Middle Eocene climate cyclicity in the southern Pacific; implications for global ice volume

• Published in: Geology (Boulder) 2008-08, Vol.36 (8), p.651-654
• Main Authors: Burgess, Catherine E, Pearson, Paul N, Lear, Caroline H, Morgans, Hugh E. G, Handley, Luke, Pancost, Richard D, Schouten, Stefan
• Format: Article
• Language: English
• Subjects: alkaline earth metals; Australasia; biochemistry; calcium; carbon; Cenozoic; clastic rocks; clastic sediments; climate forcing; Coasts; cycles; Eocene; Foraminifera; Fossils; geochemical indicators; geochemistry; glacial geology; global; Hampden Beach; Hampden Formation; ice; ice sheets; ice volume; Invertebrata; isotope ratios; Isotopes; magnesium; Marine; metals; Mg/Ca; microfossils; middle Eocene; Milankovitch theory; New Zealand; O-18/O-16; orbital forcing; organic carbon; Oxygen; Pacific Ocean; paleo-oceanography; paleoclimatology; paleoecology; Paleogene; paleotemperature; Protista; sandstone; sea-surface temperature; Seawater; sedimentary rocks; Sediments; silt; South Island; South Pacific; Southwest Pacific; stable isotopes; Stratigraphy; Temperature; Tertiary; West Pacific
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G24762A.1

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 approximately 55°S in the Pacific Ocean. The cyclicity was studied in detail over a approximately 4 m section corresponding to an interval of approximately 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 approximately 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 (δ18OSW). These indicate bottom water temperatures of approximately 11-13°C and sea surface temperatures of approximately 23-25°C with good agreement between the proxies. Temperature cyclicity with a magnitude of approximately 1.5°C occurs in both surface and bottom waters, approximately in phase with the sedimentary cycles. Estimates of δ18OSW have a mean value of -1.2 per mil 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.