Variations in sea surface temperature reconstructed by algal biomarker thermometry in the Neogene equatorial Pacific sediments

The eastern equatorial Pacific Ocean today sustains significant amounts of global marine productivity, and the region is one of the largest marine sources of CO2 to the atmosphere. However, geological time-scale variations of marine environment and ecological / biogeochemical systems in the equatorial Pacific have been still unclear. In this study, we reconstruct the variations of sea surface temperature (SST) by long chain alkenone and the newest long-chain diol thermometers from the equatorial Pacific sediments, and discuss fluctuations in paleoceanographic and paleoclimatic systems in this region during the Neogene. Integrated Ocean Drilling Program (IODP) Expeditions 320/321 (Pacific Equatorial Age Transect; PEAT) recovered a Cenozoic sediment record from the equatorial Pacific by coring above the palaeoposition of the Equator at successive crustal ages on the Pacific plate. We used a core U1337 in the present study. We identify C37 - C38 alkenones as well as saturated C28 and C30 1,13-diols, C28 and C30 1,14-diols, and C30 1,15-diol from almost all the Neogene sediments (23 - 0.23 Ma) in a core U1337. This indicates that diatom, haptophyte and eustigmatophyte algal productions were consistently significant in the equatorial Pacific throughout the Neogene. The UK'37 values were converted to temperatures by using the calibrations reported by Prahl et al. (1988) and Kienast et al. (2012). The alkenone-based SSTs in a core U1337 were nearly constant over the past 25 Ma, ranging from 26 to 28 C, although there were two much lower spikes of 15 - 20 C in 13.2 - 12.5 Ma and 6 Ma. The Long chain Diol Index (LDI; Rampen et al., 2012) values were converted to SSTs by using the calibrations reported by Rampen et al. (2012) and Sawada et al. (unpublished data). The LDI values were estimated to be 7 - 30 C and 12 - 27 C by the Rampen et al. and Sawada et al. calibrations, respectively. The decreasing spikes of SSTs in U1337 core are observed in the horizons of 12.5Ma, 11Ma, 7Ma and 2.5 - 1 Ma. It is found that the decreasing spikes of diol-based SST correspond to the increasing peaks of the Diol indices. These results suggest that the much lower diol-based SSTs were presumably attributed to intrusion of cooler deep waters into the sea face layer by enhanced upwelling system. Variations in diol-based SSTs were quite different from those in alkenone-based SSTs in U1337 core. Thus, the diol-based temperatures might reflect water temperatures in not only sea surface but also subsurface layers in upwelling region of the equatorial Pacific. Furthermore, decreasing spikes of diol-based SSTs in the horizons of 12.5Ma, 11Ma, 7Ma and 2.5 - 1 Ma correspond to the East Antarctica Ice Sheet event (EAIS), Carbonate crash event, onset of the Biogenic bloom event, and the Northern Hemisphere Glaciation (NHG), respectively. Hence, the trends of variation in the diol-based temperatures were possibly concordant with that of the Neogene global paleoclimatic fluctuation.