COOPER, ERIN E.* AND JEFFREY A. MYERS. Department of Earth and Physical Sciences, Western Oregon University, Monmouth, OR 97361. - Paleoclimate of the middle Eocene John Day Gulch Flora, central Oregon.
Leaf physiognomic data from the 45-44 Ma John Day Gulch Flora, Clarno
Formation, indicate that central Oregon experienced a frost-free, warm
subtropical climate with high, non-seasonal rainfall during the middle
Eocene. These data were used to test two hypotheses to explain
climatic cooling trends during the middle Eocene. The traditional
model (Wolfe, 1992), proposes that the John Day Gulch Flora grew
during a late Eocene "Late Eocene Cool Interval", which was
followed by a return to near-tropical conditions in the latest Eocene.
An alternative hypothesis (Myers, 1996; Manchester et al, 1998)
proposes that the John Day Gulch Flora a series middle to late Eocene
cooling steps. Morphologic/anatomical (physiognomic) analysis of
the leaves of extant woody dicot assemblages provides quantitative
information about climate that can be applied to ancient leaf floras.
Two leaf physiognomic approaches, Climate Leaf Analysis Multivariate
Program - CLAMP (Wolfe, 1993) and Leaf Litter Analysis (Greenwood,
1992), were used to estimate the paleoclimate of the middle Eocene
John Day Gulch Flora. The CLAMP method and the Leaf Litter Analysis
yield essentially identical MAT estimates of 17.5oC and 17.8oC,
respectively (+/- ~ 1oC), compared to the ~13oC CLAMP MAT estimated
from the flora by Wolfe (1992). This leaf physiognomic MAT estimate,
combined with new age information, indicate that the John Day Gulch
assemblage grew under climate conditions similar to estimates from
other assemblages of the Clarno Flora, and refute the hypothesis that
John Day Gulch assemblage grew during a late Eocene cool interval.
Key words: CLAMP, Clarno Flora, middle Eocene, paleobotany, paleoclimate