Clumped isotope thermometry and oxygen isotope composition of seawater of key climate events during the Oligocene

点击翻译按钮获取中文摘要

Understanding past climate changes and their underlying causes requires reconstructions of past ocean composition and temperatures. However, traditional proxies of ocean temperatures and ice volume (e.g., stabile oxygen isotopes and Mg/Ca values of foraminifera) are undermined by biological processes or changes in ocean chemistry. Clumped isotope thermometry, a promising, novel proxy, circumvents these problems. Carbonate clumped isotope thermometry is entirely thermodynamically based and therefore provides temperature estimates independent of the original isotopic or chemical composition of the source water, in contrast to other carbonate temperature proxies (oxygen isotopes and Mg/Ca ratios). Paired oxygen-clumped isotope measurements on carbonate samples thus enable the calculation of the oxygen isotope composition of the water, from which the carbonates precipitated. That means, clumped isotope thermometry also provides insights into global ice volume changes. The clumped isotope method used here allows measurement of small sample sizes and has an analytical error as small as +/-1-2°C, which is comparable to other temperature proxies. The Oligocene is an important climate epoch as it marks the inception of the icehouse world following the warm greenhouse climate of the Palaeocene and Eocene. The onset of the Oligocene is one of the most fundamental reorganisations of global climate, as the first permanent, large continental ice sheets formed on Antarctica. However, a recent study using clumped isotopes across the Eocene-Oligocene transition (EOT) at a Southern Ocean site indicates no evident temperature change across this ice growth event. This unexpected lack of temperature change could arise from uncertainties in the clumped isotope-T calibration at low temperatures. Therefore, this proposed study aims to establish a clumped isotope-T calibration of modern surface-, thermocline-dwelling and benthic foraminifera of high latitude Southern Ocean sites to improve published clumped isotope calibration equations on the low-temperature end. Finally, this study will reconstruct reliable bottom and surface seawater temperatures, using paired measurements of foraminiferal clumped isotope values and Mg/Ca ratios, at sites spanning different latitudes and ocean basins. These analyses will target specific time intervals of the Oligocene, such as the EOT (to reassess the findings of the Southern Ocean study) and key glaciation phases (Oi-2b, Mi-1). The Oi-2b glaciation occurred during maximum ice volume of the entire Oligocene. During the Mi-1 glaciation, benthic foraminiferal oxygen isotope records illustrate an increase similar to the EOT interval, raising the question of comparable estimates of ice growth. In summary, investigating these intervals will evaluate the seawater temperatures, seawater oxygen isotope composition and global ice volume changes during a time when global climate was mainly driven by Southern Hemisphere climate mechanisms.