DEVELOPING AN UNDERSTANDING OF MODERN AND PAST HYDROCLIMATE VARIABILITY IN PHILIPPINES KARST ENVIRONMENTS (Invited Presentation)

Tropical islands rely on freshwater from rainfall to recharge groundwater. Access to this valuable resource, a basic human need for health and well-being, impacts the livelihood of billions of people. As a result, understanding hydrological parameters that modulate freshwater supply and groundwater storage across tropical island nations, many of which lie in the Western Pacific Warm Pool, demands active research efforts to constrain modern as well as past trends in rainfall delivery. Further, tropical island nations are already facing climate change extremes, sea-level rise, ocean acidification, and strengthening of typhoons without the infrastructure to combat it.

We present results from our work in the Philippines, a nation of over 7,000 islands in southeast Asia with complex hydroclimate dynamics. Rainfall is seasonally dominated by the southwest (summer peak) and northeast (winter peak) monsoon systems, on an interannual scale by phases of the El Niño Southern Oscillation (ENSO), and sporadically by typhoons. In this presentation we: 1) present analyses demonstrating the impact of ENSO on hydrology using a 100 year long dataset of river discharge and rainfall; 2) report on results from a continuous and active monitoring network of multiple caves across almost 10° of latitude; and 3) describe efforts to develop terrestrial records of paleo-hydroclimate variability using speleothems.

Modern datasets show the potential for multi-year drought conditions following El Niño events using superposed epoch analysis. Results of our cave monitoring effort suggest that the caves experience seasonal ventilation, minimal changes in cave-air temperature, and transient drip rate changes responding to rainfall events, all suitable conditions for favorable growth of speleothems that will track hydroclimate variability. Further, stable isotope analysis of cave-drip water and trace element analysis suggest minimal evaporative effects, a clustering across winter-biased and summer-biased regions, and prior calcite precipitation dictating changes in the geochemistry of modern cave drip water and farmed plate cave-calcite. Finally, we will present two stable isotope and trace element records from two speleothems that grew between 28 to 51 ka collected from Bohol, Central Visayas and from 1 to 4.5 ka collected from Puerto Princesa Subterranean River National Park, Palawan.