A High-resolution Speleothem Record of Western Equatorial Pacific Rainfall: Implications for Holocene ENSO Evolution

The El Niño-Southern Oscillation (ENSO) is the primary driver of interannual climate variability in the tropics and subtropics. Despite substantial progress in understanding ocean-atmosphere feedbacks that drive ENSO today, relatively little is known about its behavior on centennial and longer timescales. Paleoclimate records from lakes, corals and deep-sea sediments suggest ENSO variability was generally weaker during the mid-Holocene (4-6 kyr BP) than the late Holocene (0-4 kyr BP) [1-3]. However, discrepancies amongst the records preclude a clear timeline of Holocene ENSO evolution and therefore attribution of ENSO variability to a specific climate forcing. Here we present δ18O results from a U-Th dated speleothem in Malaysian Borneo nominally sampled at seasonal resolution. The δ18O of Borneo rainfall is a robust proxy of regional convective intensity and precipitation amount, both of which are directly influenced by ENSO activity [4-5]. Replicated δ18O records from multiple cave locations in Borneo indicate stalagmites from the region represent regionally coherent changes in atmospheric convection [6-7]. Our estimates of stalagmite δ18O variance at ENSO periods (2-7 years) show a significant (p < 0.01) reduction in interannual variability during the mid-Holocene (4580-4760 yr BP) relative to both the late Holocene (2400-2600 yr BP) and early Holocene (6620-6720 yr BP and 8100-8300 yr BP). The Borneo results are generally consistent with Holocene records of interannual variability from the eastern equatorial Pacific [2-3], indicating that long-term changes in ENSO were likely the primary driver. Reduced mid-Holocene δ18O variability in Borneo coincides with an overall minimum in δ18O from 3.5 to 5.5 kyr BP, suggesting the low-frequency signal is driven by persistent atmospheric convection in the Western Pacific Warm Pool (WPWP). Additional high resolution time slices are necessary to fully characterize interannual-scale δ18O variance in Borneo and to determine whether ENSO or another mechanism (e.g. migration of the Inter-Tropical Convergence Zone) is the primary driver of long-term changes in convective activity in the WPWP during the Holocene. 1. Conroy, J.L. et al., Quaternary Science Reviews 27, 1166-1180, 2008 2. Koutavas, A., Joanides, S., Paleoceanography 27, doi: 10.1029/2012PA002378, 2012 3. Cobb, K.M. et al., Science 339, 67-70, 2013 4. Cobb, K.M. et al., Earth and Planetary Science Letters 263, 207-220, 2007 5. Moerman, J.W. et al., Earth and Planetary Science Letters 369-370, 108-119, 2013 6. Partin, J.W. et al., Nature 449, 452-456, 2007 7. Carolin, S.A. et al., Science, 10.1126/science.1233797, 2013