Impacts of Climate Change and People
The climate of the Midcontinental United States from circa 7000 to 2200 BC was generally warmer and drier than today's. Forested areas shrank and open grasslands expanded. This period is called the Hypsithermal Climatic Optimum. In deposits at archaeological sites, this period is represented by evidence for increased soil erosion and deposition.
Archaeologists encountered similar evidence for the Hypsithermal Climatic Optimum in the soils at Ireland Cave. The graph to the right illustrates the cave’s sedimentation record over time.
The steep line shows that erosion near the cave, and thus soil deposition within it, increased dramatically from 5600 BC to 4000 BC. The less steep line at the top of the graph indicates that deposition slowed from 4000 to 1000 BC. This suggests that a more stable landscape had developed near the cave, and thus, that less erosion occurred.
The information from Ireland Cave suggested that perhaps natural or human-set fires created open vegetation on the hillside surrounding the cave. This would have lead to increased erosion. But the extensive human use of the cave over time also could have impacted the surrounding vegetation. People likely trampled the grasses, plants, and shrubs during their stays, and this could have contributed to erosion around the cave opening, too.
From these data, researchers inferred that environmental fluctuations and human use of the cave during the Middle Archaic caused soil erosion in the area around the cave and thus soil deposition inside the cave itself.