Hadeen, Xennephone, Rowe, Cassandra, Brand, Michael, Comley, Rainy, Das, Sourav, Wurster, Chris, Zwart, Costijn, and Bird, Michael (2025) Comparing pollen and n-alkane carbon isotope records in a tropical lacustrine environment. Quaternary Science Reviews, 351. 109204.

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Abstract

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-C29-31 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-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.

Item ID:	90901
Item Type:	Article (Research - C1)
ISSN:	1873-457X
Keywords:	Biomarker; Compound specific isotope; Palaeoecology; Palaeolimnology; Photosynthetic pathway; Tropical;
Copyright Information:	© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Funders:	Australian Research Council
Projects and Grants:	CE170100015, FL140100044
Date Deposited:	09 Mar 2026 07:13
FoR Codes:	37 EARTH SCIENCES > 3703 Geochemistry > 370302 Inorganic geochemistry @ 50% 37 EARTH SCIENCES > 3709 Physical geography and environmental geoscience > 370905 Quaternary environments @ 50%
SEO Codes:	28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280111 Expanding knowledge in the environmental sciences @ 100%
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