Middle Eocene climate cyclicity in the southern Pacific: Implications for global ice volume

We use a multiproxy approach to determine surface and bottom water temperatures off the coast of New Zealand during the middle Eocene and to constrain the delta(18)O of seawater. We use these data to place constraints on the size and variability of global ice sheets at that time. The Hampden Section in South Island is characterized by exceptionally well preserved microfossils and clear sedimentary cyclicity, providing a remarkable window into conditions at palleolatitude similar to 55 degrees S in the Pacific Ocean. The cyclicity was studied in detail over a similar to 4 m section corresponding to an interval of similar to 70 k.y., ca. 41.7 Ma. The sedimentary cycles are defined by fluctuations in the sand (> 63 mu m) component, occurring on a wavelength of similar to 1 m, corresponding to Milankovitch-scale frequency. Analyses of foraminifer oxygen isotopic (delta(18)O) and Mg/Ca composition, combined with TEX(86) analyses from organic carbon, are used to generate records of seawater temperature and oxygen isotopic composition (delta(18)O(SW)). These indicate bottom water temperatures of similar to 11-13 degrees C and sea surface temperatures of similar to 23-25 degrees C with good agreement between the proxies. Temperature cyclicity with a magnitude of similar to 1.5 degrees C occurs in both surface and bottom waters, approximately in phase with the sedimentary cycles. Estimates of delta(18)O(SW) have a mean value of -1.2 parts per thousand throughout the study section. Taken together, the data suggest a largely ice-free world with orbital-scale cycles expressed as temperature and hydrological variation with little or no ice volume change.