Bel Antar 2018

by Jean-Louis Tison.

To answer this question, you first need to know the length (and therefore the time span) of the ice core that you wish to extract from the ice sheet.

If you are only interested by a short core a few to 10 meters long, for example to study the properties of the snow and firn (transition from snow to ice) you can use a simple and lightweighted system, entrained manually or with a small motor at the top.

At the other extreme, if you want to recover 3000 meters of ice, all the way to the bottom of the ice sheet, covering up to 800.000 years, you need to gather all your friends internationally (and a lot of money!) for an international drilling program that might last more than 5 years (summers in the field).

In that case, the drill is much more complex and send down the hole with the motor on top of the drill barrel, which is made of an upper container (tube) to collect the chips from the cutting of the ice, and a lower one to collect the core. At the bottom of the latter is the drilling head, equipped with 2-3 knives, that digs an annular hole, isolating the core from the ice sheet.

The cutting chips are pushed up an “archimede’s screw” on the outside of the inner tube, where the core is collected. The chips are contained by an outer tube and are therefore driven up, to end up in the upper container. When up to 5 meters of core are drilled, the tubes are full, and the drill is brought back to the surface, the chips emptied from the upper container, and the ice core recovered from the inner tube.

Everything is controlled from the surface by a computer connected to the drill motor by way of electrical cables within the hauling metallic cable. The drilling is performed within a drilling fluid slightly denser than the ice (once we are in the ice, ca. 100 m depth), in order to prevent the hole from closing down from one year to the other (the ice deforms under its own weight and closes the hole!). It also prevents mechanical damage of the core by the drill, when the pressure of the ice around gets too big compared to the atmospheric pressure in the hole.

For intermediate depths, like at our Mass2Ant locations (300 m core, several centuries), we use a light-weighted version of the deep drilling system: the Eclipse drill (Canadian made). Because the quality of the cores was degrading under 100m depth last year, we will use a “wet drilling” version of the system, for the first time this year! Cross fingers and wait for the pictures of this year!

In the meantime, some pictures of last year! (photos: T.J. Young and Emmanuel Potvin)

The drill head with 3 knives, in rotation

Extracting the inner barrel (containing the core) from the outer barrel. In this light-weighted version, there is no dedicated chips chamber. The chips reaching the top of the spiral fall on top of the core in the inner barrel.

The drill trench and operators (Emmanuel and Etienne)

A good core

…and a bad (broken in) core!

 by Jean-Louis Tison

One of the most crucial information that we can extract from an ice core is the temperature that existed at the time the snow deposited at the surface of the ice sheet. We will make an attempt today to better understand the principle of this “thermometer of the Past” (Paleo-thermometer).

A little bit of basic chemistry (good old school days!): Ice is solid water (H₂O). It is made of an assemblage of atoms of Oxygen and Hydrogen. Atoms are characterized by a core, made of protons and neutrons, and electrons spinning around it. The number of protons (atomic number) in the core define the “name” of the atom: Oxygen has 8 protons in its core and Hydrogen only 1 proton in its core. However, there exist several varieties of atoms of Oxygen and Hydrogen, differing by the number of neutrons in their core. Less neutrons in the core will make the atom lighter, more will make it heavier. We call these various atoms of oxygen or Hydrogen “isotopes”.

To make a long story short, let’s talk about Oxygen only. A similar story could be told for Hydrogen in the water molecule.

A molecule of water containing lighter isotopes of Oxygen will be lighter than a one containing heavier oxygen isotopes. Nature is such a nice friend to scientists that there is a simple (linear) relationship between the temperature at which the snow accumulates at the surface of the ice sheet and the proportion of light and heavy water molecules in the snow: the colder the temperature, the higher the amount of light water molecules that will be present in the snow and vice-versa. This is illustrated in Figure 1 for the Antarctic and Greenland. Scientists quantify the proportion of light and heavy isotopes in water molecules by the symbol d¹⁸O (for Oxygen, where the most abundant heavier version is ¹⁸O, as compared to the dominant lighter isotope ¹⁶O). When d¹⁸O is negative, the snow sample contains less heavy oxygen isotope than the “reference”, which is standard ocean water (the most abundant at the surface of the Earth) and vice versa. So, in brief, the lower the d¹⁸O of the snow, the lower the temperature when it fell at the surface of the ice sheet, and the relationship is linear (see Figure 1). The same will be true for d²H (also written dD, D is for Deuterium, the heavy form of Hydrogen, with 2 neutrons in its core).

Figure 1 : the linear relationship between snow d¹⁸O (or dD) and the air temperature when it falls at the surface of the ice sheet: the principle of the paleo-thermometer.

For each successive layer in our ice core, we can then measure the proportion of light and heavy oxygen isotopes (d¹⁸O) or Hydrogen isotopes (dD) in the snow (that became ice) to reconstruct the temperature that existed at the time that layer of snow was at the surface of the ice sheet: this is the principle of the paleothermometre.

The beauty of the technique is that it works at all timescales: we will for example clearly see the difference between a glacial period (cold period with very negative d¹⁸O or dD) and an interglacial period (warm period with less negative d¹⁸O or dD) on long time scales. Not only that …we will also be able to discriminate summer (warmer) from winter (colder) each year. This is crucial, since we will then be able to “count the years”, and therefore date the core from those seasonal fluctuations of the water stable isotopes signals (d¹⁸O or dD), as shown in the example of Figure 2.

By measuring the thickness of ice that exists between two peaks (or two troughs) of the seasonal d¹⁸O or dD signal we will be able to reconstruct the annual snow fall at the surface of the ice sheet, one of the major goals of the Mass2Ant project, as you already know by now!…

Figure 2 : An example of d¹⁸O (black curve) and dD (light blue curve) profiles between 20 et 30 m in the Derwael Ice Rise ice core, in the vicinity of the ice coring locations of Mass2Ant (Philippe et al., 2015). This depth interval covers 10-11 years of accumulation, as shown by the seasonal fluctuations of the water stable isotopes (d¹⁸O.et dD)….”Rendez-vous” in one of our next blogs to understand what the other profiles mean !

 by Jean-Louis Tison

Ice cores are the memory of our Climate. They are not the only one, though!… You can read the climate of the past in ocean sediments, corals, tree rings, stalactites in caves, lake deposits, peat deposits and many others. Not all of these archives give you the same amount of information though. Also, some are easily dated (a must for an archive, of course!), other less easily.

Ice cores are extremely wealthy in information, as you will discover here and in the following blogs. They can give you a very detailed record of the Climate and the Environment on several ten thousands of years. However, they have only gone back in time for less than a million year until now, because snow that turns into ice as it is slowly buried in the ice sheet, deforms under its own weight, and flows both downwards to the bedrock and outwards towards the ocean. It thins up so much under these processes, that no ice is left beyond that age limit! The oldest ice dated today in an ice core goes back 800.000 years in time. However, a new project funded by the European Union aims at finding ice older than 1.5 million years old in Antarctica, and this new drilling project will be the next challenge of the coming decades. Other countries are also involved in this challenging task: Russia, Australia, Japan…

In our Mass2Ant project, as you already know, we will be more focused on the last centuries, to document the transition of the Climate into this new geological period that some of us refer to the Anthropocene: a period where the Climate increasingly feels the influence of Mankind (mostly since the Industrial Revolution).

A climate archive, such as ice cores, always works the same way: you measure a variable/property in the medium (ice for example), what we call a “proxy”, to pull out information on a climate or environmental variable (Temperature, precipitations, winds, air masses circulation, greenhouse gases content of the atmosphere a.s.o..). The Figure here below lists some of these information that you can retrieve from an ice core

Stay with us, and you will learn more on the most important ones as we drill deeper into the ice!…