T194 Height, the Elusive Third Dimension of Terrestrial Ecosystems: New Approaches for Determining the Structure and Complexity of Ecosystems on Land in Deep Time

Tuesday, 27 September 2016
8:00 AM - 12:00 PM
Mile High Ballroom 3C

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

Paper number: T194-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

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 (δ13Cleaf) 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 δ13Cleaf measured from Ginkgo adiantoides showed a larger than expected variance in δ13Cleaf. 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 δ13Cleaf, otherwise known as the canopy effect. However, such a high variance in δ13Cleaf 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 (δ13Cn-alkane) to determine if they show the same high variance as δ13Cleaf. Extant Ginkgo biloba is morphologically identical to Ginkgo adiantoides, and makes for an ideal comparative analog. Thus, we also measured the variance in δ13Cleaf and δ13Cn-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 δ13Cleaf and δ13Cn-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 δ13Cleaf is a primary signal, then Ginkgo adiantoides may have lived along streams, but also within forests with highly closed canopies.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287905

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