GEOL 102 UNLV Lecture 16 - Snowball Earth

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Life on Neoproterozoic Earth ~630 Ma

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  • mesoproterozoic stromatolites China

  • mesoproterozoic eukaryotes Canada

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Two glaciation periods in Neoproterozoic Earth

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  • Marinoan

  • Saturnian

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18 Terms

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Life on Neoproterozoic Earth ~630 Ma

  • mesoproterozoic stromatolites China

  • mesoproterozoic eukaryotes Canada

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Two glaciation periods in Neoproterozoic Earth

  • Marinoan

  • Saturnian

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Three models proposed to solve low latitude glaciation puzzle?

  • high-obliquity model

  • plate tectonic model

  • snowball earth modeldd

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High-obliquity model

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

  • glaciation occurred preferentially at low latitutdes

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Plate tectonic model

  • glacial diamictite was formed at moderate to high latitude

  • plate movement brought them to low latitude ocean

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Snowball Earth model

glaciation was global, including the ocean

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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

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

Oxygenation

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Two important greenhouse gases

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

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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

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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

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Evidence for greenhouse CO2 vs. CH4

cap carbonate:

  • thin, laterally-persistent, fine-grained carbonates

  • overly directly glacial deposits

  • carbon isotopes as low as -5%

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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

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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

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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

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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

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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

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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