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So we want to consider the Earth as a series of spasmodic,
catastrophic, terrible events actually that have taken place.
And the reason we want to understand that is because those are the kind of events
that life has responded to.
The organisms that are alive at the time these events happen.
Some of them have the right characteristics,
they are able to reproduce in a certain way, they can get food in a certain way.
They are actually in an environment that was less effected than others.
But they have characteristics that they've evolved that then would
allow them to be more successful than others when the environment changes.
So one of the most striking ones, and
especially relevant to the modern day discussion in our society about global
warming is the idea that the Earth has been much warmer than it is now,
and that it was especially warm during the time of the dinosaurs in the Cretaceous
but another time period that is equally dramatic but in the opposite direction is
about 0.6 billion years ago at the end of the Proterozoic the entire Earth froze.
We had ice sheets that moved all the way from the poles down to the mid latitudes
and ended up with sea ice on the equator and we call that Snowball Earth.
Now there's discussions about Snowball Earth about whether or not it happened and
then to what degree it took place.
Some scientists now are suggesting that maybe we had a slush ball Earth where
instead of having true ice at the equators we had slushy ice at the equator but
the bottom line is that the Earth's temperature had dropped dramatically.
And in that process as the Earth cooled, then water was drawn out of the oceans and
then formed ice sheets, both on the continents but
then it spread out onto the open oceans as well.
So the Snowball Earth is dramatic, and
I want us to visit just briefly what some of the evidence for this is.
One of the things we can look for
in the geological record that are a fingerprint of a glacier?
and one of the most easy to find and
less controversial, least controversial of these, are what we call dropstones.
And dropstone is simply the idea that a piece of rock that made up
a continental land mass, was gouged and ground and
brought up into an ice sheet that formed a continent, which we call a glacier.
Then that ice moves and creeps slowly out towards the open coast and into the ocean.
And when it gets to the ocean, it breaks into pieces and forms icebergs.
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So when those icebergs that have the rock material that they derive from
the eroding continent they were on when those icebergs float from the northern
latitudes into the equator then they melt.
And as they melt, this load they have of rocks,
it drops out of the bottom of the iceberg and it falls down onto the sea floor.
So that deposit we call a dropstone, and
it'll have the rock types of the land mass on which the original glacier was formed.
So those dropstones at the equator indicate
that we had a very much colder Earth.
Now we have other lines of evidence that has to do with the chemistry of some of
the rock deposits that had time and
some of the chemistry such as the oxygen isotopes allow us to reconstruct what
the temperature was directly of the sea water at that moment.
But, taken as a whole, the dropstones, along with this other data is indicating
that yes, we had a very cold planet, and yes we had ice sheets at the equator's,
temperate ice sheets and then ice covering the equator.
So that's one thing, good dropstones at equatorial zones in the oceans.
Now the next one is at the same time that all of us have some
strong familiarity with metal, iron ore.
Every one of us who go into a building in which there's iron gurters,
every one of us who ever rode in a car, every one of us who have ever taken
a plane ride, we're all familiar with iron and different types of ores.
So this is something that might not be a unknown to you but
you might not of thought about where some of the metal comes from that let's
just say it makes up your car.
So it turns out that during this Proterozoic Era time period, and
actually back into the Archean as well, in the deep oceans
we had fluctuations in the amount of oxygen,the amount of nutrients and
also the amount of dissolved metals, things like iron.
[COUGH] Iron become dissolved when the oxygen drops to low levels within
the ocean, whereas when the ocean becomes well oxygenated, then the iron
chemically bonds with the oxygen and it precipitates a metal ore.
So the thing that we want to think about there is that we have a fingerprint of
microbial mats and microbial activity on the deep sea floor in a very
unique kind of deposit called a BIF,a banded iron formation.
And banded iron formations are ubiquitous around the planet in these aged rocks.
Now if they're ocean rocks,
and a lot of the rocks preserved at this time were deposited in the deep ocean.
So banded iron information is composed of the following.
Imagine you're very deep down in the ocean,
far below where sunlight is able to penetrate.
So on the order of kilometers in water depth.
And on that sea floor, you have organisms that are forming these gooey-ooey
microbial mats and those organisms they don't use oxygen and
they also are not able to photosynthesize so they have to have a lifestyle and
that lifestyle for them is that they basically breathe iron.
So iron three is taking into their cells and then they use that iron to actually
generate proteins and actually create energy, ATP, the basic currency of life.
So they're going about their lifestyle just trying to survive and using the raw
materials that they can get their hands on, which in this case is iron.
So in that process of bringing iron into their cells and utilizing it,
they can drive the precipitation of the iron into a solid mineral.
And those solid minerals are forms of hematite and
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curetite and some of the other iron rich minerals that we see forming.
So when you have microbial mats, and
these organisms are going about their lifestyle, they bring iron to the cell and
the end result is they precipitate the iron ores so you get a pure iron ore
that's driven by the metabolic activity of microbes that are living on the sea floor.
And these pure iron ores come out and
finally layered stratified rock deposits, called pattern iron formations.
Now they're different from sematolites in that sematolites
are cetementary rock deposits.
These are also sedimentary but they're not made out of sand grains.
They're not made out of individual grains and clay components.
They're actually composed out of metal minerals that are precipitating
out of seawater and driven by the metabolic activity of the organisms.
Now, banded iron formations are very beautiful.
Oftentimes, the iron oxidizes to a bright red.
So if you see it in a rock outcrop, it has these beautiful layers around the order of
tens of millimeters, to maybe a single millimeter and even thinner in thickness.
And you have layer, upon layer, upon layer.
And so you see these bright red layers.
That are intermixed with oftentimes a grey to a black.
And so those also have some metals in them,
but it's also other types of sediments or mineral precipitation on the sea floor.
In the end, it makes these beautiful banded iron formations, BIFs.
And the BIFs are important because they're a good indicator of ocean circulation in
the ancient oceans, and also the activity of these ancient microbes.
So we find BIF's, primarily concentrated in the Archean time period,
then up thru the Proterozoic.
So this is the type of fingerprint we see in the rock record that tells us
definitively we had a deep ocean, lack of oxygen, iron was mobile and
available to these organisms to breathe, take into their cells,the inproginis
precipitate these iron ore mats and drop a stone from a melting iceberg that's just
above it a few kilometers in water depth, and then you continue the process forward.
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Bruce W. Fouke, Ph.D.Director of the Roy J. Carver Biotechnology Center
