Climate of New Mexico during the Last Glacio-Pluvial

Abstract

On the basis of Hans Ahlmann's determinations of the temperature at the glaciation limit, the altitude of the mean June-September isotherm of 36° F. is taken as the modern climatic snow line in New Mexico. This plane naturally rises southward. The actual snow line of the last glacial maximum, probably the Cary of more than 24,000 years ago, is determined mainly from data presented by Louis Ray. The Cary snow line is found to have been practically horizontal from Pikes Peak in central Colorado to Cerro Blanco (Sierra Blanca) in southern New Mexico. The altitudinal difference between the snow-line planes, the Cary snowline depression, ranges from 3,000 feet at Pikes Peak to 4,300 feet on Cerro Blanco. Since the temperature lowering probably was slightly greater at Pikes Peak than at Cerro Blanco, the smallest snow-line depression, 3,000 feet at Pikes Peak, is the largest amount referable to lower temperature. Therefore, the mean June-September temperature during the Cary maximum was, at most, 10° F. (5?5 C.) lower in the area. The snowline depressions in excess of 3,000 feet are the minimum lowerings caused by heavier snowfall. Thus the Cary glaciation in New Mexico was a product of both lower temperature and greater snowfall, and the latter increased in importance southward. Lake Estancia and other pluvial lakes in New Mexico and contiguous regions postulated both heavier rainfall and lower temperature. Over Lake Estancia the rainfall can have been 9 inches greater. The exceptional precipitation in November, 1940—October, 1941, demonstrates that an altered and favorable circulation of the atmosphere can produce a greater precipitation in the region during all seasons, even in winter. Therefore, the glaciers and the lakes must have been caused by the same temperature lowering and precipitation increase and may have culminated at the same time, for the lakes did not receive any water from melting glaciers. The pluvial maximum was probably attained slightly earlier than in the Great Basin, where glacial meltwater contributed to most lakes. This dating of the pluvial culmination in New Mexico is new, for I earlier referred it to a late stage of the deglaciation. The revised dating is based mainly on the finding that the Cary glaciation was in part produced by heavier snowfall and on the realization that the abnormal precipitation of November, 1940—October, 1941, might be a sample of the normal one during the glacio-pluvial maximum. At the latitude of Santa Fe, the Cary snow-line depression was 4,000 feet and equaled that of the life- and climatic zones deduced by Charles Stearns from the present and one-time altitudinal distribution of the dusky marmot or western woodchuck. The present semiarid pinon-juniper woodlands and short-grass steppes in New Mexico were then subhumid timberlands and tall-grass prairies.