Abstract
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×CBTpeat+8.07 (n=51, R2=0.58, RMSE=0.8) and the degree of methylation of brGDGTs is positively correlated with MAAT, MAATpeat (°C)=52.18×MBT5me′−23.05 (n=96, R2=0.76, RMSE=4.7°C). 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 (∼15.2kyr), 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 climate.
Highlights
Reconstructions of terrestrial environments are crucial for the understanding of Earth’s climate system, suitable depositional archives are rare on land
Using 470 samples from 96 peatlands from around the world we explored the environmental controls on the bacterial branched glycerol dialkyl glycerol tetraether (GDGT) (brGDGTs) distribution in peats
We demonstrate that brGDGT distributions are correlated with peat pH and especially mean annual air temperature (MAAT)
Summary
Reconstructions of terrestrial environments are crucial for the understanding of Earth’s climate system, suitable depositional archives (especially longer continuous sequences) are rare on land. Peat-based proxies include those based on plant macrofossils, pollen, and testate amoebae (e.g., Woillard, 1978; Mauquoy et al, 2008; Valiranta et al, 2012), inorganic geochemistry (e.g., Burrows et al, 2014; Chambers et al, 2014; Hansson et al, 2015; Vanneste et al, 2015), (bulk) isotope signatures (e.g., Cristea et al, 2014; Roland et al, 2015) and organic biomarkers (e.g., Nichols et al, 2006; Pancost et al, 2007; Pancost et al, 2011; Huguet et al, 2014; Zocatelli et al, 2014; Schellekens et al, 2015; Zheng et al, 2015) These proxies can be used to provide a detailed reconstruction of the environment and biogeochemistry within the peat during deposition, an accurate temperature or pH proxy for peat is currently lacking (Chambers et al, 2012). The aim of this paper is to develop peat-specific pH and temperature proxies for application to peat cores as well as ancient peats from the geological record preserved as lignites
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