Reconstructing tropical African hydrology using hydrogen isotope ratios of lacustrine sedimentary biomarkers: a calibration transect across Cameroon
Final Report Abstract
Hydrogen isotope compositions (δD) in ancient lipid biomarkers potentially record isotopic properties of ancient source water, which may indirectly reflect isotopic signatures of rainfall. In the other hand, carbon isotope compositions (δ13C) in ancient lipid biomarkers help to differentiate between the metabolic pathway and growth form of organisms. There is a tight relationship between biomarker δD and source-water δD as demonstrated for material collected in several mid- and high-latitude lacustrine environments. In contrast, for the tropical realm, where both ecosystems and processes associated with isotope fractionation in the hydrologic system strongly differ from those at higher latitudes, calibration studies for these molecular and isotopic proxies are not yet available. Here, we propose to analyse the compound-specific hydrogen and carbon isotope ratios of lipid biomarkers deposited in lake sediments and soils from tropical Africa. The overall aim of this project is to (i) identify the environmental factors (e.g., hydro-climatology) and physiological processes that determine the hydrogen isotopic composition of lipid biomarkers in the tropical realm and (ii) to develop a robust framework for the quantitative application of compound-specific hydrogen and carbon isotopes in tropical Africa. For this purpose, δD and δ13C values of lipid biomarkers from lake surface sediments, soils, living plants, and phytoplankton, were analyzed along a N-S transect spanning Cameroon. This 10°-long latitudinal transect was chosen because it encompasses a wide range of contrasting vegetation classes and climates that are representative for tropical Africa. The biomarker δD values were compared with the δD values of lake water, water from tree sap, and rainfall along this transect. We found that the δD values of leaf-wax lipids such as the n-C29 alkane in lake surface sediments and the n-C27, n-C29, and n-C31 alkanes in soils were significantly correlated with surface water δD values (i.e., local river water and groundwater). This information validates the use of these molecular and isotopic proxies preserved in lake sediments and soils to quantitatively assess the hydrogen isotopic compositions of past surface waters in tropical Africa. The δ13C values of leaf-wax n-alkanes, which inherit carbon isotope ratios that are representative of the plant’s carbon fixation pathway (either C3 or C4), were compared to the C3/C4 composition of vegetation to provide a novel framework for the reconstruction of vegetation cover. Since leaf-wax n-alkane composition from modern plants showed differences in abundance/production between C3 and C4 plants, we developed a simple non-linear modelling approach to reconstruct changes in C3/C4 vegetation cover from the δ13C values of sedimentary n-alkanes. Furthermore, we measured the δD values of source specific biomarkers such as the pentacyclic triterpene methyl ethers (mainly derived from graminoids) in topsoils and demonstrated their use to estimate ecosystem evapotranspiration. This dual isotope (δD and δ13C) calibration effort is unique and is the first study of its kind in the tropical realm. The tropics are the main source of heat and water vapour for Earth’s atmospheric convection, and the accurate reconstruction of their past climate dynamics as well as vegetation changes strongly relies on new, adequately calibrated proxies.
Publications
- (2012) Hydrogen isotope ratios of lacustrine sedimentary n-alkanes as proxies of tropical African hydrology: Insights from a calibration transect across Cameroon. Geochim. Cosmochim. Acta 79, 106–126
Garcin Y., Schwab V. F., Gleixner G., Kahmen A., Todou G., Séné O., Onana J.-M., Achoundong G. and Sachse D.
(See online at https://doi.org/10.1016/j.gca.2011.11.039) - Reconstructing C3 and C4 vegetation cover using n-alkane carbon isotope ratios in recent lake sediments from Cameroon, Western Central Africa. Geochimica et Cosmochimica Acta
Volume 142, 1 October 2014, Pages 482-500
Garcin Y., Schefuß E., Schwab V. F., Garreta V., Gleixner G., Vincens A., Todou G., Séné O., Onana J.-M., Achoundong G. and Sachse D.
(See online at https://doi.org/10.1016/j.gca.2014.07.004) - Dinosterol δD values in stratified tropical lakes (Cameroon) are affected by eutrophication. Org. Geochem., Volume 88, November 2015, Pages 35-49
Schwab V. F., Garcin Y., Sachse D., Todou G., Séné O., Onana J.-M., Achoundong G. and Gleixner G.
(See online at https://doi.org/10.1016/j.orggeochem.2015.08.003) - Effect of aridity on δ13C and δD values of C3 plant-and C4 graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa). Organic Geochemistry
Volume 78, January 2015, Pages 99-109
Schwab V. F., Garcin Y., Sachse D., Todou G., Séné O., Onana J.-M., Achoundong G. and Gleixner G.
(See online at https://doi.org/10.1016/j.orggeochem.2014.09.007)
