Archean Eon Friday 21 March 2008

The Eoarchean Earth?

Archean rocks Greenstone belts:Typically found in belts which have a synclinal form. Show a sequential transition from ultramafic volcanics at the bottom to felsic volcanics and capped by sediments. Mafic Volcanics - pillow lavas are common (environment?) Sedimentary Rocks - composed dominantly of graywackes ("dirty sandstones"), conglomerates and sandstones. - deposited in shallow water deltas, tidal flats, shallow marine shelf environments.

Archean Making and keeping crustal fragments

Arcs collide due to subduction to form larger land mass

Land masses continue to grow

Precambrian shields and cratons Shield: area of exposed Precambrian rocks Craton: buried and exposed Precambrian rocks

Superior Craton of Canada Small fragments before 3.0 billion Explosion of crustal growth billion (end of Archean)

Model for assembly of cratons Continental accretion: plates collide with volcanic arcs and other plates. Continents grow along margins

Archean greenstone pillows

Archean conglomerate: more evidence for water

Archean sediment layers: more evidence for water

Canadian Shield Gneiss

By end of Archean –Earth cooler –Easier to keep crust –Easier to make and keep granite

Proterozoic to the Cambrian explosion Rocks, life and climate

~ Ga--first red Banded Iron Formations (BIF's) - red and gray zones of oxidized iron layers of silica. Responsible for world's most important iron deposits. As soluble iron was used up, O2 began to increase in atmosphere, and CO2 decreased continued until ~2 Ga=first appearance of red beds (and appearance of ozone!) Red beds = shallow-water, river, or soil deposits in which the iron has combined with O2 to form red iron oxide Proterozoic rocks

Banded iron formation 92% of BIF are Proterozoic ( Ga)

BIF’s Banded Iron Formation, Alternating bands of red jasper and black hematite, about 2250 million years old (2.55 billion years old) Jasper Knob, Ishpeming, Michigan

Model for origin of BIF Oxygen-rich upper ocean Oxygen-poor deep ocean Upwelling brings Fe and Si-rich water up Iron oxide formation occurs (rust-like minerals form)

red beds=O2 increase in atmosphere BIFs decline, an indication that reducing compounds are disappearing from the oceans ( weathering as we know it was probably set up at this time Less CO2 more CO2 Proterozoic rocks

Life in Proterozoic Until about 1.9 Ga we only have evidence of the simplest kinds of life - the PROKARYOTES – bacteria and blue green algaePROKARYOTES Requirements to be Life –self-replicating (DNA) –metabolism (chemical processes that convert food into energy) But at about 1.9 billion years we start to see fossils of much larger cells. These cells belong to the Eukaryotes - of which we are membersEukaryotes First sexual reproduction!

Life in Proterozoic ~650 mya (0.65 billion) the first multicellular forms are present called the Ediacara assemblages seem to be elaborations of forms with –large surface areas –living in shallow relatively high energy environments –often in red beds.

Life in Proterozoic Originally discovered in Pound Qtzt, Ediacara Hills, S. Australia; later found worldwide (including Piedmont area of NC) at low paleolatitudes by ( my) impressions and molds of animals (associated with trace fossils)

Mawsonites, similar to jellyfish, Australia Smithsonian Institution, Museum of Natural History Dickensonia costata, segmented worm, from Australia Smithsonian Institution, Museum of Natural History Unnamed "spindle-shaped organism" from Newfoundland Smithsonian Institution, Museum of Natural History Tribrachidium heraldicum, Echinoderm?, from Australia Smithsonian Institution, Museum of Natural History EDIACARA FAUNA

Cambrian explosion—545mya What happened at the P-Tr boundary? What about all of the other “dips” in the diversity?

Paleogeography and climate

Animals with hard-shells appeared in great numbers for the first time during the Cambrian. The continents were flooded by shallow seas. The supercontinent of Gondwana had just formed and was located near the South Pole. NA=Laurentia N.Europe=Baltica

Proterozoic climate Cold multiple times!! Evidence: –Till –Polished, striated bedrock

N.A. Orogenies 8 major Archean-aged crustal blocks: Hearne, Nain, Penokean, Rae, Slave, Superior, Wopmay, and Wyoming. Collisions between these blocks resulted in orogenic (mountain building) events. - continent-continent collisions