EAPSI: Reconstructing Past Coastal Groundwater Discharge and Implications for Future Water Resource Management

Broach Kyle H, Santa Cruz CA

Investigators

Abstract

As climate change continues to add heat energy to the oceans and the atmosphere, the water cycle may undergo significant changes in the amount and timing of precipitation. For regions that depend on seasonal rainfall for water resources such as California and the Caribbean, such changes may drastically alter the freshwater resources available, impacting a wide array of agricultural and municipal activities. In order to predict and plan for these changes, it is necessary to understand the variability of rainfall and groundwater resources in the past to accurately replicate the hydrologic cycle in climate models. This study will use fossil shells from a coastal lagoon in the Yucatan Peninsula as archives of past water chemistry, including the element boron. These fossils record groundwater discharge which is directly related to the amount of precipitation falling over the Yucatan. The researcher will collaborate with Dr. Chen-Feng You, a world expert on boron, at the National Cheng Kung University in Tainan, Taiwan, which has the clean lab facilities necessary to measure boron in fossil shells. Two sets of samples will analyzed: one from the Yucatan and one from Taiwan. The Yucatan field site is a brackish lagoon influenced by groundwater springs and ocean water that result in seasonally persistent salinity and pH gradients. Samples consist of several 3 meter long sediment cores covering the past 5,000 years. Foraminifera picked from the core tops and downcore centimeter intervals will be analyzed for the boron isotope 11B, a proxy for pH. Microfossil and water samples will be analyzed on a multi-collector inductively coupled plasma mass spectrometer at Dr. You's lab. Variations in 11B suggest changes in water pH and thus changes in the amount of rain-recharged groundwater entering the lagoon. The Taiwan field site is a similar estuary where modern samples will be collected to establish baseline 11B and compared to the Yucatan as a proof of concept for the 11B groundwater discharge proxy. This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Ministry of Science and Technology of Taiwan.