Trace element geochemistry of Belizean and Bermudan stalagmites: new tools, proxies and applications.

Abstract

Speleothem trace elements are an important and effective tool for palaeoenvironmental reconstruction. They can be used to reconstruct a plethora of climate variables, and are a vital tool for improving our understanding of the climate system. This is particularly important given the on-going challenges of comprehending and tackling anthropogenic climate change. Only by thoroughly understanding controls on climate variability can we attempt to predict future change. This thesis presents a broad study of current speleothem trace element proxies. In addition to reviewing the current state of knowledge, this thesis presents several additions and developments to the speleothem trace element toolkit.
The 22 year ATM-7 trace element record from Belize has a greater than seasonal resolution, and a highly precise chronology. As a result of this exceptional chronology, combined with extremely high resolution LA-ICP-MS analysis, it is possible to detect the geochemical indicators of volcanic ash deposition. Principal Component Analysis identifies a clear signal of a multi-elemental input of trace elements at the beginning of the wet season following volcanic eruptions with ash reaching the cave site.
U/Ca variability in aragonitic speleothems is strongly influenced by the occurrence of Prior Aragonite Precipitation. The U/Ca record in Belizean stalagmite YOK-G strongly suggests that modern drying has occurred in Belize, primarily caused by a reduction in wet season rainfall. This is consistent with published stable isotope data from YOK-G, previously interpreted as the result of southward ITCZ displacement. These results strongly suggest that U/Ca values in aragonitic speleothems are excellent proxies for rainfall variability. This new tool, combined with the exceptional chronological control characteristic of aragonitic stalagmites and the high spatial resolution afforded by modern microanalytical techniques, should facilitate the construction of new exquisitely resolved rainfall records, providing rare insights into seasonality changes as well as long-term changes in local recharge conditions.
In the Bermudan stalagmite BER-SWI-13, magnesium concentrations record, via varying prior calcite precipitation, changes in local rainfall which appear to correspond to variation in the North Atlantic Oscillation (NAO). Through a different mechanism, phosphorous also correlates with changes in the NAO. We infer that local effective rainfall changes, influenced by NAO state, influence bioproductivity above the cave and thus the amount of phosphorous in dripwaters. Surprisingly, for a location such as Bermuda, we see no evidence of clear direct anthropogenic influence on speleothem chemistry. These results
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suggest that Bermudan speleothems are well situated to record basin scale climate changes in the North Atlantic.
Together, these three separate studies demonstrate the strength and versatility of high- resolution trace element analysis of speleothems. They establish new techniques of data analysis, new proxies, and the applications of existing proxies in new contexts to reconstruct palaeoenvironmental variables. Looking forward, these discoveries demonstrate that speleothem trace elements continue to have a great deal to offer to the field of palaeoclimate reconstruction, and that there are still new techniques and applications to be developed.