SWEET Team Blog 6, July 2019

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Gordon – Reconstructing ocean temperatures using lipid biomarkers

My expertise

My name is Gordon Inglis and I am an organic geochemist based at the University of Bristol. I use lipid biomarkers (organic compounds which are derived from living organisms) to understand how the earth system behaves under higher CO2 concentrations. I have specific expertise in the development and application of lipid biomarker proxies and have used these to reconstruct temperature, hydrology and biogeochemical change over the last 65 million years.

Reconstructing ocean temperatures using lipid biomarkers

During the Early Eocene Climatic Optimum (EECO; 49 to 53 million years ago), atmospheric CO2 concentrations were much higher than today and likely exceeded 1000 ppm. As a result, the Earth was much warmer. However, quantitative records of temperature are required to test the latest generation of climate model simulations.

Here, I will analyse glycerol dialkyl glycerol tetraethers (GDGTs; see figure) in marine sediments to reconstruct sea surface temperatures during the EECO. GDGTs are membrane-spanning lipids which are inferred to be mainly derived from a group of single-celled organisms (Thaumarchaeota). Thaumarchaeota change their membrane lipid distribution depending upon the temperature of the ocean, and experimental and field-based studies indicate that GDGTs with fewer rings are produced in cooler waters. In contrast, GDGTs with more rings are produced in warmer waters. As such, a simple index, based upon the number of rings (TEX86), can therefore be used to estimate the temperature of the ocean in the past.

The TEX86 paleothermometer has been widely used to reconstruct SST during the Eocene (e.g. Inglis et al., 2015). However, TEX86 SST estimates are typically restricted to a few, well-sampled regions (e.g. SW Pacific). In the SWEET project, I will generate new quantitative temperature estimates from the EECO using the TEX86 paleothermometer and compare my results alongside other SST proxies (e.g. Mg/Ca, δ18O). This data will later be assessed alongside IPCC AR6-class model (UKESM) simulations of a greenhouse climate to understand the causes of EECO warmth.

Beyond temperature…

This work will also have implications for other elements of the climate system. Specifically, what is the impact of higher CO2 and a warmer climate upon the water cycle? A recent modelling study investigating environmental change during the early Eocene found an increase in global precipitation and evaporation rates and an increase in heat transport from the equator to the poles. Proxy records support these first-order predictions; however, additional records are required to understand regional variability. To resolve this, I will analyse the hydrogen isotopic composition (δ2H) of leaf wax biomarkers preserved within the same marine sediments. This proxy can provide insights into atmospheric moisture transport and will help refine our understanding of the hydrological cycle during the EECO.

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