Effect of aridity on delta 13C and delta D values of C3 plant- and C4 graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa)

Abstract The observation that the hydrogen isotope composition (δD) of leaf wax lipids is determined mainly by precipitation δD values, has resulted in the application of these biomarkers to reconstruct paleoclimate from geological records. However, because the δD values of leaf wax lipids are additionally affected by vegetation typeand ecosystem evapotranspiration, paleoclimatic reconstruction remains at best semi-quantitative. Here, we used published results for the carbon isotope composition (δ 13 C) of n -alkanes in common plants along a latitudinal gradient in C 3 / C 4vegetation and relative humidity in Cameroon and demonstrated that pentacyclic triterpene methyl ethers (PTMEs) and n -C 29 and n -C 31 in the same soil, derived mainly from C 4 graminoids(e.g. grass) and C 3 plants (e.g. trees and shrubs), respectively. We found that the δD values of soil n -C 27 , n -C 29 and n -C 31 , and PTMEs correlated significantly with surface water δD values, supporting previous observations that leaf wax lipid δD values are an effective proxy for reconstructing precipitation δD values even if plant types changed significantly. The apparent fractionation ( e app ) between leaf wax lipid and precipitation δD values remained relatively constant for C 3 -derived long chain n -alkanes, whereas e app of C 4-derived PTMEs decreased by 20‰ along the latitudinal gradient encompassing a relative humidity range from 80% to 45%. Our results indicate that PTME δD values derived from C 4 graminoidsmay be a more reliable paleo- ecohydrologicalproxy for ecosystem evapotranspirationwithin tropical and sub-tropical Africa than n -alkane δD values, the latter being a better proxy for surface water δD values. We suggest that vegetation changes associated with different plant water sources and/or difference in timing of leaf wax synthesis between C 3 trees of the transitional class and C 3 shrubs of the savanna resulted in a D depletion in soil long chain n -alkanes, thereby counteracting the effect of evapotranspirationD enrichment along the gradient. In contrast, evaporative D enrichment of leaf and soil water was significant enough to be recorded in the δD values of PTMEs derived from C 4 graminoids, likely because PTMEs recorded the hydrogen isotopic composition of the same vegetation type.