Clumped isotope thermometry in foraminifera - From calibration to Plio-Pleistocene temperature reconstructions in the Indo-Pacific Warm Pool
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Clumped isotopes thermometry on foraminifera holds the potential to accurately reconstruct ocean temperatures on million-year timescales. In contrast to most other paleothermometers, clumped isotopes do not rely on prior knowledge regarding ocean chemistry changes thus evading a major source of uncertainty inherent to most other paleothermometers. This thesis aims to amend our understanding of the clumped isotope signal in foraminifera and provide improvements to this paleothermometer for application to ocean sediments (Paper I). The knowledge gained is applied to Plio-Pleistocene sequences from two locations within the Indo-Pacific Warm Pool in order to address discrepancies among other temperature proxies and shed new light on long-standing debates regarding the long-term temperature evolution of this crucial region in the global ocean (Papers II and III). The results displayed in this thesis include a clumped isotope to temperature calibration dataset for planktonic foraminifera that was combined with several existing foraminifer-based calibrations in order to elucidate potential laboratory differences as well as species effects on the paleothermometer (Paper I). Our combined calibration highlights the excellent agreement among various analytical approaches and different foraminifera species. This work thus provides a robust tool to reconstruct past ocean temperatures using various species and on million-year time scales. The results of papers II and III constrain the Plio-Pleistocene temperature evolution of the Indo-Pacific Warm Pool independent of ocean chemistry changes. The knowledge gained can be used to disentangle the influences of these reservoir changes and the temperature signal recorded in other proxies. The paired Mg/Ca and clumped isotope records comprise evidence for the validity of Plio-Pleistocene Mg/Ca-based sea surface temperature reconstructions and argue against a systematic bias of Pliocene Mg/Ca temperature estimates by past Mg/Ca changes of seawater. The long-term temperature evolution of the Indo-Pacific Warm Pool across the Plio-Pleistocene is illustrated by the combined mixed layer and thermocline records from IODP Sites U1488 (Paper II), U1482 and U1483 (Paper III). The evidence presented in viii this thesis portrays a vertical and lateral expansion of the Indo-Pacific Warm Pool during the Pliocene relative to its modern extent, albeit without significant warming of surface waters in the central part. Our results document the strengthening of glacial periods from the Pliocene to present, while interglacial temperatures are not subject to long-term trends. Moreover, our results corroborate Mg/Ca records from the equatorial Pacific and thus support the hypothesis of a “permanent El Niño-like state” or “El Padre” during the Early Pliocene. The potential of clumped isotope thermometry on foraminifera for the reconstruction of past ocean temperatures at different depths in the water column is demonstrated by the results included in this thesis. Applied on Cenozoic time scales this paleothermometer ideally complements other, higher resolution methods by providing the opportunity to test these proxies and deliver independent constraints on non-thermal effects such as ocean chemistry changes.
Postponed access: the file will be accessible after 2021-06-25
Består avPaper I: Meinicke, N., Ho, S.L., Hannisdal, B., Nürnberg, D., Tripati, A., Schiebel, R. and Meckler, A.N. (2020). A robust calibration of the clumped isotopes to temperature relationship for foraminifers. Geochimica et Cosmochimica Acta 270, 160-183. The article is available in the main thesis. The article is also available at: https://doi.org/10.1016/j.gca.2019.11.022
Paper II: Meinicke, N., Reimi, M.A., Ravelo, A.C., and Meckler, A.N. Coupled Mg/Ca and clumped isotope measurements confirm stable Western Pacific Warm Pool sea surface temperatures over the last 6 million years. The article is not available in BORA.
Paper III: Reimi, M.A., Meinicke, N., Meckler, A.N., and Ravelo, A.C. A multi-proxy study of changes in the Indonesian Throughflow since the Pliocene. The article is not available in BORA.