1. Ore Deposits* and the Early Earth (*and other bits of geology, too) George H. Shaw Geology Department Union College Schenectady, NY 12308 shawg@union.edu

2. “The present is the key to the past” But not exactly… If you think about BIF or detrital uraninite/pyrite

6. Can we apply this to Earth’s earliest atmosphere and oceans? Rubey did, using volcanic emissions as the “present key”, that is, the surface volatiles can be accounted for in terms of primary release of water, carbon dioxide and nitrogen from volcanos. What’s wrong with this? Is there a better answer to the nature of Earth’s earliest atmosphere?

7. Which model of the early atmosphere is most consistent with the geological data? Geology Planetary accretion/meteoritics CO 2 -N 2 CH 4 -NH 3

8. Planetary accretion - meteoritics Antarctic meteorite summary Typemass (g)mass fractionmetal fraction in classoverall metal fraction CC352540.0100 H9148980.2550.1430.037 L16671450.4660.0270.013 LL2843380.07900 other stony891140.02500 stony-iron2023480.0570.50.028 iron3877990.1081 Total3580896 0.186

9. Which model of the early atmosphere is most consistent with the geological data? Geology Planetary accretion/meteoritics Prebiotic organic synthesis CO 2 -N 2 CH 4 -NH 3 X

10. Which model of the early atmosphere is most consistent with the geological data? Geology Planetary accretion/meteoritics Prebiotic organic synthesis Archean rock record (esp. CO 3 = ) CO 2 -N 2 CH 4 -NH 3 X X

11. “Archean carbonates are of relatively infrequent occurrence”(Veizer, 1983) “Archean (> 2400 m.y.) sediments of the Canadian Shield are principally composed of greywacke and shale of flysch facies that are preserved within slightly metamorphosed "greenstone" belts. These Archean sedimentary sequences contain only trivial proportions of carbonate rocks compared to those of Proterozoic and Phanerozoic age. Archean sediments from other parts of the world appear to have a similarly low proportion of limestone and dolomite.” (Cameron and Baumann, 1972) Archean rock record (esp. CO 3 = )

12. Which model of the early atmosphere is most consistent with the geological data? Geology Planetary accretion/meteoritics Prebiotic organic synthesis Archean rock record (esp. CO 3 = ) Faint young Sun problem CO 2 -N 2 CH 4 -NH 3 X X X

13. Which model of the early atmosphere is most consistent with the geological data? Geology Planetary accretion/meteoritics Prebiotic organic synthesis Archean rock record (esp. CO 3 = ) Faint young Sun problem Oxygenation history of atmosphere CO 2 -N 2 CH 4 -NH 3 X X X X

14. Oxygenation history of atmosphere Advent of cyanobacteriaFree oxygen in atmosphere

15. Why was oxygenation delayed? Limiting nutrients: –Iron … not likely given BIF –Nitrogen … not likely since Cyanobacteria are main nitrogen fixers. If they produce at the current rate of oceanic nitrogen fixation they can make all of the oxygen in the present atmosphere in a mere 2.5 Ma. –Phosphorus … using current rates of erosion to give approximate rate of delivery of P to ocean margins gives oxygen production sufficient to make the oxygen in the atmosphere in about 3.5 Ma. Therefore, CO 2 was in short supply!

16. Which model of the early atmosphere is most consistent with the geological data? Geologic information Planetary accretion/meteoritics Prebiotic organic synthesis Archean rock record (esp. CO 3 = ) Faint young sun Oxygenation history of atmosphere CO 2 -N 2 CH 4 -NH 3 X X X X X

17. Concluding statement “Much of this account of atmospheric evolution is therefore speculative. Many of my guesses may be wrong, but if my version of atmospheric history stimulates the development of more accurate histories I shall be well pleased” Walker, 1977