CHEMICAL LIMITATIONS ON A SIMILARLY SUITABLE MOTHER (CLASS-M): ARE SNOWBALLS A MANDATORY PHASE IN THE DEVELOPMENT OF AN OXYGENATED ATMOSPHERE?

The development of marine plankton large enough to sink to the deep ocean during the Proterozoic Eon linked, for the first time, the oxygenation state of the deep ocean with the global carbon cycle. Prior to the development of marine plankton with sufficient ballast to sink to the bottom of the ocean, the oxygenation state of the deep ocean was a comparatively insignificant component of the global carbon cycle, but afterwards it became a sink for organic carbon being exported from the surface ocean (and atmosphere) with no pathway for recycling that carbon aside from subduction-related volcanism. Current estimates of the timing of this development suggest that by the start of the Neoproterozoic Era, plankton with sufficient ballast to sink to the bottom were commonplace and that the oceanic ‘biological pump’ was fully active.

The development of an oxygenated atmosphere, although linked to the production of oxygen by the plankton, took considerably longer and levels of atmospheric oxygen remained considerably below Present Atmospheric Level (PAL) throughout the Proterozoic Eon. Lower levels of atmospheric oxygen impacted the oxygenation state of the deep ocean throughout the Proterozoic and current estimates suggest that the deep ocean was likely anoxic/euxinic during most, if not all of, the Protoerozoic.

If a low- or no-oxygen deep ocean allowed a constant loss of carbon to the deep ocean sediment reservoir that could not be recycled back into the ocean-atmosphere system. The net result of this would be a runaway draw down of atmospheric CO₂ that could not be countered through actions of the oceanic component of the global carbon cycle. The only component of the global carbon cycle remaining to counteract this runaway cooling would be the silicate weathering cycle and its long-term control on atmospheric CO₂. Therefore, we suggest that widespread glaciation (i.e. Snowball Earth) is likely a mandatory restorative mechanism for global atmospheric CO₂ levels during the time between the development of ballasted organic matter and the development of a fully oxygenated atmosphere.