Late Miocene thermal gradients in the equatorial Pacific during prolonged El Niño- and La Niña-like states

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About the Project

Late Miocene (11.63-5.333 Ma) climate was characterised by warmer temperatures and CO2, similar to the 4°C warming and CO2 levels predicted by the IPCC for the year 2100. There has been an ongoing question about whether or not permanent El Niño-like conditions existed during the Late Miocene, with the equatorial Pacific characterised by warm waters spread across the entire region. As the modern El Niño causes significant global climate anomalies, understanding the potential presence and impacts of permanent El Niño-like conditions in the past is important.

There are currently three main hypotheses about which conditions dominated the equatorial Pacific during the Late Miocene: 1) a Late Miocene permanent El Niño-like state (e.g., Ravelo et al., 2014), 2) alternating El Niño-like (9.6-6.5 Ma) and La Niña-like (6.5-6.1 Ma) states (e.g., Nathan and Leckie, 2009; Drury et al., 2018), or 3) modern thermal gradients since ~12 Ma (e.g., Zhang et al., 2014). However, the lack of high-resolution records in the Western Pacific Warm Pool (WPWP) was a hurdle to resolving which hypothesis was more likely. A recent project generated new high-resolution stable oxygen (δ


O) isotope data in the WPWP (Drury, MIONIÑO, 2019-2022), but independent temperature records are nonetheless needed to establish E-W equatorial Pacific temperature gradients, as foraminiferal δ

18O is also influenced seawater δ18O changes (e.g., global ice volume, salinity), as well as temperature.

This PhD project would focus on generating high-resolution sea surface and thermocline temperature records in the WPWP using planktonic foraminiferal Mg/Ca analysis paired with clumped isotope (Δ47) thermometry on coccolith-rich fine fraction sediment. This would make it possible to reconstruct the WPWP thermocline structure, as well as E-W thermal gradients, to understand the thermal impact of alternations between prolonged El Niño-like and La Niña-like conditions. As the records will be high-resolution, it will make it possible to test whether the sensitivity to astronomical forcing changes between these climate states. The impact of these different climate states and associated thermal shifts on the biosphere will be investigated using multi-species planktonic foraminiferal geochemistry records.



The proposed PhD project will focus on generating Mg/Ca major trace element analysis of planktonic mixed layer and thermocline foraminifera to reconstruct upper water column temperatures in the WPWP, using material recovered by the International Ocean Discovery Program (IODP). Clumped isotope Δ47 thermometry on coccolith-rich fine fraction will help constrain long-term temperature changes that may be impacted by changes in seawater Mg/Ca concentration. Scanning electron microscopy to establish specimen preservation and the composition of the isolated sediment fractions. Spectral analysis techniques on the new data will be used to test for any sensitivity changes of sea surface temperatures to astronomical forcing during the changing climate states. Multi-species δ18O and δ13C analyses will be used to trace links between ecological and environmental changes during prolonged warm intervals. 

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