Reconstructing tropical Pacific climate change using alkenones as sea-surface temperature proxies
This research was performed as part of a University of Durham funded PhD studentship investigating the potential causes of mid-Pleistocene climate change (~ 1 million years ago).
At the present day, variations to tropical Pacific circulation, expressed as the El Niño/Southern Oscillation (ENSO), have impacts over heat and moisture fluxes to the past ice-sheet source regions. Using the modern sea-surface temperature distributions associated with ENSO as an analogue, the hypothesis that tropical Pacific circulation change drove the mid-Pleistocene ice-sheet expansion was tested using the alkenone biomarkers in marine sediments from the east and west equatorial Pacific (McClymont and Rosell-Melé, 2005).
Alkenones are long-chain ketones synthesised by some members of the Prymnesiophyceae class of algae. The relative abundance of the C37 alkenones, expressed by the UK37' index, records mean annual sea-surface temperatures. The upper limit of the proxy, near 29ºC, is reached at the present day at the chosen West Pacific site ODP 806, because the relative abundance of the ‘warm’ C37:2 alkenone far exceeds that of the C37:3 alkenone. To determine UK37' values at ODP 806 a selection of samples was analysed using GC/CIMS at the NERC LMSF. As GC/CIMS is more sensitive than GC-FID quantification of C37:3 alkenone could be undertaken with less error (Rosell-Melé et al. 1995).
Figure 1 shows the results from ODP 806 compared to those from the east Pacific site, ODP 849. The data reveal that although moderate cooling occurred in the west Pacific during the mid-Pleistocene, much stronger cooling occurred in the east. By comparison with modern sea-surface temperature distributions, this suggests a strengthening of tropical Pacific circulation after 1.15 million years ago, prior to the ice-sheet expansion ~0.9 million years ago. By reducing heat flux and increasing moisture transport to the ice-sheet source regions, it is proposed that these events were important in driving the onset of the mid-Pleistocene climate transition (McClymont and Rosell-Melé, 2005).