Wednesday, June 5, 2013

The Living Earth – Rocks and All

Biology concepts – Gaia Hypothesis, photosynthesis, biogeology, plate tectonics, biological weathering, oxygen crisis

The physical form of Earth definitely influences how life evolves on Earth. You can't argue that ice ages and the birth of shallow seas in the middle of continents changed what life forms survived and thrived. But what about the other possibility?

Question of the Day – Does life have an effect on the physical form of Earth?

This is a small town in Kansas about to be inundated by a dust storm. 
The dust would reach miles high and would deposit hundreds 
of tons of Kansas topsoil in the Atlantic Ocean. Unfortunately, these 
were nearly daily occurrences. Every morning people would 
have to dig their houses out from under the dust that blew in overnight.
Obvious examples include humans. We have had a recent effect on Earth, probably coinciding with the Industrial Revolution. Our activities, to a greater or lesser degree, have changed the climate. Reasonable people can disagree on how much, but could we really have an effect on something as big as the physical Earth?

What about the Dustbowl? In the 1930’s, the bottom fell out of the wheat market and farmers in Oklahoma, Texas, and Kansas abandoned their land. Farmland that isn't farmed is no longer held in place by roots. The farmers had plowed under the grasslands that had kept the soil in place for thousands of years, but a couple of years of drought and a surplus of corn and wheat led to a national disaster. By 1938, 5 inches of topsoil had been lost from more than 26 million acres of farmland.

Other life has also had an effect on the physical Earth. Organic rich sedimentary rocks are formed when living things die, decay, and under the influences of pressure heat, and time come to form specific products. The rocks and other end products are considered organic rich only if they are greater than 3% organic material. You might have heard of these end products - coal and oil shale.

These are interesting examples, but allow me to relate a newly written story. It's still a hypothesis, but there is a lot of data that supports this fresh idea of the massive effects which life has had on the planet. How massive you ask? Let me give you this hint – it doesn’t get much bigger.

The earliest life on Earth appeared more than 3.8 billion years ago. Oxygen was very scarce in the atmosphere, so these organisms did not use oxygen as an electron acceptor in the production of cellular energy. The earliest bacteria necessarily used those things that it had at hand, things like methane or other chemicals.

The early Earth atmosphere was affected greatly by erupting volcanoes. 
These would spew hydrogen gas, carbon dioxide and water into 
the air. Some helium and hydrogen would be lost to space since they 
are so light, but some oxygen would combine with the hydrogen 
to form more water. Notice that no free oxygen is indicated.
About 3.8 billion years ago, photosynthetic bacteria evolved to make use of sunlight and carbon dioxide. These photosynthetic bacteria didn’t look a lot like the cyanobacteria of today, using much more iron (Fe2+) and sulfate (SO42-) than is made use of in modern organisms.

But these photosynthetic bacteria did release oxygen – something that had not happened on Earth previously. The oxygen that was produced didn’t just float around in the atmosphere; there were other compounds that were ready to react with the O2.

One thing that the Earth had in abundance 3.8 billion years ago was hydrogen. But hydrogen is light. A thin atmosphere could be made thinner by losing hydrogen to outer space, so combining hydrogen and oxygen to produce water was a fortunate way to keep some of this hydrogen on the planet and to increase the water supply. We will see just how important this was in a few paragraphs.

Other things early Earth had in great supply were iron and basalt. Basalt is the rock formed by the eruption of volcanoes and interaction of the magma with the atmosphere. The oxygen produced via photosynthesis (and much of the oxygen already present) quickly reacted with iron in the earth and lava and with the basalt.

The basalt and oxygen underwent a much longer process in the earth’s crust, changing from igneous rock to metamorphic rock (meta = change and morph = form). Basalt or clay + minerals + terrestrial O2 + water, form granite. Is it a coincidence that the only place in the universe we have seen granite is right here on Earth? Sounds like photosynthetic life had an influence on the kinds of rocks located on this particular planet. But wait, it gets bigger.

Plate tectonics theory explains how continental crust and
oceanic crust floats on top of the mantle. When two plates
crash into one another one plate can slide (subduct) under
the other. Since the granite of the continental crust is lighter,
it usually ends up on top. Or, the two plates can pile up to
form mountains (like the Himalayas), but remember that
the Smokey Mountains used to be much taller than the
Himalayas, so weathering also plays a role in what we see.
Granite is dense, but the addition of oxygen makes it less dense than basalt. Because of the density difference, basalt tends to sink underneath granite; granite sort of floats on basalt. When Earth’s plates of crust meet, the basalt tends to subduct under the granite. Also, basalt morphs into a much more dense rock called eclogite – but granite doesn’t.

What were the results of this density difference? Floating granite formed continents, while heavier basalt and eclogite formed ocean floors. Yes, photosynthetic bacteria influenced the formation of the continents! Photosynthesis had effects on water, on terrestrial oxygen, on granite formation and density – and therefore they effected the physical form of Earth.

This theory was proposed by Minik Rosing in his 2006 paper. I actually read about it in a thriller paperback called The Last Good Man, by Anders Klarland and Jacob Weinreich. Intrigued by the author’s discussion, I checked on it and learned a whole bunch. Strange, but a murder mystery was the beginnings of this post.

I did another literature search, looking for papers that talked about the photosynthesis-granite theory and paper. I couldn’t find any. This made me wonder if the hypothesis had been refuted, or if the scientific community just didn’t buy it. I contacted a couple of well-known geologists and asked about the state of the theory.

I laid out the idea as I understood it and asked if I was getting the point and if the point was worth getting. They both agreed that the theory is alive and well and has been well accepted. Amazing – almost nothing this revolutionary is well received at first. And it shouldn’t be easy; every step forward should be scrutinized and tested to make sure it isn’t a step backward.

So photosynthetic bacteria are responsible for the continents. Does the story end there? Nope, there’s more.

Basalt crystals are large and hexagonal. Columns of basalt
crystals form in nature, as shown on top. In Micronesia,
an island city called Nan Madol was constructed in the 12th
and 13th centuries, completely from basalt columns. The city
was abandoned by 1628 (bottom picture), but before that
it was used as a residence for the nobility of the civilization.
After stable continents were formed and plate tectonics was working, photosynthesis continued and expanded. About 2.6 billion years ago, most of the rock became saturated with oxygen, and the excess was then released to the atmosphere over the next 200 million years. This wasn’t necessarily an easy thing with which to deal.

Most forms of life on Earth at this time were unable to deal with higher concentrations of oxygen. What makes oxygen so great for cellular respiration is that it easily takes electrons from other atoms, or can combine easily to share electrons. This is also what makes it dangerous. It can damage other molecules are hinder their function by combing with them or altering their structure. This is oxidative damage.

The increased oxygen in the environment starting killing most of the forms of life. This was such an important happening that biologists gave it a name – the Oxygen Crisis, the Great Oxygenation Event, the Oxygen Catastrophe. The Great Oxidation, or the Big Bad Breath (OK, I made that one up).  Only the organisms that evolved a mechanism to deal with oxidative damage continued to survive and change. Eventually, some came to use the oxygen in their metabolism.

So life has affected the physical form of the earth, and of course life has affected the later forms of life. But there is even more to this story, including how both processes are at work at the same time.

Before the oxygen crisis, iron-rich hematite recorded seasonal
changes in ocean waters. The iron oxide precipitated when
warm, oxygen-bearing water from the surface mixed with cold,
iron-rich water beneath it. When there was sufficient oxygen
in the rocks, the excess oxygen could be released to the
atmosphere. Later, the iron from these formations could be
used by surviving organisms to employ more complex
Over time, granite and other surface rocks were weathered by physical means (water, wind, chemicals, and temperature), and by biological means. Some things grow inside rocks and expand them or dissolve them, like lichens, and other things grow in cracks and physically break down rock. The color of plankton can affect temperature changes and wind or thunderstorm formation. Life can affect the climate and the weathering of rocks in many ways.

Just recently a study indicated that trees in the rainforest can sense when they are receiving too much sunlight and are getting too hot. They will then release chemicals that promote cloud formation by acting as seeds for vapor to form into water droplets. The clouds reduce the amount of sunlight reaching the trees and cool them down. Smart guys those trees.

So early life built up Earth’s continents, and then proceeded to help tear them back down. There is a balance between the weathering of rock and the formation of rock. And this, in and of itself, has also affected life.

When granite weathers, it releases some of the minerals it contains, heavy minerals that were brought up from the mantle. These heavy metals and minerals are able to act in chemical reactions, including the banded iron forms that were made during the initially increase in oxygen formation. Some organisms managed to find ways to use them as they were spread over the ground and the surface of the shallow seas.
In Greek mythology, Gaia was the Earth goddess who gave birth
to Heaven and ocean (the two children above). When James
Lovelock came up with the idea of Earth as a single organism,
his neighbor, William Golding (author of The Lord of the Flies),
suggested the name Gaia.

A new study indicates that the weathering of granite and the release of minerals was a crucial event in the development of life on Earth. Carbon release and heavy metal release, especially iron, apparently stimulated and increase in complexity of life forms on Earth. What was the most crucial increase in complexity brought about by weathering? The evolution of eukaryotes about 2.0-1.6 billion years ago!

In the 1970’s, the Gaia Hypothesis was introduced, stating that Earth and life were inextricably linked. The hypothesis was adopted by some non-scientific types who started to talk about the earth as a living organism, with a soul and the ability to die. Too touchy feely for me, but these new studies definitely bring us back to the hypothesis as it was presented by Lynn Margulis and James Lovelock. Earth and life – life and Earth, just one system after all.

Next week, life on a smaller scale. Can a plant as small as a grain of salt really be considered a whole plant? 

Parnell, J., Hole, M., Boyce, A., Spinks, S., & Bowden, S. (2012). Heavy metal, sex and granites: Crustal differentiation and bioavailability in the mid-Proterozoic Geology, 40 (8), 751-754 DOI: 10.1130/G33116.1  

Paasonen, P., Asmi, A., Petäjä, T., Kajos, M., Äijälä, M., Junninen, H., Holst, T., Abbatt, J., Arneth, A., Birmili, W., van der Gon, H., Hamed, A., Hoffer, A., Laakso, L., Laaksonen, A., Richard Leaitch, W., Plass-Dülmer, C., Pryor, S., Räisänen, P., Swietlicki, E., Wiedensohler, A., Worsnop, D., Kerminen, V., & Kulmala, M. (2013). Warming-induced increase in aerosol number concentration likely to moderate climate change Nature Geoscience DOI: 10.1038/NGEO1800  

Rosing, M., Bird, D., Sleep, N., Glassley, W., & Albarede, F. (2006). The rise of continents—An essay on the geologic consequences of photosynthesis Palaeogeography, Palaeoclimatology, Palaeoecology, 232 (2-4), 99-113 DOI: 10.1016/j.palaeo.2006.01.007

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