PHYSICAL RESPONSE OF AEOLIAN SAND SEAS TO CHANGES IN WIND REGIME OVER THE LAST 100,000 YEARS - AN APPROACH FROM SYSTEM SELF-ORGANIZATION

The reconstitution of wind regimes has impacted aeolian sand seas throughout the last 100,000 years of Icehouse Milankovitch and smaller-scale climatic cycles. Approached as self-organizing systems, the physical response of dunes within sand seas can be partly understood from models and theory. Given a sediment supply, the availability of this supply, and wind energy, dune fields develop over relatively short periods of time. Through the process of merging with migration, dunes increase in size, crest length and spacing, with a corresponding decrease in defect density. Steady state is approached when all bedforms are similar in size, migrate at the same speed, and crestline orientation is as perpendicular as possible to all constructive winds. Dune type is a function of the direction and magnitude of these winds. With a change in wind regime, crest reorientation occurs at defects, and at a rate inversely proportional to defect density. Because the rate of change of spacing and orientation within well-developed dune fields with few defects is slow, most relict dune-field patterns do not form templates for new patterns, but rather new patterns form congruent with the new wind regime. Linear dune fields especially have a typically low defect density, and the rate of formation of a new pattern of dunes exceeds the rate of crest reorientation for the existing dunes, leading to the superimposition of generations of dunes. Although a large array of other complex dune-field patterns exists that are poorly understood, many of these probably originate through superposition of dune generations. Complex dune-field patterns are characteristic of accumulation surfaces where the frequency of wind-regime change exceeds any burial rate. In contrast, much of the aeolian rock record results from dune fields in which the accumulation surface was exposed for relatively short periods of time. Complexity of aeolian cross-strata in the geological record is best attributed to large dune size, which fosters the formation of superimposed dunes, and to main bedform crest orientation that is oblique to shorter frequency wind cycles, allowing the occurrence of superimposed lee-face dunes.