A global Bayesian temperature calibration for lacustrine brGDGTs

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

“…The Meth set, which allowed only methylation number to vary, provided a MAF calibration (R 2 = 0.90, Fig. 10b) that was on par with other recent global and regional lake sediment calibrations (R 2 = 0.85 to 0.94; Dang et al, 2018;Russell et al, 2018;Martínez-Sosa et al, 2021). The MI, MC, and Full sets, which additionally allowed for changes in cyclization number and/or methylation position, added little to the calibration performance (R 2 = 0.90 to 0.91, Fig.…”

“…However, studies employing sub-seasonal sampling of sediment traps and suspended particulate matter in two midlatitude lakes have shown that brGDGTs are produced within the water column throughout the year, despite the presence of ice cover (Loomis et al, 2014b;Woltering et al, 2012). These and other studies employing similar sampling techniques (van Bree et al, 2020;Hu et al, 2016;Miller et al, 2018;Weber et al, 2018) have additionally found production of brGDGTs to be dependent on the degree and timing of lake mixing vs. stratification. Though heightened biological activity may still be the underlying driver of the observed warm-season bias, these depth-and time-resolved studies paint a complex picture of brGDGT production in lakes that precludes a simple explanation.…”

“…Without a clear mechanistic understanding of the dependencies of brGDGTs on environmental parameters and no brGDGT-producing model organisms currently available for laboratory experimentation, researchers have relied on statistical methods to construct empirical brGDGT calibrations. The majority of recent calibrations have employed a variety of statistical techniques to construct linear or polynomial regressions using brGDGT fractional abundances (FAs;De Jonge et al, 2014a;Martínez-Sosa et al, 2021;Pérez-Angel et al, 2020). The fractional abundance f x i of a compound x i in a set of n compounds is defined as…”

“…Since the methodological advances that allowed for the separation of 5-and 6-methyl isomers (De Jonge et al, 2014b) and the development of new calibrations, both modern studies (Cao et al, 2020;Dang et al, 2018;Hanna et al, 2016) and paleo studies (Crump et al, 2019;Harning et al, 2020;Super et al, 2018;Thomas et al, 2018) have continued to support a warm-season bias. Additionally, a recent Bayesian calibration found the mean temperature of months above freezing (MAF) to be the only mode to significantly correlate with brGDGT distributions in a global lake sediment dataset (Martínez-Sosa et al, 2021). However, as the source organisms of brGDGTs remain incompletely known, other measures of warm-season temperature such as peak warmth or cumulative heat may prove to be more biologically relevant.…”

Supplemental Material: Differences between soil and air temperatures: Implications for geological reconstructions of past climate

“…Similarly, several report that brGDGTs in lakes record temperatures above freezing (e.g., Dang et al, 2018;Martínez-Sosa et al, 2021;Raberg et al, 2021;. brGDGTs that accumulate in sediment in lakes, however, bring the additional complication that some are produced by bacteria in the lake column and some are washed in from surrounding soils (e.g., Colcord et al, 2015;Günther et al, 2014;Li et al, 2016Li et al, , 2017Loomis et al, 2011;Russell et al, 2018;Tierney and Russell, 2009;van Bree et al, 2020;Wang et al, 2012;Weber et al, 2015).…”