Introducing global peat-specific temperature and pH calibrations based on brGDGT bacterial lipids

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Glycerol dialkyl glycerol tetraethers (GDGTs) are membrane-spanning lipids from
Bacteria and Archaea that are ubiquitous in a range of natural archives and especially
abundant in peat. Previous work demonstrated that the distribution of bacterial
branched GDGTs (brGDGTs) in mineral soils is correlated to environmental factors
such as mean annual air temperature (MAAT) and soil pH. However, the influence of
these parameters on brGDGT distributions in peat is largely unknown. Here we
 investigate the distribution of brGDGTs in 470 samples from 96 peatlands around the
world with a broad mean annual air temperature (−8 to 27 °C) and pH (3–8) range and
present the first peat-specific brGDGT-based temperature and pH calibrations. Our
results demonstrate that the degree of cyclisation of brGDGTs in peat is positively
correlated with pH, pH = 2.49 x CBTpeat + 8.07 (n = 51, R2 65 = 0.58, RMSE = 0.8) and
the degree of methylation of brGDGTs is positively correlated with MAAT,
MAATpeat (°C) = 52.18 x MBT5me’ – 23.05 (n = 96, R2 67 = 0.76, RMSE = 4.7 °C).
3
These peat-specific calibrations are distinct from the available mineral soil
calibrations. In light of the error in the temperature calibration (~ 4.7 °C), we urge
caution in any application to reconstruct late Holocene climate variability, where the
climatic signals are relatively small, and the duration of excursions could be brief.
Instead, these proxies are well-suited to reconstruct large amplitude, longer-term
shifts in climate such as deglacial transitions. Indeed, when applied to a peat deposit
spanning the late glacial period (~15.2 kyr), we demonstrate that MAATpeat yields
absolute temperatures and relative temperature changes that are consistent with those
from other proxies. In addition, the application of MAATpeat to fossil peat (i.e.
lignites) has the potential to reconstruct terrestrial climate during the Cenozoic. We
conclude that there is clear potential to use brGDGTs in peats and lignites to
reconstruct past terrestrial climateThis research was funded through the advanced ERC grant “the greenhouse earth
system” (T-GRES, project reference 340923), awarded to RDP. All authors are part of
the “T-GRES Peat Database collaborators” collective. RDP also acknowledges the
Royal Society Wolfson Research Merit Award. We thank D. Atkinson for help with
the sample preparation. We acknowledge support from Labex VOLTAIRE (ANR-10-
22
 LABX-100-01). Peat from Patagonia and Tierra del Fuego were collected thanks to a
Young Researcher Grant of the Agence National de la Recherche (ANR) to FDV,
project ANR-2011-JS56-006-01 “PARAD” and with the help of Ramiro Lopez,
Andrea Coronato and Veronica Pancotto (CADIC-CONICET, Ushuaia). Peat from
Brazil was collected with the context of CNPq project 482815/2011-6. Samples from
France (Frasne and La Guette) were collected thanks to the French Observatory of
Peatlands. The Canadian peat was collected in the context of the NSERC-Discovery
grant of L. Rochefort. Peats from China were obtained under a National Natural
Science Foundation of China grant (No. 41372033), awarded to Y. Zheng