ore deposits* and the early earth (*and other bits of geology, too) george h. shaw geology...
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Ore Deposits* and the Early Earth(*and other bits of geology, too)
George H. ShawGeology Department
Union CollegeSchenectady, NY 12308
• “The present is the key to the past”
• But not exactly…
• If you think about BIF or detrital uraninite/pyrite
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?
Which model of the early atmosphere is most consistent with the geological data?
• Geology
• Planetary accretion/meteoritics
CO2-N2 CH4-NH3
Planetary accretion - meteoritics
Antarctic meteorite summary
Type mass (g) mass fraction metal fraction in class overall metal fraction
CC 35254 0.01 0 0
H 914898 0.255 0.143 0.037
L 1667145 0.466 0.027 0.013
LL 284338 0.079 0 0
other stony 89114 0.025 0 0
stony-iron 202348 0.057 0.5 0.028
iron 387799 0.108 1 0.108
Total 3580896 0.186
Which model of the early atmosphere is most consistent with the geological data?
• Geology
• Planetary accretion/meteoritics
• Prebiotic organic synthesis
CO2-N2 CH4-NH3
X
Which model of the early atmosphere is most consistent with the geological data?
• Geology
• Planetary accretion/meteoritics
• Prebiotic organic synthesis
• Archean rock record (esp. CO3=)
CO2-N2 CH4-NH3
X
X
• “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. CO3=)
Which model of the early atmosphere is most consistent with the geological data?
• Geology
• Planetary accretion/meteoritics
• Prebiotic organic synthesis
• Archean rock record (esp. CO3=)
• Faint young Sun problem
CO2-N2 CH4-NH3
X
X
X
Which model of the early atmosphere is most consistent with the geological data?
• Geology
• Planetary accretion/meteoritics
• Prebiotic organic synthesis
• Archean rock record (esp. CO3=)
• Faint young Sun problem
• Oxygenation history of atmosphere
CO2-N2 CH4-NH3
X
X
X
X
Oxygenation history of atmosphere
Advent of cyanobacteriaFree oxygen in atmosphere
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, CO2 was in short supply!
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. CO3=)
• Faint young sun
• Oxygenation history of atmosphere
CO2-N2 CH4-NH3
X
X
X
X
X
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