Investigating the application of organic geochemical techniques to tropical Anjohibe (Madagascar) stalagmites

Abstract

Speleothem stable carbon isotopes (δ¹³C_carb) are used to reconstruct past environments, but are a complex signal of karst, soil and plant processes. To help untangle these signals, we used plant waxes, their carbon isotopic values (δ¹³C_wax) and polycyclic aromatic hydrocarbons (PAHs) extracted from stalagmites to evaluate plant photosynthetic pathway (C₃ vs C₄) and biomass burning above a cave. Our test case investigates stalagmites from Anjohibe in Madagascar where at around 1000 CE multiple δ¹³C_carb records increase by ∼ 8–10 ‰. This suggests that humans transformed the local landscape from C₃ vegetation to C₄ grasses through agropastoral practices, which rely on burning to promote grass growth. We evaluated different protocols to remove contamination, finding higher biomarker yields after polishing off the surface of the stalagmite versus ultrasonic pre-cleaning in solvent. Anjohibe stalagmites include n-alkanes from trees and grasses; however, bulk organic δ¹³C and δ¹³C_wax from samples dated to after the transition to the modern C₄ landscape yield values suggesting C₃ vegetation. This is likely due to a disproportionally higher contribution of C₃ waxes to the overall n-alkane signal. PAHs are present in the stalagmite but do not match the types found in overlying soils and further testing is required to determine their source. We find that δ¹³C values of bulk organic carbon, or plant waxes extracted from stalagmites, should be interpreted with caution as the proportion of plant matter on the landscape does not necessarily equate to the proportion of organic molecules produced by those plants or preserved in the sedimentary record.