Luke Handley

Tropical sea temperatures in the high-latitude South Pacific during the Eocene

Sea-surface temperature (SST) estimates of ~30 °C from planktic foraminifera and archaeal membrane lipids in bathyal sediments in the Canterbury Basin, New Zealand, support paleontological evidence for a warm subtropical to tropical climate in the early Eocene high-latitude (55°S) southwest Pacific. Such warm SSTs call into question previous estimates based on oxygen isotopes and present a major challenge to climate modelers. Even under hypergreenhouse conditions (2240 ppm CO2), modeled summer SSTs for the New Zealand region do not exceed 20 °C.

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 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 (δ18OSW). 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 δ18OSW 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.

Early Paleogene evolution of terrestrial climate in the SW Pacific, Southern New Zealand

[1] We present a long-term record of terrestrial climate change for the Early Paleogene of the Southern Hemisphere that complements previously reported marine temperature records. Using the MBT′-CBT proxy, based on the distribution of soil bacterial glycerol dialkyl glycerol tetraether lipids, we reconstructed mean annual air temperature (MAT) from the Middle Paleocene to Middle Eocene (62–42 Ma) for southern New Zealand. This record is consistent with temperature estimates derived from leaf fossils and palynology, as well as previously published MBT′-CBT records, which provides confidence in absolute temperature estimates. Our record indicates that through this interval, temperatures were typically 5°C warmer than those of today at such latitudes, with more pronounced warming during the Early Eocene Climate Optimum (EECO; ∼50 Ma) when MAT was ∼20°C. Moreover, the EECO MATs are similar to those determined for Antarctica, with a weak high-latitude terrestrial temperature gradient (∼5°C) developing by the Middle Eocene. We also document a short-lived cooling episode in the early Late Paleocene when MAT was comparable to present. This record corroborates the trends documented by sea surface temperature (SST) proxies, although absolute SSTs are up to 6°C warmer than MATs. Although the high-calibration error of the MBT′-CBT proxy dictates caution, the good match between our MAT results and modeled temperatures supports the suggestion that SST records suffer from a warm (summer?) bias, particularly during times of peak warming.