The history of El Niño in the Eastern Pacific from single-specimen chemistry of planktonic foraminifera

The El Ninõ-Southern Oscillation (ENSO) system has a strong impact on weather patterns globally, yet there is no consensus how modern global warming will affect the strength and frequency of ENSO events. One approach to test the accuracy of climate models is through reconstruction of ENSO cyclicity in pre-instrumental climate records. There is general agreement that the ENSO system is sensitive to the changes in insolation that occur as part of orbital (Milankovitch) cycles. However, palaeoclimate reconstructions of changes in the intensity of ENSO cycles during the past ~25,000 years show conflicting results, which could mean that changes in the ENSO system are not spatially uniform. Unfortunately, annually resolved climate archives like corals or annually laminated sediments, which are the prime material to reconstruct palaeo-ENSO cycles, are sparsely distributed. As a result, a denser grid of palaeo-ENSO observations is hard to achieve.

The aim of this project is to improve the ability to reconstruct ENSO variability from a wider array of climate archives. Analysis of combined stable isotope and Mg/Ca ratios in planktonic foraminiferal populations is an established method to reconstruct long-term average temperature and salinity from marine sediment cores. However, planktonic foraminifera are short-lived organisms (~one month), and the chemical composition of each individual provides a snapshot of past conditions. The range of variability between individuals from a single sample therefore provides a measure for the extent of climate variability. Stable isotope analysis of single specimens has been used successfully to reconstruct changes in ENSO variability during the Holocene and last glacial. In contrast, it is technically feasible to measure Mg/Ca ratios in single individuals, but up to now it has not been possible to relate results to ambient temperature.

The project will focus on sediment cores from the Eastern Pacific Ocean to document changes in thermocline depth and upwelling intensity since the Last Glacial. Standard micropalaeontological and sedimentological methods will be applied to assess long-term shifts in mean climate conditions. Stable isotopes will be measured on single specimens of multiple species of planktonic foraminifera to reconstruct changes in seasonality as well as interannual climate variability. A protocol will be developed to reduce sample size for the analysis of Mg/Ca ratios ideally down to the level of single specimens. Results will be integrated to improve understanding of the temporal and spatial variability of the ENSO system during the past 25,000 yr.