Date of Award

6-2020

Document Type

Restricted (Opt-Out)

Degree Name

Bachelor of Science

Department

Geology

First Advisor

David Gillikin

Keywords

Paleoclimate, Stable Isotopes, stalagmites, Portugal, Caves

Abstract

Stable isotopes archived in calcium carbonate cave deposits such as stalagmites have been widely used to reconstruct past climates. However, various processes can influence the isotope signal recorded in speleothems, potentially obscuring climate signals. Stalagmite oxygen isotopes can record the 𝛿18O values of regional precipitation and hence hydroclimate, but processes such as soil and karst water evaporation, mixing, and seasonally-biased carbonate precipitation may complicate the target signal. Here we compare high-resolution (sampled every 20-40 μm) oxygen isotopes in three small stalagmites with modern tops from southern Portugal. One stalagmite was taken from Companheira cave (C-18-1) and two were taken from Ibn Ammar cave (GIA-19-1 and GIA-19-2) ~2.3 km away. Precipitation isotope databases suggests precipitation 𝛿18O values are more impacted by changes in temperature and less impacted by the amount effect. All samples were dated using U/Th disequilibrium techniques, however had low uranium concentrations and low 230Th/232Th ratios making dates unreliable but suggest the three stalagmites grew over the last 2,000 years or less. After data from the two Ibn Ammar stalagmites were manually age fitted both samples display δ13Ccycles that repeat approximately every 40 years and align with each other. This correlation between the two records suggests stalagmite isotope records from Ibn Ammar are reproducible. Carbon isotope cyclicity observed in the Ibn Ammar stalagmites indicate that over the last 500 years southern Portuguese climate has see-sawed between warmer/drier and wetter/cooler climates every 40 years possibly due to NAO forcing. Further development of an age models will allow for a more comprehensive evaluation of dynamical changes in the regional climate system through time.

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