Archean-Proterozoic Boundary

• Marks approximate time of changes in style of crustal evolution.

• Archean-style crustal evolution was not completed at the same time in all areas.

Archean Rock Groups (4 G’s)

• Gneiss: Metamorphic rock; early cratons, strongly heated and deformed granite

• Greenstone: Metamorphic rock; metamorphosed proto-oceanic crust (early oceanic crust)

• Graywacke: Sedimentary rock; sandstones and muds from early oceans

• Granite: Igneous rock; late-stage intrusions into the other

• These rocks continued to form during the Proterozoic at a different rate.

Crustal Evolution

• Archean plate tectonics → everything was hotter, thinner, and moving faster when it comes to the crust (like holding a cookie fresh from the oven)

• Not long after Proterozoic began, a new and more modern style of plate tectonics began:

  • Sea-floor spreading, convergence, rifting became more common

  • Encounter rocks that could only be formed by thick/cool crust

  • Proterozoic rocks are generally less altered than Archean and are easier to interpret.

Changes in Climate and Sedimentary Rock

• Atmosphere begins to take shape (stromatolites)

• Widespread sedimentary rock assemblages (rare in Archean)

• More water and increased tectonic deformation and rock formation, more material to weather, erode, and transport.

Paleoproterozoic Era (2.5-1.6 Ga)

Early Plate Tectonics

• Orogenic belts developed around margins of older Archean provinces.

• Collisions among various plates formed several orogens (Laurentia)

  • Linear or arcuate deformation belts

  • Metamorphism (protolith is changing)

  • Magma creates igneous intrusions, thus forming plutons, sills, dikes, and batholiths.

• Laurentia: A large landmass that was part of the ancient supercontinent Rodinia and later became the core of North America.; old paleo N. America

Wopmay Orogeny (<2.0 Ga)

• Wopmay Orogeny: a geological event that involved a major episode of mountain building and tectonic activity in what is now northwestern Canada. Led to formation of NW Canadian Shield.

• This orogeny played a key role in the formation of the ancient continental crust in the region.

• Wilson Cycle: A theory that explains how ocean basins form and close over time; repeats over millions of years.

  • It describes the process of a landmass breaking apart to create an ocean, and then eventually coming back together as the ocean closes, forming a new supercontinent. This cycle repeats over millions of years.

Laurentia’s Southern Margin

• Following initial episode of Archean cratons union, continental accretion continued in what is now the southwestern and central United States.

• Successively younger belts were sutured to Laurentia forming orogens from 1.8-1.6 Ga.

Supercontinents

• Continents: Areas of land above sea level; consist of granitic crust (thicker and less dense than oceanic crust).

• Supercontinent: At least 2 continents merge into one, but usually includes all or most of Earth’s landmasses.

Nuna

• Nuna: First recognized supercontinent

  • Tectonostratigraphic and paleomagnetic reconstruction

• ~1.8-1.4 Ga

• Completed ~1.6 Ga

• Begins to break apart during Mesoproterozoic

Ancient Glaciers

• Very few instances of widespread glacial activity have occurred during Earth’s history.

  • The most recent (Pleistocene) is best known

  • Evidence of late Carboniferous glaciers

  • Evidence of major episodes of Proterozoic glaciation

First Snowball Earth at 2.3 Ga

• Snowball Earth: A period about 2.3 billion years ago when Earth's surface was covered in ice from the poles to the equator, making it look like a giant snowball. This extreme ice age was caused by changes in the atmosphere and climate.

• Called the Huronian Glaciation (Gowganda Ice Sheet)

  • First snowball hypothesis

• Glacial dropstones melted out of icebergs and dropped into fine-grained marine and lake sediments

• Glacial deposits sandwiched between limestones suggest sea-level glaciers or icebergs in the tropics.

• Tillites, striations, and polish in bedrock on tropically-located continents.

• Implies that glaciers covered almost the entire Earth, and the planet froze over for some time.

• Active tectonism was likely the reason Earth warmed back up.

Glacial Evidence

• Evidence of ancient glaciers, based on sparse rock record.

• Varves: mudstone seasonal laminations; may include dropstones

• Tillites: unsorted, lithified, glacial debris

• Striations and polish

• Absolute age dating of rock below, and intrusions cutting across, sets the date at Paleoproterozoic.

Glacial Deposits

• Glacial deposits or till (tillite)

• Massive, poorly sorted, angular

• Dropstones: large rocks that are found embedded in sediments that were deposited by glaciers or ice sheets.

Polish and Striations

• Striations: parallel lines or grooves carved into rocks by the movement of glaciers; show direction and strength of glacial movement.

• Polish: describes the smooth, shiny surface left on rocks by the abrasion of a glacier dragging sediment and debris over them.

The Evolving Atmosphere

• Archean atmosphere contained very little free oxygen, it was not strongly oxidizing.

• The amount of free oxygen at the beginning of the Proterozoic was no more than 1% of that present now.

• Cyanobacteria, like green algae, were present during Archean, could perform photosynthesis.

• Stromatolites: layered, rock-like structures formed by cyanobacteria (; these organisms build up layers of sediment and minerals over time.

  • Were not common until about 2.3 Ga (Paleoproterozoic)

  • These photosynthesizing organisms added free oxygen to the evolving atmosphere.

BIFs (Banded Iron Formations)

• BIFs: Units of sedimentary rock → alternating layers of iron rich material and chert.

  • Implies photosynthesis probably was in vigorous operation at this time.

  • They form when iron dissolved in the ocean reacts with oxygen produced by early photosynthesizing organisms.

• Paleoproterozoic → most of Earth’s BIFs were deposited

BIFs and the Atmosphere

• Rocks consisted of iron oxides (hematite, magnetite)

• If oxygen is absent in the atmosphere, iron easily dissolves.

• In an oxidizing atmosphere (as indicated by stromatolites), iron precipitates out and combines with oxygen to form rust-like oxides that are not readily soluble in water.

• As iron became depleted, oxygen built up and poisoned photosynthesizing plants

• Silicate layers would deposit on previous iron oxides

• Iron builds up again and the cycle repeats

• Source of iron and silica was submarine volcanism and hydrothermal vents.

Mesoproterozoic Era (1.6-1.0 Ga)

Nuna Separates

• Nuna begins to break apart ~1.6-1.4 Ga

• One of the first large crustal segments that broke away was Laurentia → the craton that eventually evolved into North America

• > 1.0 Ga, another land mass collided with Laurentia (Grenville)

Grenville Orogeny

• Present day Appalachian Mountains, E. Canada

• Resulted from the collision of continental plates and forming part of the supercontinent Rodinia.

• Carbonates and sandstones caught between the collision were thrust upwards and metamorphosed (~1.1 Ga)

  • Thrust fault - low angle where the fault moves up relative to the footwall.

• Igneous intrusions

• Some of the oldest rocks in the Northwest

• Significant compression and convergence.

• We see the typical landforms and the processes that occur today resemble the rock that formed during that time.

Midcontinental Rift

• ~1.1-1.2 Ga, tensional forces opened a long narrow continental trough bounded by faults cutting through Precambrian rocks (all rock prior to 542 Ga)

• Lake Superior southwest into Kansas and southeast into Ohio

• Terminates along rock of the Grenville orogeny (there must have been some kind of divergence or spreading)

• Most of rift buried beneath younger rocks (except Lake Superior, which has exposed igneous and sedimentary rock)

• The central part of the rift contains numerous overlapping basalt lava flows forming a volcanic pile several kilometers thick.

• The ancient rifting that occurred ultimately resulted in the formation of Lake Superior

Atmosphere

• There is very little that occurs in the atmospheric evolution over the Mesoproterozoic; little oxygen production.

• Referred to as the “Boring Billion”

• Through Paleo-, Meso-, and some of the Neo-

Neoproterozoic Era (1.0-0.54 Ga)

Rodinia Supercontinent

• Rodinia: 2nd supercontinent of Proterozoic formed (~900 Ma)

• Smaller than Nuna

• Consisted of crustal segments of Antarctica, Australia, Laurentia, and S. China.

• ~750 Ma it broke up again forming the ocean of Panthalassa.

Pannotia Supercontinent

• Pannotia: 3rd supercontinent; formed from the reassembly of the separated pieces of Rodinia.

  • Another Wilson Cycle

• Fragmentation was underway again around the end of the Proterozoic and beginning Paleozoic.

Neoproterozoic Glaciation

• Neoproterozoic is divided into 3 geologic periods:

  • Tonian

  • Cryogenian (Cryo- means frozen)

  • Ediacaran (Fossils)

• Glaciation was not continuous during this entire time but was episodic with three major glacial episodes

• Tillites and other glacial features (900-600 Ma) are found on all continents except Antarctica.

• Neoproterozoic glaciers seem to have been present even in near-equatorial areas.

Possible Causes of Glaciation

• Evidence for glaciation at this time is extensive

• Found on all continents of the time

• Three models proposed:

  • High-obliquity model: where Earth’s spin axis was tilted (> 54°)

    • Obliquity: the angle between a planet's rotational axis and its orbital plane; the tilt.

    • Current tilt of the Earth's axis (obliquity) is approximately 23.4°

  • Plate tectonic model: glacial diamictite was formed at moderate to high latitude; plate movement brought them to low latitude ocean.

  • Snowball Earth model: glaciation was global, including the ocean.

End of Glacial Conditions

• Super-volcano (dust into atm)

• Changes in solar energy output

• Reduction in atmospheric gases that tend to warm the atmosphere (methane, CO₂)

Snowball Earth 2.0

• Whatever the cause, a positive feedback loop took place

  • Albedo feedback loop - continued ice formation, continued cooling, brings more ice, and so on, the loop continues.

• Once glaciation has begun, the Earth would rapidly freeze toward the lower latitudes.

• Continents reflect energy, but oceans absorb it. Once frozen the planet would be highly reflective and not warm up again easily.

• Once the earth is completely frozen over and trapped in albedo feedback loop, how does it escape?

• End of Glacial Conditions:

  • Loop continues until slow build up of greenhouse CO₂ and methane emissions from volcanoes continuing to erupt beneath the ice would result in a super-greenhouse,

    then rapid melting of ice.

  • Life could survive during Snowball earth glaciations if the ice was thin enough to photosynthesis and if organisms lived near active volcanoes or hydrothermal vents.

  • Warming climates and melting ice would eventually lead to massive radiation of organisms (sudden growth in diversity).

Proterozoic Sedimentation (Meso/Neo)

• Orogenies and accretion of material were important processes to Earth, in particular to rock types (igneous and metamorphic).

• Other important geologic events, such as sediment deposition occurred.

• Failed mid-continental rift: a significant geological feature in North America that represents an ancient, incomplete rift system.

Proterozoic Life

Simple Single Celled Organisms

• Archean fossils not very common, and all of those known are varieties of archaea (microorganisms similar to bacteria) and bacteria, although undoubtedly existed in excess.

• The Paleoproterozoic fossil record has mostly bacteria and stromatolites.

• Little diversification took place; all organisms were prokaryotes (single cell organisms).

• Lack of organic diversity in Paleoproterozoic not surprising because prokaryotic cells reproduce asexually.

• Most variation comes from sexually reproducing populations that shuffle genes from generation to generation.

  • Mutations introduce new variation into a population, but their effects are limited in prokaryotes (simple, single-celled organisms that lack a nucleus).

Evolution of the Eukaryotic Cell

• Appearance of eukaryotic cells marks a milestone in evolution.

• Eukaryotic cells have a cell nucleus containing the genetic material and organelles.

Eukaryotes

• Eukaryotes: organisms with cells that have a nucleus; they reproduce sexually and are aerobic, requiring oxygen for metabolism.

• Eukaryotes produce sexually, prokaryotes produce asexually.

• Nearly all are aerobic, meaning they depend on free oxygen to carry out metabolic processes.

• Must have evolved in time where there was some free oxygen in the atmosphere.

• Eukaryotic cells appear in fossil record between 2.7-2.2 Ga, the prevailing view is ~2.1.

• Bitter Springs Formation in Australia

  • Fossils of single celled eukaryotes showing evidence of cell division, processes carried out only by eukaryotic cells.

Metazoans

• Metazoans: multicellular animals that possess more than one kind of cell and have their cells organized into tissues and organs.

• First fossil evidence of multicellular life was found in the Proterozoic.

• After improved discovery methods and the impressions that these organisms made, more were found during the Neoproterozoic.

The Ediacaran Fauna

• Ediacaran Fauna: refers to a group of ancient, soft-bodied organisms that lived on Earth; some of the earliest known complex life forms and are mostly known from fossil impressions in rocks.

• R.C. Sprigg discovered impressions of soft-bodied animals in the Hills of South Australia (Quartzite, metamorphic).

• Additional discoveries by others turned up what appeared to be impressions of algae and several animals many bearing no resemblance to any existing now.

• Before these discoveries, geologists were perplexed by the apparent absence of fossil-bearing rocks predating the Phanerozoic.

Represented Phyla

• Three present-day phyla may be represented in the Ediacaran fauna:

  • Jellyfish and sea pens

  • Segmented worms

  • Early stage members of the phylum Arthropoda (the phylum with insects, spiders crabs, and others).

Distinct Evolutionary Group

• Some scientists think these Ediacaran animals represent an early evolutionary group quite distinct from the ancestry of today’s invertebrate animals.

Review Questions

What were the tectonic differences between Archean and Proterozoic eons?

• Archean Eon: Earth’s crust was hotter and thinner, faster tectonics due to higher internal heat, smaller and more numerous plates, and the formation of the first stable continental crusts (cratons).

• Proterozoic Eon: Slower tectonics with fewer and larger plates as the Earth's crust cooled, leading to the formation and stabilization of larger continents.

What was the Earth’s atmosphere like during Proterozoic? What was the evidence? What are BIFs?

• Proterozoic Atmosphere:

  • The Earth's atmosphere had more oxygen compared to the Archean.

  • Oxygen levels gradually increased, allowing more complex life forms to develop.

• Evidence:

  • Presence of Banded Iron Formations (BIFs) indicates rising oxygen levels.

  • Changes in rock formations and fossils show increasing oxygen.

• Banded Iron Formations (BIFs):

  • Layers of iron-rich rock mixed with silica.

  • Formed in oceans due to oxygen reacting with dissolved iron, precipitating out as iron oxides.

  • Provide evidence of the gradual increase in atmospheric oxygen.

What is a Wilson Cycle?

• Wilson Cycle: A theory that explains how ocean basins form and close over time; repeats over millions of years.

  • It describes the process of a landmass breaking apart to create an ocean, and then eventually coming back together as the ocean closes, forming a new supercontinent.

  • This cycle repeats over millions of years.

  • Rifting: Continents start to pull apart, forming a new ocean basin.

  • Expansion: The ocean basin widens as new oceanic crust forms.

  • Subduction: Eventually, the ocean basin starts to close as one plate slides under another.

  • Collision: Continents collide, leading to mountain building and the closing of the ocean basin, sometimes creating a supercontinent.

What was the Grenville Orogeny and when did it occur? What was the Midcontinent Rift and when did it occur?

• Grenville Orogeny: A major mountain-building event where ancient continental plates collided, forming the Grenville Mountains. (Mesoproterozoic, ~1.3-1.0 Ga)

• Midcontinent Rift: A large geological rift where the North American continent began to split apart but failed to fully open into an ocean (Mesoproterozoic, ~1.2-1.1 Ga).

What was deposited during the late Proterozoic and where was it deposited?

• Glacial deposits like poorly sorted sedimentary rocks, tillites, and dropstones.

• They were deposited in old ocean basins, shallow seas, continental shelves, and sedimentary basins.

What is a supercontinent? What were the supercontinents during this time?

• A supercontinent is a massive landmass formed by the merging of multiple continents.

• Nuna (Paleoproterozoic), Rodinia (Neoproterozoic), and Pannotia (Neoproterozoic)

Were there glaciers during this time? What was the evidence?

• Yes, there were glaciers.

• Evidence: varves, tillites, striations, polish, and other glacial deposits.

What evidence of life is there? How complex were the organisms and did they change from the Archean to the Proterozoic (e.g., prokaryote to eukaryote to metazoans)?

• Fossils contain the oldest signs of life include microfossils and stromatolites.

• In the Archean Eon, life was mostly simple, single-celled organisms called prokaryotes (e.g., bacteria).

• In the Proterozoic Eon more complex life forms developed, including eukaryotes (cells with a nucleus) and eventually multicellular organisms (metazoans).

• From Archean to Proterozoic: Life evolved from simple prokaryotes to more complex eukaryotes with a nucleus and then to multicellular metazoans, showing increasing complexity.