One last concept for the Archean:
Shen et al. 2001 Nature completed sulfur-isotope analyses of sulfides trapped in 3.5 BYA barite evaporite deposits in NW Australia, the average 6 per mil fractionation indicates sulfate reduction must have been evolved by this time. Then place this date on the Universal Tree of Life
Proterozoic Eon 2.5 to 0.5 BYA (time scale figures)
Development of large craton areas, thus extensive shallow marine limestone deposition
Included major intervals of:
mountain building
glaciation
transition from single- to multi-celled organisms
Proterozoic Life
Stromatolites the predominant features of Proterozoic carbonate
rocks, but are uncommon in the modern (i.e. Shark Bay, Australia,
and Bahamas)
Mats of cyanobacteria grow across sediment surfaces, trapping and binding fine particles supplied by waves and current, with carbonate crystals forming cements in the lower parts of the mat
The repetitive process of colonization, trapping, binding, and carbonate precipitation make fine layers of limestone, with shapes that include planar, domal, conical, and cylindrical
(pictures of modern and ancient stromatolites)
Early Eukaryotes simple eukaryotes evolved in the Archean, arising from the joining of bacterial or archaea cells, forming mitochondria, permiting cell energy to be derived from respiration (Fig. 12.5) evolved in the Archean
Another union of cells, this time protozoan consuming a cyanobacteria, created chloroplasts, which is the site of photosynthesis, which probably evolved several times
Algae multicelluar protests seaweed-like algae date to 2.1 BYA
single-celled algae called acritarchs that are the dominant algal plankton in the Precambrian and Paleozoic, probably the cysts (resting stages) of dinoflagellates
Biomarkers organic compounds that are preserved in the geological record (i.e. steranes)
Buildup of Atmospheric Oxygen
Photosynthesis began ~ 3.5 BYA
Significant buildup by ~ 2 BYA
O2, and NO3 built up
Chemical sinks had to be saturated prior to significant buildup,
kept oxygen at 1 2% of modern levels, examples:
uraninite UO2 and pyrite FeS2 rare in sandstones less than 2.1
BYA
banded iron formation formed 3.5 to 1.9 BYA
Neoproterozoic (Early Proterozoic ~ 1 0.5 BYA)
Glaciation
varved glacial lake sediments
Snowball Earth 750 to 580 MYA, the entire planet may have been covered with ice, subject of next lecture
Explosion of Animal Life
The beginnings of modern life took place in the late Neoproterozoic
Explosive animal evolution in the last 30 MYA of the Neoproterozoic
trace fossils
imprints of soft bodied animals most related to jellyfish
(Phylum Cnidaria) Ediacaran Fauna but also included annelid
worms and arthropods (crabs, lobsters, insects)
skeletal fossils calcium carbonate vase- and tube-shaped
skeletons