Early Earth History

Key Reference: Nisbet and Sleep, The habitat and nature of early life, Nature, 409, 1083-1091, 2001.

Solar system began about 4.6 Gy ago

Started with several supernova explosions in the local neighborhood.

Sun formation from accretionary disk.

Roughly 500 planetoids (about size of moon) in region of inner planets.

Collisions of these planetoids produced Venus, Earth and Mars, all with inventories of water vapor and carbon.

There may have been early oceans on all three of these planets.

In discussing geological time,

1 Gyr is 109 years,

1 Myr is 106 years (the ‘ago’ is implicit and often omitted, such that Gyr and Myr refer to both time before present and duration).

There are four aeons.

The Hadean is taken here as the time from the formation of the Solar System and early accretion of the planet (4.6–4.5 Gyr), to the origin of life (probably sometime around 4.0±0.2 Gyr).

The Archaean, or time of the beginning of life, is from about 4–2.5 Gyr;

the Proterozoic from 2.5 Gyr to about 0.56 Gyr;

and the Phanerozoic since then.

Earth in the Hadean

• Earth formed about 4.6 billion years ago from coalescing interstellar gasses. • For 500 to 800 My, Earth was bombarded by large meteorites adding to

earth’s mass (also adding heat).• Hot spinning pre-earth mass melted, caused differentiation of materials

according to density.• Distinct earth layers begin to form

– Dense iron and nickel migrate to center (core) – silicate material moves out to mantle

The Hadean was a time of heavy boloid bombardment of the earth.

No terrestrial geology record of this: data taken from dating of lunar impacts.

Many impacts had sufficient energy to boil-off the oceans.

From: Bada, JL, How Life Began, Earth and Planetary Science Letters 226 (2004) 1 – 15.

Boil oceans

A palaeotemperature curve for the Precambrian

oceans based on silicon isotopes in chertsFrancois Robert & Marc Chaussidon, Nature, 2006.

NOW THEN

hot

cold

Cretaceous hot house, 100 My ago

Relative temperature of the earth, for last 600 My

The Faint Young Sun Paradox.

The Sun’s interior through out the history of its existence (4.55 Byr) has been the site of ongoing nuclear reaction (H to He fusion). This nuclear reaction process has caused our Sun to expand and gradually become brighter. These models indicate that the earliest Sun shone 25% to 30% more faintly than today.

This is a problem for climate scientists.

A decrease of just few percentage in our Sun’s present strength would cause all the water on Earth to freeze, despite the warming effect of our present day greenhouse gases.

A positive feedback would be caused by their high albedo and it would never get warm...

Climate models suggest that an early Earth with so weak Sun and present level greenhouse gases would have remained frozen for the first 3Byr of its existence.

Adding greenhouse gases to the atmosphere solves this problem.

Evidence of running water in sedimentary rocks formed during the early Earth’s history (zircons) , means it was not frozen solid.

First evidence of ice-deposited sediments occurs in rocks datedto about 2.3 Byr ago, probably due to glaciations localized in polarregions, as on Earth today, and are not an indication of completelyfrozen planet.

This conclusion also supported by the continued presence of lifeon Earth. Primitive life forms date back at least 3.5 Byr ago.

The Problem:

With so weak a Sun, why wasn’t Earth frozen for the first two-thirds of its history? This is known as

The faint young Sun paradox.

The early weaker sun (but similar/warmer temperatures) indicate that the

Greenhouse gas concentrations in the early atmosphere much have been much higher than present time.

Early Earth CO2 and O2 levelsNOTE: these are determined from proxies, like BIFs and redbed formation,

isotopes of soil minerals and the presence of partially unoxidized iron minerals.

Earth’s early environment thought to have active volcanism, causing extreme loss of volatile gases (including CO2) from its interior.

- Earth’s surface may have been entirely molten for a few 100 million years after its formation (the ‘magma ocean’ period seen on the moon),

- Craters on moon and other planets suggests that Earth was once under heavy bombardment by asteroids, meteors, and comets, triggering greater volcanism.

- radioactive elements deeper in Earth’s interior released more heat, increasing volcanism

- increased volcanic activity would have delivered more CO2 to the atmosphere and may have helped to make Earth hot.

What happened to all the CO2 that was in the atmosphere?

•Carbon is removed by weathering and buried in sediments and turned to rocks.•Today, CO2 removed by weathering is deposited in ocean sediments and becomes rocks.•Same would have worked in the past, with a slow transfer of CO2 from the atmosphere to the rocks.•Most of early Earth Greenhouse atmosphere is in rocks and not in the•atmosphere like on Venus

OXYGEN IN THE ATMOSPHERE

Microorganisms are responsible for the production of nearly all of the oxygen we breathe.

Oxygen is produced during photosynthesis by the reaction

CO2 + H2O = CH2O + O2.

Where “CH2O” is a geochemist’s shorthand for more complex forms of organic matter.

Most photosynthesis on land is (now) carried out by higher plants, not microorganisms; but

Terrestrial photosynthesis has little effect on atmospheric O2 because it is nearly balanced by the reverse processes of respiration and decay.

By contrast, marine photosynthesis is a net source of O2 because a small fraction (about 0.1%) of the organic matter synthesized in the oceans is buried in sediments. This small ‘leak’ in the marine organic carbon cycle is responsible for most of our atmospheric O2.

The ‘small leak’

Early Earth CO2 and O2 levelsNOTE: these are determined from proxies, like BIFs and redbed formation,

isotopes of soil minerals and the presence of partially unoxidized iron minerals.

GLOBAL CLIMATE - in briefest summary

• 4.6 to 4.0 (or 3.8) By: The Hadean; massive boloid bombardment periodically

boils ocean. Earth's core forms; geomagnetic field preserves atmosphere.

• 3.8 to 2.5 By: Archean earth was ice-free and warm (60°C?) in spite of lower

sun luminosity. Must have had a very strong GHG effect (CO2, H2O, probably

methane). Occasional boloids must have made life uncomfortable. BUT life

was present at 3.8 By.

• 2.3 By: End of Archean. 1st evidence of surface glaciation, continents form,

traditional rigid plate sea floor spreading begins. Oxygen is present.

• 2.3 to 0.9 By: Proterozoic. Warm (30°C) during Early and Middle Proterozoic.

Life abundant.. Mega-continent Rodinia forms. Atmospheric now with lots of 02

present. Megacontinent Rodina breaks-up. Massive global glaciation starts in

Late Neo-Proterozoic (Snowball Earth).

• 0.9 to 0.6 By: Neo-Proterozoic. Four possible periods of 'Snowball Earth',

where glaciation was - at sea level, at the equator. Intervals 10 My long with

ice-covered surface are followed by extremely elevated atmospheric CO2

levels, followed by (very) warm periods of inorganic carbonate precipitation.

Brief Climate Summary – continued.

• 600 My to 210 My: Climate warm to temperate, but punctuated with 2

periods of major global glaciation. Mega-continent Pangea forms from

Laurentia, Baltica and Gondwana. Development of multi-celled life, land

plants/animals.

• 210 to 145 My: Jurassic climate was ‘temperate’. Pangea breaks-up.

• 150 – 65 My: Cretaceous. Warm, high atmospheric CO2, high sea levels,

fast seafloor spreading, Large Igneous Provinces and mantle plumes.

• 55 My: Eocene Climate Maximum. High temperature excursion within the

general cooling from the Cretaceous-warmth. Brief period of extreme

warmth followed by general cooling toward present time (methane!).

• 14 My: Formation of Antarctic Ice sheet.

• 3.0 My: Oscillations between periods of major glaciation and inter-glacial

warm periods. Emergence of Central American connection may have

changed global ocean circulation patterns. Mostly (90%) cool, only 10% of

the last 3 My were as warm as present.

• 22,000 to 18,000 years: Last glacial maximum.

• 6,000 years: present Holocene (generally) stable climate.

Some of these glacial periods may be related to changes in greenhouse gases, driven by biology

- OR by plate tectonics.

Quaternary

Permo-Carboniferous

Ordovician

Neoproterozoic

Paleoproterozoic

Major glaciations

HOW continents form – and when they did it.

PLATE TECTONICS – a major player in global climate

The Wilson Cycle

The Wilson Cycle

The Wilson CycleThe Wilson Cycle

The Wilson CycleThe Wilson Cycle

The Wilson CycleThe Wilson Cycle

The Wilson Cycle

The Wilson Cycle

Rodinia – the SuperContinent before Pangea (1 By ago)

Most of North America “Boxed-In”

Note orientation and neighbors of “North America”

N

The Appalachian Mountains – form by the closing of the paleo-Atlantic Ocean;

after Rodinia broke up and Pangea reformed.

The world we live in.

The break-up of

the Mega Continent Pangea

Weathering of rock: CO2 + XSiO3= XCO3+SiO2

Oceanic crust

Weathered sediment from continents

Ocean

Burial of organics: CO2 + H2O = CH2O+O2

Weathering of organics: CH2O+O2 = CO2 + H2O

Metamorphism of rock;

XCO3+SiO2=CO2 +XSiO3

Why is this important to CLIMATE? Long term (>105 year) concentrations of atmospheric CO2, O2, CH4 are set by many different interactions, including plate tectonics.

Fast seafloor spreading – high CO2

input: both at mid-ocean ridges (new CO2) and at subduction zones (re-cycled

CO2). Slow SFS, low CO2.

And this variation in CO2 input has both positive and negative feedbacks.

BOTH examples => are negative feedback

As the Wilson cycle happens, seafloor spreading and subduction are speeding up – and slowing down

(i.e., fast during breakup, slow during final stages of Mega- continent formation).

This puts variable amounts of CO2

into the atmosphere,

changes the latitude of the continents, and

changes the ratio of coast (wet) to continental interiors (dry).

All of these are major climate processes.

THE 'BLAG' HYPOTHESIS - WHAT IS IT?

LARGE-SCALE CLIMATE CHANGES ARE CONTROLLED BY THE AMOUNT OF CO2 IN THE ATMOSPHERE (the thermostat), AND ATMOSPHERE CO2 CONTENT IS, IN TURN, CONTROLLED BY THE PROCESS OF PLATE TECTONICS, i.e.,

seafloor spreading rates, rates of subduction, mountain-building and weathering.

HIGH RATES OF SEA FLOOR SPREADING

bring more new CO2 up from the mantle,increase the amount of old CO2 emitted from subduction

volcanoes,increase the rate of mountain building and the exposure of new rocks to weathering,raise sea level, changing earth's albedo.

Fast seafloor spreading – high CO2

input: both at mid-ocean ridges (new CO2) and at subduction zones (re-cycled

CO2). Slow SFS, low CO2.

And this variation in CO2 input has both positive and negative feedbacks.

BOTH examples => are negative feedback

Tracing the pathway of CO2.

(1) MOR eruptions: (2) transfer to atmosphere: (3) combined chemically during weathering: (4) transfer to ocean via rivers: (5) incorporated in biology

(6) Eventually sinks to seafloor as sediment: (7) seafloor is subducted: (8) mantle heat released CO2 in subduction zone: (9)

emitted by subduction volcanoes back into atmosphere.

Then, the cycle starts over.