The talk was focusing on the dating technique called luminescence dating and how it is being used in Patagonia, in combination with other methods, to date the retreat of ice after the last glacial maximum. Although this talk did discuss the use of boulders and grains for these dating methods, the geology component is very brief, focusing instead on how the techniques were applied. Dating the ice retreat in Patagonia is important as it will help scientists understand how the remaining ice there will respond to climate and precipitation shifts occurring there today.
Blog Theme: reconstructing the past climate and ice retreat in Patagonia to understand current glacier behaviour due to climate changes
Ongoing Blog Theme: climate change on Earth: past and present – the things that have and will change and what we can do about it
What was the talk about?
The main aim of Rachel’s research was to reconstruct the past climate in Patagonia and to reconstruct the temperatures in Patagonia around the time of the Last glacial maximum (LGM). From the results she is piecing together, Rachel wants to understand the effect that temperature and precipitation will have on modern day glaciers in Patagonia (see Figure 1) to predict and understand ice retreat due to climate shifts.
Why Patagonia?
Patagonia is a natural lab to study past climate and because of major ice retreat there is lots of exposed rock for dating techniques to be applied. Rachel wants to get more robust empirical data for temperature and precipitation to fill in the blanks on the map of the world as the mid-latitudes are all blanks, especially in Patagonia.
There is ice at the low latitudes today due to high precipitation rates due to the South Westerly winds. However, there has been dramatic ice retreat and widespread drought as precipitation has been reduced by 50% (Figures 2 and 3). Poleward shifts in precipitation have occurred – and the South Westerlies are predicted to shift 1-2˚ in the coming century. This shift in the winds will redistribute the precipitation and storms in Patagonia, drastically affecting the ice there.
Rachel wants to reconstruct the past ice extents in Patagonia to reconstruct the climate and ice retreat that occurred due to precipitation changes, something that parallels with the changes taking place today. Reconstructing the past ice retreat data in Patagonia can provide information about the influence of glacier behaviour due to changes in temperature and precipitation related to the Southern Westerlies. All of this links in with studying the past to understand the future changes that will take place in Patagonia.
Methods
The main methods used were:
· Luminescence dating
· Cosmogenic nuclide dating
Luminescence dating
Luminescence dating was one of the main methods that Rachel utilised for her project (see Figure 4).
Luminescence dating is a technique that takes sand and silt grains to understand the burial dating of rocks (see Information Box 1). Simply put, a grain of sand is like a battery – over its life in the world it is exposed to the sun and gradually, due to this exposure, it loses its charge. Eventually, the grain will be deposited and buried, and the charge will once again start to build up due to radiation exposure while buried. A date is calculated from this grain by determining the charge of the grain “battery” and working out how much radiation the grain was exposed to per year. The radiation that a grain can be exposed to varies: it could be any of U, Th, K or Rb (Uranium, Thorium, Potassium and Rubidium, respectfully).
The grains/rocks are collected by Rachel and her team in the field in a very careful way so as to not expose the rock to any sun, as this could affect the charge of the “battery” and impede the dating technique. The natural light data is then calibrated against laboratory doses of light to calibrate the right dose to work out the exposure and charge. This is done using the formula:
The brightest grains (the most charged) are the oldest grains. Aeolian settings (like Patagonia) are the best places to sample grains for luminescence dating as grains tend to be very bleached and almost completely reset before burial (they glow very bright) due to lots of sun exposure (see Information Box 2).
The age for the grains is based on the average of the minimum dose of light exposure that it takes to recharge a grain. This is called single grain dating.
Cosmogenic nuclide dating
Cosmogenic nuclide dating dates the bedrock surface and any boulders. It is the opposite to luminescence dating as it focuses on the exposure age of a sediment (see Information Box 3).
Once a boulder is exposed to the atmosphere (i.e. after it is dropped by a glacier) the boulder will start to accumulate cosmogenic nuclides (see Figure 5). As long as the boulder remains in the same position after deposition, it will give an exposure age for the maximum extent of the ice sheet – this is the length of time that the rock has been exposed for at the Earth’s surface.
Results
The talk discussed two sites in Patagonia that Rachel had taken samples from:
1. Lago Buenos Aires (published)
2. NE Patagonia (not yet published)
Lago Buenos Aires
The site was to the East of the Andes and receives a precipitation gradient coming down from the mountains. It is located east of the contemporary Northern Patagonian Icefield (-46˚S). The site has existing cosmogenic dates from the glacial outwash plain – Rachel’s work here wanted to tie together the cosmogenic and luminescence data. She focused on dating the single-grain feldspar for the luminescence data using the well-preserved glacial outwash plains and took a total of 13 sedimentary samples for dating in the lab.
Cosmogenic nuclide dates were recalculated by Rachel’s team and found that the ages from these dates put the age ranges from 108 ± 6 ka to 209 ± 11 ka, with weighted mean and standard deviations of 148 ± 33 ka and 154 ± 35 ka (Smedley et al, 2016).
Rachel compared the ages using the dating methods and found that the luminescence dates new luminescence ages indicate that major outwash accumulations formed around 110 ± 20 ka to 140 ± 20 ka, corresponding with the cosmogenic dates. This meant that the last glacial advances of the Patagonia ice sheet occurred during this time, and the sediments she tested in the outwash plain were deposited then.
If you would like to read Rachel’s paper, follow this link: https://livrepository.liverpool.ac.uk/3016722/1/Smedley%20et%20al.%202016%20%28QSR%29%20-%20Patagonia.pdf
NE Patagonia
This area of Patagonia is located in the northern threshold of where the changing precipitation levels due to the movement of the south Westerlies will have drastic effects.
Rachel’s team used luminescence and nucleotide dating to try and reconstruct the former ice limits in the area. In the area she sampled, Rachel found lots of cobbles and sand, and so sampled cobbles (something that hadn’t been done before) to try and get luminescence dates from them.
Rachel found from her cobble dating that the maximum extent of the Patagonia ice sheet occurred about 27 ka years BP, with deglaciation starting about 17 ka years BP.
While in NE Patagonia, Rachel’s team also studied the palaeoclimate by taking peat cores. The peat cores told a story of ice cover during glacial cycles followed by a change to deglaciation with a cold spell before returning to record warming temperatures in the Holocene. The peat also recorded lots of tephra layers and contained lots of pollen. Further work with this data could provide accurate dating using tephrochronology (see Information Box 4) and palynology (see Information Box 5) for a record of changes in the vegetation and climate. This data, if combined with the luminescence and cosmogenic dates could potentially constrain the dates for ice extent and deglaciation in Patagonia.
Suitability of the techniques
Luminescence dating
Luminescence dating usually requires sand grains, yet sand is not always preserved at a site due to weather conditions and climate. At the sites in NE Patagonia, there was little sand but lots of cobbles. Rachel collected cobbles to date with the luminescence method and considered each pebble to be the equivalent of a large grain of sand.
Luminescence dating is limited mainly by the quality of the samples – if sampling in the field is not carried out according to the correct methodology (i.e. the sample is exposed to light) then the sample will be compromised and the date perturbed (see Figure 6).
As long as Rachel’s team carried out good sampling practice in the field and lab, their use of the technique and their subsequent dates will be reliable.
Cosmogenic Nuclide Dating
Cosmogenic nuclide dating, like luminescence dating, requires a good sampling practice and lab methodology to ensure the dates are suitable and correct. Several factors can affect cosmogenic dating:
· Rock type
· Oscillation/reduction of cosmic rays
· Topographic shielding
· Post-depositional movement
· Burial by snow, earth or vegetation
Rocks chosen for sampling need a high percentage of quartz grains i.e. granite or sandstone boulders are a common choice. Cosmic rays can only penetrate the first few cms of the rock’s surface and so sampling requires the use of chisels and saws in the field to remove blocks, which can be time consuming. If a boulder has tumbled or been moved from its original depositional position, the surface that is sampled might not have been exposed to cosmic rays, and so will yield null results. Before sampling, it must be certain that a rock has been in its position for a long period of time and that there is no snow or vegetation cover on it that could block cosmic rays. A large mountain nearby an exposed boulder, something Rachel’s team will have had to encounter, will block or reduce the cosmic rays reaching a rock. Researchers must account for these variations when calculating the cosmogenic dates (Antarctic Glaciers, 2014). If these considerations are taken into account, then the technique can be very reliable.
Data Set
For the results from Rachel’s studies to have more validity she needs to incorporate a larger data set with many more samples for dating. She will also need to visit more field sites so that she can eventually fill in the gaps in the ice sheet data from Patagonia.
How has the talk advanced our knowledge?
The talk was important as it highlighted how important it is to study the past to understand the changes that could take place in the future. Sometimes in can be unclear why scientists study certain and specific things (like the climate in Patagonia) in the past and hopefully it has become clear that this is being done for the good of the planet. Since the retreat of the ice sheets in Patagonia was influenced by fluctuating levels of precipitation in the Quaternary and that the South Westerly winds bringing precipitation to Patagonia are set to move, understanding how ice behaviour changed then can help us predict how it will be affected in the coming century.
All of this information may one day allow us to understand how the Earth responds to long term climate changes, something incredibly relevant, so that we can prepare and plan ahead for future consequences of climate change i.e. sea level rise etc.
Thanks for reading!
If you’d like to read more about Rachel’s work please follow this link to her research papers: https://www.liverpool.ac.uk/environmental-sciences/staff/rachel-smedley/research/ and here: https://www.liverpool.ac.uk/environmental-sciences/staff/rachel-smedley/publications/
My next blog will be focusing more on current changes that humans and the climate are having on the freshwater availability on Earth, something that is vital for life.
References
Antarctic Glaciers. (2014). Cosmogenic Nuclide Dating. Available at: http://www.antarcticglaciers.org/glacial-geology/dating-glacial-sediments-2/cosmogenic_nuclide_datin/ [Accessed: 28/03/2019]
Bladon C. (2018). The life of a glacier in the Patagonian ice fields. Available at: https://www.pura-aventura.com/pothole/the-life-of-a-glacier-in-the-patagonian-ice-fields [Accessed: 30/03/2019]
Cambridge Dictionary. (2019). Aeolian. Available at: https://dictionary.cambridge.org/dictionary/english/aeolian [Accessed: 28/03/2019]
Dugmore A.J, Newton A. (2009). Tephrochronology. Available at: https://link.springer.com/referenceworkentry/10.1007%2F978-1-4020-4411-3_218 [Accessed: 29/03/2019]
English Heritage. (2008). Luminescence Dating. Available at: https://www.aber.ac.uk/en/media/departmental/dges/pdf/english_heritage_luminescence_dating.pdf [Accessed: 29/03/2019]
Florida Museum. (2018). Palynology. Available at: https://www.floridamuseum.ufl.edu/paleobotany/palynology/ [Accessed: 29/03/2019]
Flueck, W.T, Smith-Flueck, J.A.M. (2011). Recent advances in the nutritional ecology of the Patagonian huemul: implications for recovery. Animal Production Science, 51, 311-326.
Koch, J. (1999). Available at: http://facultyweb.kpu.ca/~jkoch/older_stuff/fieldtrips/Patagonia/pataveget/pataveget.htm [Accessed: 28/03/2019]
Smedley, R.K, Glasser, N.F, Duller, G.A.T. (2016). Luminescence dating of glacial advances at Lago Buenos Aires, Patagonia. Quaternary Science Reviews, 134, 59-73.
ThoughtCo. (2018). Luminescence Dating. Available at: https://www.thoughtco.com/luminescence-dating-cosmic-method-171538 [Accessed: 28/03/2019]
UofD. (2019). Luminescence Dating Research Lab (D136). Available at: https://www.dur.ac.uk/archaeology/facilities_services/laboratories/136/ [Accessed: 28/03/2019].
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