SCHERER, JACQUELINE*, GARLAND R. UPCHURCH, JR., GREG H. MACK, AND MARGARET J. KNAUS. Department of Biology, Southwest Texas State University, San Marcos, TX 78666, and Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003. - Foliar physiognomic estimates of climate for the latest Cretaceous: comparative sensitivity to transfer function.
Estimates of climate derived from the foliar physiognomy of modern
vegetation indicate that, for modern test sites, the calculated
climates can differ from actual climates and are sensitive to the
equation used to quantify the leaf/climate relationship (or transfer
function). To determine how estimates of past climates can vary in
response to transfer function, we analyzed a latest Cretaceous
(Maastrichtian) leaf assemblage from the Jose Creek Member of the
McRae Formation, southern New Mexico, in part because of its relevance
to climatic change across the Cretaceous-Tertiary boundary. The Jose
Creek assemblage represents probable in situ leaf litter
preserved in volcanic ash and was derived from vegetation growing in
well-drained soil. The analyzed assemblage consists of 132 specimens
of dicot leaves belonging to 43 species, which are associated with a
comparable number of ferns, conifers, and thermophilic monocots
belonging to at least 10 species. Estimates of mean annual
temperature (MAT) and mean annual precipitation (MAP) were made using
published univariate and multivariate transfer functions derived from
the CLAMP database of Wolfe or regional floras. Calculations were
made using raw physiognomic data and data adjusted to account for the
over-representation of small leaves in modern leaf litter. Calculated
MAT for the Jose Creek assemblage generally ranges from 16-21°C, with
multivariate transfer functions tending to give cooler values than
univariate transfer functions. Calculated MAP ranges from under 800
to over 1500 mm per year, with an average of 1100 mm. Variation in
calculated temperature and precipitation, relative to the mean of all
estimates, can exceed ±35%, underscoring the potential bias introduced
by choice of transfer function. However, the range of values is
congruent with more qualitative estimates of climate derived from
fossil soils and plant life form, which indicate warm moist conditions
and above-freezing cold-month means for the Jose Creek Member.
Key words: Cretaceous, paleobotany, paleoclimate, physiognomy