GEOL 102 UNLV Lecture 16 - Snowball Earth

Life on Neoproterozoic Earth ~630 Ma

• mesoproterozoic stromatolites China

• mesoproterozoic eukaryotes Canada

Two glaciation periods in Neoproterozoic Earth

• Marinoan

• Saturnian

Three models proposed to solve low latitude glaciation puzzle?

• high-obliquity model

• plate tectonic model

• snowball earth modeldd

High-obliquity model

• Earth’s spin axis was tilted (>54 degrees)

• glaciation occurred preferentially at low latitutdes

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

What caused the melting of pervasive ice cover on Earth?

If >50% of the earth surface was covered by ice, the albedo feedback becomes unstoppable

Did the melting of ice sheets play roles in triggering animal evolution? If so, in what manner?

Oxygenation

Two important greenhouse gases

• carbon dioxide and methane could potentially lead to global warming before anthropogenic CFCs and food production

Warming up the Earth by CO2

• possibly low CO2 during glaciation

• source for additional CO2 is volcanic outgassing

• seawater - atmospheric CO2 exchange prevents fast warming

Reasoning for “snowball” Earth

• earth was covered entirely by ice sheets

• seawater - atmospheric exchange was blocked by ice cover

• catastrophic melting of ice when CO2 reached a level of 0.12 bar

Evidence for greenhouse CO2 vs. CH4

cap carbonate:

• thin, laterally-persistent, fine-grained carbonates

• overly directly glacial deposits

• carbon isotopes as low as -5%

Snowball Earth Hypothesis/Theory

• volcanic outgassing w/o air-seawater exchange for millions of years

• atmospheric CO2 as high as 350 times the present level

• extremely high CO2 caused catastrophic greenhouse effect-deglaciation

• cap carbonate records transfer of excess atml CO2 to sedimentary resevoir

• carbon isotopes of cap carbonates reflect kinetic effects of postglacial weathering

Testing snowball Earth

• GCM modeling conflicts w/ complete frozen ocean and accumulation of CO2

• continental to basin scale cycles in glacial successions indicate active hydrological system during ice ages

• positive carbon isotope values in glacial successions and basal cap carbonates

• microbial fossils and biomarkers within glacial strata conflict with a frozen ocean

• cap carbonates track sea level; contain stromatolites and peloids, indicating normal photosynthetic origin

carbon isotopes

• modern ocean seawater excursion is 0%

• methane or carbon oxidation could also produce the negative carbon isotopes value

• both CO2 and CH4 can explain the isotopes in cap carbonates

• CO2 requires a hard snowball but CH4 does not

Warming up Earth by CH4

• methane is a stronger gas than CO2 (20x)

• methane can be generated by methanogenesis and stored in sediments

• large quantity of methane has been found in modern continental shelves

• during Neoproterozoic glaciation in the CH, hydrates pool may be larger than today

Problems with methane hypothesis

• source of alkalinity

  • cannot be the carbon source for all cap carbonates

• methane-derived carbon excursion signal (< -35%)

  • only in south china and africa were extremely negative carbon excursion values found

Evidence supporting Methane hypothesis

• oxidation of methane consumes oxygen, and may cause ocean anoxia, increasing organic carbon burial and leading to oxygenation

• a -5% carbon isotope excursion needs the release of ~30% of the modern methane hydrate reservoir

• carbon isotopes as low as -41% can only be methane-derived carbon