TY  - JOUR
A1  - Schwab, Valerie F.
A1  - Garcin, Yannick
A1  - Sachse, Dirk
A1  - Todou, Gilbert
A1  - Sene, Olivier
A1  - Onana, Jean-Michel
A1  - Achoundong, Gaston
A1  - Gleixner, Gerd
T1  - Effect of aridity on delta C-13 and delta D values of C-3 plant- and C-4 graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa)
JF  - Organic geochemistry : the international journal for rapid publication of current research in organic geochemistry and biochemistry
N2  - 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 C-13) of n-alkanes in common plants along a latitudinal gradient in C-3/C-4 vegetation 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 delta D values of soil n-C-27, n-C29 and n-C-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(app)) between leaf wax lipid and precipitation delta D values remained relatively constant for C-3-derived long chain n-alkanes, whereas eapp of C-4-derived PTMEs decreased by 20 parts per thousand along the latitudinal gradient encompassing a relative humidity range from 80% to 45%. Our results indicate that PTME delta D values derived from C-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-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 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-4 graminoids, likely because PTMEs recorded the hydrogen isotopic composition of the same vegetation type.
KW  - Evapotranspiration
KW  - D-enrichment
KW  - Pentacyclic triterpene methyl ethers
KW  - PTMEs
KW  - n-Alkane
KW  - Compound-specific isotope
KW  - Paleo
KW  - Climate
KW  - Proxy
Y1  - 2015
U6  - https://doi.org/10.1016/j.orggeochem.2014.09.007
SN  - 0146-6380
VL  - 78
SP  - 99
EP  - 109
PB  - Elsevier
CY  - Oxford
ER  -