Post from a Scientist: The Climate Crystal Ball

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Have you ever looked at a landscape and thought to yourself… How did this happen?! … Why is this hill here? …  Why do these rocks look like this? … What does it all mean?

My job is to look for clues in the landscape that indicate the former presence of glaciers. Glaciers are masses of ice that flow downhill from mountain tops, where it is cold enough for snow to remain on the ground all year round. Rocks fall on the surface of glaciers and are transported to the glacier margins where they accumulate in elongated, sharp-crested ridges (as in the photo). These ridges are called moraines, and they can remain in the landscape long after the ice melts away. When I find moraines in a location where no glaciers currently exist, I ask myself these questions: (1) Why did a glacier used to be here? (2) When was it here? (3) Why is it no longer here?

I am here at a glaciology summer school in Alaska, USA, to learn about what causes glaciers to grow and shrink. Temperature is a key control as it determines how much precipitation falls as snow rather than rain in winter, as well as how much snow and ice melts in summer. When temperatures remain stable for a many years, glaciers eventually reach an equilibrium state, where input of snow equals output by melt, and the glacier length remains stable. If temperatures increase, then less snow may fall and/or more melt may occur, causing the glacier to retreat. If temperatures decrease, then more snow may fall and/or less melt may occur, causing the glacier to advance. As moraines represent former glacier lengths, they contain important information about past climate change.

My research focuses on two volcanoes in central North Island, New Zealand – a long, long way from Alaska! I have multiple moraines in several catchments on these volcanoes, which document former glacier length changes. I have dated these moraines using a technique known as “cosmogenic surface exposure dating.” This technique measures the accumulation of rare elements, produced by exposure to high-energy particles that come from outside our solar system and reach the Earth surface. The moraines on these volcanoes range in age from 200 to 60,000 years old. Now I am using a computer model that simulates the growth of glaciers under different climatic conditions. I input different temperatures to try to recreate the former glacier lengths, as indicated by the moraines I have found. Putting the moraine ages and model results together, I end up with estimates of how much colder it must have been in New Zealand at a certain point in time.

I know what you’re thinking … you’re thinking “who cares how cold it was thousands of years ago?” Well, that’s a great question. These results are important for several reasons. For example, they provide an important test for computer models that try to predict future climate change. If these models can recreate past changes, then we have increased confidence in their predictions for the future. You can think of this as cleaning the climatic crystal ball, making it easier to look into the future and see what Earth’s climate may be like for your children and grandchildren.

- Shaun, Victoria University of Wellington, New Zealand

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Here I am on a moraine at the side of a former glacial valley that drains away from the active volcanic cone of Mt. Ngauruhoe in New Zealand.

 

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2 Responses to Post from a Scientist: The Climate Crystal Ball

  1. Mary W says:

    Hi, Shaun. Your research is very interesting. I’d never heard of cosmogenic dating. Are these particles from space cosmic rays (high energy protons) or something else? Are they forming isotopes of elements when they collide with atoms on earth? Does cosmogenic dating work something like carbon dating, where you compare the expected ratio to the measured ratio of stable carbon-14 atoms to unstable carbon-16 atoms in an organic sample and then estimate the age using the half-life of the unstable isotope?

    • Shaun says:

      Hi Mary, thanks for your questions.
      You are correct, the high energy particles are predominantly protons (about 85%) and alpha-particles (about 15%), originating mostly from within the Milky Way. Those with sufficient energy to overcome Earth’s geomagnetic field collide with atomic nuclei in the Earth’s atmosphere creating secondary and tertiary products, including neutrons and muons. You are again correct in that these high-energy spallation reactions create isotopes (cosmogenic nuclides), both in the atmosphere and terrestrial materials. In fact, this mechanism is the primary source of carbon-14 in the atmosphere – together with that produced by nuclear weapons testing in the mid 20th century!

      There are many terrestrial cosmogenic nuclides and each has unique properties that make it useful for different scientific applications. 10-Beryllium is the most commonly used as it is produced and retained in quartz, which is a very common mineral on Earth. For surface exposure dating applications, we simply measure the concentration of cosmogenic nuclides atoms in a sample and use that as a proxy for time. For example, cosmogenic nuclide concentrations in moraine boulders represent the length of time since the boulder was deposited by a glacier (i.e. when the ice was last at that location). In order to convert the measured concentration of a cosmogenic nuclide to time units (e.g. calendar years), we also need to know the rate at which the nuclide is being produced and its decay rate (if any). Production rates vary across the globe according to atmospheric depth (altitude) and position relative to the geomagnetic field, as well as varying in time according to changes in solar activity and geomagnetic field strength. These processes can be modelled, to a point, but most useful is to ‘calibrate’ production rate estimates by measuring cosmogenic nuclide concentrations in samples with a ‘known’ exposure history. For example, a common approach is to use rock avalanche deposits. These can often be dated using radiocarbon (if the rock avalanche event takes out some trees, for example) and cosmogenic nuclide concentrations can be measured in rocks exposed by the same event. Using this approach, we can derive a time-integrated production rate for that specific location, which we can then apply to nearby samples of unknown exposure duration.

      For my own research, I am using the cosmogenic nuclide 3-Helium. This is mainly because I am working with igneous rocks that contain very little quartz. 3-Helium is produced and retained in pyroxene minerals, which are abundant in the local andesites. 3-Helium has some useful properties that make it an attractive tool for surface exposure dating. For example, it has one of the highest production rates (approx. 120 atoms of 3-Helium are produced in every gram of pyroxene, per year – compared to just 4 atoms/gram/yr for 10-Beryllium) – this means we can accurately measure 3-Helium concentrations in samples with very short exposure histories. It is also stable (does not decay with time), therefore can be used to date surfaces of infinitely long exposure – in Antarctica, measurements of 3-Helium in some rocks has shown they have been exposed for millions of years!

      I hope this is helpful.
      Kind regards, Shaun

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