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

Postponed access: the file will be accessible after 2022-01-27This 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

Southern Ocean bottom-water cooling and ice sheet expansion during the middle Miocene climate transition

The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT.publishedVersio

Coupled Southern Ocean cooling and Antarctic ice sheet expansion during the middle Miocene

The middle Miocene climate transition (~14 million years ago) was characterized by a dramatic increase in the volume of the Antarctic ice sheet. The driving mechanism of this transition remains under discussion, with hypotheses including circulation changes, declining carbon dioxide in the atmosphere and orbital forcing. Southern Ocean records of planktic foraminiferal Mg/Ca have previously been interpreted to indicate a cooling of 6–7 °C and a decrease in salinity that preceded Antarctic cryosphere expansion by up to ~300,000 years. This interpretation has led to the hypothesis that changes in meridional heat and vapour transport along with an early thermal isolation of Antarctica from extrapolar climates played a fundamental role in triggering ice growth. Here we revisit the middle Miocene Southern Ocean temperature evolution using clumped isotope and lipid biomarker temperature proxies. Our records indicate that the Southern Ocean cooling and the associated salinity decrease occurred in phase with the expansion of the Antarctic ice sheet. We demonstrate that the timing and magnitude of the Southern Ocean temperature change seen in previous reconstructions can be explained if we consider pH as an additional, non-thermal, control on foraminiferal Mg/Ca ratios. Therefore, our new dataset challenges the view of a thermal isolation of Antarctica preceding ice sheet expansion, and suggests a strong coupling between Southern Ocean conditions and Antarctic ice volume in times of declining atmospheric carbon dioxide.acceptedVersio

North Atlantic surface ocean warming and salinization in response to middle Eocene greenhouse warming

Quantitative reconstructions of hydrological change during ancient greenhouse warming events provide valuable insight into warmer-than-modern hydrological cycles but are limited by paleoclimate proxy uncertainties. We present sea surface temperature (SST) records and seawater oxygen isotope (δ18Osw) estimates for the Middle Eocene Climatic Optimum (MECO), using coupled carbonate clumped isotope (Δ47) and oxygen isotope (δ18Oc) data of well-preserved planktonic foraminifera from the North Atlantic Newfoundland Drifts. These indicate a transient ~3°C warming across the MECO, with absolute temperatures generally in accordance with trace element (Mg/Ca)–based SSTs but lower than biomarker-based SSTs for the same interval. We find a transient ~0.5‰ shift toward higher (δ18Osw), which implies increased salinity in the North Atlantic subtropical gyre and potentially a poleward expansion of its northern boundary in response to greenhouse warming. These observations provide constraints on dynamic ocean response to warming events, which are consistent with theory and model simulations predicting an enhanced hydrological cycle under global warming

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

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

Middle Miocene bottom water carbonate clumped isotope temperatures, ODP Hole 120-747A, Kerguelen Plateau

This table includes temperatures calculated from carbonate clumped isotope measurements performed on middle Miocene benthic foraminifera from Ocean Drilling Program (ODP) Hole 120-747A, Kerguelen Plateau, and is an accompaniment to the manuscript "Southern Ocean bottom water cooling and ice sheet expansion during the middle Miocene climate transition", Climate of the Past, submitted. A complete set of individual measurement data, including sample labels, depth and raw mass spectrometer data, can be found on the EarthChem database (Leutert et al., 2021 doi:10.26022/IEDA/111808)

Temperatures from TEX86 measurements performed on middle Miocene sediment material from ODP Hole 189-1171C

This table includes temperatures calculated from organic biomarker (TEX86) measurements performed on middle Miocene sediment material from Ocean Drilling Program (ODP) Hole 189-1171C, South Tasman Rise

Southern Ocean bottom-water cooling and ice sheet expansion during the middle Miocene climate transition

The middle Miocene climate transition (MMCT), around 14 Ma, was associated with a significant climatic shift, but the mechanisms triggering the event remain enigmatic. We present a clumped isotope (Δ47) bottom-water temperature (BWT) record from 16.0 to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean and compare it to existing BWT records from different latitudes. We show that BWTs in the Southern Ocean reached 8–10 ∘C during the Miocene climatic optimum. These high BWT values indicate considerably warmer bottom-water conditions than today. Nonetheless, bottom-water δ18O (calculated from foraminiferal δ18O and Δ47) suggests substantial amounts of land ice throughout the interval of the study. Our dataset further demonstrates that BWTs at Site 747 were variable with an overall cooling trend across the MMCT. Notably, a cooling of around 3–5 ∘C preceded the stepped main increase in benthic δ18O, interpreted as global ice volume expansion, and appears to have been followed by a transient bottom-water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom-water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Feedbacks required for substantial ice growth and/or tectonic processes may have contributed to the observed decoupling of global ice volume and Southern Ocean BWT

Discrete bin temperatures from carbonate clumped isotope measurements performed on middle Miocene planktic foraminifera from ODP Hole 189-1171C

This table includes temperatures calculated from carbonate clumped isotope measurements performed on middle Miocene planktic foraminifera from Ocean Drilling Program (ODP) Hole 189-1171C, South Tasman Rise. A complete set of individual measurement data, including sample labels, depth and raw mass spectrometer data, can be found on the EarthChem database (doi:10.26022/IEDA/111547)