Rainfall Variability and the Rise and Collapse of the Mississippian Chiefdoms: Evidence From a DeSoto Caverns Stalagmite

A newly acquired, absolute U/Th dated, delta O-18 record archived in a stalagmite from DeSoto Caverns in Alabama renders highly resolved time series for the past four millennia. Two principal states of variability are discerned in the delta O-18 record: (1) stable state spanning the intervals before Common Era 2350 to A. D. 400 and A. D. 1700 to 2008 that exhibits significant periodicities of 30 and 60 years and (2) unstable state in between containing six major discontinuities alternating with rapid delta O-18 positive excursions. The two contrasting states are likely the manifestations of extreme rainfall events established on the basis of the imprints of anomalously high/low drip flow rates discerned in the fabrics of the discontinuities. The proxy rainfall record offers valuable insights on whether climate variability may have been implicated in the rise and demise of the Mississippian chiefdoms (A. D. 800 to 1700) in the southeastern United States. The time of emergence and growth of the Mississippian chiefdoms and their subsistence transition to a dependency on corn, a warm and wet weather crop, coincides with a period of increased rainfall over the A. D. 450-1000 interval. Overall decline in rainfall between A. D. 1000 and 1500, alternating with droughts, is contemporaneous with evidence of abandonment of towns and villages and downstream movement of populations. Thus, warmer, wetter conditions than present may have promoted corn agriculture during the rise and growth of the chiefdoms, whereas food shortages, caused by failed corn crops under drought conditions, may have played a much greater role in the demise of the Mississippian chiefdoms than previously recognized.