Variation of branched tetraethers with soil depth in relation to non-temperature factors: Implications for paleoclimate reconstruction

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that have been extensively used as tools for paleoclimate reconstruction. Temperature and pH were thought to be two major factors affecting the distributions of brGDGTs; however, the effects of other factors on the distribution of brGDGTs and related temperature and pH proxies in soil environments remain unclear. Here, we investigated iso- and br-GDGT distributions in three soil profiles (SPs) developed on bedrocks consisting of hornfels (HP), granite (GP), and basalt (BP), respectively. The downward environmental factors in these SPs differ, thus providing a chance to evaluate their effects on the distribution of brGDGTs. In the three SPs, the methylation degree for 5-methyl and total brGDGTs (MBT′5ME and MBT′), respectively, varied significantly downwards despite minor changes in soil temperature, highlighting a non-temperature effect. The depth-dependence of these proxies appears to be caused by the oxygen level. The MBT′ for 5- or 6-methyl brGDGTs (MBT′5ME and MBT′6ME), or total brGDGTs (MBT′), increased as the oxygen level decreased, suggesting tetramethylated brGDGTs (Ia, Ib, and Ic) would increase in abundance with decreased oxygen level. The two pH proxies, CBT and IR6ME, both decreased at lower oxygen levels. Such regulation of brGDGT compositions in response to oxygen availability could be explained by the changing bacterial communities that have different oxygen preferences. The possible oxygen level impact on the distribution of GDGTs could result in brGDGT-inferred temperature skewed towards higher values and pH towards lower values with decreased oxygen level. This finding may also have implications for the mechanism of soil aridity impact on the brGDGT-based proxies in semi-arid and arid regions, which is of profound importance for paleotemperature and hydroclimate reconstruction in loess-paleosol sequences. Increasing soil aridity could lead to changes in brGDGT-based proxies similar to that observed at the higher oxygen level, i.e., lower MBT′, higher CBT, and higher IR6ME. As soil aridity is generally associated with better aeration and higher oxygen level, we argue that changes in oxygen level might be the real cause of the bias in brGDGT-based proxies in soils from semi-arid and arid regions.