Des-A-lupane in an East African lake sedimentary record as a new proxy for the stable carbon isotopic composition of C3 plants

We studied the high-resolution and well-dated 25,000 year sedimentary record of Lake Challa, a deep tropical crater lake in equatorial East Africa, to explore new proxies for paleoenvironmental and paleohydrological change. Sedimentary biomarker analysis revealed the presence of des-A-triterpenoids with oleanane, ursane and lupane carbon skeletons, microbial degradation products of angiosperm plant triterpenoids. Their increased influx from 16,000 years ago corresponds with previously documented changes in the terrestrial vegetation of the Lake Challa basin during postglacial warming, in particular the relative increase in C-3/C-4 plant ratio inferred from the stable carbon isotopic signature (delta C-13) of sedimentary n-alkanes derived from plant leaf waxes. In contrast to this n-alkane delta C-13 signature, the delta C-13 of des-A-lupane maintains a constant value of -27.4 +/- 1.1% across the glacial-interglacial transition. Since des-A-lupane is derived from C-3 plants, its delta C-13 signature is here proposed to represent a novel and independent proxy for the time-variable carbon isotopic composition of local terrestrial C-3 plants, which can improve estimates of the C-3/C-4 plant ratio based on two-end member mixing models of n-alkane delta C-13 values. (C) 2016 Elsevier Ltd. All rights reserved.