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

Abstract

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 δ¹⁸O 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 micro fossils and clear sedimentary cyclicity, providing a remarkable window into conditions at paleo latitude ~55°S in the Pacific Ocean. The cyclicity was studied in detail over a ~4 m section corresponding to an interval of ~70 k.y., ca. 41.7 Ma. The sedimentary cycles are defined by fluctuations in the sand (>63 μm) component, occurring on a wavelength of ~1 m, corresponding to Milankovitch-scale frequency. Analyses of foraminifer oxygen isotopic (δ¹⁸ O) and Mg/Ca composition, combined with TEX₈₆ analyses from organic carbon, are used to generate records of seawater temperature and oxygen isotopic composition (δ¹⁸O_SW). These indicate bottom water temperatures of ~11–13 °C and sea surface temperatures of ~23–25 °C with good agreement between the proxies. Temperature cyclicity with a magnitude of ~1.5 °C occurs in both surface and bottom waters, approximately in phase with the sedimentary cycles. Estimates of δ¹⁸O_SW have a mean value of −1.2‰ 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.