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How hot is hot? Tropical ocean temperatures and plankton communities in the Eocene Epoch

Piga, Emanuela 2020. How hot is hot? Tropical ocean temperatures and plankton communities in the Eocene Epoch. PhD Thesis, Cardiff University.
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Sea surface temperatures approach but rarely exceed 30°C in the Indo-Pacific Warm Pool, the hottest region of the modern open ocean. Climate models with reconstructed continental configurations and elevated CO2 concentrations suggest that an Indo-Pacific Warm Pool has existed since the Mesozoic but virtually no reliable paleotemperature proxy data from the region exist, partly due to the rarity of finding exceptionally preserved foraminifera from deep time. Models suggest that temperatures in the Warm Pool may have become extremely hot, up to 40°C, during warm climate phases such as the early Eocene, potentially exceeding the tolerance limit of eukaryotic life. As part of this study, a thorough exploration of “legacy” samples archived in the British Petroleum (BP) foraminiferal collections at the Natural History Museum, London, was conducted to search for material suitable for geochemical analysis. This yielded an important sample from the early Eocene of Papua New Guinea which was analysed as well as material from the late Eocene of Java that was collected in a previous expedition. New oxygen and carbon isotope data are presented from both the early and late Eocene planktonic foraminifera, alongside Mg/Ca for paleotemperature reconstructions from the early Eocene. The study samples contained exceptionally well-preserved (“glassy”) foraminifera but suffered from diagenetic infilling by late stage calcite. By the innovative method of crushing the samples and carefully separating foraminiferal test fragments from the infill it was possible to obtain reliable geochemical data from a variety of mixed layer and thermocline species, as well as one data point from benthic foraminifera for both the early Eocene and late Eocene. The successful outcome of the separation was confirmed by the infill consistently having lower δ18O values than its associated original test, which is indicative of the influence of meteoric waters. Eocene sea surface temperatures were derived from Papua New Guinea, and measured ~32-35°C, which is several degrees warmer than comparable early Eocene data from a similar latitude in Tanzania. Thus, the data support the presence of a moderately hot Warm Pool in the early Eocene which supported an abundant and diverse eukaryotic plankton community. Considering that modelled palaeogeographies positioned early Eocene Papua New Guinea at ~29°S, the results are still several degrees warmer than modern Warm Pool temperatures. This suggests there were even higher temperatures at the core of the early Eocene Warm Pool. Late Eocene temperatures were retrieved from Java which was positioned in the tropics (0.54°N), and recorded sea surface temperatures of 37°C while still hosting a diverse and abundant foraminiferal assemblage. When simulating the Eocene temperatures on the palaeoclimate models under 1120 ppm of atmospheric CO2, the results from late Eocene Java matched the model outputs better than early Eocene Papua New Guinea, as the temperatures of the latter were underestimated to a greater extent by the models. This may be due to either the presence of higher than 1120 ppm atmospheric CO2 concentrations in the early Eocene, which would have increased the simulated temperatures of the climate model, or a higher climate sensitivity during the early Eocene due to the intensified hydrological cycle typical of this interval which would have increased the amount of tropical cyclones transferring heat from the tropics to higher latitudes.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 14 December 2020
Last Modified: 14 Dec 2021 02:30

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