6 December 2012
Shelter from the Snowball
Posted by kramsayer
Bacteria dependent on light may have found refuge from encroaching glaciers in inland seas some 600 million years ago, when Earth was a giant ice ball.
The idea of an icy Earth, called a “Snowball Earth,” is controversial. One question surrounding the theory: If the Earth was covered in miles-thick snow and ice, how did photosynthetic bacteria, which need sunlight to create nutrients, survive?
That’s where inland seas, bodies of water mostly surrounded by land, but connected to the ocean by a narrow channel or strait, come in. Adam Campbell, a graduate student at the University of Washington in Seattle presented findings about the theory on Monday at the American Geological Union’s Fall Meeting in San Francisco.
Geologists estimate the last Snowball Earth event happened about 600 million years ago during the Marinoan ice age, and lasted about 10 million years. Glaciers covered land and oceans, but Campbell speculated there may have been inland seas that served as the last refuges for photosynthetic bacteria because they remained liquid, were replenished with ocean water, and were protected from advancing glaciers.
The key is to have a narrow opening leading to the sea and a quick rate of ice loss from the glacier. Glaciers flowing inward from the ocean had to rub against the banks of the channel, which slowed them down. The narrower the opening, the more the glacier is slowed. At the same time, if ice vanished from the glacier rapidly enough through evaporation of ice molecules directly into water vapor, or sublimation, the already slowed glacier might not have gained further ground.
A model that Campbell has developed answers “yes” to the question “Is there a sweet spot where the rate of sublimation is faster than the flow rate of the ice?” he said.
When a narrow sea the width of the modern-day Red Sea (about 200 km wide for a channel about 1,300 km long) has a strait connecting it to the ocean, much like the Bab el Mandeb strait that separates the Red Sea from the Gulf of Aden, the constricted channel could potentially slow the advance of the glacier, Campbell found.
In his model, if the strait was one-tenth the width of the sea behind it, there was only a 5 percent chance that the inland sea at the end of the channel would be completely iced over. But if the strait was one-fifth the width of the sea, the chance of complete ice-over rose to 50 percent. A strait half the width of the sea meant an 85 percent chance of complete glaciation.
Just because an inland sea escapes glaciers approaching from the ocean doesn’t mean it can escape icing over – cold atmospheric temperatures can still freeze the water.
Campbell plans to look more closely at what conditions keep inland seas warm enough to stay liquid or at least keep surface ice thin enough so photosynthetic bacteria have ample light.
-Rina Shaikh-Lesko is a science communication graduate student at UC Santa Cruz