Palaeoclimate and palaeoenvironment of the middle Eocene southern Pacific: insights from New Zealand

The Hampden Section of South Island, New Zealand, is used to generate a multiproxy record of middle Eocene palaeoclimate. The Hampden Formation is a calcareous clay-rich siltstone deposited in a shelf edge environment, containing exceptionally well-preserved micro- and nannofossils. A range of sedimentary, geochemical and fossil assemblage records from this formation are combined to provide new insights into middle Eocene climate in the southern Pacific Ocean. A palaeoclimate record generated through the formation spans the period from -42.1 to -39.3 Ma and shows clear cooling in ocean mixed-layer temperatures from -18 C to -14 C, with long period cyclicity (likely -405 k.y.) superimposed. This cooling trend is punctuated by a transient warm excursion of - 2.5 C lasting -450 k.y. that may represent the Middle Eocene Climatic Optimum. There is a brief influx of the tropical planktonic foraminifer Hantkenina australis during this time and the excursion is also marked by a substantial drop in the sand fraction of the sediment, indicating that the warm interval had a significant impact on the terrestrial realm. The water temperatures produced from this record are warmer than those previously estimated for similar latitudes, likely due to the excellent preservation of the microfossils reducing the effect of diagenesis on their geochemistry. These temperatures suggest that during the middle Eocene, the site lay in the path of a warm southward flowing current rather than a cold Antarctic gyre. They also support the hypothesis that the global latitudinal temperature gradient was reduced during the middle Eocene. A shorter high-resolution record of climate was also obtained. This shows -18 k.y. cyclicity in a range of palaeoclimatic and palaeonvironmental proxies that is considered to have been orbitally forced. The Mg/Ca derived bottom water temperatures range from -11 to 13 C and the TEXg6 derived surface water temperatures from -22.5 to 24 C through these cycles. The combination of 8 O and Mg/Ca in foraminiferal carbonates indicates that there was little or no ice present globally. Despite the lack of ice available to amplify the cycles they had a major and complex effect on both the marine and terrestrial environments