Altitudinal climatic index changes in subtropical China indicated from branched glycerol dialkyl glycerol tetraethers proxies

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) have been widely applied in climate parameter reconstructions over the past few years. MBT′5ME (the methylation of branched tetraethers based on the 5-methyl isomers) and CBT′ (cyclisation of branched tetraethers that includes 6-methyl brGDGTs in the numerator) have proven to be useful brGDGT proxies for interpreting the mean annual air temperature (MAT) and soil pH of different climate condition areas in China. However, the application of MBT′5ME and CBT′ for indicating altitudinal climate conditions is still open to discussion, e.g., the lack of comparisons between attitudinal results from mountain areas of different climate conditions. In this study, we compared soil brGDGT along three mountain transects in subtropical China, i.e., Mt. Gaoligong, Mt. Fanjing and Mt. Shennongjia. The brGDGT distributions of Mt. Gaoligong and Mt. Fanjing were similar to the subtropical surface soil brGDGT distribution pattern, featured by the dominance of chemical structures of brGDGTs Ia and IIa. However, Mt. Shennongjia was characterised by increasing brGDGTs IIIa′, IIa′, IIb′ and Ib, much like the semi-humid/humid temperate climate area brGDGT distribution pattern. These results indicate that altitudinal climate indicators such as temperature and precipitation could be recorded by varied brGDGT distributions. Also, MBT′5ME-inferred MATs were negatively correlated with mountain altitude (R2: 0.79–0.87). The MAT lapse rate increased from Mt. Shennongjia to Mt. Fanjing to Mt. Gaoligong (0.5 °C/100 m, 0.9 °C/100 m and 1.3 °C/100 m, respectively). The influence of the Qinghai Tibetan Plateau ice cover and differences in latitudinal temperature and/or moisture conditions might be the reasons for the various lapse rates. The soil pH was less correlated with altitude (R2: 0.58–0.63) than to the regression of MAT and altitude, showing slightly different relations from those of horizontal studies. At Mt. Gaoligong and Mt. Fanjing, soil pH and altitude demonstrated positive linear regressions but with different slopes. The different reducing rate of soil pH as a function of altitude for the two mountains is probably caused by their dominating summer monsoons. For Mt. Shennongjia where the climate condition is less humid than Mt. Gaoligong and Mt. Fanjing, the soil pH did not significantly change with altitude, and thus no relationship was observed. This might probably be partially induced by the weakly alkaline parent rocks of Mt. Shennongjia.