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Title: BrGDGT lipids in cold regions reflect summer soil temperature and seasonal soil water chemistry
The distribution of brGDGT lipids produced by soil bacteria has been used to reconstruct temperatures in marine and terrestrial settings as far back as the Cretaceous period. However, modern calibrations of this proxy have primarily relied on air rather than in situ soil temperatures, which can differ by more than 10 ◦C. Furthermore, the influence of other parameters such as temperature seasonality and soil chemistry on brGDGT lipids is not fully understood. We measured brGDGT distributions, in situ soil temperatures, pH, soil water content, and electrical conductivity on soils from the Eastern Canadian Arctic and Iceland. We compiled our results with those of published soil brGDGT studies that also provide in situ soil temperatures and ancilliary measurements and generated global temperature and pH calibrations from the resulting dataset. Soil temperatures outperformed air temperatures in these calibrations, with mean summer soil temperature providing the highest-performing fit among the 10 tested soil temperature parameters. When applied to a loess/paleosol sequence from the Chinese Loess Plateau, these new calibrations produced paleotemperature and paleo-pH histories consistent with the results of previous studies, encouraging the application of our new calibrations on a broader scale. We also detected 7-methyl and IIIa’’ brGDGT isomers in our Eastern Canadian Arctic and Iceland soils, which have been shown in lakes to relate to salinity and anoxia, respectively. While neither correlated with bulk soil properties such as conductivity, soil water content, or pH, these brGDGT isomers did correlate with seasonality and winter soil temperature. We hypothesize that these compounds are generated in winter by bacteria in habitable niches of more saline, sometimes anoxic liquid water in the otherwise frozen soil matrix. Finally, we report the presence of overly branched GDGTs with m/z = 1064 and suggest that these heptamethylated tetraethers should be investigated as a potential tool for improving brGDGT calibrations. Overall, our results expand our understanding of the seasonality of brGDGT production, especially at high latitudes, and provide in situ soil temperature and pH calibrations for global use.  more » « less
Award ID(s):
1836981
PAR ID:
10513988
Author(s) / Creator(s):
; ; ; ; ; ;
Publisher / Repository:
Geochimica et Cosmochimica Acta
Date Published:
Journal Name:
Geochimica et Cosmochimica Acta
Volume:
369
Issue:
C
ISSN:
0016-7037
Page Range / eLocation ID:
111 to 125
Subject(s) / Keyword(s):
brGDGT Paleoclimate Biomarker Soil temperature
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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