THE BIOGEOMORPHIC RESPONSE MODEL AND LATE PLEISTOCENE LANDSCAPE EVOLUTION IN CENTRAL PORTUGAL

“Biogeomorphic response” implies complex behavior introduced to geomorphic systems by the lag time of ecosystem response to climate change. The concept was developed by James Knox in a 1972 paper, with particular reference to valley alluviation in southwestern Wisconsin. His model predicted that maximum hillslope erosion and fluvial sediment yield would follow a transition from arid to humid climates, when erosive rains fall on minimally vegetated slopes. The model has been widely cited and extended to interpretation of floodplain stratigraphy and other geomorphic systems.

The Estremadura region of central Portugal illustrates biogeomorphic response to late Pleistocene climate fluctuations, specifically the high-frequency changes of Heinrich events and Greenland stadials/interstadials. Located along a steep gradient between Atlantic forests and Mediterranean steppes, this maritime landscape is highly sensitive to changes in temperature, precipitation, and storminess. Pollen and deep-sea sediment records show that Heinrich events 1-6 produced sharply-defined cold and arid phases between 10-70 ka BP in western Iberia. Each event lasted about 1,000 years and was associated with sudden reductions in sea surface temperature and reduced forest cover. This chronology is remarkably similar to the hypothetical conditions of the biogeomorphic response model, which predicts that accelerated upland erosion and valley aggradation should occur at the end of each Heinrich event. Thus, the Estremadura study area represents a rare opportunity to test a long-standing geomorphic model over Pleistocene time scales.

Our suite of more than 20 OSL ages on alluvial, colluvial, and aeolian deposits from Estremadura is consistent with the biogeomorphic response model, although our interpretations are limited by the precision of the luminescence technique and the uncertain ages of Heinrich events. Most late Pleistocene sections demonstrate periodic episodes of rapid sedimentation in previously stable valley bottoms. Currently, the 1-sigma OSL age range of nearly all late Pleistocene surface deposits in the region encompass a Heinrich event, suggesting a non-random link between geomorphic activity and Heinrich-driven bioclimatic fluctuations.