PREHISTORIC HUMAN INFLUENCE ON EROSION AND SEDIMENTATION IN THE IN THE CONTEXT OF GLOBAL CLIMATE CHANGE

Human influence on sediment yield has been heavily studied over historical timescales, but research is shifting toward prehistoric time and has shown the ancient human imprint to be quite significant. However, prehistoric distinction between human- versus climate-driven changes can be subtle and difficult to prove. This paper illustrates these points with three diverse case studies of bottomland sedimentation from highlands of Oaxaca in southern Mexico, the western French Pyrenees Mountains, and the Southern Blue Ridge Mountains (USA).

In the semi-arid Mixteca Alta of Oaxaca sediment check dams were constructed from stones (lama-bordos) by 3400-3500 years ago. These lama-bordos provide obvious clues about human influence on sedimentation, validating the “Anthropocene” epoch. However, there may have been a climatic impetus for people to construct the dams in order to conserve and use soil and water for agriculture.

In the humid-temperate Pyrenees the use of fire for deforestation and maintenance of grazing land from Neolithic (ca. 7500 years ago) to historic time is reflected in sedimentary records containing charcoal in zero-order hollows. Radiocarbon dates not only point to the time of initial deforestation by burning in the Pyrenees, but also to changes in sedimentation rates and patterns. A unique change is peat-bog formation and accretion possibly resulting from human-induced vegetative changes in the water balance (i.e. reduced evapotranspiration by the absence of trees).

In the Southern Blue Ridge Mountains vertical accretion of alluvium fails to indicate any prehistoric human influences on sedimentation rates, but sandy signatures of large floods correlate with the Medieval climate anomaly and known periods of wetness. However, the Blue Ridge alluvium hints at human modification of vegetation communities by around 1500-2000 years ago, coeval with emerging agriculture in the region. A trend toward less forested conditions is indicated by progressively less negative trends in delta ¹³C upward through the profile, and human use of fire is indicated by upward increasing charcoal concentrations.

These studies illustrate the need for non-anthropic background data to isolate effects of climatic when seeking to understand human influence on prehistoric sediment yield.