Our aim is to investigate past climate and environmental changes in Greenland using mineral and sedimentary deposits founds in caves. In doing so, we are constructing the first cave-based climate records for Greenland and the wider High Arctic, as well as improving our understanding of climate change during times and in places that are beyond the limit of the ice cores.
Why is this important?
The Arctic region is expected to experience some of the greatest climate and environmental changes in the next centuries as a result of climate change, and the consequences of these changes will be experienced worldwide, for instance through rising sea-levels or changes to Northern Hemisphere weather systems. Improving understanding of how the Arctic will develop in a warmer world is therefore of paramount importance, and one way to achieve this is to look at periods of warmer climate in the recent geological past.
Understanding how the climate has changed in the past can be achieved by investigating a number of different geological archives, as well as by running sophisticated climate models on computers. In Greenland, deep ice cores drilled from the interior of the ice sheet have provided extremely high-quality data that has revolutionised our understanding of past climate change, both in the Arctic and further afield, particularly during our present warm climate period (known as the Holocene interglacial), as well as during the last ice age (glacial). Through our work in the caves of Greenland, our aim is to address some knowledge gaps, for instance warm climate periods further back in time than the ice cores can reach, as well as in places where the ice sheet is not present, such as arid coastal regions. To achieve this, we have undertaken three expeditions to Greenland, two to the northeast in 2015 and 2019, and one to the east in 2018.
Cave-based climate records
Calcite cave deposits such as stalagmites and stalactites are collectively known as speleothems, and these form from drip waters that have percolated from the surface, through soil and limestone, and into a cave. Since the drip waters were once connected with the atmosphere and soil above the cave, they contain valuable information related to temperature, moisture, and vegetation processes, which become locked layer upon layer within the cave deposit.
Palaeoclimatologists (people who study past climate change) are able to analyse the chemical signature that is contained within a speleothem, and from this they can construct a record of how the climate and environment has changed during the time that the speleothem was growing. The chemical element that is most commonly analysed for climate interpretation is oxygen, whilst the method most commonly used to find out when the speleothem was growing is called uranium-thorium dating.
Oxygen records in speleothems
Isotopes
In order to understand how the climate has changed in the past, we use the element oxygen. The nucleus of an oxygen atom contains 8 protons and either 8,9 or 10 neutrons, which, when added together, gives a mass of either 16,17, or 18. These oxygen atoms with different masses are forms of isotopes (but they can be found in other elements too).
In palaeoclimatology studies, we are interested in the relationship between oxygen-16 and oxygen-18 (there is so little oxygen-17 in the world that we don’t worry about this one). Because oxygen-18 is heavier than oxygen-16, it requires more energy to evaporate water (H2O) that contains an oxygen-18 atom than it does one that contains an oxygen-16 atom. During times of warmer climate, precipitation therefore contains a higher proportion of oxygen-18 relative to oxygen-16, and since speleothems ultimately form from rainwater, the isotopic signature of the rainfall gets locked within the calcite layers of the speleothem.
For analysis of oxygen isotopes, the Innsbruck Quaternary Research Group typically takes between 4-10 samples per millimetre, enabling the greatest chance of capturing rapid climate change events. Understanding how fast the climate is capable of changing from one state to another is currently one of the key questions that climate-change scientists are working to answer.
For a really fun explanation that uses cats to show how oxygen isotopes are used in palaeoclimate studies, see here.
Uranium-thorium dating of speleothems
In speleothem-palaeoclimatology, it is important to establish when the speleothem was growing so that the climate record can be placed in a wider context. To do this, a method called uranium-thorium dating is typically used, and this can be applied to speleothems that grew over the last 650,000 years. Uranium is highly soluble, consequently the water that the speleothem forms from will contain some very small quantity that is locked into the speleothem at the time of formation. Uranium is radioactive however, and will decay at a known rate with time and turn into thorium, which was not present in the speleothem at the time of formation. The ratio of thorium to uranium can be analysed to very high accuracy and precision, enabling the date of formation to be calculated.