Why should we look specifically at volcanic activities? In the present day Earth, there is only one mechanism of supplying NEW components to the atmosphere and that is volcanic degassing. Other mechanisms, including biologic activity (w/c of course was non-existent in Early Earth) only RECYCLE components into the atmosphere. THEREFORE, it is not a bad idea to look at volcanic degassing as the source or control of the earliest inputs into the atmosphere.

(1) high in H₂ (but has H2O),

(2) high in CO/CO₂ (relative to present),

(3) H₂S dominant S (not SO₂),

(4) N₂ dominant N (as always).

These are initial estimates of speciation of ATMOSPHERIC INPUT.

- Over time, we expect H₂ to escape into the atmosphere leaving H₂O as the

dominant H compound.

S (FeS to SO2) and carbon (graphite to oxidized carbon and methane).

The classic Miller-Urey experiment was performed in a methane-ammonia-water plus hydrogen soup using electrical discharge. The result "stunned" the scientific world because it was shown that the fundamental building blocks of life as we know it were formed. These include amino acids (building blocks of proteins), sugars (saccharides), aldehydes, nucleosides (subunits of cells carrying genetic information).

- BUT, CO₂ must have been 200 to 1000 times PAL (6 to 31 % !!!). Ammonia also has a a problem staying early atmosphere for long (photolysis or photodissociation). Methane and ammonia atmosphere cannot have persisted for long for these additional reasons:

- Ammonia dissolves in water and falls back to sea:

CH₄ + OH = CH₃ + H2O

CH₄ + 4O₂ = HCHO (formaldehyde) + H₂O + 2O₃ (not equilibrium !!)

- Clearly then, the combination of hydrogen escape and above would have resulted in evolution to an N2-CO2 atm (although bear in mind that CO2 in the presence of an ocean will get sucked in again - not as efficiently as now because of fixation now and larger fluxes then).

PRIMITIVE OCEANS:

We did not explicitly talked about the primitive oceans in class but evidence points to massive formation of the Earth's ocean during the first 1.5 by of Earth's history (along with the Earth's atmosphere). A gas also released by degassing that we have not explicitly discussed is HCl. During volcanic degassing, most Cl partition into the volatile phase as HCl. All the Cl in the present ocean must have been released in this way

Classification by Forms:

prokaryotes - primitive single celled (bacteria, blue-green algae or

cyanobacteria) organisms nothing more than a bag of protoplasm (no nucleus to

store genetic material -loosely distributed in protoplasm).

eukaryotes - organized cells with nucleus (all other algae and higher plants) mostly capable of sexual reproduction (mostly aerobic).

Classification by Metabolism:

heterotrophs - incapable of building their own

organic material (use what is available in surrounding (bacteria even now are

heterotrophs).

autotrophs - photosynthesizers (photoautotrophs, CO2 + water

= CH2O + O2; CO2 + 2H2S - CH2O + H2O + 2S),

- chemoauthotrophs - methanogens (CO2 + H2 = CH2O)

• Earliest organisms are prokaryotes that heterotrophically survived by fermenting early biomolecules. At some point the consumption of abiotically produced organic and the changes in the environment will favor the autotrophs which can produce their own food. Genetic studies apparently suggests that evolution from prokaryotes to eukaryotes may have involved early dominance of methanogens (use H₂ or H₂S instead of H₂O) then photoautotrophs). Autotrophic green, purple and blue green algae (chlorophyl-a) evolved eventually using water as the hydrogen source. It is the predominance of the latter that eventually produced a more oxygenating atmosphere.

• Stromatolites - symbiotic blue-green algae ppting CaCO₃ at the top and shielded anaerobic bacteria (decomposers) in the bottom. Oldest one known are 3.5 by old in Warroona, Australia. In shales, siltstones, sandstones - also preserved are acritarchs - suspected to be largely composed of spores of unicellular algae - phytoplankton. Middle and late proterozoic carbonaceous compressions of probable algal affinity. After 680 Ma - a distinctive assemblage of soft-bodied animals impressions, cast and molds have been found in many continents (Proterozoic to Phanerozoic).

• Other Evidence of O₂ Record (not covered in class):

1. Paleosols - soil profiles that are geologically preserved (e.g., Elliott Lake Ontario, 2.5-2.7 Ga, Athabaska Basin). The basic idea is that in contrast to podsols or laterites, etc., the Fe profile shows depletion of Fe throughout the entire soil horizon. This means that Fe is not oxidized (which makes it insoluble) OR the atmosphere is reduced ordefficient in O₂.

2. Detrital Deposits: Here again, certain minerals are not stable in oxidizing conditions. Take the case of uraninite.

Photodissociation of H₂O: bring pO₂ to 4 E-3 atm (<1 % PAL)

Eukaryotes require > 0.5 % PAL because they're primarily aerobic

Eukaryotes need ozone shielding but by 1 % PAL, there is as much ozone shielding as now

(optimum at 10 % PAL). Photosynthesis eventually overcame the reducing reaction at the surface of the Earth, and excess O₂ began to build up to present day levels.

Conditions:

(1) The high latitude regions are some 12 deg warmer - evidence provided by the oxygen

isotopic composition of deep ocean waters (benthic forams) formed in high

latitudes. Equatorial waters just a few deg warmer then (planktonic fauna). Consistent with

warm pole flora/fauna and no evidence of ice (oxygen isotope balance). Globally average temp is

6 to 12 deg warmer.

(2) Continental interiors with no seasonality !

(3) Different distribution of continents (lots in northern hemisphere)

(4) High sea-levels (no polar ice cap) hence diff. land/sea ratio

- If everything is held the same other than plate geometry and sea-level, warm Cretaceous simulation shows 1.6 to 4.8 warming of the polar region. BUT, equatorial region must be warmer (cannot reproduce the low gradient) than what is known. CONCLUSION: By itself, purely geographically induced warming is not likely.

• Can the ocean do the trick? With less land (less barrier to circulation), more efficient redistribution of heat (or just speculate better circulating ocean). PROBLEM: this mechanism cannot warm the high latitude continental interiors enough to prevent it from freezing in winter. No evidence of this seasonality was found (fauna). CONCLUSION: geography + ocean cannot do it either.
• Can CO2 do it ?? YES, and this will have warming effects on both coast and interiors. However, SIMILAR problem of maybe not enough warming of the poles ! Role of ocean?

Evidence:

(1) Detailed record of mid and high latitude waters

(2) Growth of the Antarctic Ice.

(3) Glacial episodes 60, 45, 38, 28 (disputed 15) 2.5 (last series)

(4) Greater continental seasonality evidenced by fauna.

One essentially has to reverse the "Cretaceous Warming"). The growth of the ice is a consequence, not an initiator of cooling (although it is a positive feedback). Geography is changing, but this again is a relatively small forcing factor, and the timing of the overall cooling and the specific cooling episodes does not correspond with dramatic geographic shifts.

The most logical choice is the removal of CO₂ from the atmosphere leading to an "ice-house" effect. CO₂ residence time is quite long, how do you accomplish this? Volcanoes won't suck them back in. Massive consumption by weathering is our primary mechanism. Is this consistent with what we know? The overall scheme that seems to fit the bill is the enhanced weathering that accompanied major mountain building episodes (e.g., Himalayas). This will have various regional effects but also a global effect of cooling the Earth.

When the earth cooled own enough, the Milankovich cycles (eccentricity, obliquity, precession) became capable of cycling conditions between glacial and interglacial. Class hand-out diagrams show Milankovich cycles. It cannot be the simple story though because amplitudes of cooling and warming do not completely match. Feedback effects (ocean, atm etc.). Clouds and volcanisms can also perturb the system enough to cause glacial episodes.

Not just glacial cycles but tropical monsoons - warming enhances the contrast between land and sea encouraging lower atmosphere inland transport (monsoon). This causes lake levels to rise whenever we are in a warming phase (eccentricity, 100 ky) of Milankovich cycles.

Retreating glaciers caused changes in wind directions (jet stream marks boundary of cold and warm air masses) particularly due to glacial anticyclones that develop as a result of glacial.

Not discussed in class is the occurrence of Younger Dryas (named after Dryas flower) - signifying return to cold dry climate in Northern Atlantic and western Europe during the time of overall warming from the last glacial stage. Cessation of NADW conveyor belt (see Ocean circulation notes) was suggested by Broecker. This cessation of ocean conveyor belt circulation during has to do with cold melt water of the ice sheets mixing with the salty water coming from south diluting its salinity. This caused the water density to be not high enough for it to sink (or form the NADW).