Sr/Ca and δ18O seasonality in a Porites coral from the MIS 9 (339-303 ka) interglacial

Past changes in the seasonal distribution of insolation across the Earth's surface are thought to play a critical role in Quaternary climate cycles. In this study we use Sr/Ca and delta O-18 as geochemical proxies in the skeleton of a fossil Porites coral to reconstruct the seasonal cycle of sea surface temperature (SST) at Henderson Island, southeast Pacific (24 degrees S, 128 degrees W) during the Marine Isotope Stage (MIS) 9 interglacial (similar to 339-303 ka). Previously-published closed-system U-series ages provide broad age constraints for the timing of reef growth for this unit on Henderson Island, ranging between 334 and 306 ka. We apply published delta O-18-SST slope relationships to the stacked 6180 annual cycle in the fossil Porites, and find the amplitude of the seasonal cycle of SST recorded by the MIS 9 coral to be -4.1 +/- 0.57 degrees C, which agrees within error with the modem seasonal cycle of SST (similar to 4.1 degrees C). Sr/Ca-SST slope relationships applied to the fossil Porites stacked Sr/Ca annual cycle suggest the amplitude of the seasonal cycle of SST was similar to 4.7 +/- 0.75 degrees C, exceeding the modern cycle by similar to 15%, but within error of the modem value. Taken together, these results suggest the seasonal cycle of SST at Henderson Island during MIS 9 equaled or exceeded the modem amplitude. Using modem latitudinal relationships between insolation seasonality and SST seasonality; we present a new application for SST amplitudes reconstructed from fossil corals that can be used in conjunction with U-series ages to provide additional geochronological constraints on the development of open-ocean, interglacial reefs. (c) 2006 Elsevier B.V. All rights reserved.