#isua

Ayrine Barlier

Concept Art | [email protected]

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Were mud volcanoes the first habitat?

Many solutions have been offered over the years to the puzzling problem of the geological environment in which rock and chemistry somehow turned into life. They include between layers within clays in altered volcanic rocks and black/white smokers deep under the ocean surface (see http://tinyurl.com/o99wjxv). A study from the French CNRS published last year in the Proceedings of the National Academy of Sciences advances mud volcanoes (see[http://tinyurl.com/ojo3vov) as another possible location.

Geochemical analysis of highly metamorphosed Archaean eon (2-2.5 billion years ago) mud volcanoes in a rock called the Issua Gneiss that outcrops in Greenland has revealed that they released chemical elements essential to the formation of early biomolecules 3.8 billion years ago, and that the chemical environment was favourable to life's emergence back in that misty epoch.

The crucial mineral in this case is serpentine (a green mineral often used to imitate jade), originally formed when seawater in giant convection cells infiltrates the upper mantle through the extensively fractured young oceanic crust, or is carried down on a subducting slab. The mineral is also precipitated in hot springs and veins of hydrothermal origin. They used zinc isotopes to indicate whether the environment of the fluids that permeated the serpentinites was acidic or alkaline. Alkaline environments are much more favourable to the development of stable biomolecules such as amino acids, and the Issua serpentine was clearly permeated by carbonate rich alkaline fluids. Black smokers are less likely as a source of life as they are quite acidic, while white smokers are slightly alkaline and another strong contender.

The zinc signature is similar to that of serpentine mud volcanoes in the Marianas trench rather that from oceanic spreading ridges where black and white smokers occur. It is the first time that an environment suitable for life has been proved in such ancient rocks. Such mud volcanoes are rarer now, since they congregate seemingly in places where one oceanic plate dives under another back into the mantle. Back in the Archaean, the continental crust was much smaller than today's, which has been built up by continental collisions and subduction volcanism over the past several billion years. This would have meant more oceanic crust, and possibly a greater number of life nurturing mud volcanoes.

Loz

Image of mud volcano in Azerbaijan credit: Rietje

http://www.livescience.com/16580-mud-volcanoes-life-ingredients-greenland.html

Abstract: http://www.pnas.org/content/early/2011/10/10/1108061108.abstract

The beginning of all life, in the palm of my hand.

Went a fossil expo today, and i came across these rocks from the Greenstone belt in Isua (Greenland). The rocks from this formation has been dated to be about 3,7 billion years old, and it’s in these rocks that you can find the oldest traces of life on Earth.

Archaean Life

Follow the arrows. The arrow in the upper image points to a tiny garnet grain – the 200μm scale bar is 1/5 of a millimeter wide. The lower image zooms in on a tiny inclusion of dark material trapped inside that garnet. Those inclusions are about 1 μm in size. See them? They might be your ancestor.

The Isua Supracrustal Belt is a sequence of extremely ancient rocks found in west Greenland, near the heart of what was eventually assembled into the North American craton. It contains what were once igneous and sedimentary rocks formed about 3.7 billion years ago – when the planet Earth was less than a billion years old. Virtually every rock that can be found there today has been metamorphosed, including the ones you’re looking at in thin section here. This garnet crystal is a porphyroblast, a larger crystal that grew during metamorphism of the surrounding rock.

The surrounding rock was once a sedimentary rock, a fine-grained shale deposited in quiet water. The original rock would have had grains so small you couldn’t see any of them in the upper microscope image; everything big enough to see grew during metamorphism.

Scientists have heavily studied one particularly interesting part of these metamorphic rocks. The rocks contain graphite; the low pressure form of pure carbon. If you took organic material, the kind that makes up organisms like bacteria or you and I, and put it under the same metamorphic conditions as the rest of the rock, you’d probably end up with graphite. Those graphite inclusions, therefore, might be a signal of ancient life existing in the shales.

Life also has a distinctive isotopic character. Carbon has 2 stable isotopes: carbon with 6 neutrons (total mass 12) and carbon with 7 neutrons (total mass 13). Life strongly prefers carbon 12 and thus fossilized organic matter should be very rich in carbon 12. This graphite is in fact rich in carbon 12, supporting the argument that it was once an organism.

However, because it is turned to graphite, there are still other sources of carbon that could explain these inclusions other than life. This image comes from a new study that strongly supports the argument that the Isua carbon was originally organisms.

When a crystal gets big enough, like the garnet porphyroblast, it can overrun surrounding minerals and lock them inside its structure as an inclusion. The larger mineral can then protect things in inclusions from some alteration during metamorphism. The arrow in the second image points towards one of the inclusions trapped inside this garnet – the researchers, led by a team from the University of Copenhagen, found some of the Isua carbon-rich inclusions trapped inside this garnet. However, in this case, they weren’t turned to graphite – the garnet protected some remnant of what was there!

Using tiny infrared spectrometers, the researchers identified carbon-nitrogen, carbon-oxygen, and even phosphorus-oxygen bonds in the sample. With the exception of hydrogen that was still somewhat affected by the metamorphism, the compounds in this inclusion are the same ones required to form life.

This work therefore shows that the graphite found in Isua was once much closer to organic matter. It had nitrogen, oxygen, and phosphorus – all the compounds you need to make amino acids. In the surrounding rock many of these elements were burned off during metamorphism, but there’s still a record of them trapped deep inside this tiny garnet crystal. This stuff might well be some of our ancestors.

-JBB