The aim of this collaborative project is to reconstruct long-term and short-term changes in the El Nino Southern Oscillation (ENSO) using high-growth rate, precisely dated speleothems from caves in Borneo. Using the speleothems, the researchers would produce 150- to 300-year time series of changes in delta 18-Oxygen for five key time periods during the Holocene.
The research addresses several science questions, such as: 1) Was ENSO absent or just weaker in the early Holocene? 2) Did the frequency or magnitude of El Nino events change on centennial time scales during the late Holocene? 3) Were low-frequency changes in western equatorial Pacific precipitation driven by ENSO or by migration of the Inter-Tropical Convergence Zone (ITCZ)? and 4) Can the impact of ENSO on tropical rain forest productivity be detected using speleotherm stable isotopic records?
The broader impacts include the training off a female postdoctoral researcher and the mentoring of undergraduates involved with the research project. Data would be archived at the National Climatic Data Center.
The El Nino Southern Oscillation (ENSO) is the primary source of interannual climate variability on a global scale. Although the atmospheric and oceanic processes that govern El Nino behavior today are well understood, it is nevertheless difficult to predict future ENSO behavior in response to global climate change. Computer model simulations of ENSO behavior in response to global warming currently give us conflicting predictions due to complex and counteracting climate feedbacks and responses. To better understand ENSO processes and their potential for change in the future under differing global climate states, we have to turn to records of ENSO behavior in the past, when climate drivers such as insolation differed. Reconstruction of El Nino from geologic archives such as continuously-growing stalagmite carbonate deposits can provide a unique perspective on long-term ENSO evolution, clarify the roles of global and local climate, and provide improved data for testing model simulations. The primary focus of this study was to reconstruct ENSO behavior at key intervals during the Holocene epoch of the last 10,000 years, when global climate has broadly resembled modern climatic conditions. Estimates of past ENSO variability provide a baseline against which future changes can be judged and they also allow us to determine how sensitive ENSO behavior is to modest changes in background climate during the Holocene. We constructed a record of past precipitation from a stalagmite from Malaysian Borneo. Stalagmites can be accurately dated using uranium and thorium isotopes, and their oxygen isotopic composition can be used to infer rainfall variability over long timescales. Borneo sits in the midst of the West Pacific Warm Pool (WPWP), a region of the ocean strongly affected by ENSO. In Borneo, interannual rainfall variations are due primarily to ENSO; conditions are wet during La Nina and dry during El Nino. Thus, the oxygen isotopic composition of the stalagmite, recording rainfall variations, can be used to infer ENSO activity in the geologic past. We sampled five segments of the stalagmite that grew over spans of decades to centuries, from five times during the last 10,000 years. Using a computerized drill, we were able to drill carbonate samples along growth bands in the stalagmite at sub-millimeter scale resolution, corresponding to temporal resolution of 2-4 samples per year. This high sampling resolution allows us to identify individual events in the stalagmite record that occur at 2-7 year intervals, the typical frequency range of ENSO events. We also drilled carbonate samples from the stalagmite for U-Th dating, to improve our age model and growth rate estimates. Our record from the Western Pacific Warm Pool can be compared to records from the Central and Eastern Pacific to provide an understanding of ENSO-scale climate variability across the entire equatorial Pacific and across the regions most affected by ENSO. Our records from the mid-Holocene show little oxygen isotope variability, suggesting that ENSO variability was at its weakest during the mid-Holocene, a time when previous studies indicate there was an overall increase in the mean atmospheric convection and rainfall in Borneo. Our data are consistent with coral records from the central and eastern equatorial Pacific that also show a mid-Holocene reduction in ENSO activity. The simplest explanation of common changes in these two independent archives is a reduction in ENSO variance, reducing the strength of ENSO events affecting the equatorial Pacific from east to west. Although records from lake sediments in the eastern Pacific suggest little ENSO activity in the early Holocene, our stalagmite record suggests substantial ENSO-related precipitation variability in Borneo at this time, in agreement with Pacific marine records such as oxygen isotopes of corals and microfossils. Our stalagmite record shows the greatest oxygen isotope variability in the 2-7 year frequency band corresponding to ENSO events during the late Holocene, also in agreement with Central and Eastern Pacific records. Our results suggest that during the generally warm Mid-Holocene, increased atmospheric convection in Borneo associated with the warm mean climate state resulted in persistent easterly winds in the tropical Pacific, which in turn limited the development of El Nino events. Based on the timing of changes in the mean oxygen isotopic content of the stalagmite, it appears that the overall increase in atmospheric convection and resulting precipitation is related to variations in the earth's orbit and the associated seasonal changes in solar energy received in the tropics. Thus, the available long-term records imply that ENSO is highly sensitive to modest changes in the seasonality of solar energy received in the tropics.
