Different paleotemperature reconstructions using fluid inclusions

I have applied two different methods to reconstruct paleotemperatures using the stable isotopes of fluid inclusions. Firstly by means of the classic application of the carbonate thermometer and secondly by means of the δ2H/T relationship. However, both appli- cations can only be used under certain circumstances. For the application of the classic carbonate thermometer the calcite precipitation must take place in equilibrium, which proves to be problematic for several stalagmite studies. Kinetic fractionation occurs for fast growing stalagmites or for rapidly changing growth rates. This is partly the case for Stam 4 from Cloşani, where the traditional carbonate thermometer leads to unrealistic temperature variations of 10 °C for the last 100 years. Another factor that enhances kinetic fractionation is a drip rate that is too fast or too slow. In case of a very slow or even interrupted drip rate, evaporation in the cave can be enhanced and causes kinetic fractionation during calcite precipitation. For this reason I excluded a large number of samples for the temperature reconstruction of LA - 1 (Puerto Rico). I have found that the use of the carbonate thermometer can only be applied under a suitable pre - selection and exclusion of fractionation effects. I could show that for the westerly tropical Atlantic during the Heinrich stadial a significant cooling of about 3.5°C has occurred, which is more pronounced than expected by SST reconstructions. For the interstadials, temperatures were reconstructed that are comparable to the present day cave air temperature and the reconstructed sea surface temperatures.

In contrast, the application of the δ2H/T relation is a suitable approach for stalagmites that have grown under continental climatic influence. This means that the isotopic composition of the precipitation is mainly controlled by the Rayleigh rainout effect and not by other isotopic effects such as the amount effect, which excludes tropical stalagmites. If the relationship between the δ18O value of precipitation and surface temperature is well characterized, the deuterium signal in the fluid inclusions can de- termine temperatures with a precision of ± 0.45°C. With this I was able to resolve the temperature increase due to anthropogenic climate change for southern Romania for the second half of the 20th century using the fluid inclusion measurement. The recon- structed temperature increase of 1.02 ± 0.63 °C for the period between 1950 to 2010 is in excellent agreement with local temperature records. Thus, the newly applied deu- terium - temperature relationship proves to be a suitable method to determine mean annual temperatures for mid - latitude stalagmites.

7.1 Conclusion

7.1.3 Climatic imprint on isotopic compositions of fluid inclusions

I was able to demonstrate the extent to which a climate signal is imprinted in stable isotopes of fluid inclusion in addition to the temperature information. For two different climatic regions (continental and tropical) I have found significantly increased isotope values in comparison to the isotopic composition of the present day drip water. Frac- tionation was more pronounced for oxygen than for hydrogen with a slope (δ2H/δ18O) of + 2.27 ± 1.12 for Bu4 (Germany) and + 3.73 ± 0.22 for LA-1 (Puerto Rico). This clear deviation from the GMWL with slopes of 2.3 and 3.7 indicates fractionation due to evaporation. This effect has never been reported before in stable isotope mea- surements of fluid inclusions. In fact, evaporation effects in caves are usually only marginally discussed due to the typically high humidity in the cave. However, that the same evaporation pattern can be found for these two different stalagmites shows, that evaporation in caves is an important phenomenon that can lead to disequilibrium conditions during calcite precipitation. The findings obtained in this thesis on the analysis of fluid inclusions make an important contribution to the better characteriza- tion of kinetic fractionation due to evaporation. For both stalagmites it was further shown that evaporation was favoured by dry climatic conditions, so the position of the stable isotopes of the fluid inclusions relative to the GMWL can be used as an indicator for dry or humid climatic conditions.

With the simultaneous measurement of δ18O and δ2H, the deuterium excess can be determined, which provides information about the conditions of formation or possible moisture recycling of the precipitation [Froehlich et al., 2002]. Here I could show that with respect to the δ2H signature very dry climatic conditions can lead to a strongly varying deuterium excess. Furthermore, the varying deuterium excess shifts some sam- ples that experienced evaporation and should follow the evaporation line towards the GMWL, which may lead to misinterpretation. Although deuterium excess has no in- fluence on the δ18O signal and thus does not concern the use of the classic carbonate thermometer, it should be discussed in detail as it can provide important information on the formation conditions of precipitation.

A further conclusion is, that the source of precipitation can be derived from the fluid inclusions. I was able to identify δ18O values for the tropical stalagmite that are sig- nificantly higher than the isotopic composition of today’s drip water, which indicates a change in the type of precipitation. The measured fluid inclusion data of LA-1 indicates that there was a shift from convective rain to more orographic rain during the Green- land interstadials. For the stalagmite from Cloşani, I could show that significantly increased (more positive) isotope values and concurrently positively correlated NAO and EAWR modes indicate a shift of the precipitation source towards Mediterranean influence. Regarding the interpretation of circulation patterns (Cloşani - Romania), kinetic effects during calcite formation are excluded and it is assumed that the signal stored in the fluid inclusions is purely atmospheric.