Soil chemistry, temperature and bacterial community composition drive brGDGT distributions along a subarctic elevation gradient

• Published in: Organic Geochemistry
• Main Authors: Halffman, Robin, Lembrechts, Jonas, Radujkovic, Dajana, De Gruyter, Johan, Nijs, Ivan, De Jonge, Cindy
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Physics; Chemistry; Norway; Subarctic
• Online Access: https://hdl.handle.net/10067/1854630151162165141; https://repository.uantwerpen.be/docstore/d:irua:10566

Abstract: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids which can be used to reconstruct past terrestrial mean annual air temperature (MAAT) and soil pH values. To reconstruct these environmental conditions in geological archives, we make use of the brGDGT ratios MBT'(5ME), CBT' and IR, which summarize the structural variation in brGDGT lipids. However, the most recent calibrations between the brGDGT-based temperature proxy MBT'(5ME) and temperature on a global scale are characterized by a residual error between 3.8 degrees C and 6.0 degrees C. This lack of accuracy of the MBT'(5ME)-MAAT calibration is often attributed to the difference between soil and atmospheric temperatures. Also, previous studies suggested that the variation of the MBT'(5ME) along chemistry and temperature gradients reflects a possible influence of bacterial community changes. Here, we analyzed the effect environmental variables have on brGDGT signatures collected along five elevation gradients in the north of Sweden and Norway, where MAAT changed between -4.7 degrees C and 2.7 degrees C and soil pH varies between 3.3 and 5.7. Specifically, we determined the impact of air and soil temperature, the bacterial community composition and soil chemical characteristics. The range of MBT'(5ME) values encountered (0.30-0.70) results in a wide range of reconstructed temperatures (1-13 degrees C). The use of in situ soil temperature data spanning one year did not improve the correlation with brGDGT MBT'(5ME) values, compared to using a longterm MAAT dataset. Although a temperature gradient was present, soil chemistry apparently determined brGDGTs concentration and distribution on this local scale. Specifically, soils with high cation exchange capacity (CEC) showed an increased concentration of brGDGT Ia, resulting in increased Community Index (CI) and MBT'(5ME) values, both brGDGT ratios that reflect the degree of methylation. Soils with increased pH (pH range 5-6) had a distinct brGDGT fingerprint with ...