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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Bachelor Thesis

Tropical hydroclimate variability from 217 – 144 ky BP – reconstructed by stable isotopes and vapor-liquid homogenization temperatures of fluid inclusions of a stalagmite from northern Borneo

Lukas Scharf (01/2020-04/2020)

Support: Britta Planer-Friedrich

Speleothems are terrestrial calcareous depositions that archive environmental information and can be dated by U-Th dating. To reconstruct former climatic conditions, paleoclimate proxies like stable isotopes of oxygen and carbon and paleothermometers, like vapor-fluid homogenization temperatures of fluid inclusions or clumped isotope thermometry, are used. In this study 18O and 13C values along the central axis of a stalagmite (FC-18-2) from Gunung Mulu National Park, northern Borneo were measured. The stalagmite was roughly dated back to 217 - 140 ky BP. In addition, grayscale values along the transects and vapor-liquid homogenization temperatures of one growth layer were measured to test their methodical relevance in paleoclimate research. 18O and 13C showed a general trend towards more positive values during growth and were positively correlated (R = 0.6). Grayscale values showed high resemblance to 18O values and therefore also seem to reflect hydroclimate variability. Homogenization temperatures showed values around 20.7 C with a small standard deviation, which can help to assess hydroclimate variability in a quantitative way. The primary factors which influenced 18O values in stalagmite FC-18-2, were temperature (23%), changes in moisture source 18O (38%) and atmospheric hydrological processes dominated by the amount effect (39%). Therefore, 18O and 13C values indicate a trend from warm and wet climate towards cooler and drier climate with a change in temperature of approximately 23 C to 20 C. Furthermore, precessional insolation seems to be a dominant factor that influences hydroclimate variability. Changes of 13C can not be interpreted as changes in vegetation type (C3/C4) because prior calcite precipitation likely plays a crucial role. Next steps for this study would be a refined age model, additional records to test the reproducibility of stable isotopes, and measurements of vapor-liquid homogenization temperatures of fluid inclusions along the whole transects. Furthermore, comparison to other stalagmites and to precipitation models would improve the understanding of tropical hydroclimate.

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