Preprints
https://doi.org/10.5194/bg-2021-16
https://doi.org/10.5194/bg-2021-16

  28 Jan 2021

28 Jan 2021

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Revised fractional abundances and warm-season temperatures substantially improve brGDGT calibrations in lake sediments

Jonathan H. Raberg1,2, David J. Harning1,2, Sarah E. Crump1,a, Greg de Wet1,b, Aria Blumm1,c, Sebastian Kopf1, Áslaug Geirsdóttir2, Gifford H. Miller1, and Julio Sepúlveda1 Jonathan H. Raberg et al.
  • 1Department of Geological Sciences and Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, 80309, USA
  • 2Faculty of Earth Sciences, University of Iceland, Reykjavík, Iceland
  • acurrently at: Ecology and Evolutionary Biology Department, University of California Santa Cruz, Santa Cruz, CA 95064
  • bcurrently at: Department of Geosciences, Smith College, Northampton, MA 01063
  • ccurrently at: Department of Geosciences, University of Arizona, Tucson, AZ 85721

Abstract. Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperatures from lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding a full understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding in three ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, filling an important gap in the global dataset. Second, we introduce a new approach for analyzing brGDGT data in which compound fractional abundances (FAs) are calculated within structural groups based on methylation number, methylation position, and cyclization number. Finally, we perform linear and nonlinear regressions of the resulting FAs against a suite of environmental parameters in a compiled global lake sediment dataset (n = 182). We find that our approach deconvolves temperature, conductivity, and pH trends in brGDGTs without increasing calibration errors from the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating the biological underpinnings of their structural diversity. Warm-season temperature indices outperformed MAT in our regressions, with Months Above Freezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 °C, n = 182). The natural logarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R2 = 0.83, RMSE = 0.66, n = 143), notably outperforming pH in our dataset (adjusted R2 = 0.73, RMSE = 0.57, n = 154) and providing a potential new proxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detail the advantages and disadvantages of each.

Jonathan H. Raberg et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-16', Anonymous Referee #1, 15 Feb 2021
    • AC1: 'Reply on RC1', Jonathan Raberg, 02 Apr 2021
  • RC2: 'Comment on bg-2021-16', Anonymous Referee #2, 15 Feb 2021
    • AC2: 'Reply on RC2', Jonathan Raberg, 02 Apr 2021
  • EC1: 'Comment on bg-2021-16', Sebastian Naeher, 28 Feb 2021
    • AC3: 'Reply on EC1', Jonathan Raberg, 02 Apr 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-16', Anonymous Referee #1, 15 Feb 2021
    • AC1: 'Reply on RC1', Jonathan Raberg, 02 Apr 2021
  • RC2: 'Comment on bg-2021-16', Anonymous Referee #2, 15 Feb 2021
    • AC2: 'Reply on RC2', Jonathan Raberg, 02 Apr 2021
  • EC1: 'Comment on bg-2021-16', Sebastian Naeher, 28 Feb 2021
    • AC3: 'Reply on EC1', Jonathan Raberg, 02 Apr 2021

Jonathan H. Raberg et al.

Jonathan H. Raberg et al.

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Short summary
BrGDGT lipids are a proxy for temperature in lake sediments, but other parameters like pH can influence them and seasonality can affect the temperatures they record. We find a warm-season bias at 43 new high-latitude sites. We also present a new method that deconvolves the effects of temperature, pH, and conductivity and generate global calibrations for these variables. Our study provides new paleoclimate tools, insight into brGDGTs at the biochemical level, and a new method for future study.
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