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Understanding what is happening to the global climate system is a scientific issue we have been hearing plenty about in the news recently. For scientists, a crucial step towards understanding the changes in the climate system has been the development of reliable climatic models. These models can simulate the past, using proxy data obtained in the field. Furthermore, the models can project what might happen in the future, based on past and current observations. The more we know about past conditions, the more information we have to reliably base future projections on.
Palaeoclimate reconstruction plays a key role in stimulating and guiding model development(3). As climate models become more sophisticated, it is necessary to produce quantitative data with which their output can be directly compared (e.g. temperature, rainfall amount/seasonality, wind strength/direction, evaporation rates). Such data is difficult to obtain because most Quaternary (i.e. the last ~2.6 million years) geological proxies can only be interpreted quantitatively after passage through various forms of transfer function which generally introduce quite large uncertainties.
One variable that can be measured both directly and also modelled is the isotopic composition of palaeo-precipitation.
All water is composed of hydrogen (H) and oxygen (O). However, there are three different isotopes of oxygen that can make up water. An isotope of an element is one with the same atomic number, but a different mass. Of the three oxygen isotopes, 16O has the lightest mass, followed by 17O and then 18O. Of all oxygen present on our planet, 99.762% of it is in the form of 16O; 18O accounts for 0.2%; and 17O is negligible at 0.038%(6). By analysing the ratio of 16O:18O it is possible to detect changes in rainfall composition, these changes can help in interpreting what has happened to an air mass before precipitation occurred, although, subsequent modifications to the water can occur post-precipitation (e.g. evaporation), which complicates any interpretation. The modern isotopic distribution of rainfall is quite well known and can be related to atmospheric circulation patterns(5).
When calcite is formed in caves, in certain conditions, it will directly reflect the isotopic composition of the parent drip water it precipitated from. Under these circumstances the calcite can act as a proxy data source for the above ground atmospheric oxygen isotope ratios &endash; owing to the fact that cave drip waters originate from precipitation (rain, snow etc) above ground.
Modern cave seepage waters have isotopic compositions very close to the mean values of annual precipitation above cave sites, with little seasonal variation(1,2,4,7). As cave calcite precipitates, forming features such as stalagmites and flowstones, microscopic amounts of drip water may be trapped and preserved inside. Analysing this water, known as fluid inclusions, can reveal information on what precipitation was like, above ground, at the time of calcite deposition. However, it is important for the calcite to have formed in stable cave conditions, where rapid degassing of drip water did not occur. Where rapid degassing of drip water occurs, the preserved isotopic properties of calcite are no longer indicative of the palaeo-drip waters.
My project aims to measure the isotopic compositions of calcite and fluid inclusions (ancient drip water trapped in the calcite) in stalagmites and determine the isotopic gradient of palaeo-precipitation across Europe along a transect from Britain to Slovakia (Fig.1). That is, my intention is to see how calcite from different parts of Europe has recorded ancient precipitation signals and to examine how these signals differ from one another for the same time period of calcite growth, asisotopic compositions of precipitation have become modified through movement from a maritime climate (U.K.) to a continental one (Slovakia). The time period my project focuses on covers the last 12,000years, i.e. the end of the Late-Glacial and the Holocene (which is the geological period we are in now).
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Figure 1. Transect from Britain to Slovakia. My PhD involves working on samples from Yorkshire, Belgium and the Low Tatra Mountains in Slovakia.
When fluid inclusion data is used in conjunction with stalagmite calcite isotopic information, it is possible to compile a record of palaeo-cave temperatures, which in turn reflect mean annual surface temperatures. Through this I hope to present climate reconstructions for the areas involved in my study. |
