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

41st Saas-Fee Course From Planets to Life 3-9 April 2011

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

Ref.: M. Brasier et al., Nature 416, 76 (2002)

• Martin Braiser reexamined the Apex Chert samples usingconfocal microscopy

• What he saw was highly revealing…

after Bassinot et al. 1994

O isotopes—the last 900 k.y.

• Dominant period is ~100,000 yrs during this time• Note the “sawtooth” pattern..

Marine carbonate 18O vs. time (detailed, time axis reversed)

Shields & Veizer, G3, 2002

Warm

Cold

• When one looks at 18O over longer time scales, however, a pronounced trend towards lighter (lower) values is seen

18O of modern and ancient cherts (SiO2 )

P. Knauth, Paleo3 219, 53 (2005)

Warm

Cold

• 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

(SM

OW

)

(SM

OW

)

Chert data:• Mean surface temperature was 7015oC

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 E. coli to allow them to be expressed as proteins

• Protein melting points werethen measured in the lab

• Ancestral elongation factorproteins (EF-Tu) of allorganisms (panel a) andeven of mesophiles (panel b)indicate a thermophiliccommon ancestor for extantlife (40-80oC)

Nature, 2003

• More recent work by this group proposes a detailed time scale for surface temperature evolution, based on two different molecular clock techniques

• “Our results are further supported by a nearly identical 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.”

O isotope data

Nature, Feb., 2008

Glaciations

• 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 hydrothermal activity

E. M. Moores, GSA Bull. (2002)

Ga

0 3.5