41st Saas-Fee Course From Planets to Life 3-9 April 2011 Lecture 4.5: Hot early life and the hot early Earth The Apex Chert microfossils/ Oxygen isotopes and paleoclimate/ Gaucher et al. and the hot early Earth J. F. Kasting
Apex Chert microfossils (3.5 Ga) • Possible fossils of cyanobacteria in rocks dated at 3.5 Ga were reported in 1993 • Were these real microfossils, though? J. W. Schopf, Science (1993)
Apex Chert microfossils reexamined • Martin Braiser reexamined the Apex Chert samples using confocal microscopy • What he saw was highly revealing… Ref.: M. Brasier et al., Nature 416, 76 (2002)
O isotopes—the last 900 k.y. • Dominant period is ~100,000 yrs during this time • Note the “sawtooth” pattern.. after Bassinot et al. 1994
Marine carbonate 18 O vs. time (detailed, time axis reversed) Cold Warm Shields & Veizer, G 3 , 2002 • When one looks at 18 O over longer time scales, however, a pronounced trend towards lighter (lower) values is seen
18 O of modern and ancient cherts (SiO 2 ) Cold (SMOW) (SMOW) Warm • Cherts, which are better preserved than carbonates, tend to show the same trend, i.e., they get isotopically lighter (in O) as they get older P. Knauth, Paleo 3 219, 53 (2005)
Chert data: • Mean surface temperature was 70 15 o C at 3.3 Ga – Ref.: Knauth and Lowe, GSA Bull., 2003 Carbonate data: • Surface temperatures remain significantly elevated until as recently as the early Devonian (~400 Ma)
• Ancestral genes were synthesized and cloned into Nature, 2003 E. coli to allow them to be expressed as proteins • Protein melting points were then measured in the lab • Ancestral elongation factor proteins (EF-Tu) of all organisms (panel a) and even of mesophiles (panel b) indicate a thermophilic common ancestor for extant life (40-80 o C)
Nature, Feb., 2008 • More recent work by this group proposes a detailed time scale Glaciations for surface temperature evolution, based on two different molecular clock techniques • “Our results are further supported by a nearly identical O isotope data cooling trend for the ancient ocean as inferred from the deposition of oxygen isotopes. The convergence of results from natural and physical sciences suggest that ancient life has continually adapted to changes in environmental temperatures throughout its evolutionary history.”
• How can one explain the O (and Si) isotope data which show that the early Earth was warm? 1. The O isotope ratios in the ancient cherts have all been reset by interactions with seawater during burial and diagenesis 2. The oxygen isotope composition of seawater has varied with time 3. Hydrothermal activity was widespread on the ocean floor (A. Hoffman, Precambrian Res., 2005; Van den Boorn et al., Geology (2007) Plate tectonics was operating differently at that time
Models for Archean Plate Tectonics • Geothermal heat flow was higher in the past • Archean oceanic crust was thicker —in some models, at least— because of greater partial melting beneath the midocean ridges • This thick crust would have cooled very slowly as it moved away from the ridges, possibly creating widespread Ga hydrothermal activity 0 3.5 E. M. Moores, GSA Bull. (2002)
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