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

• Published in: Geochimica et Cosmochimica Acta
• Main Author: Leutert, Thomas Jan
• Other Authors: orcid:0000-0002-1714-0080
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: The University of Bergen 2022
• Subjects: Antarctic; South Tasman Rise; Southern Ocean; The Antarctic; Antarc*; Ice Sheet
• Online Access: https://hdl.handle.net/1956/21397

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 ...