Video 1.7: Using stable isotopes to understand Earth processes - Dr. Ray Burgess

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Our Earth: Its Climate, History, and Processes

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University of Manchester

Our Earth: Its Climate, History, and Processes

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Develop a greater appreciation for how the air, water, land, and life formed and have interacted over the last 4.5 billion years.

À partir de la leçon

Building Blocks of Earth’s Climate System

Video 1.0: Introduction and Philosophy7:45

Video 1.1: How does science work?6:21

Optional Video: How do scientific papers get published?10:50

Video 1.2: Introduction to the Earth's climate system8:49

Video 1.3: How do we measure geologic time?9:50

Video 1.4: Geological Time Scale Song2:50

Video 1.5: Minerals and Rocks2:59

Video 1.5.1: Igneous Rock3:03

Video 1.5.2: Sedimentary Rock6:15

Video 1.5.3: Metamorphic Rock5:39

Video 1.6: Using radioactivity to date rocks - Dr. Ray Burgess7:48

Video 1.7: Using stable isotopes to understand Earth processes - Dr. Ray Burgess6:15

Video 1.8: How do we know how old the Earth is?5:46

Video 1.9: What are those rocks doing lying around?10:48

Rencontrer les enseignants

Prof. David M. Schultz
Professor of Synoptic Meteorology
School of Earth, Atmospheric and Environmental Sciences
Dr Rochelle Taylor
Postdoctoral Research Associate
School of Earth, Atmospheric & Environmental Sciences
Dr Jonathan Fairman
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences

So, I'm with Professor Ray Burgess, who's professor of isotope geochemistry here

at the University of Manchester, and I wanted to find out about isotopic ratios,

and how we can use the ratios of different isotopes within a substance

to understand physical processes within the Earth's system.

So first of all, what are these isotopic ratios, and how do we use them?

>> Okay, so some isotopes are what we call stable isotopes.

They don't undergo radioactive decay.

And the variations in the isotope ratios that we measure for these

group of isotopes result from chemical and physical processes, and

the reason why we get isotope variations is related to the mass of the isotopes.

So these are what we call mass dependent isotope variations.

So if you think of an example, the hydrogen isotopes.

There are two hydrogen isotopes, hydrogen-1 and

then hydrogen-2, which has a special name, deuterium.

So there's hydrogen-1 and hydrogen-2, and

the relative mass difference between them is 100%.

So the magnitude of the isotope variations that we measure is related

to the mass difference.

So the hydrogen isotopes that we measure are variations, naturally occurring,

which are probably larger than any other element.

>> So when I look at the literature, I see people write about and say, so

much percent of the hydrogen atoms in the universe are composed of hydrogen-1.

So much percent are composed of hydrogen-2, but seems to me what you're

saying is that these ratios, I mean that's a kind of a global ratio, but

within individual substances those ratios may differ from the average.

>> That's right and the reason for

that is that they will change in response to chemical reactions.

Particularly, biochemical processes will fractionate.

Changing the ratio we call fractionation.

They will fractionate the isotopes.

Reactions that go on in cells in our body will tend to concentrate

the low mass isotope into the products of any biochemical reactions, and

that's one of the major ways of fractionating isotopes on Earth.

But you can also fractionate isotopes during physical processes as well, so for

example during the evaporation and condensation of water.

The oxygen and hydrogen isotopes in the water will become fractionated.

And also, probably most important,

is that all isotope fractionations that occur like this are temperature dependent.

And curiously, the magnitude of that change,

that fractionation, is larger as the temperature decreases.

And this makes isotopes very good thermometers.

>> Hm. So one way that I've seen this displayed

in the literature is taking an ice core, say, and

then there's a graph with depth of the relative ratios,

of I think it's the oxygen-18.

>> Yeah. >> And then so the wiggles in this curve

from either high oxygen-18 concentrations to low oxygen-18 concentrations

indicate something about the temperature of that ice core or the environment?

>> Yes, that's right.

So probably the most important application of stable isotopes in geology has

been in the area of climate science, particularly working out past climates.

So they can apply to a whole range of climate archives,

similar to what you suggested.

Ice cores, but also ocean sediment cores, cave deposits, lake deposits.

Well, a whole range of things, really.

And this is probably the most important way of interrogating

the earth's past climate using stable isotopes over

thousand-year to billion-year time scales.

>> And so by looking at these records then, we can see these as you said,

the relative highs, relative lows.

And then these relate to warm,

cold periods in the Earth's climate.

And if we look at the biological things,

we can determine the presence of active or inactive biological periods as well?

>> Yes, that's right.

And we can also establish the links between what's happening in

the terms of how much biological activity there is

at any point in the past, with the amount of inorganic activities going on and

there's a very fine balance between the two.

So when you start increasing the levels of biological activity,

it has affect on the sort of inorganic process, for example,

that go on in the ocean or in the earth's atmosphere.

So you can interrogate these records, you can work out the relative

balance by looking at the isotope variations through time.

>> What's an example of the way that you've used these stable isotopes

in your research?

>> So, an example here is on the screen just behind me here where

we've been looking at some climate records,

which have been contained in fossil hydrothermal systems.

So these are essentially hot water that's been

circulating around hot igneous rocks in the geological past.

>> In the mid ocean ridges or, >> No, these are in Antarctica.

So here we're looking at Antarctic climate conditions

between about 20 million years ago and about 50 million years ago.

Which is a critical time, because it's the time when we went from being fully

non-glaciated, to conditions where we now have polar ice caps.

>> Excellent, so thank you so much for your time and good luck.

>> Thank you.

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