International audienceGlycerol 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 = 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 = 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 (∼15.2 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 climate

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A monolith of peat taken from an upland blanket mire at Talla Moss, southern Scotland, was subjected to peat humification and pollen analyses to produce both a proxy-climate record and a vegetational history covering the last 5500 years. While the peat showed little visible stratigraphy, with no intimation of major peat humification changes, colorimetric data indicate a markedly oscillatory climate record, which is apparently largely independent of, or out of phase with, major vegetational changes. The raw data imply particular wet shifts in climate at c. 3455 BP, c. 2600 BP, c. 1930 BP, c. 1095 BP, with a markedly wet (or cool and wet) episode commencing at c. 540 BP. (These are central age estimates, and should not be regarded as precise dates for the inferred climate shifts.) Other wet shifts apparently date from c. 3070 BP, c. 2265 BP and c. 1700 BP, although the first of these corresponds with pollen evidence for significant prehistoric human activity in the locality. Spectral analysis of the peat humification data, when expressed on an interpolated calibrated age-scale, suggests a cycle of c. 210 years; this is dependent on the accuracy of the radiocarbon chronology and should be treated with caution. The upland site is amenable to tephrochronology, which, if also applied to ombrotrophic mire sites elsewhere, might then permit more precise correlation and comparisons of proxy-climate data between sites.

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