Stalactites and stalagmites (also called speleothems) are found in cave environments across the planet (see Figure 1). They form as water from the surface percolates into the ground and drips into the cave. Speleothems can be extremely sensitive to changes in climate and environmental conditions and they record these changes as variations in their chemistry. An excellent review of speleothem formation and their use for palaeoclimate reconstruction by Prof. Ian Fairchild can be found here, so this article focuses on the way that oxygen is stored in speleothems and how we can use this as a record of past climate conditions.
Why and how do we use oxygen isotopes?
Oxygen levels in the environment are closely linked to temperature, rainfall amount, and atmospheric circulation. When speleothems form, oxygen is trapped in their crystal structure. If we analyse the levels of oxygen in the speleothem, they can tell us about past environmental conditions. In fact, oxygen isotopes are so useful, that they are the most commonly analysed chemical in speleothems.
What is an isotope of oxygen?
Oxygen has two main isotopes: 16O which has an equal number of protons (8) and neutrons (8) in its nucleus; 18O has two more neutrons (10) than it does protons (8). This difference in the number of neutrons causes a change in the atomic mass, so that 18O is isotopically heavier than 16O. When we analyse the oxygen in speleothem carbonate we consider the ratio of the light and heavy isotopes (18O/ 16O). We report this ratio as a change in parts per thousand (parts per mil – ‰), and we use the symbol delta ‘δ’ to display this, for example, δ18O).
What can isotopes in speleothems tell us about climate?
This depends upon where in the world the speleothem was growing. All speleothems are formed from cave drip waters. These waters are sourced from rainfall, which falls on the land surface and percolates through the ground into the cave system. This means that, in the first instance, the oxygen isotopic composition of any speleothem is influenced by the oxygen isotopic controls over the rainwater. Depending upon the location of the cave, the oxygen isotopes in rainwater may be controlled by the source of rainfall, by temperature, or by the amount of rainfall. If only one of these controls dominates in a region then the oxygen isotope value of the underlying speleothems can record a long duration record of changes in this process (for example the amount of rainfall that has fallen in a particular location over thousands of years).
Is it really that simple?
No…. Whilst it appears logical that oxygen isotopes in all speleothems should record changes in climate there are several processes which can act to complicate the oxygen isotope signal in the speleothem. Some examples are:
Karst water mixing
Caves are characteristics features of ‘karst’ environments. Karst is a type of landscape that is dominated by rocks such as limestone or gypsum, which are highly soluble in water. This means that they are the ideal environment for the development of caves. In areas where there is a lot of limestone between the surface (where the rainwater infiltrates the ground) and the cave (the point of speleothem growth), large aquifers of water build up. These aquifers can allow water to be stored for tens of years allowing old and new waters to mix and exchange their isotopic value. In this case the isotopic value of waters entering the cave and forming the speleothem become mixed, leading to an average isotopic value recorded in the speleothem over many years.
Speleothem deposition is controlled by two major chemical processes; the release of carbon dioxide (CO2) from the drip water and the deposition of calcium carbonate to form the speleothem (CaCO3). During both of these reactions oxygen isotopes can fractionate. This process leaves speleothem carbonate with higher δ18O values than the water from which the speleothem is formed. If this has occurred, we need to quantify the extent to which it has affected the osygen values in the speleothems. This is so that our reconstructions of past oxygen levels, and therefore past climate, are reliable. If this can’t be done, then it is difficult to establish if the oxygen isotope value is a result of changing climatic conditions or a product of fractionation. It is for this reason that all speleothem scientists now aim to make detailed analyses of the cave environment before they interpret the oxygen isotope records as archives of climate change (Fig.3).
An example of oxygen isotopes as an archive of palaeoclimate change
Soreq Cave, Israel
Climate reconstructions from Soreq Cave (Israel) over the past 7,000 years use oxygen isotope values as tracers of past rainfall amounts. This is done by assessing how much the oxygen isotope values of modern rainfall change dependent upon an annual change in rainfall amount. In years when Soreq Cave receives more rainfall, speleothem δ18O value decreases, in dryer years the δ18O of speleothem carbonate increases (Fig. 4). The Soreq oxygen isotope record identifies periods of drying at 6650–6600 yr BP, 6250–6180 yr BP, 5700–5600 yr BP, 5250–5170 yr BP, and 4200–4050 (Fig. 4). Importantly, several of these can be linked to significant cultural upheavals in the region; including importantly the last two events which correspond to the collapse of civilisations in Mesopotamia and the Akkadian Empire. This shows how scientists can start to look at past changes in climate and relate them either collapses or growths in civilisation.
The full paper from this example can be found here.