Doctoral lecture in Geology - Jonathan Henrik Råberg

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Ph.D. student: Jonathan Henrik Råberg

Dissertation title: Lipids at high latitudes: investigation of sources, environmental controls, and new potential applications of brGDGT-based paleoclimate proxies

Advisor: Dr. Áslaug Geirsdóttir, Professor at the the Faculty of Earth Sciences, University of Iceland

Doctoral committee: 
Dr. Gifford H. Miller, Professor at the University of Colorado, Boulder, USA
Dr. Julio Sepúlveda, Professor at the University of Colorado, Boulder, USA
Dr. Sebastian Kopf, Professor at the University of Colorado, Boulder, USA
Dr. Thomas M. Marchitto, Professor at the University of Colorado, Boulder, USA
Dr. Noah Fierer, Professor at the University of Colorado, Boulder, USA

Chair of Ceremony: Dr. Freysteinn Sigmundsson, Research Scientist at the Institute of Earth Sciences, University of Iceland, and the Head of the Faculty of Earth Sciences, University of Iceland

This is a joint doctoral degree between the University of Iceland and the University of Colorado, Boulder and the defence took place at the University of Colorado, Boulder on December 15th 2021.

Abstract:
As high latitude regions continue a decades-long trend of warming at roughly twice the rate of the global average, an understanding of their climatic histories becomes increasing important for predicting their future. Organic molecular proxies preserved in lake sediment archives offer one avenue for reconstructing key elements of such past climates, including their temperature, precipitation, and vegetation regimes. In particular, a class of bacterial membrane-spanning lipids called branched glycerol dialkyl glycerol tetraethers (brGDGTs) form the basis for a paleothermometer that can be applied to reconstruct temperatures as far back as the Cretaceous in sedimentary archives across the globe.

Despite these successes, challenges remain that complicate the development and application of brGDGT-based proxies. First, while they correlate best with temperature and pH, other environmental parameters can influence brGDGT distributions, including seasonality, conductivity, and oxygen availability. Second, it is unknown whether these empirical correlations are the result of a direct physiological response of brGDGT-producing organisms to their environment or an indirect effect resulting from variations in bacterial community composition. Finally, an incomplete understanding of where brGDGTs are produced on the landscape and how they contribute to the sedimentary record hinders our ability to interpret proxies in mixed-source archives.

Herein, I present research addressing each of these three challenges with an emphasis on the Eastern Canadian Arctic and Iceland. First, I develop a technique for grouping brGDGTs based on structural characteristics and show that it can be used to deconvolve the effects of temperature and pH/conductivity. I further find a warm-season bias in brGDGT-derived temperatures and develop calibration equations for temperature and conductivity. Next, I compile >2500 samples from a dozen sample types across the globe and find near-universal trends in the relationships between brGDGTs and temperature, pH, and one another. These commonalities support a physiological basis for observed environmental trends. Finally, by measuring brGDGTs in their intact, polar form, I find that lipid sources in lake catchments can be distinguished and suggest novel applications down core. By advancing our understanding of brGDGTs, my results further our ability to reconstruct key climatic variables from sedimentary archives, especially at high latitudes.

About the doctoral candidate:

Jonathan Henrik Råberg is a Swedish-American researcher from Lexington, Massachusetts in the United States. Jonathan received his B.A. with distinction in Chemistry at Carleton College in 2012 and his M.S. in Chemistry from the University of California Berkeley in 2015, where he used spectroscopic methods to study solid-liquid interfaces for new battery technologies. Jonathan was drawn to the geosciences and paleoclimatology through his love of the outdoors and his interest in climate change. His graduate studies at the University of Iceland and the University of Colorado Boulder have focused on the development and application of lipid-based proxies for paleoclimate reconstructions in Iceland and Baffin Island. Jonathan is also an avid musician, traveller, and outdoorsman who has lived and worked in Myanmar and hopes to continue to blend his love for science and the outdoors to address climate change issues in the future.