2 00:00:02,787 --> 00:00:07,537 One of the real workhorse paleo-tracers, 3 00:00:07,537 --> 00:00:12,537 and one of the very first to be developed, was based on 4 00:00:12,537 --> 00:00:17,585 what are called isotopes, which are 5 00:00:17,585 --> 00:00:22,985 the same element 6 00:00:22,985 --> 00:00:28,060 but they have different masses. 7 00:00:28,060 --> 00:00:34,430 Oxygen 18 has two more neutrons in the nucleus than oxygen 16. 8 00:00:34,430 --> 00:00:36,830 A neutron doesn't have any charge. 9 00:00:36,830 --> 00:00:41,470 It doesn't mean that you have to have more electrons. The oxygen 18 and 10 00:00:41,470 --> 00:00:44,320 16 have the same number of electrons so it means they have the same chemistry. 11 00:00:44,320 --> 00:00:47,760 They are both oxygen, but they have different masses. 12 00:00:47,760 --> 00:00:53,880 Different processes in a hydrological cycle tend to separate 13 00:00:53,880 --> 00:00:55,350 these two isotopes. 14 00:00:55,350 --> 00:00:57,670 And so we can measure the relative abundances 15 00:00:57,670 --> 00:01:00,790 of the two isotopes either in calcium carbonate (CaCO3) 16 00:01:00,790 --> 00:01:03,740 on the seafloor, or in ice (H2O) to get 17 00:01:03,740 --> 00:01:07,090 clues about how the earth worked at earlier times. 18 00:01:08,900 --> 00:01:13,680 The planet acts like a giant still, 19 00:01:13,680 --> 00:01:18,880 distilling the light isotope, to form the great ice sheets. 20 00:01:18,880 --> 00:01:24,100 When water evaporates it tends to favor the light isotope because 21 00:01:24,100 --> 00:01:27,928 the lighter one can go into the vapor phase more easily. 22 00:01:27,928 --> 00:01:31,970 And then as that water vapor gets 23 00:01:33,300 --> 00:01:38,580 selectively rained out, the rainfall tends to choose for the heavy isotope. 24 00:01:38,580 --> 00:01:41,130 And so that means that the water vapor that's left 25 00:01:41,130 --> 00:01:43,980 in the atmosphere, as you go further and further north where 26 00:01:43,980 --> 00:01:48,370 it's colder and colder, tends to get lighter and lighter and lighter in its 27 00:01:48,370 --> 00:01:52,660 proportion of oxygen 16 to 18. 28 00:01:52,660 --> 00:01:57,940 Until finally, when it forms an ice sheet, the ice sheet is very light. 29 00:01:57,940 --> 00:02:03,270 By very light I mean something like it has 5% less 30 00:02:03,270 --> 00:02:09,270 oxygen 18 relative to 16 of what the ocean has, 31 00:02:09,270 --> 00:02:11,170 which doesn't sound like a huge change. 32 00:02:11,170 --> 00:02:15,230 But it actually is a very large change, very easy to measure. 33 00:02:15,230 --> 00:02:22,180 and the way geochemists describe this is to do it in "per mil" instead of percent. 34 00:02:22,180 --> 00:02:28,600 5% is the same as 50 per mil. And the fact that it's 35 00:02:28,600 --> 00:02:34,960 lighter means put a minus sign here. The ice sheets have an isotopic 36 00:02:34,960 --> 00:02:38,480 signature of minus 50 per mil relative to 37 00:02:38,480 --> 00:02:42,200 the ocean which starts out at zero per mil. 38 00:02:42,200 --> 00:02:48,270 And then when you take enough water out of the oceans to make this isotopically 39 00:02:48,270 --> 00:02:53,270 light ice sheet, the water that's left behind is a little bit enriched. 40 00:02:53,270 --> 00:02:56,470 It turns out that the ocean can be 41 00:02:56,470 --> 00:03:00,070 about one or two per mil heavier during a time 42 00:03:00,070 --> 00:03:04,890 when there's a lot of ice on the Earth. 43 00:03:06,390 --> 00:03:12,640 We can measure the isotopic composition of the ice just 44 00:03:12,640 --> 00:03:17,325 by putting it in a mass spectrometer and counting the different kinds of atoms. 45 00:03:17,325 --> 00:03:21,390 But we can also figure out what the isotopic composition of the ocean 46 00:03:21,390 --> 00:03:25,500 was in the past from sediment cores by looking at the oxygen in 47 00:03:25,500 --> 00:03:27,290 calcium carbonate. 48 00:03:27,290 --> 00:03:31,595 Here is an equilibrium reaction where we're changing this heavy 49 00:03:31,595 --> 00:03:36,740 oxygen for the two light oxygens in this calcium carbonate here. 50 00:03:36,740 --> 00:03:43,610 And it turns out that which chemical the oxygen 18 wants 51 00:03:43,610 --> 00:03:50,530 to be bound to depends on the temperature 52 00:03:50,530 --> 00:03:53,040 at which the calcium carbonate forms. 53 00:03:53,040 --> 00:03:58,272 Its actually a bit of a trick because, measuring the proportion of oxygen 18 to 16 54 00:03:58,272 --> 00:04:03,640 in calcium carbonate in sediments, there could be 55 00:04:03,640 --> 00:04:08,090 a component of the signal that comes from the global ice volume, that can 56 00:04:08,090 --> 00:04:11,740 tell us about the shrinking and growing of ice sheets through the ice age, 57 00:04:11,740 --> 00:04:16,540 But it can also be a function of the local temperature, because when 58 00:04:16,540 --> 00:04:21,410 you change the temperature it changes how the oxygen wants to be distributed. 59 00:04:21,410 --> 00:04:26,900 No paleo-proxy is ever used in isolation. 60 00:04:26,900 --> 00:04:29,669 There are other proxies that can be used to help 61 00:04:29,669 --> 00:04:33,858 constrain and pin down the local temperature, for example, and then 62 00:04:33,858 --> 00:04:36,627 from suite of many of these tracers we can begin 63 00:04:36,627 --> 00:04:39,660 to put together a picture of the world in the past.