Growth-ring analysis of Lower Paleocene gymnosperm woods from the central Patagonia, Argentina.

BREA, M., ZUCOL, A. F., MATHEOS, S. AND M. S. RAIGEMBORN.

Puerto Madryn

Otro; Reunión Anual de Comunicaciones de la Asociación Paleontológica Argentina; 2005

CENPAT

The analysis of growth rings in Lower Paleocene fossil forests is discussed here. The José Ormaechea fossil forests are located southward of Musters and Colhué Huapi lakes (Chubut province, central Patagonia, Argentina 45 48' 53'' S, 69 04' 01'' W). These fossil woods are found in fluvial-deltaic sediments of the Salamanca Formation. They are composed by a large number of petrified parautocthonous trunks. Nine sample and 111 rings were measured and analyzed, all of them representing woods of gymnosperms. The mean ring-width is 2.10 mm, the minimum ring-width is 0.28 mm and the maximum ring-width is 6.59 mm. The values of mean sensitivity (MS) are concentrated in a range from 0.340 to 0.949. The average MS is 0.593. This indicates that all trees were sensitive, showing growth under more variable conditions, a fact which suggests that there were episodes during growth when climatic influences were more marked. The histograms of annual sensitivities (AS) show difference in growth among successive years. All samples have very narrow latewood, consisting of only two or five small cells. Narrow latewood indicates very rapid cessation of growth. The earlywood is very well developed. The trees grew fast during the growing season, what resulted in wide growth rings. The characteristics of the annual rings, wide growth rings, narrow latewood, annual sensitivities (AS) and high mean sensitivities (MS), indicated that the climate was warm-temperate and humid, with dry summers and rainy springs.   This work is a contribution to the project PICT 07-08671 (ANPCyT).