Tuesday, September 3, 2013

Snowball Earth: Refugia for Life

At around 600 to 800 Mya, during the Neoproterozoic era, the Earth underwent at least two global-scale glaciation events, more commonly known as Snowball Earth events (Trindade and Macouin, 2007). During a Snowball Earth event, the ocean was completely covered by thick ice right down to the tropics. At low latitudes, the equilibrium ice cover over the ocean is estimated to be hundreds of metres thick and only got thicker at higher latitudes. The ice cover over the ocean would tend to flow from higher latitudes towards the Equator in the form of a global sea glacier (Goodman and Pierrehumbert, 2003; Pierrehumbert et al., 2011). Any unprotected area of open ocean surface is likely to be overrun by it.

The existence of photosynthetic eukaryotic algae predates the Snowball Earth events of the Neoproterozoic era. At that time, life has yet to evolve from ocean to land and a completely ice covered ocean poses a problem for the survival of photosynthetic life. Nevertheless, a number of ways have been proposed that allow photosynthetic life to survive through a Snowball Earth event. It has been suggested that small pools of open water can exisit above geothermal hotspots on coastlines of volcanic islands (Hoffman and Schrag, 2000) and can serve as refugia for photosynthetic life on a frozen planet. Photosynthetic life can also survive around deep-sea hydrothermal vents by using the faint trickle of optical photos coming off from hot water to perform photosynthesis, albeit at very low rates (Beatty et al., 2005; Cardenas et al., 2013).

A paper published by Campbell et al. (2011) in the Geophysical Research Letters show that an inland sea analogous to the present-day Red Sea at low latitudes can serve as a refugium for photosynthetic life during the Snowball Earth events in the Neoproterozoic era. The study examines a long narrow inland sea similar to the present-day Red Sea. One end of the inland sea is connected to the ocean while the rest of it is surrounded by non‐glaciated desert land. The reason for non-glaciated desert land is due to net sublimation expected at low latitudes which prevents ice from accumulating. Such an inland sea can be formed by continental rifting, much like how the present-day Red Sea was formed by rifting of the African and Arabian plates.

Figure 1: An image of the present-day Red Sea taken on 22 June 2013 from on board the International Space Station.

Several conditions must be met in order for an inland sea to serve as a refugium for photosynthetic life during Snowball Earth events. The conditions mentioned in the paper are: (1) the inland sea must not be fully penetrated by a sea glacier; (2) the climate on the inland sea must be such as to maintain it either ice‐free or covered by an ice layer sufficiently thin to allow photosynthesis below the ice; (3) the depth of the sea at its entrance, and throughout its length, must be great enough that seawater is able to flow under the sea glacier to replenish water loss from the refugium by evaporation/sublimation, and (4) water circulation in the inland sea must be adequate to allow nutrients to be delivered to organisms living in the bay at the landward end.

A necessary criteria for the inland sea to serve as a refugium for life is that the sea glacier coming in from the ocean must not reach the landward end of the inland sea. On a Snowball Earth, net sublimation is expected to occur at low latitudes and the inflowing sea glacier will lose mass as it penetrates into the inland sea. Net sublimation will cause the sea glacier to shrink till it reaches zero thickness after penetrating some distance into the inland sea. At the low latitudes where net sublimation is expected, the estimated zonally‐averaged mean annual surface temperatures ranges from -50°C to -20°C. Since ice is softer at warmer temperatures, a warmer sea glacier is expected to penetrate further into the inland sea.

Figure 2: The solid contours represent the penetration length to width ratio L/W as a function of net sublimation rate (vertical axis) and surface ice temperature (horizontal axis) for a sea glacier with an initial thickness of 650 m. A warm sea glacier with low net sublimation rate will have a large penetration L/W, while a cold sea glacier with high net sublimation rate will have a small penetration L/W. The dashed line represents the L/W of 6.5 for the Red Sea. Conditions to the left of the dashed line allow a Red Sea analogue to serve as a refugium for life without being overridden by ice cover.

To determine if an inland sea similar to the present-day Red Sea at low latitudes can remain ice free during a snowball event, the penetration length of a sea glacier coming in from the ocean is estimated. The present-day Red Sea is approximated by a rectangle 200 km wide and 1300 km long, a length to width ratio L/W of 6.5. At the mouth of the inland sea, the initial thickness of the sea glacier coming in from the ocean is 650 m. The study shows that the length of the present-day Red Sea can easily exceed the penetration length of the sea glacier (Figure 2). As a result, a Red Sea analogue during a Snowball Earth event can be long enough to remain ice free and serve as a refugium for photosynthetic life on a largely frozen planet.

- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Goodman and Pierrehumbert (2003), “Glacial flow of floating marine ice in Snowball Earth”, Journal of Geophysical Research 108(C10), 3308, doi:10.1029/2002JC001471
- Pierrehumbert et al., “Climate of the Neoproterozoic”, Annual Review of Earth and Planetary Sciences 39 (2011) 417-60
- Hoffman and Schrag, “Snowball Earth”, Scientific American (January 2000), Volume 282, pp. 68-75
- Beatty et al. (2005), “An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent”, Proceedings of the National Academy of Sciences 102 (26): 9306-10
- Cardenas et al. (2013), “The potential for photosynthesis in hydrothermal vents: a new avenue for life in the Universe?”, arXiv:1304.6127 [astro-ph.EP]
- Campbell et al., “Refugium for surface life on Snowball Earth in a nearly‐enclosed sea? A first simple model for sea‐glacier invasion”, Geophysical Research Letters Volume 38, Issue 19, October 2011