Mike Hall: how to determine climate history from cores of deep sea sediment

Senior technician and laboratory manager Mike Hall describes work pioneered with Nick Shackleton on the use of mass spectrometers to determine climate change from deep sea sediment cores, containing shells of foraminifera.

My name’s Mike Hall, and I am the manager of the Godwin Laboratory of Paleoclimate Research, Department of Earth Sciences, Cambridge University. I started work in this department in 1963, with working in the radio carbon dating laboratory originally. And then in 1969 I moved over to work with Nichola Shackleton, after he’s got his PhD, as his research assistant. And I’ve been doing work with – in this field of climatology since 1969 up until the present time. And I worked with Nick from 2000 – until 2006 when he died. When I started here there was just two of us working in an old Nissen hut near to the Cambridge [railway] station, and with one very old mass spectrometer. And when I joined him in ’69, we managed to improve that mass spectrometer so it was able to analyse smaller samples, and analyse them more quickly, and more precisely. And then subsequently we developed other equipment so we were able to analyse much smaller samples as well. And basically that method that we produced in the 1970s and ‘80s is still used today in most cases for doing this particular sort of analysis. We managed to get back in time for thousands and millions of years, looking at past climate change. And a lot of Nick’s work has been related to the Milankovitch theory, with the Earth’s orbit round the sun we’ve been able to prove the Milankovitch theory. We looked a mapping of the oceans at the last glacial, and Nick continued doing lots of work with regard to that, right up to his death. These are cores which we’re going to analyse. And what happens is when the boat goes out it drills into the ocean bed. So as it drills into the ocean bed it brings up a very long core, often something like ten metres long. But it’s difficult to handle a ten metre long core, so this is cut into sections of one and a half metres. And then on the boat they are split in half. So you’ll see that it came out as a cylinder, but what you’ll be seeing is like a piece of guttering with the sample laying in the gutter, and that is half the core. You’ll be able to see some sort of stratification of where the sediment has been settling over the years on the core. It’s a general sediment that’s been in the oceans and it will contain both planktic and benthic foraminifera, which is the thing that we want to look at in order to get the isotope ratios out of this, in order to look at climate change. While the shells are living, then they are taking in this chemical composition, and then they die after they’re three or four weeks and drop to the bottom of the ocean, so they build up a sediment. So by drilling into the core you’re going back in time. So as you go back in time, you can then go down a core and take samples from it and then you get a relationship between time and past climate change. Once we have some good relationship between the past climate then it’s hopeful that by knowing what happened in the past, we’ll be able to predict what’s going to happen in the future. There are three main parts to the mass spectrometer. There’s the source, the flight tube and the magnet, and the collector arrangement. And you can see that they go round in an arc. And so your CO2 gas, which we’re going to produce from our samples, is put into the mass spectrometer in what’s called the source. And here the gas is ionised. And this is done by heating a filament and a beam of electrons goes in a sort of spiral across a box and it ionises or knocks off electrons from various molecules that are inside the box. These molecules are then accelerated with a potential difference, or a voltage, of about 4,500 [4 and a half thousand] volts, so that they will be drawn out of the source by this potential difference and into the flight tube of the mass spectrometer. They go through the magnetic field. Without the magnetic field they would continue in a straight line, but with the magnetic field it will bend them round depending on the mass of the molecule or mass of the ion. So the heavier masses will be bent less than the lighter ones. So you would get a spectrum of mass numbers. And so we need to pick on three of these, mass numbers forty-four, forty-five and forty-six to look at the oxygen and carbon isotopes, and these are collected by the collector arrangement which are three gold buckets. And these gold buckets actually measure the instant number of molecules on them, and they turn this into a current. So you’re looking at very small currents which you are comparing, depending on the number of instant molecules from each particular mass number. If we look as mass numbers forty-four and forty-six, these are the two isotope ratios – isotope numbers O16 and O18, and we look at the ratio between these two, which give us an indication of the temperature of the sea water where this animal was living when it died. Forty-four and forty-five look at the carbon ratio between C12 and C13, and this bears a relationship to the atmospheric CO2 while the animal was living. Samples can be taken down the core, and we can go back in time as we go down the core, and so we’ll be able to plot out the values that we’re getting from the machine, the mass spectrometer, the isotope as we’re getting, against time. So as we go back you’ll be able to see how the climate has changed. We can go back to millions of years looking at past climate change. And the printout comes out at the end of a run of each particular sample and this will give us an idea of the difference, ten differences, between a reference and the sample. So we have a list of differences that are taken by the machine automatically, and it automatically takes a mean value of these differences over the run of ten differences, and prints us out that value and a standard deviation so we can see that the measurements are good. This value that we’ve got then has to be calibrated by using our standard samples which we’ve put in throughout the run, to the international standard VPDB so that this bit can be compared with other people’s results, and also so we can compare our results from day to day. So as we go down a core, using samples consecutively down the core, we can see how the climate may have changed.

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