GRASS-CUTICLE ANALYSIS: A NOVEL APPROACH TO RECONSTRUCTING ENVIRONMENTAL CHANGE WHILE EXPLORING THE PHYSIOLOGICAL - BIOGEOCHEMICAL RESPONSES OF AMAZONIAN PLANTS AND BIODIVERSITY

Amazonian biodiversity has been a very contentious issue, with debate stemming from reconstructions based on a range of environmental indicators. This study combines the latest palaeobotancial and biogeochemical analytical techniques, the complex biological and physiological responses can be explored, permitting the effects of CO₂ on earth system processes to be better understood in Amazonia.

This research assesses how rainforest-savanna boundaries have shifted in response to past environmental changes, which is crucial to understanding the present-day vegetation mosaic. Stable carbon isotopes act as a tracer of past changes in carbon cycling. Individual plant leaf-wax compounds can be extracted from lake sediments and their isotopic values measured by compound-specific ¹³C analysis, permitting the evaluation of past changes in the abundance of C₃ and C₄ graminoids, and thereby clarifying ambiguities in the pollen record. Grass-cuticle analyses provide even finer taxonomic resolution, enabling the identification of individual C₄ subtypes (which have distinct environmental tolerances).

In this study, a multiproxy approach, using grass-cuticle assemblages and stable carbon-isotope analyses of total organic carbon), is being applied to a lake-sediment core from Laguna Chaplin, situated in Noel Kempff Mercado National Park (which straddles a climatic transition zone between Amazonian moist evergreen forest, semi-deciduous forest and savanna). The δ13C values of TOC ranged from 17 per mille at the LGM to 26 per mille in the late Holocene, suggesting an increased representation of C₄ graminoids during glacial times. This finding is consistent with the corresponding glacial-age uncharred fossil cuticle coverages of Salviniaceae as well as pollen assemblages, which are dominated by grasses and palms (Mauritia/Mauritiella) with low levels of rainforest taxa, implying an expansion of seasonally-flooded savannas at the expense of seasonally-flooded forest. Compound-specific ¹³C analyses of leaf-wax n-alkanes and n-alkanoic acids shows a dominance of algal and aquatic macrophytes biomarkers in the last glacial period lending additional support to this inference. The inferences made from previous studies are tested by applying novel techniques permitting overall conclusions to be drawn at a greater taxonomic resolution.