Impact of weathering processes on n-alkane pattern in badlands

Abstract

The aim of this study is to investigate the influence of weathering processes on the distribution of n-alkanes in unweathered and weathered samples of badland material. Organic matter (OM) of both the weathered and unweathered zones in badland environments is derived from the same sources such any differences between the n-alkane pattern of the two zones can be attributed to the impact of weathering processes under different climate conditions. Organic geochemical and mineralogical characterizations were employed to investigate twenty-one samples from weathered badland environments in Italy, Spain, and Canada that originally derive from marine, distal alluvial, and fluvial depositional settings, respectively. According to the distribution of n-alkanes and diterpanes, unweathered samples from Italy have an original mixed aquatic-terrestrial origin of OM, with a variable (low to moderate) contribution of terrestrial plants as precursors, while unweathered samples from Spain predominantly contain OM originating from submerged/ floating macrophytes deposited under reducing conditions. Unweathered samples from Canada predominantly consist of terrigenous OM. Intensity of changes in n-alkanes distribution when subjected to weathering depends on the type of precursor biomass and quantity of smectite. Predominantly terrigenous OM under semi-arid conditions shows the highest affinity for adsorption is in contact with smectite surfaces. Mid- and long-chain odd n-alkanes have a tendency to build colloidal particles with polar organic compounds such as fatty acids by strong binding and thus their preservation from degradation is possible.