CLIMATIC CHANGES IN THE SOUTHWESTERN UNITED STATES AROUND THE YOUNGER DRYAS

The study of late Pleistocene to early Holocene climate change is interesting for a number of reasons including the earliest well-documented cases of human colonization of North America and dramatic changes in the megafaunas. Speleothem growth in arid regions such as the southwestern United States is moisture-limited, and therefore changes in growth rate, including hiatuses, may be used to infer changes in regional precipitation. We obtained precise uranium-series dates on speleothems using thermal ionization mass spectrometry. Stalagmite growth in the Guadalupe Mountains of New Mexico beginning in the middle Younger Dryas and ending abruptly in the earliest Holocene indicates wetter conditions persisted in the southwestern United States during this period. Uranium-series dating of seven speleothems from three caves (two of which are couplets) show normal stalagmite growth from 12,300 to 10,800 years BP. Lack of, or less stalagmite growth from at least 14,000 to 12,300 years BP suggests somewhat drier conditions during the early Younger Dryas and Allerod chronozones. These stalagmites have tops or hiatuses at approximately 11,000 years BP, indicating a distinct shift to significantly drier conditions after the earliest Holocene. The apparent wet period starts in the middle of the Younger Dryas and persists for about 800 years after it ended. These climate changes occurred during the earliest well-documented human occupation of North America illustrating the potential of stalagmite studies for resolving past climate over the paleo-indian period (Clovis and Folsom for example). For instance, the most significant stalagmite growth from 12,300 to 10,800 years BP represents wetter conditions during the latter part of Folsom occupation in the southwestern United States. Early Folsom times were apparently drier.