Biomarkers record environmental changes along an altitudinal transect in the wettest place on Earth

Abstract

The combined application of plant wax δD (δDwax) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) has been suggested as proxy for paleo-elevation. In some of the altitudinal transects studied so far, nonlinear precipitation gradients, large variations in seasonality, soil and vegetation types introduced substantial amounts of scatter in the relationship between these proxies and altitude. To further evaluate the principal functioning of the proxies, δDwax and brGDGTs were analysed in surface soils along an altitude gradient (from 28m up to 1865ma.s.l.) in Meghalaya, India. The transect experiences limited seasonal temperature variation and receives very high monsoonal precipitation along the whole elevation gradient. The δDwax show a significant relation with altitude (r2=0.72). The additional fits with stream water δD (r2=0.72) and modelled precipitation δD (r2=0.72) indicate that δDwax tracks the altitude effect on precipitation. Also the brGDGT distribution shows a correlation with altitude, reflecting the decrease in temperature with higher elevation (r2=0.65, or r2=0.66 using the original and recalibrated methylation of branched tetraethers-cyclisation of branched tetraethers (MBT–CBT) proxy). Application of the original MBT–CBT calibration generates calculated air temperatures that overestimate measured air temperature by ∼6°C, whereas temperatures similar to measured are obtained with the revised calibration. These results indicate that δDwax and brGDGTs may faithfully and accurately track environmental changes with altitude for transects where seasonal and diurnal temperature variability is relatively limited. Furthermore, proxy application to locations that experience high rainfall increases the suitability as climatic indicators, as it excludes soil moisture availability as a limiting factor.