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Title: Trace-metals and isotopes in carbonates : understanding past climate records
Author: Day, Christopher Charles
ISNI:       0000 0004 2722 1604
Awarding Body: Oxford University
Current Institution: University of Oxford
Date of Award: 2009
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An increasing number of studies report stable-isotope and trace-element records in speleothems. This variation is controlled by diverse environmental variables including the climatically important variables, temperature and rainfall. There is, however, a paucity of laboratory studies attempting to understand the influence of these environmental controls on stalagmite geochemistry. Quantitative data from such studies would dramatically improve our ability to reconstruct palaeoclimate from stalagmites. This study produces a new calcite growth setup, which closely mimics natural processes (e.g. precipitation driven by C02 degassing, low ionic strength solution, thin solution film) but with a tight control on growth conditions (temperature, pC02, drip rate, calcite saturation index and the composition of the initial solution). Results of a series of experiments (12 experiments), which investigates synthetic calcite growth at four different temperatures (7, 15, 25 and 35°C) and three different drip rates (1.6,5.8 and 10.4 drips/min) provide important, new information on speleothem chemistry and how it relates to environmental conditions. A relationship between temperature, drip rate and growth rate is derived and shows that temperature is the dominant control on growth rate, although the effect of saturation index is not yet known. Varying degrees of δ180calcite equilibrium/disequilibrium are observed throughout the range of temperatures and drip rates and demonstrate that faster drip rates are more likely to produce 8180calcite in equilibrium with the drip solution. Trace metal results provide cave-analogue partition coefficient values for Mg, Co, Sr, Cd, Ba and U. In agreement with previous studies, Mg/Ca is shown to respond to both temperature and the percentage of calcite precipitated (%cp). D(Cd), not previously used in stalagmite studies, shows a more sensitive response to %cp and therefore has the potential to act as a proxy for effective rainfall with increases in Cd/Ca corresponding to wetter conditions. The near unity value of D(Co) limits the response of Co to %cp and therefore variation in Co/Ca should be indicative of a different kind of environmental change. In addition to the synthetic calcite experiments, early results were presented from palaeoclimate reconstruction studies in Southern Chile and Northern Morocco. U-Th chronology on the two Chilean stalagmites indicate growth during the second half of the Holocene period. The stable isotope records of both samples are comparable at 4ka BP but the records diverge after that period. Robust palaeoclimate interpretations are hampered by this divergence, by a lack of modern drip water data and of a duplicate zero-age stalagmite record. Results from Northern Morocco represent an inItial investigation of the suitability of Moroccan caves for future palaeoclimate work. Results indicate that Ghar Cahal, located close to Tetouan, may be suitable for retrieving palaeoclimate data that will be directly comparable with archaeological records from that same location.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available