Age and origin of leaf wax n -alkanes in fluvial sediment–paleosol sequences and implications for paleoenvironmental reconstructions

[摘要] Leaf wax n -alkanes are increasingly used for quantitative paleoenvironmental reconstructions. However, this is complicated in sediment archives with associated hydrological catchments since the stored n -alkanes can have different ages and origins. 14 C dating of the n -alkanes yields independent age information for these proxies, allowing their correct paleoenvironmental interpretation. This also holds true for fluvial sediment–paleosol sequences (FSPSs) that integrate two different n -alkane signals: (i) a catchment signal in fluvial sediments and (ii) an on-site signal from local biomass that increasingly dominates (paleo)soils with time. Therefore, the age and origin of n -alkanes in FSPSs are complex: in fluvial sediment layers they can be pre-aged and reworked when originating from eroded catchment soils or from organic-rich sediment rocks in the catchment. In (paleo)soils, besides an inherited contribution from the catchment, they were formed on-site by local biomass during pedogenesis. Depending on the different relative contributions from these sources, the n -alkane signal from an FSPS shows variable age offsets between its formation and final deposition. During this study, we applied compound-class 14 C dating to n -alkanes from an FSPS along the upper Alazani in eastern Georgia. Our results show that preheating the n -alkanes with 120  ∘ C for 8 h before 14 C dating effectively removed the shorter chains ( C 25 ) that partly originate from n -alkanes from Jurassic black clay shales in the upper catchment. The remaining petrogenic contributions on the longer chains ( ≥ C 25 ) were corrected for by using a constant correction factor that was based on the n -alkane concentrations in a black clay shale sample from the upper catchment. Due to different degrees of pre-aging and reworking, the corrected leaf wax n -alkane ages still indicate relatively large age offsets between n -alkane formation and deposition: while intensively developed (paleo)soils showed no age offsets due to a dominance of leaf wax n -alkanes produced on-site, less intensively developed paleosols showed much larger age offsets due to larger proportions of inherited leaf wax n -alkanes from the fluvial parent material. Accordingly, age offsets in nonpedogenic fluvial sediments were largest and strongly increased after ∼4  ka cal BP. The leaf wax n -alkane homolog distribution from intensively developed (paleo)soils indicates a local dominance of grasses and herbs throughout the Holocene, which was most likely caused by anthropogenic activity. The leaf wax n -alkanes from fluvial sediments show a dominance of deciduous trees and shrubs as well as grasses and herbs in different parts of the catchment between ∼8  and ∼5.6  ka cal BP. Since no older deciduous tree- or shrub-derived n -alkanes were dated, this seems to confirm a delayed regional postglacial reforestation of parts of the catchment compared with western and central Europe.

[学科分类] 妇产科学