Comparing pollen and n-alkane carbon isotope records in a tropical lacustrine environment

Abstract

Both pollen and the carbon isotope (δ¹³C) compositions of long chain n-alkanes (n-C_29-31) are widely used to reconstruct changes in past vegetation in the tropics. Both approaches are able to infer the proportions of tree/shrub (C₃) and grass (C₄) biomass, with changes in these proportions generally interpreted to be driven by changes in hydroclimate. Both pollen and n-alkane δ¹³C records are subject to biases in production and transport to a site of deposition that may influence the interpretation of the record. Here we compare detailed 150,000-year pollen and n-alkane carbon isotope (δ¹³C) records from a lake in monsoonal northern Australia. There is a broad agreement between the two records at some times, with both identifying major wet periods of high tree/shrub C₃ terrestrial biomass during marine isotope stages 5 and 1. There are significant differences between the two records at other times. At times during glacial marine isotope stages 2 and 6, C₄ grass pollen comprised >80% of terrestrial pollen, whereas the n-C_29-31 δ¹³C values indicate dominantly C₃ terrestrial biomass.

These differences are the result of changes in the relative abundance and δ¹³C values of alkane inputs from within the lagoon itself that impact the δ¹³C value of the n-C_29-31 ‘terrestrial’ alkanes. The drivers of these changes include (i) changes in the biomass and δ¹³C value of aquatic vegetation (floating and submerged) that result from dramatic changes in lake level (ii) the changing importance of groundwater-derived dissolved inorganic carbon as a substrate for photosynthesis in the lake, (iii) changes in the proportion of sedge biomass in and around the lake that also accompany changes in lake level, and (iv) changes in the mix of C₃ and C₄ species comprising the sedge biomass in and around the lake. The significance of contributions of n-alkanes from aquatic and sedge sources to chain lengths usually considered to derive from terrestrial plants, may be underestimated in small, high productivity lacustrine environments, particularly in carbonate terranes.