Fire and Ice

This account of a field trip taken to Death Valley for a sedimentology class I was taking dates back to Spring 2004, about halfway through my graduate career.  I originally wrote this for Livejournal back when it was the big social network thing and hadn’t yet been eclipsed by Facebook.

This version has been modified slightly to be able to stand on its own as a combination travelogue and discussion of the geological past.

______________________________________________

The Death Valley region is at once the hottest spot and lowest point in North America. The two are related; since the temperature of air is related to its height above sea level due to the change in pressure, air sunk below the level of the sea at the bottom of the valley compresses and warms (a process known as adiabatic heating). Death Valley is an ancient lakebed, where all that remains of Pleistocene Lake Manly today are the harsh mineral salts lining the bottom. The rocks that surround it are older still, the remains of ancient seas. We’re far enough inland to find very old strata, originally laid down on a world radically different than the one we know now. Atmospheres without oxygen, hot worlds without ice – or frozen worlds trapped beneath it, where North America bows under the weight of the Laurentide ice sheet, or the young Earth is buried under a kilometer of glacial ice. Hothouse and icehouse, cycling throughout the Phanerozoic through to the present day.

Whether our world will end in fire or ice, it was born in both.

________________________________

Day One

Modern Day – Fire

The initial jumping-off point for this trip isn’t even in Death Valley proper, which we never actually spent time in, but the next valley over. We’re supposed to meet up in a little town called Shoshone. We are accompanied by an undergraduate student, Vicks, who isn’t in the class but is coming along as a combination of moral support and photographer.  She is lucky enough to fly in with the ringmaster of this whole trip (one Martin Kennedy by name) but the rest of us are driving, and somewhere along the line half our little car caravan managed to inexplicably disappear.  I spend as much time during this as possible asleep. What we’re driving through looks something like this, at least from the air:It’s counterintuitive, but desert landscapes such as this one are primarily shaped by water. Deserts don’t see much rain, so the land and the plants are not well adapted to large-scale rainfall. Rocks weather locally into clays that carpet the ground and make it hard for water to soak in. Plants are sparse, with deep instead of spreading roots, and they don’t do much to break up the soil. When it does rain in Death Valley, the rains become floods. Only heavy boulders and sticky clay, which require a huge amount of water energy to lift and transport, are left behind in the canyons between the mountains as a storm becomes a torrent down the slopes, spilling its sediment at the bottom as broad alluvial fans once it becomes unchannelled and loses steam.

This desert even has a river, the Amargosa. For reasons unknown it makes a sharp u-turn to the north just southeast of Death Valley, never reaching the sea, spilling out its last drops onto places like Badwater and Devil’s Golf Course. It’s too late in the year for there to be water in the Amargosa, so right now it’s dry.
_________________
14,000 years before present – Ice First stop is at a relatively young sedimentary environment, at least in terms of the rocks we’ll be looking at for most of this trip. We’re at the edge of a hot spring wetland – quite possibly the last thing I expected to be standing on the edge of, at least out here. Reeds and green algal mats grow in water heated from below by mantle warmth, and shade a shoreline crusted with mineral salts. Inexplicably, there are tiny fish living in this bubbling brew of microbial mats and geologic heat, probably desert pupfish. There’s also a horrific amount of flies, some of which are easily large enough to carry off the pupfish.
Death Valley is a pull-apart basin, a rip in the crust formed at the edge of two sliding crustal plates where they don’t quite shear smoothly against each other. Such tears reach very, very deep, more so than most geologic basins. There’s a vaguely inorganic smell in the air, the odor of mineral-rich water. It’s mostly dampened by the smell of rotting algae.

The hummocky rolling hills around the existing lake are the soft sediments that remain of a much larger body of water. Glacial lake Tecopa was born in the last glacial maximum, when the creeping ice sheets elsewhere in the country forced the jet stream south, covering the southwest with rain. The storms this region saw during that time were similar to those that fall over Northern California and Oregon in our modern interglacial climate. Death Valley was drowned by an Ice Age body of water known as Lake Manly, and the area was flush with greenery.
Onwards to more lake deposits near China Ranch. These are older, Miocene and Pliocene in age, somewhere between 24 and 2 million years old. We’re getting into some serious Wile. E. Coyote territory once we arrive:

We’re not looking at that right now, however. At the moment we’re poking at claystone lake sediments, chewing on the rocks and showing Vicks how to tell various clays apart. Smectites, when placed on the tongue, will stick; others do not; this leads a bit of a Dumb Geologist Trick whereby you stick bits of the outcrop to our tongue and then stick your tongue at someone else both to prove the type of clay it is primarily composed of and to gross them out slightly. The lake sediments are softer and more homogenous than the rocks making up the arches and the hoodoos we had seen before.  These are soft claystones laid down in the bottom of an ancient lake, while the others are alluvial fan conglomerates (fanglomerates).

The claystones are packed chock full of gypsum, indicating a lake subject to periodic drying, further reinforced by the fact that elsewhere, the fibrous gypsum can be seen filling in v-shaped mudcracks. We’re on the edge of what may have been a desert playa lake. Perhaps the ice sheets were retreating, and the region was drying out.

This place may have seen more than one lake, perhaps related to another pulse of glacial advance. Dry out a lake, uplift its sediments and warp them from tectonic stress so they no longer lie horizontal, erode them down and put more sediments on top, and you have yourself an angular unconformity such as this one, made up of lake sediments both above and below.

To lithify, uplift, tilt and erode strata takes enormous amounts of time.  The deposition of soft claystones, which settle as millimeter-thick layers in still, quiet water, takes more time still. James Hutton, the father of modern geology, once said he could see the dizzying depths of geologic time in an angular unconformity such as this one – “No vestige of a beginning, no prospect of an end.”  This one’s not quite as dizzying, but that’s because the west coast of North America is geologically very active compared to the beaches of Scotland.

We break for one of China Ranch’s famous date shakes before leaving behind sediments left by a modern-looking Earth. Enough of these young Cenozoic deposits; back we go in time, skipping most of the Phanerozoic in the process.

_________________

545 million years before present – Fire

Most of the west coast of North America disappears, as it has not accreted yet. The young North American craton stops much farther east than it does today (roughly around here) and most of it is flooded by warm, shallow oceans.The entire continent rests near the equator. We stop on the road at the boundary between Phanerozoic and Proterozoic, early in the Cambrian, at the bottom of a very long slope leading up to this:

The steel-gray cap atop the hillside is the early Cambrian Bonanza King Formation. It represents the remains of an ancient tropical carbonate platform – think Bahama Banks, I suppose, with clear blue water and white carbonate sands. Where scleractinian corals build tall, complex reefs today, ancient calcified sponges called archaeocyathids built their slender branching frameworks, held together by calcified microbes. They’re too small to be called reefs, so they’re typically known simply as “bioherms.”

But that formation and its shallow seas lie far upslope. Right now we’re down in the Stirling Quartzite, the boundary between the Cambrian and the Precambrian, cursing and arguing and trying to measure the first 300-odd meters that represents the initial shoaling cycle, where deepwater sediments give way to progressively shallower, lower-energy ones as the sedimentary basin becomes filled over time. Vicks wandered off and found evidence that someone else had been out here before.
Sun’s going down. We take a break back at Shoshone for dinner and beer at the Crow Bar, since it’s too dark to get much done in the way of work and we’re all tired anyway. While the other students (and the teacher) play pool, Vicks and I crouch out back to alternately photograph and annoy a stray Western toad, Bufo borealis.

Time to go make camp.

________________________________

Day Two

Ugh. It’s too damned early to be awake, but we’re up anyway. Now that there’s actual light to see it becomes obvious that we camped among cliffs of soft ash-fall tuff, probably generated by Pleistocene volcanism. I’m not sure we’re actually supposed to be camping here, but as they say, it’s easier to beg forgiveness than permission. This seems to have been the approach everyone else took, as people have been busily carving caves out of the cliffs here for some time now.

A a bit of coffee and breakfast later, and we’re off.

I see too many damned sunrises in this line of work. Don’t know why people tend to make a big deal out of them, but I guess that’s because other people aren’t expected to be crawling on rocks just after dawn. Then again, it may also have something to do with the fact that I take to mornings like a lead duck to water.

_________________

1 billion years before present – Fire

Once again we stand on the remains of an ancient carbonate shelf, but this one is much older than yesterday’s Bonanza King. Tiny photosynthetic microbes built reef-like bioherms on what was then a tropical passive margin – think modern-day Florida and you probably have something about right, at least in terms of tectonics and water temperature. The air that far back would have been even more oxygen-poor than the Cambrian; in the Paleozoic, after the rise of molecular oxygen, atmospheric oxygen levels were thought to be half what they are today. These reefs are much older. The land was still lifeless, the atmosphere starved of oxygen, but the oceans held the first stirrings of life. Then came the rifting, pulling this passive margin apart, sinking it deep as the basin expanded and tore. Dark, iron-rich lava called diabase came up and pooled in the carbonate country rock, providing the only numerical age for the entire succession: 1.09 Ga. This is the Crystal Spring Formation.

Multiple shoaling cycles are recorded above it as the basin subsides, begins filling to the surface, and subsides again. The resistant beds that stand out are shallow-water carbonates, while the softer stone in between is deeper-water mud. Notice how the cycles are getting shorter upsection. We past a karst surface where the carbonate has been aerially exposed and silica has replaced the limestone; apparently, this is where the silicified remains of the first microbial life on land was found. The eroded karst surface marks the end of the Beck Springs Formation, and now we’re in no-man’s land.

Siliciclastic shoaling cycles. The section has been tilted so that younger sediments are to the left.

Sometime after the rocks were exposed, the sea returns but the warm-water carbonates do not. Nobody’s entirely sure why, but we end up climbing on fissile, sharp-edged sandstones for much of the rest of the succession. Carbonates weather to razor edges (a process called karstification) so they’re terrible to climb around on, but I’m not sure these are better:
I spent much of my time at this succession chasing undergrad students with band-aids.

_________________

900 million years before present – Ice


Atop the Beck Springs Dolomite and the unnamed siliciclastic succession lie rocks generated in cold, glacial waters. Back at this morning’s stop we saw them exposed as a glacial diamictite, a jumbled mess of mud and cobbles and pebbles that showed the striations of grinding by ice. Here the same formation shows its glacial colors in the form of dropstones – boulders and cobbles dropped by glaciers into deep-water sediments as they calve off and melt. But this is no ordinary diamictite. It’s red.
What you are looking at now is one of the last of Earth’s banded iron formations – enormous deposits of oxidized iron laid down as ocean beds. In the modern-day ocean iron is scarce (it’s actually a limiting resource for phytoplankton), and the scarcity is related to the oxygenated water and atmosphere. In the presence of molecular oxygen, iron oxidizes and becomes solid, precipitating out of the water. Reducing conditions are necessary to keep iron in solution in seawater, but how do you keep this much iron out of reach of the atmosphere?

Cap it with ice, or so the theory goes. The glacial sediments we saw elsewhere as a diamictite record one of the last of the great Neoproterozoic glacial episodes, which some scientists (such as Paul Hoffmann) think represent a glacial advance that covered the entire planet, all the way up to the equator.  Paleomagnetic data does indicate that the continent was in the lower latitudes when it was freezing over, but that doesn’t necessarily mean the entire planet was frozen. After all, once the entire Earth is frozen over there is no way to break the positive feedback loop of increasing albedo and falling temperature and force back the ice – yet somehow it must have retreated, or else this planet would to this day resemble Europa, the frozen ocean moon of Jupiter.  At any rate, the banded iron formations represent the glacial retreat and exposure of the waters to the atmosphere; elsewhere, similar retreats are marked by thick dolomite horizons, known as cap carbonates. This one has one too, called the Noonday Dolomite.

_________________

800 million years before present – Fire

Even if you haven’t frozen the world solid, how do you beat back glaciers that have covered most of the rest of the planet in ice? Carbon dioxide would still be outgassing from the planet, but it’s actually a fairly weak greenhouse gas compared to others present in Earth’s atmosphere. Water vapor is stronger and represents a significant amount of heat capture in the modern atmosphere, but freezing temperatures plunge the carrying capacity of air down too low to hold a significant amount of this natural greenhouse gas. But we have another, much more powerful source of greenhouse emissions locked up in the depths of Earth’s oceans. Methane clathrates are molecules of methane caged in special crystals of ice.  They lie buried in Earth’s ocean sediments, a pool of natural carbon larger than the entire biomass and all the fossil fuel emissions on Earth combined – set the entire living world on fire and you still won’t release enough carbon to match the methane hydrate pool. Methane is also 27 times more effective an atmospheric greenhouse gas than is carbon dioxide. The clathrates are metastable at modern-day deep ocean conditions, balanced on a razor’s edge of cold temperatures and high pressure, and destabilizing any of this pool is liable to bring about a positive feedback that cannot be stopped. More methane release warms the planet more, which makes remaining hydrates unstable. A world that was once flash-frozen becomes a furnace, and the ice retreats. That’s what Martin Kennedy works on, and the methane in the oceans is probably what this was growing on:

That entire little peak at the center of the image is a single stromatolite. Or rather, it’s one of them; it’s one of the odd bioherms from the Noonday Dolomite, a cap carbonate for the Kingston Peak succession that shows evidence of methane release. There are multiple bioherms growing up on the Noonday platform, but these are growing down in a deep-water succession of black, organic-rich clays, well below the reach of sunlight. These probably weren’t built by cyanobacteria (as they are thought to have grown in water to deep for sunlight to penetrate) but may instead represent methanotrophic organisms, the inhabitants of ancient cold seeps. The organic nature of the rock becomes clearer on closer examination, despite the unusual weathering that makes the surface of the rock appear to run like wax.

_________________

Modern Day – Ice

We think of our modern climate as warm and equable, but we live in the midst of an ice age. The surface of our planet is shaped in a large part by the large-scale continental ice sheets that dominate our poles. Ice creates a strong thermal gradient, equator-to-pole, that gives the wind its strength and creates the jet stream that brings storms to the temperate regions. The formation of sea ice generates the subfreezing water that stirs and ventilates the deep seas. Our human society has built itself up based on the climate of the interglacial Holocene, a world balanced on the edge of glacial ice and greenhouse warmth; but if these ancient sediments are indicators of anything, it is that such climates are themselves intrinsically unstable. The center cannot hold, and the warmth – or cold, in our case – can not and does not last. Hothouse and icehouse, glacial advance and methane release, cycling over and over through Phanerozoic time.

Our world will end in fire someday, billions of years from now when the sun becomes unstable. In between it will shift forever between the extremes, fire and ice, greenhouse and icehouse – no vestige of a beginning, no prospect of an end.

Advertisements

Green Planet

Green planet began life as a post I was going to put up on Suburbivore to talk something about the science of food production, specifically soils and how plants evolved.  However, once it was finished it didn’t fit the blog topic, and I didn’t think I could make it fit without monkeying around with it until it was about something very different.  As a result it ended up languishing on my hard drive for a while before finding its way here.

I’ve taught introductory geology courses before, and this was always one of my favorite parts of the program, where I tie together what I know about planetary astronomy and how rocky planets develop with the history of life on Earth to explain why Earth is such an anomaly in the Solar System.  Sometimes I wonder if I’d stop seeing so many claims that the Earth was made for man (which drive me bananas) if more people understood how much Earth is both like and unlike other rocky planets we’ve observed, and why.

___________________________________________________

It is a peculiar chauvinism of our species that we named our planet “Earth” when, by all rights, we probably should have named it “Ocean.” This oddity has been noted in the past by better writers than I , where they comment on the fact that 70% of our planet is covered by water and only about 30% is available as dry land.  I would go a step further and argue that our world should perhaps most rightfully been named “Life” – for the world as we know it would not be possible without life, and would indeed be a very alien place to those of us used to the modern green and blue Earth.

However, it wasn’t animal life that most changed the world into the one we have now.  We are most emotionally invested in the forms life most like ourselves, and we therefore pay the most attention to warm-blooded animals, particularly large ones.  This homeothermic chauvinism can be seen, for example, by the very different levels of outrage over the wearing of fur versus the use of insecticides.  In general, if we are averse to the poison, it is not for the sake of the insects but because it affects other mammals or birds (if not ourselves!).  But while insects are alien and “other” to us, plants represent a step still further away on the tree of life, often viewed more as a form of living scenery than as organisms in their own right.  If we deign to notice plants, it is in their usefulness to us – the food they produce that we can eat; the aesthetic value of the leaves and flowers they bear; or the spiritual calmness of the forests they create, sheltered under towering trees and spreading leaves.  Bacteria and archaea, if they impinge on our consciousness at all, are swept into the catch-all category of “germs” and become things to be annihilated with the increasingly omnipresent triclosan and hand sanitizers.

But plants are more than just food factories and living carpets for our lawns, and the bacteria and archaea – both kingdoms in their own right – are more than just the seeds of disease.  Together they created the very air we breathe, and the earth beneath our feet.

Part One: Prologue

Our world was born without an atmosphere nearly 5.6 billion years ago, for as a small planet Earth does not have the gravitational grip to hold onto the light gases that were most common in the vortex of gases available as the solar system formed.  Only larger planets could retain these early, primary atmospheres of hydrogen and helium, so the outer giant planets alone held onto them, growing large enough to keep even flighty hydrogen from escaping due to the cooler temperatures present farther away from the nascent Sun.

We acquired an atmosphere later, after the planet had cooled from an initial molten state driven by constant impacts with the dust and debris left behind from early planetary formation.  Now a different type of impact introduced the materials that the great planets of the outer solar system had used to grow so large: water, methane, ethane, and carbon dioxide, the inorganic and organic ices that could only solidify in the cold dark of  the outer solar system, far from the Sun.  Additional gases were released from their mineral prison in the interior of the Earth as began to experience volcanic activity with a heat and violence unmatched by any modern natural disaster.  From these starting materials were born our primordial oceans and a secondary atmosphere wholly alien to us today, but actually more characteristic of small rocky planets of our type.  The dominant gases would have been carbon dioxide, nitrogen, and argon (about 95%, 3%, and 2% respectively, based on the current atmosphere of Venus and Mars1), with other gases making up tiny fractions of the remainder.  Oxygen is present in the atmospheres of both of these planets, and would likely have also have been present as part of Earth’s primordial atmosphere.  There is not much, however, and it is likely that any oxygen available close to the surface would probably have been rapidly consumed by rusting minerals exposed on the rocky surface, like the interactions that produced the red surface of Mars1.

Oxygen is not produced in great quantities through normal planetary processes like those described above; it takes life to do that, and life is much older on our planet than we are used to normally thinking.  There are some possible hints (quite controversial) that life arose 5 billion years ago2, but life was definitively present by at least 3.5 billion years ago as tiny, bacterial things1.  It then proceeded to spend the next two billion years doing nothing terribly interesting, at least based on the fossil record.

The problem, of course, is that a bacteria is a bacteria is a bacteria, and they have very strong selective pressures to stay small and morphologically similar to each other2.  Internally, of course, they are more free to diversify and have done so: bacteria and their distant cousins the archaea, ancient ancestors of our own multicelluar life, have developed incredibly diverse strategies for surviving and producing energy from the environment.  They live in the scalding heat of geothermal pools, the freezing interstices between granules of glacial ice, and can generate energy from sulfur and iron.  One group of them long ago seems to have developed a method for generating energy by using the energy of sunlight to split water, and it is this group that would transform the world: the cyanobacteria or “blue-green algae”, the descendents of which are present in the cells of every green living thing.  Photosynthesis evolved only once on our planet, and it was the cyanobacteria that managed to do it2.

Part Two: Air

Some 2.4 billion years ago, give or take a few million (it is difficult to determine exact dates in rocks so old), Earth’s atmosphere began to change to more closely resemble our modern one in what is now referred to as the Great Oxygenation Event.  Levels of carbon dioxide fell, leaving nitrogen behind as the most common element in our atmosphere, as it is today.  Oxygen levels rose.  Cyanobacteria are an old group, and biomarkers that indicate their presence were found in rocks that predate this event, some 2.71 billion years ago6.  So what changed?

Here is where the narrative gets fuzzy, in no small part because it is difficult to exactly tell the ages of rocks and correlate them across continents in the absence of distinctive and recognizable fossils (fossils are present in such rocks, of course; but as I said before, most bacteria tend to look very much alike).  The current going theory is that large-scale glaciations spread multiple times across the early Earth, possibly reaching down to the equator, with the ice only driven back by the release of trapped methane gases from beneath the seafloor3,4.Even smaller-scale glaciations, however, could have been the key.  They would have scraped dust and sediment from the exposed continents and washed them into the sea, providing a means of burying the bodies of early cyanobacteria so that they  could not react with the oxygen they had produced in life to rot.  As more and more carbon was buried, less and less of it was exposed to the amounts of oxygen that the survivors were producing to generate their food, and so more of it was free to build up in the atmosphere2.

This poisonous gas would have smothered most of the early life originally present on Earth, but bacteria are nothing if not resourceful (a generational span of minutes will do that) and some of them, the purple bacteria or proteobacteria, appear to have learned how to utilize this new toxic by-product in their own metabolism to help break down food.  Much as the cyanobacteria are the original progenitors of all chloroplasts, the tiny green parts inside plant cells that allow them to photosynthesize, the proteobacteria are the ancestors of our modern mitochondria, the tiny cellular machinery that forces us to breathe oxygen in order to break down our food for energy.  Ricksettia prowazekii, an obligate parasite from this portion of the bacterial family tree and the infectious agent behind typhus, is one of the closest living cousins to the tiny endosymbionts living in our cells5.

The buildup of oxygen is recorded in the rocks in a series of iron deposits known as banded iron formations (BIFs) found throughout the world and even today still mined as our primary source of iron ore.  They are remnants of great stores iron that were once dissolved in Earth’s oceans during its early anoxic years, most of them converting to an insoluble form and settling to the seafloor as oxygen built up during the great periods of glaciation that swept the early Earth6.  Our great railroads from the turn of the previous century, and the bodies of our cars even today, are built from the fossils of the turnover of our atmosphere.

Oxygen in our atmosphere had a second side-effect besides giving us sweet cars and transcontinental railroads, billions of years later.  Oxygen high in the atmosphere interacts with the ultraviolet radiation produced by our sun and present in sunlight, creating a layer of ozone that subsequently absorbs incoming UV radiation before it can reach the Earth’s surface.  Life could exist sheltered in the deep oceans because water absorbs UV radiation very quickly, protecting aquatic life and its DNA from the damaging energy of full sun exposure.  The development of an ozone layer would allow life to spread into shallow water and to emerge from the sea in time, to reshape the land as it had the sky2,7.

Part Three: Earth

We don’t give dirt a lot of thought despite having named our planet after it, but much like our atmosphere it’s actually quite unique in the solar system.  The surface of our moon provides a current example of what Earth’s early surface probably looked like before the advent of soils.  Igneous rocks would have formed from volcanic activity much as today, forming great solid sheets of magma, and impacting comets and asteroids would have blasted great craters out and let the dust and debris settle on the surface to form regolith, the pure inorganic sediment that makes up the surface of the moon and Mars.

Soil is quite different.  Regolith is only powdered rock of varying degrees of fineness, but soil contains regolith and more: the clays, sheetlike silicate minerals that only appear after the rise of the earliest land life8, and humus, an organic component made from decomposing plant or animal matter.  As the cyanobacteria began to conquer the land, sheltered inside the cells of multicellular plants, the types of chemical interactions required to produce both could finally occur.  Living plants or even simple cyanobacterial colonies produce organic acids that they excrete onto the surfaces that they live on, and it is thought that these are part of the chemistry required to convert igneous minerals into clays.  As these early plants died they would have broken down through the actions of the ever-present, ever-busy bacteria, leaving behind a carbon component in the sediment that would allow the formation of soils.  The structure of clays attracts organic acids to them and traps water, allowing such chemical interactions to continue underground as more plants die and more soil forms above them.

And so soils build up across the land over time, the speed at which they do determined by temperature, water availability, and the presence, ultimately, of life.  The formation of soils would later allow larger plants to grow, the great coal-measure swamps and their kind, moving ever farther from the water’s edges to conquer the sterile stone of the continental interiors, allowing the buildup of oxygen in the atmosphere to even well above modern levels.  Although these levels would ultimately fall at the close of the Permian, driving the greatest extinction ever seen on this planet1, the stage was set some 360 million years ago for the Earth as we know it today: green land, blue skies, and enough oxygen for a modern organism to breathe.

So remember this, as you tend your garden soil or water your houseplants: the world as we know it today is actually very unusual, and the plants that we care for are descendants of pioneers, the first terraformers, the life that took a plain rocky planet and reshaped it into a haven for – and ultimately built by – life.  While Earth was not made specifically for life, life made the modern Earth and, in doing so, fit itself to the changes it wrought on the planet in a positive feedback loop of change and adaptation.

They say Earth is truly a blue planet due to the extent of its oceans.  I say it is a green one due to the presence of, and the changes made by, life.

 References

  1. Morrison, D. and T. Owen.  2003.  The Planetary System. 3rd edition.  San Francisco: Addison Wesley.
  2. Lane, N. 2009.  Life Ascending: The Ten Great Inventions of Evolution.  New York: W.W. Norton and Company.
  3. Hoffman, P.F. and D.P. Schrag.  2002.  “The snowball Earth hypothesis: testing the limits of global change.”  In: Terra Nova 14(3): 129-155.
  4. Kennedy, M.J., N. Christie-Blick and L.E. Sohl.  2001.  “Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals?”  In: Geology 29: 443-446.
  5. Gray, M.W.  1998.  “Ricksettia, typhus and the mitochondrial connection.”  In: Nature 396 (12 November 1998): 109-110.
  6. Basta, F.F., A. E. Maurice, L. Fontboté and P.-Y. Favarger.  2011.  “Petrology and geochemistry of the banded iron formation (BIF) of Wadi Karim and Um Anab, Eastern Desert, Egypt: Implications for the origin of Neoproterozoic BIF.”  In: Precambrian Research 187 (3-4): 277-292.
  7. Barley, M.E., A. Bekker, and B. Krapež.  2005.  Late Archean to Early Paleoproterozoic global tectonics, environmental change and the rise of atmospheric oxygen.  In: Earth and Planetary Science Letters 238(1-2): 156-171.
  8. 7. Berkner, L.V., and L.C. Marshall.  1965.  “On the Origin and Rise of Oxygen Concentration in Earth’s Atmosphere.”  In: Journal of Atmospheric Sciences 22: 225-261.
  9. Droser, M., M. Kennedy, L.M. Mayer, D. Mrofka and D. Pevear.  2006.  Late Precambrian Oxygenation: Inception of the Clay Mineral Factory.  In: Science 311: 1446-1449.