Transcript from Epic of Evolution: Life, the Earth and the Cosmos (BEP 210A)

April 14, 2000 - Lecture by Michael Wysession

Okay, I want to get started and let me finish up talking about the systems of motion of rock as part of the rock cycle and then I’m going to talk a little bit about how climate has changed over time. So let me just finish. I had started talking about glaciers and I showed you the picture of the Matterhorn and how the ice had essentially carved out huge scours in the mountainside and this has carried a lot of rock away. I mentioned that the rock gets dumped somewhere and one of the places that this happens is much of the Midwest and we are all under in some places hundreds of meters of glacial sediment that’s Canadian rock that’s been imported down here. I mentioned that Long Island and Cape Cod are also examples of these glacial deposits so the glaciers have moved a lot of rock around at the surface. There have also been some very catastrophic events. During the last ice age as the ice was beginning to melt there were huge lakes that were beginning to develop. The Great Lakes are essentially the puddles left over from those giant lakes. Sometimes the lakes would get dammed up by ice and would burst out catastrophically. There’s a great example of this. Anyone who’s ever spent anytime or plans to go to Minneapolis and St. Paul, take an hour drive east or so to the St. Croix River which runs down the border between Minnesota and Wisconsin. It’s this trickling little stream but there are some really nice places that you can go canoeing on there and the water goes so slowly you can canoe upstream as fast as you can downstream. What’s amazing is it sits in this giant river canyon with these huge steep cliffs on the side and all around the stream are these gigantic potholes some of which go down vertically about 40 feet and at the bottom of these potholes are these round boulders the size of a VW micro bus. Essentially what happened was glacial Lake Duluth burst open suddenly and the whole lake drained in about a week and as it came through it just carved out this river canyon and the swirling eddies of water tossed these giant boulders around like pebbles and they just bore these holes down into the rock 30-40-50 feet and then the water was gone and there’s not much left of it. There are places where these glacial lakes washed out across the West, in particular in the states of Oregon and Washington where there are ripple marks -- you know when you have a stream you get ripples at the bottom of the stream, those ripple marks are ridges that are 100 feet apart. And that’s this huge volume of water just rushing over the land and it’s kind of hard to imagine but it gives you some idea of anybody witnessed these sorts of things in Europe where some of our flood myths might have come from in a lot of early cultures.

Okay, the next thing I want to talk about just briefly is the groundwater systems and I showed you how there is a water table that has the flow of water essentially flowing downhill much the way water flows down in a stream. In terms of rock though the groundwater does two important things. First of all it produces our soil. Soil is formed by the decomposition of rock. This is done partly by organisms (bacteria and other critters in the ground) but partly by water flowing through that dissolves the rock and you get this progression. If you took a cross-section of any area where you have soil and then you have soil with some rock still in there and then eventually you get to solid bedrock. So these are all rocks here. As the water percolates down it dissolves away at the rock and this line here goes down over time as more and more water comes down and dissolves the rock and turns it into soil. Obviously in places where you get a lot of rain you get deeper soils. Now, this is the answers to one of the current ecological problems facing us and that is in a lot of tropical rainforest areas people have been cutting down the rainforest to grow crops and their crops fail in a year and people were initially puzzled why. Well, it turns out that the soils in rainforests are the worst soils in the world because there’s been so much rain that all of the nutrients in the rock have washed down hundreds of feet. This column here is enormous, hundreds of feet in tropical rainforests. There’s no nutrient left in the ground which is why the tropical rainforest have evolved to have all of their life up above ground. In fact, much of the rain sometimes doesn’t even make it to the ground. It gets trapped by all these plants. Some plants live in other plants and their roots are up in the trees and so there’s a whole process by which the organic matter is recycled in the rainforest without it ever getting down into the ground. Because once it gets into the ground it just washes right out and washes right down. So trying to grow agriculture in tropical areas is the worst possible thing you can do because the soil just can’t support it at all.

The other thing that groundwater is important for and that is in the development of caves. Missouri is the cave state. We have over 5,000 mapped caves here though there are some states with actually more caves than Missouri. That 5,000 is just the number that have been mapped. There are obviously probably many more. There are some cave networks like Mammoth Cave in Kentucky where there are hundreds of miles of interconnected caves. In fact, I’ve had some cavers speculate to me that a goal of cavers is to be able to make it from the East Coast to St. Louis entirely underground, that there probably is an interconnected set of caves. They just haven’t been found yet. It’s obviously very hard to explore in caves because you have to bring everything down with you and it’s cold and dark and they crawl through passages like this and they’re generally crazy people. But it is remarkable that essentially it’s just water flowing through the ground and usually it’s flowing through limestone because as I said the rain is slightly acidic and limestone gets dissolved and as the rain goes through it just dissolves out these giant caverns. If any of you ever get a chance I would strongly recommend going out to Onondaga Caverns. It’s an hour and a half, 2 hours down on Route 44. It has some of the best cave structures -- flow stones, columns, giant caverns in places that’s larger than the basketball stadium in the athletic center and it’s all carved out by streams but it redeposits the rock off in really beautiful, colorful formations. So it’s a neat place to visit.

Groundwater also poses a lot of ecological challenges. We used to think that everything that flushed into the ground we didn’t have to worry about but there are problems because petroleum products (oil and gas) float on water so we’ve got a huge problem around the country now in that we’ve got all these old gas stations that for years have been indiscriminately leaking gasoline into the ground and all that stuff sits at the top of the water table and so it contaminates drinking water and water used for agricultural processes. Agriculture is another huge problem. There are places like the Great Valley of California for instance, Sacramento where about a third of our winter fruits and vegetables are grown. They’re pumping the water out so fast and at the same time they’re using so many nitrates as fertilizers that the pumps are essentially pulling in the nitrates and there are city water supplies that are now totally contaminated because they’re filled with these fertilizers, these nitrates that are being pulled in. There are also cases where the groundwater flows so fast that we don’t understand the system. In fact, there was a town in Missouri where they had a sinkhole and they were throwing garbage in there regularly and thinking it wasn’t any big deal. Well, when geologists did a test it turned out that anything thrown into that sinkhole entered back into the streams that they got their drinking water from in about a day and a half, was the return time so a groundwater system can move water remarkably quickly in certain areas. Long Island has a severe problem because they’re pulling out the glacial water so quickly that they’re actually sucking salty sea water in to take its place and so many drinking wells in Long Island are now pulling out salty sea water. And they have a problem like everyone else, much of the water that we pull out for agriculture from the ground is leftover runoff from the ice ages so it took tens of thousands of years for that water to sink in there and we’re pulling it out in a matter of decades. And it just simply can’t be recharged over that fast of a period.

Another area of sort of moving geological rock and I don’t want to spend a lot of time on it is that of coastlines and essentially you end up with what we call primary coasts and these are coasts that develop for non-shoreline reasons like you take a mountain valley up here or fjords or glacial valleys and you raise the sea level and you flood them and you create a new coastline. Or you deposit a lot of sediment like the Mississippi River and that sediment creates a delta. Over time the shore action is very active and will alter that coastline and the process is one of turning one of these types of coasts into essentially a straight coast. Shoreline processes, waves try to level out a coastline and they do that two ways. Waves carry a tremendous amount of power. I mean anyone who’s stood by the ocean and felt the pound of the surf realizes I mean it’s like a hammer continuously banging away at the rock and you relatively quickly geologically will just tear away any parts of the coastline that stick out and create cliffs. At the same time sediment falls out of water when it slows down so if that sediment gets into a bay it begins to settle out and sediment builds up and that fills in so over time a rugged coastline will become a straight one. And there’s very active long shore currents that carry the rock around. Anyone who’s ever swum in the ocean and over time looked up and realized that your towel is 100 feet up the coast that way is aware that there are these currents that run along the coast and will just carry you down along with all the sand over time. One of the important things about coastlines -- well, first of all this erosion is a continual process. About 86 percent of California’s coasts are receding at about half a meter per year. Okay, that’s 1½ feet per year. That doesn’t sound like a lot but imagine that you’ve got beachfront property that you’ve paid $3 billion for and you’re losing it a foot and a half every year. Ten years that’s 15 feet. That’s making a lot of people very unhappy and there’s essentially no way to stop it. You can try putting jetties out of rock and breakwaters but it just makes the erosion happen over in your neighbor’s yard or coastline. However, storms can be tremendously damaging, in particular hurricanes. Hurricane Andrew that happened in 1992 cost more than $25 billion worth of damage and there are places around the world where huge numbers of people die regularly from huge storm surges along the coastlines. There was a storm in the Bay of Bengal in 1970 that killed over 300,000 people. There was one again that happened in 1991 where over 140,000 people essentially died in a day. You know that’s fairly large amounts of people. An interesting statistic I just came across when I was researching this: 6 percent of all the people that have ever lived are alive today and because our population is growing so rapidly and infant mortality rates have been so high in the past 20 percent of all the people that have ever been your age or older are alive today (1/5). So anyway, I thought that was kind of sobering. So shorelines and these natural disasters are a whole other field. They do a lot of damage to life as well as geology.

One last system I want to talk about and that is wind, and when people think of deserts the thing you most commonly think about sand dunes and so this is a picture of how essentially sand moves through a sand dune with wind. I’ve talked mostly about water moving rock but you can get wind moving rock as well and essentially moving a sand dune marching it along over time. Wind is very active in deserts because of the huge changes in temperature. I was camping in the Grand Canyon for awhile and it was amazing especially at nighttime you would hear this incredible rushing sound and it wasn’t the river. It was the sound of air, cold air coming off the Colorado Plateau because there was still snow up there at that time (it was March) and then the air in the bottom of the canyon was 80-90 degrees and so the hot air was rushing up, the cold air was rushing down in a very active convection cycle and this very strong wind could be heard over the sound of the river. And it carves out, it blows sand and actually carves rock out. Wind on a larger scale moves tremendous amounts of sediments. In fact, if you go out to the middle of the Pacific and you look at the sediment that’s building up there most of it is actually sand from the Gobi Desert in China that has been blown 10,000 miles across, you know picked up from the desert and then deposited into the ocean. All of the Midwest even south of here has very thick glacial deposits and that’s because all of the rock that got pulverized by the glacier when the water dried up that rock got blown all around. You know people may have memories of watching the Grapes of Wrath where these Oklahoma Dust Bowl storms. Wind can move dust around tremendously even in our country.

Okay, let me skip now to showing a few slides of environments so I just want to run through these. I’m not going to spend a lot of time on them but I want you to get a sense of what some of these environments look like. I can only encourage you to take as many roadtrips as possible, especially while you’re young. You don’t know what to do with a week, get in a car and drive out west and look at the incredible geology that exists in this country as well as other places.

[slide 1] Much of our rock and our land comes initially from volcanoes. This is one in Hawaii. Hawaiian volcanoes tend to be very colorful and again this is the start of the rock cycle.

[slide 2] This is Mount St. Helens after it blew. About a kilometer and a half of the top of the mountain blew away. Here’s a little cone building up again for the next eruption and you can go and visit it, it looks like Mordor in Lord of the Rings, you know just desolate, dead, ash everywhere.

[Is there a reason why we don’t tamper with those at all like to prevent another]

We can’t.

[You can’t like drill a hole in that?]

Can’t do anything. The forces are way bigger than we. We’re like tiny, little ants running around on the top of these things. We have much less impact than ants do.

[slide 3] But you get volcanism everywhere. This is in Arizona with recent cinder cones there only about 40,000 years old. You go across Idaho, the southeastern part of Idaho is all black with volcanoes. It’s the ash that probably makes the potatoes so good. It’s nice rich rock there. That’s left over from the Yellowstone Hot Spot and that volcanism, some of it’s only 2,000 years old so geologically that’s still active and you could get more volcanoes there really at any time.

[slide 4] Sometimes we see quite dramatic evidence of former volcanoes. This is Devil’s Tower in Wyoming and this is a former plug or conduit for a volcano so when the land was much higher this cooled underground and as the ground has eroded away the basaltic volcanic rock is much more resistant and it’s still here long after everything else is eroded.

[slide 5] Some places volcanic activity takes quite dramatic forms though. This is the travertine falls at Yellowstone and these are formed by the hot minerals that come out and are then deposited and cool and they form these beautiful white pools. Next is the same thing.

[slide 6] These by the way have the greatest biodiversity of any part in the world. You can go into one of these little hot springs here and you can cover three-fourths of the tree that Ursula showed of various types of one-celled organisms going from cold water to 110 degrees Centigrade water and you have these creatures living in every possible mineral composition and temperature. It’s really quite spectacular. We have several people in our department who go out there regularly to look at that.

[slide 7] I talked about the effect of glaciers carving away and perhaps the best known places in our country are Alaska so this is up near Denali State Park in Alaska and you see the very active glaciers occurring there, carrying rock eventually to the ocean.

[slide 8] Or if that’s too far to get to, just a few hours east of San Francisco you can get to Yosemite which was carved 15,000 years ago by recent glaciers and you can see for instance Half Dome here. The other half was in here and was eroded away by the glacier and quite dramatic evidence of it in here. El Capitan is just out of U. It’s three-fifths of a mile vertical rock face and a great place for rock climbing if you’re into that.

[slide 9] Other places in the U.S. in the Southwest you get some fascinating deserts. Here’s a dust storm rolling across the Sonora Desert.

[slide 10] And some places, this is the Sonora Desert actually in Mexico not far from the border, you get essentially oceans of sand. Here’s the tops of a mountain range here poking out above the sand that has essentially washed right over.

[slide 11] Or this is near the Sultan Sea in southern California. You get these little dunes and you see there are little cactus here or some sort of plant here for scale and these dunes essentially just march right across as very separate entities across the desert floor.

[slide 12] I mentioned caves are quite dramatic carved out by water flowing through limestone.

[slide 13] And you get just incredibly unusual structures as the material drips down and lands on the ground and some of these are very spectacular. The deepest cave, this is from Lechuguilla, these are huge 20-30 foot gypsum deposits. Lechuguilla Cave in New Mexico goes down over a mile and a half or that’s the deepest it’s been mapped so far by someone who’s been able to go down there and make it back up. Several people haven’t but you don’t have to. To see caves, there are many caves that are now public and you don’t have to be (I’ll use a nice word) common sensally challenged to do this.

[slide 14] We get some dramatic landscapes carved by rivers. Some of them are very dramatic like in the Rockies where you have rapid mountain streams carving away at the rock and sometimes as I mentioned depositing the sediment in large fans. This is at the front edge of the Rockies.

[slide 15] But you also get places like Missouri with very gentle, rolling streams that have a much more sort of gentle nature to them, of course except in times when they flood like what happened here in 1993. For those of you who aren’t from St. Louis it was really dramatic watching the daily reports because the flood walls in St. Louis are at an elevation of 52 feet above flood stage level and the water reached 50 feet above flood stage. And had it made those remaining 2 feet there would have been a lot of St. Louis property that would have been seriously damaged.

[slide 16] In fact here’s an aerial photograph of the Missouri and Mississippi Rivers. St. Louis is right here, Wash U is about here. Here they are normally. Here they were during the flood. They were one river about 5 miles across. It was really very dramatic.

[slide 17] Ocean coastlines are really beautiful. This is off the coast of Cape Cod and you sometimes get long spits of sand being washed around and moved about.

[slide 18] And again here’s Cape Cod and you can see that this is all glacial deposit here but the ocean waves have sort of whisked the sediment up along the coast here through continuous motion.

[slide 19] This is not the United States actually. This in the southwest Pacific but you also get some beautiful -- I don’t know, maybe it’s an American territory -- but you get places where you have living creatures like corals that greatly alter the shoreline. In this case you have very active coral reefs developing along and in fact long after the sea floor sinks and the rock sinks beneath the surface the coral keeps growing on top of itself to try to stay above sea water and you can get a couple kilometers of coral still growing. In some places all that you have left are these coral rings called atolls.

[slide 20] Take a place like Yosemite and flood it when the sea level goes up and you have a fjord and so in many cases we have very dramatic coastlines. Much of Maine has sort of mini fjords where you have again deep valleys that have since been flooded and you have arms of the sea reaching very far inland.

[slide 21] In general everywhere on the surface of the land has been tremendously shaped by erosion. This is the Grand Tetons in Wyoming and you can see their jagged nature is evidence of the active erosion that’s tearing them down.

[slide 22] This is Bryce Canyon in Utah. Again this is the level of the plateau here but as water has eroded away this red limestone it’s left these quite unusual columns sticking up.

[slide 23] This is Monument Valley in Arizona and again you can see that the different rocks erode at different rates. Here’s the plateau level kind of across here but once it breaks beneath a fairly hard cap the rest of it erodes away quickly. In fact, you can see all this material sort of deposited as fans around these large buttes and mesas.

[slide 24] Sometimes a little stream like the Missouri or the Mississippi keeps on eroding down and gets stuck in one place and here’s the Snake River canyon and you can see again here’s the plateau level up here, here’s the stream that got stuck in one channel and has now carved out these giant what they call goosenecks as the stream continues to carve further and further down.

[slide 25] Sometimes we get very dramatic topography. This is parts in China where you get the limestone that is so old and eroded that the caves essentially crumble and wash away and all that you’re left with are these large limestone towers towering around the rest of the land.

[slide 26] And of course you have areas where it’s almost all eroded away like the Grand Canyon where you can look back essentially 600 million years of history from top to bottom across the canyon. So like I said we have more courses that deal with these sorts of processes of geology but really the best way to do it is just get out and go see the country. So I’ve made my plug for that. Okay, any quick questions on that before I talk a little bit about climate?

Alright, our climate has fluctuated greatly over the past. Through known time we have seen a tremendous variation in climate. And when I say climate I’m primarily talking about the distribution of temperature. Climate is actually a very complex thing that involves a lot of things but you can boil it down to essentially temperature and precipitation. Involves a lot of other things like air and ocean currents and pressure and humidity but we’ve noticed that things have changed rapidly over time. In terms of the Earth sciences though climate is far and away the least understood process. And I mentioned this before, if we can’t predict whether or not it’s going to rain for the Thurtene Carnival tomorrow 1 day from now, there is no way we can predict what the climate is going to be 10, 100, 1000 years from now. Because we can’t even understand what this history of climate variation is going back in time. And this is simply mean global temperature (cold, warm) on a sort of funny logarithmic scale here in millions of years. And we have warm periods like 80 to 140 million years ago and periods of glaciation like recently and here 280, 440 million years ago.

[How were these temperatures determined that far back?]

The temperatures are actually determined by a process called oxygen isotope dating. The basic idea, I’m not going to get into this, don’t bother writing it down. If you want to look at it on the Web when it makes it there. The basic idea is that there’s a naturally certain percentage of oxygen-18 isotopes. The most oxygen appears is oxygen-16. Evaporation prefers oxygen-16 because it’s lighter, easier to break the bonds and easier to evaporate so evaporation is O₁₆, rain is O₁₆, ice that forms is O₁₆. So that means during periods of ice you’re taking out all this O₁₆ isotopes and your ocean is enriched in oxygen 18 relative to O₁₆. So we go back and we look at fossil shells and we grind them up and put them in mass spectrometers which count individual atoms essentially and we look at the rate, the amount of the oxygen-18 isotopes and if there are a lot of them then we know it was an ice age when a lot of ice was up on land. So that’s how this curve -- in fact now that I’ve actually said that I can go and show the next side here which talks about here’s this oxygen isotope ratio in marine plankton so you can see this curve. Now, there’s some funny other curves that are next to it and let me explain basically what those involve. One of the biggest factors of climate is the amount of sunlight -- or I shouldn’t necessarily say light but energy from the sun. And it turns out that that is determined by a set of parameters called the Milankovitch Cycles. And I will come back to this picture in a minute but let me show you what I mean by Milankovitch Cycles. The Earth is a top spinning and I didn’t bring one but picture in your minds you know you spin a top and it doesn’t just sit there and spin, it kind of does some of these wobbles a little bit. Like it’ll wobble a little bit and then it’ll come back up again. Well, these wobbles take a couple different forms and for the Earth you can break it down into three different types of changes in the Earth’s orbit. One of them is called the Eccentricity and that is a fluctuation of the orbit from being more like a circle to more like an ellipse to more like a circle again. And that variation happens with a period of about 100,000 years and that’s important because during periods when it’s more elliptical the Earth actually receives less total sunlight so it tends to have a little chilling effect on the Earth. Then you also have a change in the tilt. Today’s tilt is about 23½ degrees so our pole of rotation is tilted relative to the path that it orbits around the sun by 23 degrees, but that tilt varies back and forth between 24½ and 22½. And that has a period of repeat time of 41,000 years. And the other one is that the axis normally, think of it spinning, it normally goes like this around the sun. It’s always pointing off at the same spot off at the North Pole but that wobbles a little bit back and forth. In fact, it actually goes all the way around slowly in a circle that takes 26,000 years so that’s that top sort of processing around this way. You know as the top is spinning its tilt changes. Okay, what it means is when you change these orbital parameters you change the amount of sunlight that reaches the Earth in different places and remarkably we have noticed a very strong correlation between these cycles and the overall times of glaciers of the Earth. So for instance here you can see this is a maximum here, this goes up, this comes down, this comes down. Look at how this curve matches this curve. In general there’s a very good agreement. All three peaks are sort of lining up here and we have a large peak here in the plankton oxygen isotope record which is a record of temperature. So our temperature is directly affected by these three and how they add together.

Would that that were it we’d have a quite simple story but it turns out that that’s not at all it. If you look at the fluctuations we see there are periods of intense warm temperature like 120 million years ago and periods of really intense glaciation going back in time that lasts for tens or hundreds of millions of years sometimes. And these orbital parameters here are on the order of thousands of years so the Milankovitch Cycles affect the little scale variations but not the big scale variations. Okay, what else can do that? Well, it turns out we have to go back to class a month ago and go back to hot spots. When I talked about volcanoes I talked about there being this other type of volcanism that’s due to a hot spot plume that rises up from deep within the Earth. These high spot plumes can occur catastrophically and far dwarf anything that we see currently. To give you just an example, Yellowstone is a fairly small hot spot. It doesn’t even really rank but here’s Mount St. Helens and the ash plume from the Mount St. Helen’s eruption. Here are some recent eruptions (recent geologically -- 600,000 years, 2 million years) from the Yellowstone volcano. And again, Yellowstone is still active geologically. This ash stretched from California almost to St. Louis and probably much more so but this is only what we see in the geologic record that wasn’t washed away. And these deposits are thick deposits as far as away as the Gulf of Mexico. In fact, if you look at the size of some of these, another one is in Colorado, the Fish Canyon Tuft 30 million years ago and for scale here’s the largest volcano to happen in the last couple hundred years (Krakatoa). It’s this little volcano right here, 18 cubic kilometers. Then you go up to some other volcanoes and you get to the Fish Canyon Tuft Colorado 3,000 cubic kilometers of rock were blown up into the air and who knows how much gas and carbon dioxide. It turns out that it’s the carbon dioxide that’s very important because there’s a strong correlation between carbon dioxide and temperature and when you put carbon dioxide into the atmosphere you warm the Earth. This process is called the Greenhouse Effect. There’s nothing complicated about the Greenhouse Effect in the sense that all that happens is light hits the Earth but it happens mostly in the visible spectrum which is probably why our eyes have developed. They’ve evolved to see electromagnetic energy in this spectrum because that’s the dominant amount of energy from the sun is roughly in this spectrum. And the ground absorbs that, it heats up but when it re-emits it, it doesn’t emit it as light. A little bit maybe but the rock isn’t glowing bright. It emits that energy in the microwave spectrum (I talked a little bit about this earlier) which gets absorbed by the atmosphere and carbon dioxide plays a big role in absorbing up that radiation. It re-emits that energy and some of it goes from the atmosphere out into space but some of it goes back to the Earth, gets reabsorbs, back up to the atmosphere, back to the Earth and essentially what you do is you trap microwaves between the surface and the atmosphere. Put more carbon dioxide into the atmosphere and you raise the temperature. Well, it turns out huge amount of hot spot activity happened 120 million years ago and that corresponds to that period of time. I will add, this is a tangent here but I find it a very fascinating tangent. To show you how coupled the whole system of the Earth is this is a record of the magnetic field reversals going back 170 million years. Remember I talked about -- forget the surface, go down to the core now. The magnetic field randomly flips back and forth and this is a plot going from 0 million years to 25 million years to 50 to 75, 100, 125, 150, 175. So it should go all the way across but it’s been cut into pieces here. So here we have all these reversals happening several times per million years but look what happens between about 70 million years and 120 million years. It almost doesn’t reverse at all. In fact, it’s locked totally without reversing between about 85 and about 120 million years. It’s not that we don’t have the data because if you go back in time it begins to reverse very frequently. What happened here was that a huge amount of hot spot plumes lifted up off the core-mantle boundary, came to the surface, blew all this carbon dioxide in the atmosphere, which raised the temperature, which changed the path of evolution of life, etc, but cold rock came to the core to take its place and essentially froze the outer core in place and froze the mechanism of convection in the outer core. And however the outer core field reverses which we don’t understand, somehow that cold rock brought down to replace the plume rock froze the outer core in place and it stopped reversing. That gives you a sense of how this is all tremendously coupled.

I want to mention something about a bizarre change in climate that happened just before life began and this is between about 750 and 550 million years. We had a period of alternating freezing

[Was it before life began or …]

Before now. So “MA” means millions of years ago or mega anna. I’ll put a million years ago. About four or five times during the period 750 to 550, I mean complex life, Burgess Shale kind of stuff, Ediacaran fossils and all that. I see Ursula scowling over there. Yes, the first life is 3.9 billion years ago and we know that from isotopic evidence though we don’t actually see physical evidence of first life-forms till about 3½ billion years ago but we know that life has been around much longer. Four or five times the Earth flipped back and forth between being totally frozen (I mean the oceans were frozen) to burning times when we had a runaway Greenhouse Effect with not a speck of ice on the surface of the Earth. Now, probably it was really rough for life to exist during this time just clinging on by its knuckles. Essentially when the Earth was totally frozen you would have had thermal vents, volcanoes, maybe cracks, a few cracks in the ocean ice would have been the only place that life would have continued on and it would have been almost exclusively one-celled life. The basic idea is that you can have a weakening of the Greenhouse Effect if you pull carbon dioxide out, right? Carbon dioxide keeps things warm. Well, how do you do that? Have a really good time for life, have the plant life suck out of a lot of carbon dioxide? That can be part of it. Also you can have a period of very active plate tectonics which creates a lot of plate collisions which creates a lot of mountains. If you have a lot of mountains you have very rapid weathering and erosion and all that chemical weathering that I talked about in last class primarily involves carbonic acid and the carbon either reacts with the rocks to make new minerals which pulls the carbon out or it gets washed into the ocean to form limestone which still sucks it out of the atmosphere. Pull the carbon dioxide out, you begin to chill the temperature, you begin to form ice. The runaway effect comes when the more ice you have the more you reflect back sunlight before it gets a chance to heat up and you get this runaway effect. You get more and more ice, less sunlight is warming up things and everything freezes over. Why doesn’t it stay that way? Well, we’ve got these volcanoes churning away and the carbon dioxide builds up eventually under the ice and at some point it bursts out and then you get this period of intense warming afterwards. Between 550 and 570 million years things warmed up and they never froze over again. What’s remarkable here is that we saw a tremendous burst in life at that time. There are fossils found in the Ediacara Mountains of Australia that show this diversity of life-forms about 560 million years ago, all different and strange shapes. A little bit later, 540-535 million years ago, we see these bizarre life-forms occurring and in some places like the Burgess Shale in Canada we’ve actually found evidence for all sorts of bizarre creatures that some of them are not even classifiable, creatures that don’t have any modern counterpart. And this one originally was thought to go like this but it was wrong. It actually walks on a bunch of legs and has these big spines that stick up on the top of it. And say hi to Grandpa here. This is one of the world’s first know chordates that essentially had the beginnings of a spine that was the precursor to vertebrates like ourselves. And what’s interesting at this time is it’s been proposed that life went in all different sorts of crazy directions. We normally think of the Tree of Life as sort of starting from a few types and then branching out and most things eventually go extinct but things get more and more complex and we get more and more different life-forms. Some people have actually proposed and there is a lot of debate on this but that early on we had many different whole classes or types of life and many of these never went anywhere but the life-forms that we have now are essentially refinements of those first few life-forms.

The whole history of life since that time has just been a marvelous experiment, a variety of things but there have been periods of large extinctions. And this is the number of families of life going from 500 million years ago where you just had a few. And this is an old figure so things have gone up since then but up, and then you’ve got a couple setbacks. Well, one of them here happened right about 250 million years ago and it was incredible. Ninety-five percent of all marine species went extinct. Nearly 60 percent of all life families went extinct. It’s probably the result of a double whammy here in the sense that we had an ice age that had really chilled things for a long time and so a lot of carbon dioxide got stored in the oceans. Right about this time and these extinctions seemed to have happened within about a million years so very fast geologically, huge amounts of hot spot volcanism occurred in Siberia and we see the evidence of that in northern Siberia. That would have perhaps released all this carbon dioxide which would have just been brutal in terms of rapidly increasing the temperature very quickly. This extinction here (65 million years ago) is a little more famous though it wasn’t nearly as big. This is at the end of the Age of the Dinosaurs and almost all large vertebrates including dinosaurs went extinct. Most other life families survived okay and you can see it’s not as severe. This was probably also a doubly whammy in the sense that two things happened simultaneously. We had a huge amount of hot spot activity in India and at the same time we had a meteorite impact that hit in the Gulf of Mexico right at the top of the Yucatan Peninsula and it caused incredible rapid change in climate and that had a severe effect. Many of the other little nicks in terms of extinctions are probably also related to impacts or other catastrophes. I want to show you one last picture and realize how tenuous our whole climate system is. This is a picture from Greenland ice cores and these are relative changes in temperature and you can see there’s about a 25 degree variation in temperature that’s recorded here. This goes back from the present to 160,000 years ago and look, there are times when the climate has changed by more than 10 degrees in a matter of a few years, the global climate. Our temperature, we’re all worried about how rapidly we’re changing the temperature. Our global temperature has gone up about 1 degree in the past 100 years and it’s correlated very well with an increase in carbon dioxide and so people are thinking oh, my, catastrophe. Well, no. In fact, the last 10,000 years have been both very warm and very, very stable and that’s probably why civilization didn’t get started until 10,000 years ago, because there’s no way you can grow crops if your climate is fluctuating by 10 degrees every few years. The constant warm temperatures has led to civilizations, communities that can grow crops, sustain them and it’s been very bizarre. We see a few periods of stability, islands of stability in our climate. We have no idea why the recent years have been so remarkably stable. There’s no reason why they have to stay that way and we don’t know what it would take for this system to go back into this huge fluctuation that was so challenging. So I’ll finish with that.

[end of lecture]