CARBON ISOTOPE CHARACTERIZATION OF THE CLOSED CANOPY (?) CASTLE ROCK RAINFOREST

The large floral diversity and preservation of cuticle associated with paleosol deposits makes the Castle Rock Rainforest (CRR) an ideal place to study the carbon isotope systematics of a Paleogene forest. Carbon isotope data from this setting can be used (1) to try and describe the physical structure of this fossil forest and (2) to study the cycling of carbon in ancient soils. These kinds of studies are possible because carbon isotope ratios of C₃ plants vary in response to differences in environmental conditions of plant growth within the forest canopy, and to the recycling of CO₂ in a closed-canopy setting, while ratios of soil organic matter can be influenced by microbial processes.

For this study 139 individual leaves from 10 of the most common taxa were analyzed along with 70 paleosol samples. Carbon isotope ratios of leaves ranged from -28.8 to -24.8 ‰ with an average of -26.8 ± 0.8 ‰. Cycads, ferns, and leaves coming from low-lying shoots have lowest average ratios of -28.4, -27.4, and -27.0 ‰, respectively, while averages of other leaf types range as high as -25.9 ‰. Isotopic variability within a leaf type ranges from 0.5 to 0.9 ‰. Paleosols have an average isotope ratio of -25.6 ± 0.6 ‰.

The overall variability in carbon isotope ratios of CRR leaves is less than that observed for modern tropical forests in Africa and Brazil that are characterized by a pronounced carbon isotope ‘canopy-effect' (± 1.5 to 3 ‰ around the mean). Lower carbon isotope ratios from 3 separate kinds of understory leaves, however, suggest that the CRR was closed in its structure. If CRR trees were shorter or narrower relative to modern forests, it could help explain the smaller observed range in carbon isotope ratios from CRR. Alternatively, leaves from the top of the CRR canopy may not be well represented in the fossil leaf litter thus reducing the amount of isotopic variability ‘captured' by our sampling.

Carbon isotope ratios of soil organic matter from the CRR are higher than those of associated plants, a pattern observed in modern tropical forests. This offset is indicative of microbial fractionation of carbon in organic matter during decomposition in soils, with ¹²C becoming preferentially incorporated into respired CO₂. This CO₂ presumably contributed to the lower carbon isotope ratios/closed-canopy signal preserved in understory leaves from the CRR.