The influence of soil chemistry on branched tetraether lipids in mid- and high latitude soils: Implications for brGDGT- based paleothermometry

Near-universal trends in brGDGT lipid distributions in nature

“…Subsequent studies have since shown that changes in brGDGT distributions are also accompanied by shifts in the composition of the associated microbial community, indicating that the empirical correlations between the lipids and environmental parameters may be indirect in nature (25)(26)(27)(28)(29). Whether the variation in a brGDGT paleorecord is due to the direct physiological response of microbes to a changing environment or the indirect restructuring of a microbial community has important implications for its interpretation and fidelity (28). Unfortunately, brGDGT-producing source organisms remain incompletely identified and cultured (30), limiting our ability to test these hypotheses through controlled laboratory experimentation; to date, only two strains of Acidobacteria have been shown to produce just one of the 15 commonly measured brGDGTs (31,32).…”

“…Drawing analogies to other bacterial membrane lipid classes, early studies posited that the empirical relationships between brGDGTs and environmental parameters were physiological adaptations of source organisms to variations in external temperature and pH (24). Subsequent studies have since shown that changes in brGDGT distributions are also accompanied by shifts in the composition of the associated microbial community, indicating that the empirical correlations between the lipids and environmental parameters may be indirect in nature (25)(26)(27)(28)(29). Whether the variation in a brGDGT paleorecord is due to the direct physiological response of microbes to a changing environment or the indirect restructuring of a microbial community has important implications for its interpretation and fidelity (28).…”

“…These discrepancies have led to independent modern calibrations for different sample types [e.g., soils and peats (36), lacustrine sediments (37) or SPM (38), bones (39), speleothems (40), and marine sediments (23)] and regions [e.g., East Africa (41) and China (42)] and have necessitated much effort to disentangle the allochthonous versus autochthonous sources of brGDGTs to sedimentary archives (2,23,(43)(44)(45)(46)(47)(48). Further complicating the matter, brGDGT distributions are affected by a wide array of other environmental parameters, including oxygen levels (26,27,29,(49)(50)(51)(52)(53)(54), salinity/electrical conductivity (55,56), seasonality (27,36,46,57), nutrient availability (58,59), and soil chemistry (28), which can mask or override relationships with temperature or pH. Last, an observed bias in brGDGT-derived temperatures toward warmer seasons has proven difficult to quantify, with various studies finding summer air temperature (60), the mean air temperature of months above freezing (MAF) (36,37,55), growing degree days above freezing (61), or other temperature indices to provide the strongest correlations in modern training sets.…”

The production of diverse brGDGTs by an Acidobacterium allows a direct test of temperature and pH controls on their distribution

“…This suggests that at least some variations in environmental brGDGT distributions reflect community change rather than physiological adaptations within a single taxon. In altitudinal or latitudinal transects with a large temperature or pH gradient, the impact of these environmental factors on brGDGTs overwhelmingly exceeds the community effect, resulting in significant correlations between brGDGT distribution and temperature or pH (9, 31, 32); however, at local scales, the community effect can dominate, as De Jonge et al previously observed in high and mid-latitude soils (33, 34). It is, therefore, necessary to evaluate the community effect on existing brGDGT paleoclimate proxies in their applications to paleo-reconstructions.…”

“…In altitudinal or latitudinal transects with a large temperature or pH gradient, the impact of these environmental factors on brGDGTs overwhelmingly exceeds the community effect, resulting in significant correlations between brGDGT distribution and temperature or pH (9,31,32); however, at local scales, the community effect can dominate, as De Jonge et al previously observed in high and mid-latitude soils (33,34).…”