A dramatic story of global change

Summary of : Marine Heavens on Snowball Earth ?

Time has passed, admittedly, since the Proterozoic eon – meaning the „eon of early life“ – had reached its last phase 635 Million years ago, the  Neoproterozoic era. This  „recent era of early life“ – was a dramatic time of global change. Twice Earth was ice-covered and was looking more like a ball of slush or snow. Twice Earth was sweating in  run-away greenhouse. Life, only found in the sea went through big swings.

The  kilometre-thick layer of ice of the  Neoproterozoic ice-age made Earth looking like Jupiter’s icy moon Europa. „Snowball Earth“ looked far different from the blue ball that fascinated so much the first astronauts. However the global ocean kept active under the global ice cover. How would life survive when the entire planet, land and seas, are covered by ice?

The ocean of „Snowball Earth“ was not frozen to the bottom. Salinity of seawater, high pressure and geothermal heat-flux from the bottom prevented that. The ocean was covered by a thick ice cover that is a very good thermal isolator preventing heat loss from the ocean.  Water flows were driven by heat-flux from the sea-bottom and freezing at the surface. Water warmed at the sea-bottom by geothermal heat and rose towards the surface. Salt leaked out of the freezing water  at the bottom of the marine glaciers. Adding salt to the cooled water is making it heavier so that it sinks towards the bottom.  In the Neoproterozoic ocean, close to the equator warmer and less-salty water rose to the surface, moved poleward and sunk down again.

Temperature, salinity and density of  Neoproterozoic ocean was fairly uniform in the vertical direction but showed lateral differences. These lateral differences of density sustained, because of the rotation of earth, jet-currents along the Equator. These currents were unstable and were shedding off eddies. These eddies transported warm water away from the Equator to the ice-margin. There the water was melting ice, was cooled, partly frozen to the ice, partly enriched in salt  so that it sunk downward. A compensating upward flow of warm water occurred at the Equator to close the circulation cell. Ridges at the sea-bottom or continental margins brought the source of heat closer to the surface and were interacting with the equatorial jet-currents. This interaction caused local jets, eddies, coastal up-welling and down-welling as well as convective mixing . The weak stratification of the Neoproterozoic ocean made up-welling and down-welling far easier to happen than today in our well stratified ocean. Thus „Snowball Earth“ ocean was not a stagnant pool of cold water, it was highly dynamic; at least to the eye of the oceanographer. 

And marine life? It survived „Snowball Earth“. Both, the chemotrophic life that is using sulphur as source of energy and the phototrophic life that is using light as source of energy. Chemotrophic life would have survived in the depth of ocean under total ice-cover. Phototrophic life would have needed patches open surface water or thin ice; at least temporarily. The physics of  ocean dynamics make it likely that these patches, „marine heavens“ existed regularly in the ice-covered Neoproterozoic equatorial seas close to ridges and continental margins.

The end of  Snowball Earth likely was caused by volcanism blowing carbon-dioxide into the very dry and cold atmosphere of that time.  Rain must have been seldom, and without rain little carbon-acid weathering of rocks occurs and no carbonates are flushed into the sea. Thus carbon-dioxide accumulates in the atmosphere building up a greenhouse effect that finally caused Earth to warm again.

Earth was saved from staying frozen in snowball stage by  its active geophysical processes. To note the difference, Jupiter’s moon Europa is frozen in snowball stage; likely the moon is too small for having an active geophysical evolution as planet Earth. However, researchers are quite certain that under the icy surface of moon Europa an ocean is alive. If it bears life we don’t now. But if, then it will be of a chemotrophic form. Luckily survival of phototophic life on Earth in the global glaciations of the Neoproterozoic era has happened. The causes were: ocean dynamics on a geophysical active planet with continents moving, plate-tectonics and a robust geothermal heat-flux to create some „marine heavens“.