TIME | 3/11 Itay Halevy – The geologic time scale: Events writ in stone

All right good morning everyone I’d like to thank the organizers for the invitation to speak in this symposium which already looks to be a great day of cross-fertilization and discussion and And as Kobe mentioned what I’ll talk about is geologic time or more precisely the geologic time scale both in terms of absolute age That’s where I’ll start my my talk and in terms of the events that have shaped the geologic record And which provide a relative timing of key events in Earth history So to begin we need to Get a basic understanding of rocks which are the geologists tools to study the earth? And they’re also an archive of information on the processes and events that have occurred over Earth history And so we divide rocks into three categories igneous rocks and these metamorphic rocks and sedimentary rocks the igneous rocks form when magma cools and crystallizes into forms matrix of crystals and the magma itself is a Melting product of pre-existing rocky material We have sedimentary rocks when when when either igneous or metamorphic rocks are exposed to surface conditions They weather, and the road and the particles and solutes end up being deposited in sedimentary rocks and we have Metamorphic rocks so when either igneous rocks or sedimentary rocks are buried to great depth and exposed to high temperature and pressure They the mineral assemblages to reflect the stability under high pressure and temperature and they become metamorphosed and again when either Metamorphic rocks or sedimentary rocks are buried deeply enough and heated enough They may partially melt to give us back igneous rocks, and so this is what we call the rock cycle Now humanity has wondered from very early on about the age of the earth and according So let’s let’s have a look at some of the estimates right according to the Bible the earth is five thousand almost five thousand eight hundred And the exact number depends on how one does the the accounting this is the the Jewish? the year according to the Jewish calendar Which is said to have started when the world was created, but in the 17th century Archbishop James Ussher Made an independent estimate and came up with an age that is about two hundred and fifty years older But whichever age of creation is suggested by the Bible geologists have argued for a long time And I hope this is visible in the back Geologists have argued for a long time that the earth was actually much much older, and they based their arguments on observations of repeating sequences of sedimentary rocks like the ones that are shown here and Unfolded here and each of these layers they understood to have formed by processes that they knew to be long Lasting perhaps thousands or tens of thousands of years and so many many repetitions of these layers meant That the earth was in fact much much older than the Bible suggested in fact many geologists thought that The age of the earth was Infinite that it had been around forever There were some who tried to come up with more quantitative estimates of the age of the earth for example in the early 18th century Edmund Halley came up with the idea that we can use the salinity of the ocean to To measure how long the ocean has existed if we knew the fluxes of Material coming into the ocean because of precipitation and runoff and in groundwater as well So we have evaporation out of the ocean this rains on land dissolves material and carries it into the ocean and that accumulates as a salt in the ocean and so if we know the mass of the ocean the salinity in the ocean and the fluxes of solutes into the ocean we can calculate how long it should have taken for the ocean to become this salty and One immediate thing that that comes out of this is that the earth is not the age of the Earth is not infinite otherwise the ocean would be saturated with sodium chloride much like the Dead Sea Holly didn’t try to perform this calculation because he knew that he didn’t have good enough constraints on fluxes coming in and in rivers, but almost 200 years later, John Jolie Did the calculation with some early estimates of riverine fluxes into the ocean and came up with an age of about a hundred million years? We have now much better constraints and riverine fluxes But more importantly what they didn’t know of was that there are processes that remove salt from the ocean

So salt doesn’t just make it into the ocean just sit there until The ocean becomes saturated there are many processes that actually remove salt from the ocean or remove solutes from the ocean even when the water is under saturated and so this is a gross underestimate of the age of the oceans and the age of the earth so physicists of the time and especially William Thompson, which was later knighted and given the title Lord Kelvin and After whom the scale was later named he realized that an infinite age of the earth violated the laws of thermodynamics That I wish I mentioned a few minutes ago and he calculated the time it would take a molten sphere of rock starting at a temperature of 1700 kelvins to cool down so that it So that so that the ultimate heat flux resembled the modern heat flux which again there weren’t Good measurements of the heat flux when he first Made this suggestion, but when some measurements were made of the heat flux of the geothermal gradient the change in temperature With depth into into the earth he performed this calculation and came up And I should I should note that the assumption was that the earth cooled only by conduction of heat And so he was able to calculate that the age the age of the earth was again about a hundred million years but what Lord Kelvin did not factor into his calculations was that the earth cooled much more effectively than just by conduction And this is because the convection in the mantle The mantle is a viscous fluid. It’s not it’s not it’s not liquid It’s all but on geologic timescales its viscous enough that it flows and What happens is basically that there is heating at the core-mantle boundary and the war material rises up? Towards the surface and is replaced by cooler material that comes down from the surface And this is a much more efficient cooling Mechanism than just conduction of heat and so in order to end up with the geothermal gradient that we have today There’s a lot more heat to to mind from the interior of the earth and so this again gives an underestimate of the age of the earth a former assistant of Lord kelvins John Perry Suspected something like this he and he’s quoted as saying that It’s hopeless to expect that Lord Kelvin made an error in calculation instead He focused on Lord kelvins assumptions the assumption was the the main assumption being that there was no additional source of heat inside the earth and that the earth cooled only by conduction So he did not know just like Lord Kelvin He didn’t know about convection in Earth’s interior But he carried out the same calculation under an assumption of high high interior what he referred to as quasi conductivity and this had essentially the same effect as Our understanding today that there is in fact Convection and the result was that his estimates for the age of the earth were about the order of magnitude larger than the Lord kelvins Actually one to two billion years and and and this really The range here depends on how Fake one assumes the conducting layer to be at the Earth’s surface right because the earth is not Conducting all the way to the surface, right There is a thin conducting layer at the top Also missing from kelvins calculation But also from perry’s calculation Simply because it had not yet been discovered was radioactive heat so there They assumed that the the only source of heat was this initial molten sphere An initial temperature they did not know that there is actually additional heat generation by radioactive decay of unstable elements This Lengthens the age by some by a bit, but by not, but not much the main difference going from Lord kelvins calculation to John Perry’s calculation is actually is actually the inclusion of convection or a quasi Quasi conduction that John Perry included in his calculations a Billion is 10 to the nine years. Yes, it’s one thousand million years All right, so the entire story and all its intrigue and and scientific politics is wonderfully laid out in an article By England Bedell came out in the GSA today in 2007 if anyone’s interested So the debate was finally settled when Marie and Pierre Curie Discovered radioactivity so what they basically discovered and were jointly awarded the

Nobel Prize in 1903 was that the isotopes of some elements were unstable and that they decayed with half-lives That varied over many many orders of magnitude from seconds to billions of years Parent isotopes decay through a series of a series of alpha and beta decays into ultimately decay to stable daughter Isotopes and here we have two examples of decay of uranium 238 with a half-life of four point about 4.5 billion years and uranium 235 with a with a half-life of 0.7. Billion years and they decay through this Complex series of unstable isotopes all the way down to stable isotopes mostly two isotopes of lead Uranium-235 mostly ends up as lead 207 uranium 238 mostly ends up as lead to six So in the early 20th century Ernest Rutherford Physicist realized that this could serve as a chronometer or a clock for geologic materials And he suggested using the decay of uranium 238 and 235 to lead 206 and 207 respectively as a means of measuring time since a mineral formed So how does this work say we have a mineral that has? No lead in it initially, but some uranium and zircon Zirconium silicate is a good example it contains. No lead at the beginning. It’s an igneous mineral It forms by crystallization of magma and it includes some uranium some of that uranium is Is to uranium 238 some of it is 235 depending on? when this igneous rock formed after the the beginning of the solar system As soon as the mineral formed the uranium is locked in the mineral and and it begins to radioactively decay and Because of the decay of uranium 235 is much faster than that of 238 right it has a half-life that is that is? about seven times shorter than that of uranium 238 Its product will accumulate much more quickly in in the mineral right we started out with nothing and we end up with something And but because and because of this this decay is much faster. We end up with more lead 207 than lead 206 Now so we can measure the ratios of the daughter to parent isotopes and they both start at zero because remember we start with no lead and given the half-lives of the radioactive parents we this We can we can sketch out a curve that that evolves in a predictable way as a function of how much time has passed And you can see here for example because of decay of uranium 235 is that much faster than than uranium 238 and we go up to values of lead 207 to uranium 235 that are almost 2 orders of magnitude larger than the ratio of 2:06 to uranium-238 and so a Variety of rocks were measured on earth, but more importantly rocks that came from from the moon And and we based on all of these measurements of the oldest rocks we know that the age of the earth is about four and a half billion years so we just heard that the age of the universe is 3 13.7 billion years over that Period of time over which the universe has existed there have been cycles of the formation and destruction of galaxies and stars and about four-and-a-half billion years ago Stellar nebula or this the the the solar nebula Collapsed to form our solar system and four and a half billion woops four and a half billion years ago Essentially our Sun and all the planets and bodies that orbit it formed in the space of a few millions to tens of millions of years All right, so now we have an Answer about the age of the earth this long-standing debate of how old the earth Actually is has has been has been settled because of the discovery of radioactivity But as I mentioned before and the geologists who study the sequence of sedimentary rocks realized much earlier but without any quantitative estimates that the sedimentary rock record represented very very long time spans hundreds of millions Or even billions of years and so without being able to constrain the absolute duration That was involved in generating these sedimentary rocks So they came up with principles that allowed them to determine relative timing

And these rules are the basics of stratigraphy which is the description and study of the strata? The layers in the in the geologic record and so the principles were formalized by 17th century scientist who later became a priest Nicholas steno And he stipulated the principle of superposition, which is which is to say that? Younger layers are deposited on older layers the Principle of original horizontality which is to say that? sedimentary layers deposit horizontally or on very shallowly sloping surfaces and The principle of lateral continuity which is to say that that these units are or these layers are? Continuous over large areas and this is an aside these these latter two Principles let us infer things about tectonics about Structure about movements in the earth and about geomorphological Processes those that alter the surface of the earth based on our observations of the effects of these processes on Layers that were originally horizontal and continuous But I don’t want to go into that The first principle superposition is what lets us Or what gave rise to the stratigraphic record stratigraphic record is is a listing in order from the oldest to the youngest? Periods in Earth history so we read it in this direction starting at the beginning of the solar system four and a half billion years ago and all the way To today this is just one variation of it. There are some where where? You put all these one after the other vertically And what you can see is that it split up into major and minor divisions the largest of these of which there are four are eons the eons are the The the largest the Cambrian See it excuse me the the Hadean the Archaean the Proterozoic and the phanerozoic And you can see that the phanerozoic is is Is the most finely divided it’s hard to see the numbers over here We’re starting of course at four and a half billion years ago and ending in today but What’s interesting? Is that there are many many more subdivisions in the last 500 years Or so earth history than there are in the preceding four and a half in the preceding four billion years of Earth history So this little part here Accounts for 88% of Earth history this one this here the phanerozoic Only twelve and so then what’s the reason for this much finer division of the more recent? units well one reason is the Survival of the rock record the older Iraq is the less of a chance it has to survive and be observed today So there may have been More sediments deposited or there surely were more sediments deposited Early in Earth history that have had been eroded away, or subducted because of plate tectonics They’ve been driven back into Earth’s mantle, and we don’t see them anymore but the more But but the the reason that they were able to more finely cut these more recent sediments into finer and finer Time periods is because of the appearance of fossils or the presence of fossils so about 540 million years ago In the beginning of the phanerozoic phanerozoic is the age of visible venero life or animal is going on? Neither do I i’m not exactly sure what what these are Alright, so this is the the age of visible life. This is when? Fossilized organisms began to appear in the sedimentary record and this was originally thought to reflect the beginning of life itself that 540 million years ago it wasn’t known at at the time when exactly but that that time period represents the appearance of life on earth Now we now know that this is only the appearance of relatively abundant Macroscopic life that is life that can be seen with the naked eye We know that microscopic life existed well before this almost as soon as there were oceans About four billion years ago so microbial life existed Back to 3.8. Maybe four billion years ago the phanerozoic is the appearance of macroscopic life, and we’ll come back to that later in the Later in the talk now this was quantified by Jax gusty at paleontologists from the University of Chicago

and he compiled all of the reports from himself and others on the occurrence and characteristics of fossils to generate this phanerozoic diversity curve So he so we’re seeing just the phanerozoic just the the age of visible life, which is again divided into into Eras the paleozoic mesozoic and cenozoic and what you can see is And we I don’t want to go into the details here Is that diversity which is the number of marine families as a function of time really takes off at the Precambrian? Phanerozoic boundary the Precambrian Cambrian boundary, you can see it take off here. We’ll talk about some of the inflections here And the reason that this lets us cut the cut time more finely is because We can use the range of the occurrence of a specific fossil or an assemblage of fossils To define relative time zones so here for example we have two organisms this gastropod and then ammonite And they each have a range of occurrence right this one evolved here, or was first defined here and Is lost from the record here either because it became extinct or because it changed enough such that we would Define it as a different species and this one appeared here and disappeared here and between them With the overlap between them we can we can define three bio zones By zone a in which only this ammonite was was present by ozone C in which only the gastropod was present and bio Zone B in which both of them were present and So and this is what we can do with just two fossils if we have Many different organisms and assemblages of fossils we can really cut time quite finely And this is the reason that that the phanerozoic Is so finely cut these 540 million years are so finely cut into into relative time periods that We have been able also to pin to absolute time by radiometric dating in each of these rock sequences And so what I’d like to do next is give a few examples of how the divisions in the stratigraphic record reflect observable changes in the rocks Which geologists noted a long time before they knew about? Absolute ages and will examine the boundary between the Archaean and the Proterozoic the Archaean here in the Proterozoic We’re a major environmental change resulted in clear changes in the appearance and characteristics of sedimentary rocks And then we’ll look at the Precambrian phanerozoic boundary, which we talked about now where macroscopic life first appeared and Then will be in the phanerozoic where many of these subdivisions were defined on the basis of the disappearance of organisms basically mass extinctions and so we can see this in the sub Koski curve as these downturns and Will have and the the largest of which we call the Big Five these are the big the largest five mass extinctions Which took out a largest fraction of the species on the surface of the earth? There’s actually an ongoing debate whether we’re in the midst of a sixth major mass extinction today based on the rates of extinction of animals Which which we are? Responsible for of course and out of these we’ll look at just the largest of these which is the per mo Triassic mass extinction They took a lot a large took out a large majority of life on Earth And which also defines the boundary between the Paleozoic and the Mesozoic thank you Alright, so starting with the Archaean per tog boundary. This is a story about about iron ore more precisely about the oxidation state of iron so now I’ve taken the the Geologic time scale and turned it on its side and put them all put it all One on top of each other starting in the idea and the Archaean that I mentioned the Proterozoic and the phanerozoic this is The part of earth’s history where macroscopic life Exists as I mentioned now this is the frequency of rocks that we are named banded informations their iron rich in silica rich sedimentary rocks and The icky this is what they look like in core these are these are cylindrical cores that are Drilled into the earth and these rocks are pulled out You can see there a green and and and gray and this is because iron in them is ferrous its iron 2 plus It’s a reduced relatively reduced form of iron Now because ferrous iron is relatively oxidized in the presence of oxygen the Accumulation of such large masses of iron is basically telling us that very little oxygen existed in the oceans These are marine sedimentary rocks they form in the ocean and and for for so much ferrous iron to be to flux through the ocean The ocean can’t have had a much oxygen in it now with a few notable exceptions

And I won’t go into these rocks all but disappear about two and a half billion years ago the boundary between the archaea and the Proterozoic that I mentioned before Iron begins to show up in ferric oxide cements in solicita clastic rocks These are called red beds, so this is not now an oxidized form of iron The same happens on land iron begins to appear in oxides in soils whereas before it was washed away from soils together with soluble elements like calcium and magnesium and together this this tells us that These changes in iron mineralogy and oxidation state reflect the rise of atmospheric oxygen so something happened here woops something happened here that Led to a change in the cycling of this reduction oxidation sensitive element iron So such that it stopped appearing in its ferrous form in the geologic record and began appearing as rust in the in the geologic record And what we understand that this is is the rise of atmospheric oxygen and now this this is a curve as a function of time the atmospheric pressure of oxygen or the relative The pressure of oxygen relative to today where one is today and this down here is a millionth of Levels today of oxygen levels today and what we can see is that about two and a half billion years ago going from less than a ppm less than a part per million Oxygen in the earth we went to a few parts per thousand in the atmosphere and this caused a major reorganization Of all the cycles of any elements that could react with oxygen Iron is one of them carbon is another sulfur is another so what brought about this change microscopic photosynthetic bacteria so basically around this time cyanobacteria which were the first photosynthesizing organisms or the first ones to do oxygenic photosynthesis they Evolved and they began to split water in order to fix carbon dioxide to form their biomass and in the process they generated oxygen and this accumulated in the surface ocean until it led to this change in the oxidation state of the oceans and atmosphere And remember we learned about this change originally without knowing any of the geochemistry in any of the isotopic evidence that now has led to generation of this curve simply by observations of changes in the properties of rocks in them in the mineralogy and the color in changes in in the in the sedimentary record I’m running out of time so I go through this the Cambrian explosion quite quickly This is the next transition that I’d like to discuss which is The transition from the Proterozoic to the phanerozoic the appearance of macroscopic life you’ve seen this before This is when animal fossils began to appear in the geologic record in the sedimentary record, but these examples are of very Charasmatic fossils that are not what we observe in this transition in fact when you look at this transition The rocks don’t appear to change much at all this is where this man is pointing The this transitions is in fact Defined by the appearance of this trace fossil this trace fossil is basically a burrow This is a hole hole in the mud that a worm dug That was then in filled with material that was different and so it was fossilized Or it was lift if I’d turned into stone into rock Quite differently so you know not not very spectacular, right? But this first appearance marks the beginning of a very rapid diversification Which is sometimes called the Cambrian explosion and which led over the course of only–if few tens of millions of years To the evolution of all the major families of organisms in the animal kingdom and this includes our earliest ancestors the the stem chordates And the reason that this dramatic Well this the This dramatic diversification is still being studied. We don’t know if it’s because of the the onset of predator prey Arms race if you will or because of a change in nutrient regimes But again, this is a major evolutionary event that’s recorded in this in Sedimentary rock so that we basically learned about by looking at sedimentary rocks even before we could we could time them The last event that I’d like to mention is the largest mass extinction in Earth history, which is the promo Triassic mass extinction? here we see the number of marine species number of species and whenever this gets truncated this means that the species Basically disappeared, but we can see that about 250 million years ago Many many species both in the ocean and on land disappeared about 90% of marine species 75% of terrestrial species these are just some examples of the species that went extinct on land these are not dinosaurs This is a human for scale so

They’re scary-looking, but quite small and I won’t go into the into any details about this, but this is because of major Volcanic eruptions that emitted massive amounts of carbon dioxide into the atmosphere because these massive these what are called flood basalts the Siberian traps you can see the us For scale here. They covered a huge area, and they erupted through fissures But they came up through major coal deposits. These are Russia’s economic coal deposits They came up through major coal deposits and emitted a lot of carbon dioxide This carbon dioxide acidified the ocean it causes many clime climatic effects that ultimately ended Ended up causing the extinction and just as an interesting aside The Siberian traps are thought based on on on various estimates to emitted between 5 and 50 Giga tons of carbon per year We are currently emitting about almost exactly 37 Giga tons of carbon per year so it’s interesting to note that rates of carbon dioxide emissions similar to what we are putting in the atmosphere every year Led to one of the biggest mass extinctions in in Earth history And so I’ll leave you with my summary only to say that we know the earth is four-and-a-half billion years old and Before any such knowledge the stratigraphic record told us about the major events in Earth history And with this thank you for your attention and take questions Am I okay, so the question is about where did the water come from I talked about water, where did the water come from? Water so the the elements hydrogen and oxygen or hydrogen and helium were Were generated during the Big Bang Oxygen was gen is then generated during Nucleosynthesis in stars once these stars explode in supernovae for example and their material gets dispersed ultimately that forms Solar or stellar nebulas that end up collapsing to form additional stellar systems like our solar system once things cooled down enough things begin once temperatures decrease enough materials begin to condense this starts with Various oxides ends up with very various silicates, and then the volatile Compounds condense once temperature is low enough and water is one of them That water is a part of any body in the in the solar system Depending on the distance from the Sun and and where the temperature is is low enough Such that water condenses, which we call that the ice line The earth got its water either Originally as as as it condensed from the solar nebula Or there are some ideas that the water came from from comets in any case it came early, and we have evidence for large scale water bodies essentially an ocean as early as About four billion years ago. There are sedimentary rocks that are about four billion years old That were deposited in large masses of water so where exactly the water came from whether it was primordial or its primordial in any case whether it was a Part of the original endowment of the earth or whether it came when comets is still being debated But in any case it It was present from very early on in our history