GEO130 – OCEANOGRAPHY

ORIGIN OF THE ATMOSPHERE AND OCEAN

Updated 08.26.09

 


(
Adapted from the University of Florida)

STUDY NOTE:
Use this along with the study guide for the film
How the Earth Was Made

Primitive Earth and Atmosphere:

·         early Earth probably had a molten surface or thin, unstable crust

·         sustained by heat from meteorite collisions and decay of radioactive elements and friction from migration of dense materials towards center of Earth

·         primitive atmosphere probably comprised of H2, He

·         primitive gases lost to space early in Earth's history

·         weak gravity prohibited retention of lighter gases

·         dispersed by strong solar winds

 

Formation of the Early Atmosphere:

·         early atmosphere was likely derived from volcanic outgassing and materials from meteorites/comets

·         comprised mostly of H2O (water vapor), CO2 (carbon dioxide), CH4 (methane), CO (carbon monoxide), H2 (some hydrogen remained), N2 (nitorgen), HCl (hyrochloric acid), SO2 (sulfur dioxide), Cl2 (chlorine), NH3 (ammonia), S2 (sulfur)

·         ONLY trace amounts of free oxygen (O2)  - it is not commonly found in volcanic gases

Formation of the Ocean:

·         early atmosphere was highly reflective  nearly 60% of incoming solar radiation blocked and Earth was cooling

·         resulted in planetary cooling and condensation of the water produced by outgassing

·         initial precipitation vaporized upon hitting the still hot planetary surface

·         eventually formed superheated water

·         finally collected into warm, iron rich, green seas and oceans above and around cooling crustal rock

History of the early Earth.

How do we know when the Earth's continents and oceans first formed?

·         uranium-lead (U-Pb) ages suggest first continental crust at 4.4 billion years ago 

·         oxygen isotopic composition of crustal material provides secondary evidence

·         chemistry of ancient crustal material reflects magma from which it crystallized

·         increased presence of heavy oxygen (18O) indicates water-rock interactions 

 

Evolution of the Atmosphere:

·         weak young sun - 70% of present - needed strong greenhouse effect for temperatures that could support life.

·         as sun strengthened what happened to all the carbon dioxide and methane? water vapor rained out. without removal of GHGs (greenhouse gasses) it would be too hot.

·         to remove methane - need oxyen (O2) and to get oxygen photosynthesis had to evolve.
methane + oxygen => carbon dioxide + water vapor.

·         to remove carbon dioxide- there were two processes
one was photosynthesis (it had to evolve)
creates carbon containing biomass (carbonate shells and tests) which are then buried in ocean floor sediments becoming limestone the greatest carbon reservoir on the planet.
the second was that granite rocks (continental crust) had to form
weathering of the silicate minerals removes carbon dioxide
silicate minerals + weak carbonic acid (= CO2 + rain water) => transportby running water to the sea => leads to burial of calcium carbonate in sea floor sediments.

·         modern atmosphere composed mostly of N2 (78.084%) and O2 (20.948%)

·         trace amounts of Ar (0.934%) and CO2 (0.0360%) also present

·         removal of CO2 through processes listed above left N2 as the major atmospheric constituent

 

Photosynthetic life produced most of the free O2  

CO2 + H2O + Light => Plant Bimomass

 

Development of Planetary Life:

·         Miller and Urey (1953) suggested a process known as 'biosynthesis'

·         synthesis of amino acids through energy activation (e.g., lightning strikes)  

·         early oceans contained the building blocks of amino acids  H20, CH4, H, and NH3

Artist's depiction of the ancient Earth, including stomatolites.  Living stromatolites, western Australia.

·         oldest fossils (anaerobic organisms) date to ~3.5 billion years ago

·         graphite concentrations (carbon) in many sedimentary rocks

·         stromatolites (mats of cyanobacteria or blue-green algae)

·         microbial life lacking a nucleus (prokaryotes)  each cell chemically self-sufficient

·         many microbes flourished in the hot oceans, probably around volcanic vents

·         metabolized hydrogen-rich compounds and/or organic materials to derive energy

·         sulfate reducing bacteria that produce H2S

·         fermentative bacteria that produce CO2 and alcohols

·         methanogenic bacteria

·         reduced meteoric bombardment allowed anaerobic microbes to diversify

·         many adapted to new biological niches  some on land  but stayed single celled

 

·         ~2.8 billion years ago bacteria (cyanobacteria) developed photosynthetic ability

·         photosynthesis produced O2 which was released into the oceans and atmosphere

·         rise in atmospheric O2 levels occurred between 2.4 and 1.8 billion years ago

·         initial free oxygen was consumed through the oxidation of surface materials

·         additional O2 was absorbed (oxidized) by iron dissolved in Earth's oceans

·         resulted in the precipitation of minute 'rust' particles on the ocean floor

·         now sedimentary rocks known as 'banded iron formations' or 'red beds'

Fossil stromatolites (2.2 bya) from Great Slave Lake, Canada.  Banded Iron Formation.

·         molecular oxygen in air and water became abundant by ~2.3 billion years ago

·         accompanied by conversion of a fraction of the O2 into a tri-atomic form 

·         known as ozone (O3

·         formed a protective layer in the atmosphere (reduced ultraviolet radiation)

 

·         eukaryotic metabolism began after O2 had risen ~1% of its present abundance

·         probably occurred ~2 billion years ago, according to the fossil record

Estimates of free oxygen in the Earth's atmosphere.