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Constraints on the Archaean Environment |
Isotope Geology Group, Institute of Geological Sciences, University of Bern, Erlachstrasse 9a, 3012 Bern, Switzerland kramers{at}geo.unibe.ch
The possible drawdown of a massive CO2 atmosphere in early Earth history is discussed using two working hypotheses: first, that this removal of CO2 from the atmosphere occurred mainly via silicate weathering; second, that crust-to-mantle recycling rates found from forward modelling of crust-mantle history can be used to estimate rates of this ancient silicate weathering. Previous U-Th-Pb and Sm-Nd forward modelling efforts are reviewed, from which it was concluded that an insignificant amount of continental crust existed at 4.4 Ga, i.e. so-called ‘no-growth models’ for the continental crust appear untenable. New modelling carried out is based on a crustal growth curve starting with zero mass at 4.2 Ga and reaching 75% of the present crust mass by 2 Ga. It concerns variations in crust-to-mantle recycling rates through geological time. Best fits to isotope data are obtained if it is assumed that erosion rates (mass removal per unit surface) were approximately constant from early Archaean time to the present. From the results it can be estimated that drawdown of a massive CO2 atmosphere by silicate weathering could have been completed by the end of Archaean time at the earliest, and about 1.5 Ga ago at the latest.
