USING WOOD ANATOMY TO ESTIMATE LATE CRETACEOUS PALEOCLIMATE IN NEW MEXICO

The Fruitland-Kirtland Formations of northwestern New Mexico contain abundant and well-preserved specimens of permineralized wood from a Late Cretaceous (Campanian) floodplain. In this study, we examined the anatomy of silicified wood for paleoclimate signals, since wood production is sensitive to climatic and other environmental fluctuations. A total of forty-five specimens comprising twelve angiosperms and thirty-three conifers were analyzed using cellulose-acetate peel technique in conjunction with light microscopy. Three procedures were used to estimate paleoclimate. Mean sensitivity analysis quantified the variability between adjacent rings within a ring sequence in order to determine whether cambial activity was externally influenced or interrupted. The vulnerability index determined the degree to which the angiosperms were able to tolerate water stress. Regression equations that correlate angiosperm wood anatomy to climate were used to estimate the mean annual temperature for the region. Values generated from mean sensitivity analysis of the growth rings and interruptions (0.34-0.58) indicate that these plants experienced irregular growth due to external influences. The vulnerability indices (3.76-33.82) of the fossil specimens are similar to those of extant tropical species, and suggest that the angiosperms were not capable of tolerating prolonged or extreme drought. Mean annual temperature estimates (25.59 ± 5.96^oC) derived from angiosperm wood anatomy are consistent with other studies that have quantified paleoclimate, and suggest a subtropical climate. These results support that the Late Cretaceous San Juan Basin experienced a year-round warm and wet climate. Therefore, the growth rings and interruptions in some specimens were most likely due to other environmental disturbances, such as flooding or volcanic events, capable of disrupting wood production.