Midway evaluation in Earth Sciences - Jonathan H. Raberg

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Title: Calibration program to test the origin of brGDGTs in lake sediment (soil vs. lacustrine)

Doctoral candidate: Jonathan H. Raberg

Doctoral committee:
Áslaug Geirsdóttir, Professor at the Faculty of Earth Sciences, University of Iceland
Gifford Miller, Professor at the University of Colorado Boulder, USA
Julio Sepúlveda, Assistant Professor at the University of Colorado Boulder, USA
Sebastian Kopf, Assistant Professor at the University of Colorado Boulder, USA
Thomas Marchitto, Professor at the University of Colorado Boulder, USA

Abstract:

The Holocene terrestrial paleotemperature record of Iceland is crucial for understanding the evolution of its cryosphere, changes in its vegetation patterns, and landscape evolution. Currently, such records are few and are generally qualitative in nature. Here, we aim to expand our understanding of Iceland's terrestrial temperature history using branched glycerol dialkyl glycerol tetraether (brGDGT) biomarkers in lake sediments. These bacterial membrane lipids have been shown to correlate with temperature in lake sediments and soils across the globe and have been successfully applied to reconstruct terrestrial temperatures in lake sediment archives. We will extend the application of brGDGTs to Icelandic lakes, where we will perform regional calibrations, investigate both microbial and environmental sources of the lipids, and generate paleotemperature records from lake sediment cores.

Currently, most studies use a calibration developed from East African lake sediments (Russell et al., 2018, Organic Geochemistry). While this data set spans a wide and geologically relevant range of temperatures, the low seasonality associated with its lakes means that they may not be directly comparable to sites at higher latitudes. This is especially true for its cold end-members, which experience fundamentally different conditions than most cold lakes due to a lack of ice cover. Indeed, research suggested early on that brGDGT-derived temperatures in high latitude regions were biased towards the summer. However, detailed studies have since shown that brGDGT production in lakes is not limited to summer months. In addition, analytical advances have separated previously convoluted brGDGT isomers that have distinct correlations with environmental variables. These developments motivate the re-examination of the brGDGT temperature proxy in cold regions. Here, we add surface sediments from 43 lakes across Baffin Island (Nunavut), northern Quebec, and Iceland to existing data sets and use a combination of temperature loggers and meteorological stations to examine the effects of seasonality on brGDGT-derived temperatures. In addition, we develop modified brGDGT indices that not only improve calibrations against air temperature and lake water pH, but also enhance our understanding of the relationships between these compounds and environmental variables.