Bulk carbon isotopic variability within leaves

The stable carbon isotopic composition (delta (super 13) C) of fossil leaves is a simple and common measurement that provides information about paleophysiology, paleoecology, and paleoclimate. Variance in delta (super 13) C is typically assessed across leaves; comparatively little is known about variance within leaves, a potential source of unquantified uncertainty. Here we systematically analyze the spatial patterns of bulk delta (super 13) C in fresh leaves of 10 tree species (two leaves per species; 21 or 22 analyses per leaf). We find that samples containing midvein tissue are markedly higher in delta (super 13) C than non-midvein tissue from the same leaf (median = +0.85 ppm), with samples containing only midvein tissue offset by as much as +3.01 ppm. The non-midvein samples are less variable - the typical range within a single leaf is <1 ppm - and do not show any consistent spatial patterns. In cases where whole fossil leaves cannot be analyzed, we recommend sampling as many randomized areas without major veins as is feasible.