CANOPY CLOSURE IN CENOZOIC FORESTS OF AUSTRALIA USING CARBON ISOTOPE RATIOS OF FOSSIL CUTICLE

Forests canopy structure controls the penetration of light, relative humidity, and the concentration and isotopic composition of CO₂ in the understory. These factors, in turn, influence the carbon isotope signatures of plants at different levels of the forest. Closed canopy forests have been shown to have, on average, more ¹³C-depleted leaf tissue and more variable carbon isotope ratios than open forests. Here we examine the applicability of carbon isotope ratios for reconstructing Cenozoic forest structure in southeastern Australia. We analysed the carbon isotope ratio > 40 individual angiosperm leaf cuticles each from the Eocene site of Anglesea, and the Miocene site of Kiandra. In addition, we analysed combined samples of dispersed cuticle fragments as a measure of the overall community average carbon isotope ratios from a range of additional sites spanning the Eocene to the Miocene.

Both Anglesea and Kiandra angiosperm leaves record large variation in carbon isotope ratios, and relatively negative average values, both of which are suggestive of closed canopy environments similar to modern tropical rainforests. The expected isotopic difference in angiosperms and gymnosperms is preserved in the individual leaf cuticles supporting that original leaf carbon isotope ratios are faithfully preserved. Dispersed cuticle proved difficult to interpret because of the mixing of both angiosperms and gymnosperms, which are isotopically distinct. However, the isotopic variation in the dispersed cuticle could not be fully explained by simple mixing of angiosperm and gymnosperm cuticle, suggesting additional environmental controls. In addition, the gymnosperms’ range of values suggests that they, too, occupied diverse isotopic and light environments.