DAKOTA FORMATION PALEOCLIMATE ESTIMATES FROM LATE ALBIAN ANGIOSPERM LEAVES

The mid-Cretaceous epicontinental seaway encroached upon middle North America during the Albian eventually submerging the coastal plains, burying abundant fossil plants previously deposited in clays and sands. The data used for this analysis comes from angiosperm leaves collected and studied from four localities: Hoisington in central south Kansas containing 25 morphotypes, Braun located in north central Kansas containing 20 morphotypes, Rose Creek located in southeastern Nebraska containing 21 morphotypes and Courtland located in south central Minnesota containing 23 morphotypes. These localities represent a late Albian time-sequence of several million years as the advancing mid-continent seaway buried coastal areas from the southwest towards the northeast. The estimated mean annual temperature (MAT in Celsius) and the mean annual precipitation (MAP in cm rainfall based on leaf average area as suggested by Wilf, 1998) for each locality is: Hoisington, MAT = 18.3, MAP = 12.7; Braun, MAT = 14.9, MAP = 7.9; Rose Creek, MAT = 18.6, MAP = 8.4; Courtland, MAT = 21.1, MAP = 9.9. These data suggest the MAT (potential 2 degrees C error) fluctuated by 5.2 degrees C during the latest Albian. There is a slight trend towards warming with no strong increase in precipitation. The angiosperm leaves used represent local vegetation growing near the site of deposition in proximity of water. Moisture should have been relatively abundant; however the small leaf size of most leaves preserved indicates unrealistically dry environments. This could be explained by preservation or collection bias, but neither of these satisfactorily explains away the unrealistic MAP figures predicted by these floras.