Minton, Paul; (2022) Middle to late Miocene palaeoceanography of the equatorial Pacific Ocean. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The middle Miocene saw an extended period of global warmth followed by rapid transition to cooler conditions with Antarctic glaciation. This period of contrasting climate states has been much studied, unlike the mid-late Miocene which immediately follows. Records obtained from proxies for tropical sea-surface conditions during the mid-late Miocene are scarce, reducing understanding of interactions between changing boundary conditions - such as ocean gateway constriction - and atmospheric circulation, solar insolation, and the carbon cycle. Variation in conditions in the eastern equatorial Pacific Ocean (EEP) surface waters affect global as well as regional climate on short and long timescales. The overarching aim of this project is to produce a high-resolution planktonic foraminiferal isotope record from Site U1338 in the EEP. The resulting ~2.5 Myr isotope record (~13.3 - 10.8 Ma) provides data related to the mixed-layer, lower mixed-layer and thermocline, together with Mg/Ca derived SSTs and temperatures from lower in the surface waters, the latter based on planktonic foraminiferal species not previously used for these purposes. The isotope and Mg/Ca data are compared with XRF-derived geochemical data, used to reconstruct conditions across the equatorial Pacific (EP), and to determine relationships with changing patterns of solar insolation. Application of Mn/Ca to infer ocean oxygenation is also explored. The imprint of the mid-Miocene climate transition (MMCT) is apparent in the EEP as late as ~12.5 Ma but the surface ocean responses to glaciation events are complex and not clearly coupled. Upwelling in the EEP is relatively suppressed until the end of the MMCT when the carbonate crash begins. A significant and lasting alteration in surface conditions in the EEP starts at ~12.1 Ma with freshening waters related to atmospheric zonal change and increased upwelling. This pattern is accentuated during a cool trough from ~11.6 - 11.2 Ma likely associated with the development of the West Pacific Warm Pool. East-west SST and δ18O gradients reveal a secular temperature difference but show considerable short-term variability. This period represents a complex and prolonged adjustment to a cooling climate and oceanic gateway constriction.

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