Kim M. Cobb, Jess F. Adkins, Judson W. Partin and Brian Clark
This study investigates the relationship between large-scale climate variability, rainfall oxygen isotopic composition (δ18O), and cave dripwater δ18O 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 δ18O contains prominent seasonal and interannual variability. The seasonal cycle in rainfall δ18O 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 δ18O, driving enhanced fractionation during boreal summer and less fractionation during boreal winter. Dripwater δ18O timeseries display 2‰ seasonal cycles that follow the rainfall δ18O seasonal cycles, with a mean δ18O value equivalent to the mean δ18O of rainfall. Large surveys of cave dripwaters conducted during three fieldtrips to Gunung Mulu/Buda reveal a system-wide response to rainfall δ18O 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 δ18O values that are 4 to 5‰ lighter than previous years' values, consistent with the tropical “amount effect” observed in both models and data. Dripwater δ18O values are 1 to 2‰ lighter during the weak La Niña event. The importance of the “amount effect” in driving intraseasonal rainfall δ18O anomalies at our site is supported by an 8‰ increase in rainfall δ18O 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 δ18O values remained constant. This study supports the interpretation of stalagmite δ18O 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.
Keywords: stalagmite; rainfall; dripwater; oxygen isotopes; paleoclimate; West Pacific Warm Pool