BULK CARBON ISOTOPIC VARIABILITY WITHIN LEAVES

The stable carbon isotopic composition (δ¹³C) of fossil leaves is a simple and common measurement that provides information about paleophysiology, paleoecology, and paleoclimate. Variance in δ¹³C 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 δ¹³C in fresh leaves of ten tree species (two leaves per species; 21-22 analyses per leaf). We find that samples containing midvein tissue are markedly higher in δ¹³C than non-midvein tissue from the same leaf (median = +0.85 ‰), with samples containing only midvein tissue offset by as much as +3.01 ‰. The non-midvein samples are less variable—the typical range within a single leaf is <1 ‰—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.