermal fluids?
  • What are the best biomarkers capable of recording the existence of early life? What do they tell us about the nature, distribution, abundance and evolution of this life?
  • How did interaction between biosphere, hydrosphere and lithosphere change through the Archean
  • How should we regard the evolution of life? Did all the important discoveries happen in the Hadean, to be followed by period of only modest achievement? Why did only microbes proliferate for billions of years?
  • What were conditions on the other planets and satellites and what do they tell us about the early Earth?
  • What was the impact of impacts? How do we search for evidence of impacts in the Archean; how do we evaluate their influence on geodynamic and metabolic cycles?
  • Are there modern analogues of the Archean biosphere?

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