Sunday, November 14, 2010

The Aftermath of the Snowball

Our planet covered in ice from one pole to the other. Doesn't really seem possible does it? Well, a controversial hypothesis called Snowball Earth posits that our planet was indeed covered with a thick sheet of ice for a period of its history. This thick sheet of ice lasted for millions of years and may have occurred more than once. The most severe likely occurred around 750-580 million years ago.

When an entire planet is covered in ice it is bound to have effects. A new(ish) paper in Nature takes a look at one of these effects and its relationship to life. But let's step back for a second...

Nutrients are chemicals that an organism needs to survive and grow, usually because they are vital to various metabolic and other bodily processes. What do we know about the kinds of nutrients that life needs to survive? Well, we know that life needs water (its inorganic but still counts), and if we're talking about a big melting ice sheet then we can check that one off our list. What about organic molecules? Sure. After all, we are organic and need building blocks, if you will, to help us live and grow. These building blocks, or nutrients, come in the form of carbohydrates, fats, and proteins (or amino acids), as well as various vitamins. Certain chemical elements are also important. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, calcium, etc. Of course, what type of organism you are will expand or narrow this list and vary the concentrations of these various nutrients.

The Nature paper zeros in on phosphorous. In most modern marine aquatic systems you will find that phosphorous and bioavailable nitrogen are the limiting factors. Usually when we are talking about nutrient limitations we're talking not about one being completely gone from the system but, rather, an imbalance of the nutrients. In the case of nitrogen and phosphorous you should see a nitrogen to phosphorous ratio of 16:1. It is generally thought that phosphorous limits productivity on a geologic timescale, and so it is particularly interesting to scientists to find the concentrations of this chemical over time.

In the aftermath of Snowball Earth, these researchers have found that the oceans were rich in phosphorus. Now, how in the world do you go about measuring that one? Well, these scientists looked at the rocks and sediments on the sea floor - about 700 samples of iron-oxide-rich rocks. They tracked phosphorus concentrations by analyzing the composition of iron-rich chemical precipitates which accumulated on the sea floor and took up phosphorus from the seawater. This analysis showed a spike in marine phosphorus levels in the mid-Neoproterozoic (from ~750 to ~635 million years ago). We know that the ice sheet melted right? Then it stands to reason that there was quite a bit of erosion and weathering going on at that time. These processes could explain the high concentrations of phosphorus in the seawater.

Let's take it another step, and link together what we know. Life likes phosphorus, there was lots of phosphorous in the seawater, therefore we should see more life in the oceans. Makes sense. Keep going. More life in the oceans means there's more oxygen production via photosynthesis, the oxygen is released into the atmosphere, atmospheric oxygen is available for other organisms. OK, good. Let's keep going. Oxygen is another of those molecules that life needs/likes, more oxygen availability, more animals using it and multiplying, throw in a little mutation and subsequent evolution, and bang! the emergence of more complex life on Earth.

Alright, alright, I agree, that's a lot of steps. Steps that all start at a single there-was-lots-of-phosphorus point. On their side, there is evidence that links marine phosphorus concentrations and the levels of atmospheric oxygen. And the authors aren't saying that this is definitely what happened. They are simply saying there was more oceanic phosphorus at that time and that it could have paved the way for the evolution of complex organisms and their diversification.

On a purely let's-look-at-the-chemicals level, until now scientists believed that the conditions of an iron-rich ocean would lead to low phosphorus levels. The fact that these researchers found the opposite after the Snowball Earth events is quite significant. We're talking about the finding of a possible nutrient driver behind one of the big explosions of life. Pretty neat.

Here's the source:
Planavsky, Noah J., Olivier J. Rouxel, Andrey Bekker, Stefan V. Lalonde, Kurt O. Konhauser, Christopher T. Reinhard, and Timothy W. Lyons (2010) The evolution of the marine phosphate reservoir. Nature: 467(7319): 1088 (DOI: 10.1038/nature09485)


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