The role of atmospheric CO2 in the first great ice ages: insights from very high-resolution boron isotope records

The concentration of carbon dioxide (CO2) in the atmosphere is often said to be the “control knob” of Earth’s climate. For instance, recent work at Southampton has confirmed that a decline in atmospheric CO2 played a key role in triggering the establishment of large Northern Hemisphere ice sheets 2.7 million years ago (Martínez-Botí et al., 2015; www.thefosterlab.org). On shorter timescales, ice core records have shown that CO2 also plays a crucial role in driving the well documented glacial-interglacial cycles of the past 2.7 million years (Quaternary Period) by amplifying the effects of small variations in solar radiation due to changes in Earth’s orbit around the Sun. The exact processes involved and the sensitivity of the Earth to changes in CO2, however, remain key gaps in our understanding of the Earth system with clear relevance for our warm future (e.g. Martínez-Botí et al., 2015). In this project we will examine and use the initial glacial-interglacial cycles of the Quaternary, centered on Marine Isotope Stage 100 around 2.5 million years ago (Shakun et al., 2017), as a test-bed to address these fundamental knowledge gaps.

Orbital-scale records of atmospheric CO2 change are currently limited by the availability of old Antarctic ice to the last 800 thousand years (Bereiter et al., 2015). Proxy based records extend our understanding of CO2 beyond this (e.g. www.p-CO2.org), but currently no proxy record is at sufficient resolution to document the nature, magnitude, and timing of orbital-scale CO2 change under different background climate states. Here we will apply the boron isotope pH-CO2 proxy (http://www.p-co2.org/boron) to reconstruct atmospheric CO2 at an unprecedented temporal resolution (~1 sample per 2-3 thousand years; 10x higher than any thus far published) from 2.4 to 2.7 million years ago. Coupled with new ice volume reconstructions and records of sea surface temperature, this will provide a unique window into the relationship between CO2, ice volume and climate (Martínez-Botí et al., 2015; Shakun et al., 2017), allowing us to address the following questions:

Does the sensitivity of the climate system vary as background climate state changes?
What is the magnitude of CO2 variability in the early Quaternary on orbital timescales? And how does this constrain the mechanisms responsible for orbital changes in CO2?
What are the phase relationships between the major components of the climate system in the early Quaternary? What does this reveal about how the Earth System functions when warmer and colder than today?

The SPITFIRE DTP programme 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 in Ocean and Earth Sciences at the National Oceanography Centre. Specific training will include:

· Training in state-of-the-art analytical techniques using the world-class geochemical facilities available at Southampton. This includes Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) for the measurement of boron isotopes, ICP-MS analysis of trace elements, and traditional gas source isotope ratio mass spectrometry for oxygen and carbon isotopes.

· Training in cutting edge statistical methods for combining and interpreting often noisy geochemical and isotopic time series. There is potential to undertake part of this training via a work placement at ANU in Canberra.

The student will become a member of the vibrant and diverse Geochemistry and Palaeoceanography/Palaeoclimate groups at the University of Southampton. Opportunities also exist for participation in ocean going science and cruises.