Paleoclimatic Condition Inferred from the Tertiary Plant Megafossil Assemblages from Northern Thailand

Abstract

Physiognomic analysis using CLAMP technique of Wolfe (1933) was performed to two plant megafossil assemblages from northern Thailand intramontane basins to obtain quantitative data on the mean annual temperature (MAT), cold month and warm month mean temperatures (CMMT & WMMT) and growing season precipitation (GSP). The assemblage from Ban Pa Kha (BPK), a subbasin of Li Basin, dated as Late Oligocene, consists of 30 types of woody dicotyledonous leaves along with infructescence and inflorescence of alders and winged-seed of maple, as well as conifers. Physiognomic analysis of the fossil angiosperm leaves implies a MAT of 17.60°C, CMMT and WMMT of 7.5 and 27.80°C, and GSP of 324.5cm. Its composition and physiognomic feature correspond well with Notophyllous Broad-leaved Evergreen Forest of Wolfe (1979). Assemblage from Mae Lai basin (ML) of late Early to middle Middle Miocene age consists of 36 types of dicotyledonous leaves with no component of “Laurasian montane element” proposed by Morley (1999). Climate variables obtained are a MAT of 19.20°C, CMMT and WMMT of 12.5 and 26.30°C, and GSP of 214.7cm. Although MAT estimate is slightly lower than expected, floristic composition of ML can be comparable to Paratropical Rain Forest of Wolfe (1979). MAT estimates indicate the warming trend from the BPK to the ML time. CMMT estimate from BPK is 5 degree lower than that of ML, whereas WMMT is even higher in BPK. This suggests that decrease of mean annual range of temperature, caused mainly by an increase of CMMT, result in the reduction of temperate elements and expansion of paratropical evergreen forests by at least Middle Miocene. GSP estimates indicate perhumid Late Oligocene climate and humid but much smaller precipitation in the Mae Lai time. This change might be ascribed to uplifting Tibetan plateau and the onset of monsoonal climate before the Mae Lai time.