CONSTRAINING THE DRIVERS OF VARIANCE IN LEAF δ13C IN FOSSIL GINKGO ADIANTOIDES AND EXTANT GINKGO BILOBA: CANOPY EFFECT OR DIAGENESIS?

Leaf fossils of Ginkgo adiantoides are common floral elements in North America from the Late Cretaceous to the Neogene. Their leaves are often well-preserved, with the cuticle intact. The carbon isotopes of their whole leaves (δ¹³C_leaf) should represent the isotopic values of the local paleo-atmosphere. To assess their utility, we measured carbon isotope values on leaves of Ginkgo adiantoides from the Late Paleocene and Early Eocene of the Bighorn Basin of Wyoming, USA. We found that δ¹³C_leaf measured from Ginkgo adiantoides showed a larger than expected variance in δ¹³C_leaf. If this high variance is a primary signal, it suggests an ecosystem with a relatively closed canopy, where the canopy limits available light, reduces photosynthesis, and shifts δ¹³C_leaf, otherwise known as the canopy effect. However, such a high variance in δ¹³C_leaf is in opposition to both the growth structure of ginkgo plants (open form; non-shading), and with its typical ecological habitat near streams, which have intensive light conditions. The incongruence could be a product of diagenesis at the leaf level, such as decay during burial, or through chemical weathering near the outcrop surface. Or, it is possible that some Ginkgo adiantoides plants lived in a greater diversity of habitats than previously understood, potentially including under dense forest canopies. To constrain these effects, we investigated Ginkgo adiantoides leaf-wax n-alkanes (δ¹³C_n-alkane) to determine if they show the same high variance as δ¹³C_leaf. Extant Ginkgo biloba is morphologically identical to Ginkgo adiantoides, and makes for an ideal comparative analog. Thus, we also measured the variance in δ¹³C_leaf and δ¹³C_n-alkane from a single tree of Ginkgo biloba growing outdoors at the Blandy Experimental Farm in rural Virginia (USA). Leaves were collected on the tree days before full-tree leaf drop, to simulate the state of the fossils at deposition. We sampled 8m up the trunk, and 4-6m out limbs in each cardinal direction. We quantify the variance in δ¹³C_leaf and δ¹³C_n-alkane for the Ginkgo biloba tree to determine if the variance observed in Ginkgo adiantoides is a product of a strong canopy effect or diagenesis. If the high variance in δ¹³C_leaf is a primary signal, then Ginkgo adiantoides may have lived along streams, but also within forests with highly closed canopies.