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

• Published in: Quaternary science reviews Vol. 351; p. 109204
• Main Authors: Hadeen, Xennephone, Rowe, Cassandra, Brand, Michael, Comley, Rainy, Das, Sourav, Wurster, Christopher M., Zwart, Costijn, Bird, Michael I.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2025
• Subjects: Biomarker; Compound specific isotope; Palaeoecology; Palaeolimnology; Photosynthetic pathway; Tropical; Palaeoecology; Biomarker; Tropical; Compound specific isotope; Photosynthetic pathway; Palaeolimnology
• ISSN: 0277-3791
• DOI: 10.1016/j.quascirev.2025.109204

Both pollen and the carbon isotope (δ13C) compositions of long chain n-alkanes (n-C29-31) 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-C29-31δ13C 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-C29-31 ‘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. [Display omitted] •150 kyr n-alkane δ13C and pollen record from northern Australia savanna.•Dramatic changes in past vegetation driven by regional hydroclimate change.•High aquatic biomass influences terrestrial vegetation (n-C29-31) δ13C signal.•High sedge biomass influences terrestrial vegetation (n-C29-31) δ13C signal.