Clumped isotope thermometry as a new tool for reconstructing Miocene climate change

Abstract

This PhD thesis focuses on the clumped isotope paleothermometer and its application to foraminiferal carbonates buried in ocean sediments. Based on new proxy evidence for ocean temperature, the thesis aims at improving our understanding of the mechanisms driving the climate system in a warmer world. In the first paper, the effects of diagenetic processes on clumped isotope temperatures are examined in order to assess the fidelity and robustness of the paleothermometer for applications deeper in geological time. For this purpose, clumped isotope temperature data measured on middle Eocene benthic and planktic foraminifera from six ODP/IODP sites in the Atlantic Ocean are compared. Our results demonstrate that benthic and well-preserved planktic foraminiferal carbonates are likely to yield robust temperature estimates of initial calcification, whereas temperatures derived from planktic foraminiferal tests with clear signs of diagenetic alteration appear to be biased towards cool temperatures. These observations are supplemented with end-member mixing modeling. In the second paper, we use planktic foraminiferal clumped isotope and organic biomarker-based temperature records from ODP Site 1171 on the South Tasman Rise to constrain the thermal evolution of the upper waters of the Southern Ocean across the middle Miocene climate transition, which is a large-scale climate shift towards colder conditions. Our results suggest that upper ocean cooling was gradual and coupled to the expansion of the Antarctic ice sheet. These observations contrast with previous Mg/Ca-based temperature reconstructions that indicate much more abrupt cooling preceding ice sheet expansion. We show that Mg/Ca- based paleotemperature estimates can be brought into agreement with those based on clumped isotopes and TEX86 when taking into account pH as a non-thermal influence on Mg/Ca in planktic foraminifera. Integrating our upper ocean temperature records with recent reconstructions of atmospheric CO2 indicates that the effect of CO2 forcing on southern high latitude climate may have been more important than previously assumed. In the third paper, the focus is on middle Miocene bottom water temperatures and ice volume. We present clumped as well as oxygen and carbon isotope data measured on benthic foraminiferal tests from ODP Site 747 located on the Kerguelen Plateau in the Southern Ocean. Our results suggest that Middle Miocene Southern Ocean bottom waters were substantially warmer than today, and then cooled by ~3-5°C. This cooling seems to precede ice growth during the middle Miocene climate transition, and was followed by a transient warming. We hypothesize that bottom water temperatures at Site 747 may have been influenced by regional processes, and specifically changes in heat transport between the upper and deep ocean. Taken together, the results of this thesis provide new constraints on the robustness of the clumped isotope paleothermometer towards burial diagenesis, and demonstrate the potential of the paleothermometer to provide key insights into Earth's climate history. Continued clumped isotope analyses on foraminiferal carbonates from past greenhouse climates may further improve our understanding of the impacts of future warming on sensitive regions such as Antarctica.