Signatures of the El Nino-southern oscillation on rainfall and cave dripwater oxygen isotopes in N. Borneo

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Ellis, Shelby Ann
Cobb, Kim M.
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Oxygen isotope (δ18O) records of speleothem carbonates are a critical terrestrial paleoclimate archive, providing insight into past hydroclimate variations and past changes in atmospheric circulation. Specifically, time series of rainfall and cave dripwater oxygen isotopes (δ18O) provide site-specific assessments of climate or non-climate related signals recorded in stalagmite δ18O used for such reconstructions. However, modern paired multi-year δ18O time series of rainwater and dripwater are limited in the tropical latitudes, an area known to contain regionally-specific atmospheric complexities acting on rainfall δ18O. Furthermore, karst drainage pathways vary significantly within the same cave system, altering the original climate-driven δ18O rainfall signal. In this thesis, I present an extended multi-year study of rainfall and cave dripwater δ18O time series from Gunung Mulu National Park in Northern Borneo to quantify the cloud-to-cave transformation process spatially and temporally across the Mulu karst, building on work previously presented by Moerman et al., 2013 and Moerman et al., 2014. Chapter 1 will broadly cover topics related to how stable water isotopes in rainfall, cave dripwaters, and stalagmites can detect ENSO-driven shifts in the hydrological cycle, building off almost a decade’s worth of modern (Cobb et al., 2007; Moerman et al., 2013; 2014; Partin et al., 2013a) and paleoclimate (Carolin et al., 2013; 2016; Chen et al., 2016; Meckler et al., 2012; Partin et al., 2007; 2013a) observations from a well-established research site in Northern Borneo, Sarawak, Malaysia. Chapter 2 quantifies the rainfall-to-cave dripwater transformation of isotopic climate-signals in the Mulu karst from continuous observations over the last ~12 years. These time series are the longest-running daily rainfall δ18O time series (2006 – 2018) and longest tropical biweekly dripwater δ18O time series (2007 – 2018) globally. Vadose zone mixing translates ENSO-related variations in rainfall δ18O to three monitored cave dripwater δ18O sites. Using two simple modeling techniques, we generated an ensemble of different modeled dripwater time series directly corresponding to local rainfall δ18O, estimating Mulu water takes ~3 to 18 months to transit through the karst. This transit time provides context for what resolution of climate signals can be potentially recorded in local stalagmites employed for hydroclimate reconstructions. Overall, this thesis supports previous interpretations of using the amount effect framework for Mulu stalagmite δ18O records through the multi-year, paired local rainfall and dripwater δ18O time series. This research clearly demonstrates paired rainfall and cave δ18O observations can support more minute interpretations of highly-resolved paleo-ENSO stalagmite records.
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