Regional-scale climate influences on temporal variations of rainwater and cave dripwater oxygen isotopes in northern Borneo

Abstract

This study investigates the relationship between large-scale climate variability, rainfall oxygen isotopic composition ( δ 18O), and cave dripwater δ 18O at Gunung Mulu and Gunung Buda National Parks in northern Borneo (4°N, 115°E) on intraseasonal to interannual timescales. A 3-yr timeseries of rainfall δ 18O contains prominent seasonal and interannual variability. The seasonal cycle in rainfall δ 18O is defined by lighter values of − 10‰ during late boreal summer and heavier values of − 4‰ during late boreal winter, and is poorly correlated to local precipitation, which displays very weak seasonality. Seasonally-varying moisture trajectories likely play a key role in the observed seasonal cycle of rainfall δ 18O, driving enhanced fractionation during boreal summer and less fractionation during boreal winter. Dripwater δ 18O timeseries display 2‰ seasonal cycles that follow the rainfall δ 18O seasonal cycles, with a mean δ 18O value equivalent to the mean δ 18O of rainfall. Large surveys of cave dripwaters conducted during three fieldtrips to Gunung Mulu/Buda reveal a system-wide response to rainfall δ 18O seasonality that supports a relatively short (less than 6months) response time for most drips. During the weak 2005/2006 La Niña event, sustained positive precipitation anomalies are associated with rainfall δ 18O values that are 4 to 5‰ lighter than previous years' values, consistent with the tropical “amount effect” observed in both models and data. Dripwater δ 18O values are 1 to 2‰ lighter during the weak La Niña event. The importance of the “amount effect” in driving intraseasonal rainfall δ 18O anomalies at our site is supported by an 8‰ increase in rainfall δ 18O that occurred over the course of two weeks in response to a pronounced decrease in regional convective activity. Dripwater discharge rates underwent a ten-fold decrease during the extended dry period, but dripwater δ 18O values remained constant. This study supports the interpretation of stalagmite δ 18O records from Gunung Mulu/Buda as paleo-precipitation records that are sensitive to the location and strength of deep convection in the West Pacific Warm Pool.