Innovative isotope ratio measurements of speleothem fluid inclusions

Affolter, Stéphane (2015). Innovative isotope ratio measurements of speleothem fluid inclusions (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Speleothem (cave carbonates) are recognised as a key continental archive for paleoclimate reconstructions. Stalagmites in particular often contain fluid inclusions that represent natural repositories of cave drip water and consequently relics of past precipitation. The oxygen (δ18O) and hydrogen (δD) isotopic composition of the water trapped in speleothem fluid inclusions are used as proxies for paleotemperature or for investigating changes in the moisture source and strength.
In this thesis, a new method based on laser absorption spectroscopy to measure speleothem fluid inclusions has been developed. This method uses a Picarro L1102-i instrument where the measuring principle is based on the wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) technology that has the main advantage of allowing simultaneous measurements of δD and δ18O isotopes on the same sample. The extraction line is simple and consists of basically three units: (i) a water background generator; (ii) a syringe injection unit to allow injections of standard water and (iii) a simple self-made crushing device. We can summarise the measurement procedure as follows: prior to crushing, the speleothem sample is placed into a copper tube, fixed to the line previously heated at 140°C and flushed with a nitrogen and standard water mixture. Thereafter, the speleothem sample is crushed using a simple hydraulic crushing device and the released water from fluid inclusions is transferred by a nitrogen gas stream to the analyser. This method avoids any water treatment prior to the isotopic determination such as the pyrolysis when combining gas chromatography and IRMS and it also avoids the step of water freezing treatment. Reproducibility of standard water measurements is better than 0.4‰ for δ18O and 1.5‰ for δD for an extended range between respectively at least -27‰ to 0‰ and -210‰ to 0‰. The reproducibility for real stalagmite samples is in the same range for water amounts up to 0.5 μl.
With this new method we successfully established new δD and δ18O isotopic records of stalagmites from Borneo and Switzerland. In northern Borneo (tropical West Pacific), a stalagmite from Whiterock cave covering almost two glacial-interglacial cycles from the Marine Isotope Stage (MIS) 12 to early MIS 9 (460-330 ka) was analysed, as well as two modern samples. For Switzerland, we measured a stalagmite from Milandre cave covering an interval ranging from 14000 to 9800 years including the Allerød, Younger Dryas cold phase and the early Holocene periods and a modern sample in addition. Our results show a good correlation with δ18O isotopic composition of the calcite and allow the reconstruction of paleotemperatures.
Together with the method development and to better calibrate the speleothem fluid inclusion isotope values in Milandre cave (NW Switzerland), we carried out a water isotope (δD, δ17O and δ18Ο) monitoring program to investigate how the isotope signals and their corresponding second order parameters d-excess and 17O-excess are transmitted from the precipitation above the cave to their entrapment in fluid inclusions, via the transfer in the epikarst and its dripping in the cave to feed the growing stalagmite. To achieve this goal, a further improvement of the method was achieved with the use of a new generation laser based instrument (Picarro L2140-i) and, for the first time, measurements of δ17O in speleothem fluid inclusions were performed together with δD and δ18O on a single aliquot of water released from the crushing of a sample.
This study was realised in the framework of STALCLIM – Multi-proxy climatic and environmental reconstruction from stalagmites originating from Switzerland, Turkey, Arabia and India – a Sinergia project financed through the Swiss National Science Foundation.

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