YOUNGER DRYAS "BLACK MATS" AND OTHER STRATIGRAPHIC MANIFESTATIONS OF CLIMATE CHANGE IN NORTH AMERICA

Younger Dryas (YD) age "black mats" (YDBM), while variable, represent moister conditions than before or after deposition. Higher water tables, some perched, are indicated by wet-meadow soils, algal mats, and pond sediments including marls and diatomites, some of which are white.

Geochronological study of over 50 localities from Arizona to Canada reveal that YDBMs contain the earliest post-Clovis archaeological evidence and overlie the last Rancholabrean faunas.

Upland or lowland YDBMs occur sometimes as facies or catenas. Regional YD paleosols include the Brady of Nebraska and the Leonard of the Dakotas. On uplands these are darker and thicker in swales and may reflect perched groundwater or poor drainage. In lowlands they occur deeply buried beneath floodplains; in some colluvial settings multiple black bands are separated by lighter colored slope wash. Some YDBMs are related to springfed meadows and ponds formed during increased-discharge periods.

Deposition of some YDBMs began after a period of degradation coincident with terminal deglaciation during the Bølling-Allerød (B-A). Channel deposits of the B-A period represent ~2000 years of quasiequilibrium, with Clovis arriving during the Allerød.

Climate changes in B-A strata appear to be two Allerød drought periods separated by the InterAllerød Cold Period (IACP). Clovis first appeared simultaneously with megafaunal extinctions. A few black mat deposits may be of Allerød age perhaps deposited during IACP, but major deposition began with onset of YD cooling, probably due more to watertable recharge from reduced evapotranspiration than to increased precipitation. To account for valley aggradation in low order drainages by gradual accumulation of slope-washed eolian sand and silt, rainfall may have been less intense and more general than it has been since onset of mid Holocene (Altithermal) climate and the cut and fill epicycles of the middle to late Holocene.

Clovis evidence ends with the Allerød, replaced during YD by Folsom, Plainview, and Agate Basin bison hunters in the Great Plains, western stemmed tradition in the Great Basin, Dalton in the Southeast and various post-Clovis fluted point traditions in the Midwest and East. Cultures, ecological changes, and geomorphic processes all responded significantly to changing climate over the past 13,000 years.