REDUCTIONISM VERSUS SELF-ORGANIZATION OF EOLIAN BEDFORMS - A PARADIGM SHIFT

For over half a century, the origin and development of eolian bedforms has been sought in reductionism, in which smaller-scale processes progressively define larger-scale processes. An alternative paradigm is that eolian bedforms represent self-organizing patterns that arise from non-patterned states without any underlying, smaller-scale template. In contrast to reductionism, in which the number of variables increases with increasing scale, with self-organization the number of variables decreases with increasing scale as small-scale process are slaved to larger-scale processes. Emergent behavior arises that is not predicted by the small-scale processes. Bedform self-organization is now supported by field studies, wind-tunnel experiments, computer simulations and theory. At the ripple scale, any stochastic movement of grains causes grains to clump. With migration, clumps merge, resulting in proto-ripples that continue to migrate at speeds inversely proportional to size, thereby fostering a reduction in number but an increase in spacing and crest length, and a decrease in defect density. Steady state is approached when the field is occupied by ripples of a similar size and, hence, migration speed. Dune fields follow a similar path to self-organization, beginning as random, abundant sand patches that migrate and merge into proto-dunes, then dunes. Steady state is approached as the dunes become self-similar. Dune type and crestline orientation are a function of only the number and orientation of all constructional wind directions, and the magnitude of sand transport of each. Change in dune orientation with an altered wind regime can only occur at crest terminations, accounting for the antiquity of many linear dunes, which have low defect densities. Superimposed patterns, typical of complex dune fields that have undergone multiple wind regimes, result because the formation of a new pattern of small dunes can occur more rapidly than re-orientation of existing dunes in the antecedent pattern. Because dune-field pattern development is not a direct function of smaller-scale dynamics, concepts of self-organization are exportable to other planets, such as Mars, and these offer insights into the stage of development and level of activity of Martian dune fields.