Exploring new ways of using sedimentary des-A-triterpenoids for paleoenvironmental reconstructions

The occurrence, distribution, and isotopic composition of plant leaf waxes in sedimentary archives are often used to reconstruct past environmental change. For example, the presence of non-hopanoid pentacyclic triterpenoids can be used as a proxy for the input of higher plant material in e.g. lakes. However, as these triterpenoids are generally not specific at the species level, they receive little attention in paleo-environmental reconstructions. To improve their potential for paleoclimate reconstruction, we studied the series of triterpenoid derivatives in the sedimentary record of Lake Challa, an East-African crater lake. In order to better constrain their individual sources, we analysed triterpenoids in vegetation, litter and soil, suspended particulate matter from various water depths, and sediment-trap material of Lake Challa. We found that C3-plant derived triterpenoids are dominated by des-A-lupane, whereas des-A-arborenes have an aquatic microbial source. We show that the carbon-isotopic composition of des-A-lupane can be used to refine the commonly applied crude δ13Cn-alkanes based C3/C4-vegetation box modelling. Furthermore, des-A-arborenes may serve as a proxy for chemocline stability, as their precursors are likely produced in the suboxic part of the nutrient rich chemocline. Hence, triterpenoids can provide more detailed paleoenvironmental information than previously anticipated.