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

This study investigates the relationship between large-scale climate variability, rainfall oxygen isotopic composition (delta O-18), and cave dripwater delta O-18 at Gunung Mulu and Gunung Buda National Parks in northern Borneo (4 degrees N, 115 degrees E) on intraseasonal to interannual timescales. A 3-yr timeseries of rainfall delta O-18 contains prominent seasonal and interannual variability. The seasonal 7 cycle in rainfall delta O-18 is defined by lighter values of - 10 parts per thousand during late boreal summer and heavier values of - 4 parts per thousand during late boceal 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 delta O-18, driving enhanced fractionation during boreal summer and less fractionation during boreal winter. Dripwater delta O-18 timeseries display 2 parts per thousand seasonal cycles that follow the rainfall delta O-18 seasonal cycles, with a mean delta O-18 value equivalent to the mean delta O-18 of rainfall. Large surveys of cave dripwaters conducted during three fieldtrips to Gumung Mulu/Buda reveal a system-wide response to rainfall delta O-18 seasonality that supports a relatively short (less than 6 months) response time for most drips. During the weak 2005/2006 La Nina event, sustained positive precipitation anomalies are associated with rainfall delta O-18 values that are 4 to 5 parts per thousand lighter than previous years' values, consistent with the tropical "amount effect" observed in both models and data. Dripwater delta O-18 values are 1 to 2 parts per thousand lighter during the weak La Nina event. The importance of the "amount effect" in driving intraseasonal rainfall delta O-18 anomalies at our site is supported by an 8 parts per thousand increase in rainfall delta O-18 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 delta O-18 values remained constant. This study supports the interpretation of stalagmite delta O-18 records from Gunning Mulu/Buda as paleo-precipitation records that are sensitive to the location and strength of deep convection in the West Pacific Warm Pool. (c) 2007 Elsevier B.V. All rights reserved.