Download 2.5- ~2.0 Ga

The Microbial World:
Evidence of Earth’s Earliest Life
Oldest Known Rock on Earth
It’s ours, eh !
Acasta Gneiss: Northwest Territories, Canada
(3.96 Ga)
(note: Ga = billion years, Ma = million years)
Second Oldest Known Rocks on Earth
Isua Group, West Greenland
(3.85 Ga)
Oldest Known Zircons on Earth
Oldest zircon crystals
4.4 Ga
In coarse clastic sedimentary rocks
3.0-3.7 y billion years old
Jack Hills, Western Australia
Indicates that some solid, granitic, crust existed prior to
oldest known rocks (but probably not much)
Life became established relatively early in
Earth’s history !
Oldest crust: 4.4 Ga
Oldest known life: 3.5 Ga
Oldest known evidence of life
Stromatolite
(structure constructed by cyanobacteria)
Warrawoona Series, North Pole, Western Australia
3.5 Ga
Clues to formation of stromatolites:
Modern stromatolites in Shark Bay, Western Australia
Hypersaline Intertidal Conditions
Growth of filaments
Trapping of sediment
Growth of filaments
Trapping of sediment
Growth of filaments
Trapping of sediment
Sediment Trapping by Bacterial Mats
(forms the fine laminated structure
observed in stromatolites)
Single-celled bacteria-like microfossils
Fig Tree Chert, Swaziland, South Africa
recently estimated at 3.4 Ga
Filamentous Prokaryote Microfossils
(Probably Cyanobacteria)
Apex Chert, Marble Bar, Western Australia
~ 3.4-2.5 Ga
So… there is evidence that prokaryotic life existed on Earth
by at least 3.5 billion years ago.
Carbon isotopic evidence from 3.8 Ga rocks in Greenland
suggests that microbes may have existed even earlier.
Very low δ13C signatures in these rocks suggests some
carbon fractionation by methanogens or photosynthesizers
Such organisms preferentially take up 12C over 13C, so
extreme enrichment in light carbon (12C) might suggest
primitive life…
…but this evidence is tenuous at present (perhaps 13C could
be depleted by inorganic means).
Low-oxygen conditions on early Earth
BIF production peaks here
Note: up to about 2.3-2.2 Ga, hydrosphere contains minimal free
oxygen:
-uranium occurs in solid particles (uranium dissolves in presence of oxygen)
-fluvial (river) sediments contain reduced iron (so not red)
-iron oxide is deposited in ocean in “banded iron formations” (BIF)
soils are iron-deficient
2.5- ~2.0 Ga
A time characterized by widespread
Banded Iron Formation (BIF) deposition
-interbedded chert (SiO2) and magnetite (Fe3O4)/hematite (Fe2O3)
Iron oxide formation may have precipitated inorganically (via simple
oxidation of iron by free oxygen in water)
…or assisted by metabolic activities of bacteria
2.3 - 2.0 Ga
First Definite Appearance of Redbeds
(soils and river sediments containing red iron oxides)
indicates enough free oxygen in atmosphere
to oxidize iron in river water before it reaches the sea
(note BIF production shuts off soon afterward)
The time of about 2.3-2.0 Ga is significant because
by this time, significant amounts of oxygen
occurred in the hydrosphere and atmosphere, thus
fundamentally changing the dynamics of the Earth’s
biosphere.
For this reason this time is considered to mark the
beginning of the “Oxygen Revolution.”
Earth’s “Modern” Atmosphere Takes Shape
Red
river
sediments
Red
soils
BIF production really drops off
By 2.0-1.8 Ga:
Redbeds well-established
-Soils and fluvial (river) sediments are enriched in oxidized iron
By about 1.8 Ga, BIF production really drops off (indicating that
transport of dissolved iron from land has been effectively shut off due to
oxidation on land)
But if photosynthesizers were largely responsible for
producing most of the free oxygen on Earth, why did it
take them so long ?
Possible reasons:
1. Oxygen was used by organisms (for metabolic processes) as soon
as it was produced.
2. Oxygen-bearing organic molecules were buried (and therefore was
not readily released into the atmosphere in significant quantities)
3. Oxygen was used up through oxidation of dissolved iron (thus
forming the huge deposits of BIF)
4. Nutrients such as phosphorus were in short supply (phosphorus
tends to be adsorbed onto iron oxide particles), so may have limited
the growth of cyanobacteria (and therefore oxygen production).
Stromatolites in Gunflint Chert,
Near Schreiber Ontario 1.9 Ga
Even after the initial buildup of oxygen to significant levels
in the atmosphere, things were pretty boring in terms of
the variety of living things.
Stromatolites (and associated fossil bacteria) continue to
dominate the Earth’s biosphere…
Prokaryote Microfossils
(probably cyanobacteria)
Belcher Islands, Arctic Canada
2 Ga
Cyanobacterial Filaments
Bitter Springs Chert, Northern Australia
1.5 Ga
Bitter Springs Chert, Northern Australia, 1.5 Ga
Spherical Cyanobacteria
Modern Form
Filamentous Cyanobacteria
Modern Form
Note remarkable similarity between ancient and modern forms
But there are some hints of progress…
END OF LECTURE