Late Quaternary Climates in Arizona

Abstract

AbstractThis study makes a comparative evaluation of geological and biological evidence, on the one hand, and of plant pollen data, on the other, bearing on late Quaternary climates in southeastern Arizona. The findings support the conclusions based on the former and reject the contrary claims based on pollen by Martin, Schoenwetter, and Arms. The increase of pine pollen in the Cochise-Sulphur Spring and the Cochise-Cazador beds at Double Adobe from a few per cent to 28% was much too sudden to be accounted for by the spread of pine from the mountains to Double Adobe, and does not prove a change of the climate from arid to subhumid. The beds themselves indicate a subhumid climate throughout. The low frequency of pine may be due to such causes as scanty production, decomposition, and failure of the pollen to settle in flowing water.No pollen data are at hand from the mid-postpluvial or Altithermal age of erosion. The preceding and the succeeding beds have long been referred by Antevs to the pluvial/postpluvial transition and to the relatively moist early Medithermal, respectively. The most typical features of the age, arroyo cutting and accumulation of caliche, are most competently explained by a long sharp drought. There seems to be no evidence for the view of Martin and associates that the Altithermal was an age of intense summer precipitation.The entire synthetic postpluvial pollen profile is alternately dominated by composites and chenopods-amaranths. The last shift to chenopods-amaranths occurred at times ranging from about 4000 years ago to the 19th century. The dominations are apparently controlled by unknown edaphic conditions and are therefore of little or no use for correlation, dating, or climatic conclusions. Still, because of their great numbers, these pollens together with pine pollen are used as the main basis for the division into pollen zones. This leads to incorrect dating of Medithermal deposits and to disregard of documented arroyo erosions and droughts. Schulman's finding, that the radial growth of the ponderosa (yellow) pine, which is completed by the end of July, shows best agreement with the October-July precipitation, implies that the ring width is in accord with some 75 to 80% of the annual rainfall. Thus normally a narrow ring means deficient rainfall. The Great Drought of the 1200's did exist and caused arroyo erosion.