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• 3-OH FAs and brGDGTs analyzed in soils from an altitudinal transect in the French Alps. • Investigation of the environmental factors controlling the distribution of bacterial lipids. • Influence of soil traits predominant over MAAT. • Statistical identification of the individual bacterial lipids influenced by MAAT/pH required. • Development of local calibrations with MAAT and pH based on such identification. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a family of bacterial lipids widely used for temperature and pH reconstructions in terrestrial settings. 3-hydroxy fatty acids (3-OH FAs) with 10–18 carbon atoms, produced by Gram-negative bacteria, have been recently proposed as independent and complementary proxies of temperature and pH in terrestrial environments. Nevertheless, the correlations between mean annual air temperature (MAAT)/pH and bacterial lipid (brGDGTs/3-OH FAs) distributions show a large degree of scatter, as the relative abundance of these lipids is influenced by factors other than temperature and pH. A full understanding of the environmental parameters influencing bacterial lipid distribution in soils is required to increase the reliability of the temperature and pH proxies based on these compounds in terrestrial environments. The aim of this work was to determine and quantify the cumulative effect of environmental parameters on the distribution of both brGDGTs and 3-OH FAs along a well-documented composite altitudinal transect in the French Alps (234–2748 m). Redundancy analysis revealed that the influence of local parameters (pH and to a lesser extent, soil moisture and grain size, related to vegetation and soil types) on brGDGT and 3-OH FA distributions amounted to 48.1% and 26.1%, respectively, and was predominant over MAAT. This likely explained the weak or lack of relationships between MAAT and brGDGT-/3-OH FA-based indices in this region. The identification of lipids whose fractional abundance is correlated with MAAT or pH allowed the development of local calibrations with MAAT/pH applicable in the French Alps, which are representative of highly contrasted microenvironments, reflecting different types of soil and vegetation. The present study highlighted the importance of constraining the environmental factors affecting the distribution of 3-OH FAs and brGDGTs in terrestrial settings prior to any paleoenvironmental reconstruction. Such an approach should be reproduced in other sites, where local factors could also strongly influence the bacterial lipid distribution. [ABSTRACT FROM AUTHOR]

Highlights • 3-hydroxy fatty acids analysed in soils from two contrasting altitudinal transects. • Distribution of 3-hydroxy fatty acids correlated with air temperature and soil pH. • Promising pH and temperature proxies in terrestrial environments. Abstract Three hydroxy (3-OH) fatty acids (FAs), containing 10 to 18 C and a hydroxyl group in third position are characteristic components of Gram-negative bacteria. These compounds were used previously to detect and quantify Gram-negative bacterial communities in various types of samples, from terrestrial, aquatic and atmospheric environments. The relative abundance of 3-OH FAs in soils was recently shown to vary with mean annual air temperature (MAAT) and pH in soils from Mt. Shennongjia (China). In the current study, the concentrations and abundances of 3-OH FAs were determined in soils from altitudinal transects under tropical and temperate climates: Mt. Rungwe (SW Tanzania) and Mt. Majella (Central Italy), respectively. The aim was to (i) examine and compare the distributions of 3-OH FAs in soils from different climatic zones and (ii) to investigate the potential of 3-OH FAs as temperature and pH proxies. When combined with previously published data, a moderate correlation (R2 0.62) between the summed iso and anteiso to the total amount of normal 3-OH FAs (RIAN index) and pH was obtained. We show that Gram-negative bacteria respond in the same way to temperature variations in soils from different latitudes, with a relative increase of the anteiso to normal 3-OH FA ratios of the C 15 and C 17 compounds (RAN 15 and RAN 17 indices, respectively) with decreasing temperature. Nevertheless, the intercepts of the relationships between RAN 15 and MAAT are mountain-dependent, suggesting that regional calibrations may be required to use RAN 15 as a temperature proxy. In contrast with RAN 15 , the statistical similarity of the local relationships between RAN 17 and MAAT leads to a combined calibration (R2 0.60, residual mean error 5.1 °C) covering a wide range of temperature (ca. 0–25 °C). As 3-OH FAs seem well-preserved in sedimentary archives, this strengthens the potential of these compounds as a temperature and pH proxy for paleoreconstructions in terrestrial settings. [ABSTRACT FROM AUTHOR]

The role of mountain evolution on local climate is poorly understood and potentially underestimated in climate models. One prominent example is East Africa, which underwent major geodynamic changes with the onset of the East African Rift System (EARS) more than 250 Myr ago. This study explores, at the regional East African scale, a molecular approach for terrestrially-based paleo-climatic reconstructions that takes into account both changes in temperature and in altitude, potentially leading to an improved concept in paleo-climatic reconstructions. Using surface soils collected along pronounced altitudinal gradients in Mt. Rungwe ( n = 40; Southwest Tanzania) and Mt. Kenya ( n = 20; Central Kenya), we investigate the combination of 2 terrestrial proxies, leaf wax n -alkane δ 2 H (δ 2 H wax ) and branched glycerol dialkyl glycerol tetraether (br GDGT) membrane lipids, as (paleo) elevation and (paleo) temperature proxies, respectively. At the mountain scale, a weak link between δ 2 H wax and altitude ( R 2 = 0.33) is observed at Mt. Kenya, but no relationship is observed at Mt. Rungwe. It is likely that additional parameters, such as decreasing relative humidity (RH) or vegetation changes with altitude, are outcompeting the expected 2 H-depletion trend along Mt. Rungwe. In contrast, br GDGT-derived absolute mean annual air temperature (MAAT) and temperature lapse rate (0.65 °C/100 m) for both mountains are in good agreement with direct field measurements, further supporting the robustness of this molecular proxy for (paleo) temperature reconstructions. At the regional scale, estimated and observed δ 2 H data in precipitation along 3 mountains in East Africa (Mts. Rungwe, Kenya and Kilimanjaro) highlight a strong spatial heterogeneity, preventing the establishment of a regional based calibration of δ 2 H wax for paeloaltitudinal reconstructions. Different from that, an improved regional soil calibration is developed between br GDGT distribution and MAAT by combining the data from this study (Mts. Rungwe and Kenya) with previous results from East African surface soils along Mts. Kilimanjaro (Tanzania) and Rwenzori (Uganda). This new regional calibration, based on 105 samples, improves both the R 2 (0.77) and RMSE (root mean square error; 2.4 °C) of br GDGT-derived MAAT over the global soil calibrations previously established ( R 2 = 0.56; RMSE = 4.2 °C) and leads to more accurate (paleo) temperature reconstructions in the region. [ABSTRACT FROM AUTHOR]