Effect of aridity on delta super(13)C and delta D values of C sub(3) plant- and C sub(4) graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa)

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