Atmospheric CO2 variability and climate sensitivity during past warm climates – a lesson for the future?

Project Rationale
Anthropogenic CO2 emissions are rapidly altering Earth’s climate, pushing it toward a warmer state for which there is no historical precedent. The only way to test the behaviour of the Earth in warm climate states is to examine the geological record. The mid-Miocene Climatic Optimum (MCO; ~14 to 17 million years ago) is a dynamic climate state characterised by overall warmer temperatures (> 7°C higher than today) and may be an appropriate analogue for the future1. However, CO2 estimates from this interval are much lower than expected (< 450‒550 parts per million; i.e. near-modern). This implies that: i) our CO2 estimates for the Miocene are too low, or ii) the modern climate system is more sensitive to CO2 change.

To resolve this conundrum, the student will: i) generate new high-fidelity, high-resolution CO2 records during the mid-Miocene (~14 to 17 million years ago) using several proxies preserved within marine sediments, ii) estimate the sensitivity of the climate system to warming, and iii) assess the real-world implications for future climate warming.

Methodology
Multiple proxies can be used to reconstruct atmospheric CO2 concentrations. The primary marine proxies include: i) the boron isotopic composition (δ11B) of marine carbonate and ii) the carbon isotopic composition (δ13C) of marine phytoplankton lipids (e.g. alkenones, chlorophyll-derivatives). Each proxy has its own strengths and weaknesses2 and we will use a combined approach to provide the most accurate CO2 reconstructions. For this project, the student will use two established proxies (alkenone δ13C and planktonic foraminifera δ11B; see ref. 2) and one novel proxy (phytane δ13C; see ref. 3) to determine CO2 during the mid-Miocene. This will be achieved by analysing deep-sea sediment cores in the equatorial Atlantic (ODP Site 959, ODP Site 925/925) and equatorial Pacific (IODP 1338) A knowledge of algal cell size is required for successful alkenone-based pCO2 reconstructions. This will be estimated by studying the nannoplankton assemblage and exceptionally well-preserved fossil cells (coccoliths) in a subset of samples. Taken together, this multi-proxy approach will be used to characterise the sensitivity of the Miocene earth system to warming.

Training
The INSPIRE DTP program provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the School of Ocean and Earth Science. The student will receive expert training in [1] organic geochemical techniques (Inglis/Whiteside), including compound specific isotope analysis, [2] identification of planktonic foraminifera (Foster) and nannofossils (Gibbs), and [3] inorganic geochemical techniques, including boron isotope analysis using multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) (Foster). The student will visit and obtain samples from the IODP Bremen Core Repository (https://tinyurl.com/y5gm95e9). The student will also present at national and international conferences, write peer-reviewed publications and a PhD thesis. The research training addresses practical, numerical, statistical and laboratory skills, equipping the student for a career across a range of professions.