THE DEPTH OF THE OCEAN IN THE ARCHEAN

The depth of water in the oceans depends on many factors, such as growth of continental crustal volume, growth of surface water volume, the amount of freeboard of the continents, and the rate of sea floor spreading. I have produced a model in which continental crustal volume is allowed to evolve through time, in which addition of water to the oceans can occur, where the freeboard (the amount the continents are sticking up above the sea surface) decreases through time because of the declining ability of Earth to create mountains and where sea floor spreading rates are reduced through time. Freeboard represents the balance between mountain building and erosion (the modal continental elevation is close to sea level, the base of erosional activity). Changes in sea floor spreading rate and in mountain building rate are caused by the decreasing amount of radiogenic heat being produced within Earth, which declines with an e-folding time constant of about 3.4 Ga. Freeboard is expected to decrease from 972 m 3 Ga ago to 565 m today. If large quantities of water are allowed to be added to the ocean volume, as postulated by Frank and others in their small comet hypothesis, it is found that in the early Archean the ocean depth above the ridge crest because extremely shallow and would not allow sulphide deposits to be formed above the ocean crust as the ridge crest would be well below the critical pressure of water. Growth of the volume of the continents does not alter this piecture very much if continental volume growth is limited a rate equal to about half the present day volume over the past 4 Ga.