Lunettes: A Global Inventory of Their Occurrence and Characteristics

Lunettes (including "clay dunes") form along the downwind margins of saline pans. As landforms they were little studied and poorly understood until the mid-20^th century when they attracted the attention of Australian geomorphologists, notably James W. Bowler. During the last 40 years, lunettes have been studied extensively as indicators of climate change. Their occurrence has now been documented on every continent except Antarctica. Inspection of more than 100 research sites using Google Earth has led to the recognition that lunettes occur in three definable, though somewhat overlapping, settings. Playas and lunettes are common features of closed basins having sub-humid climates. Large basins with internal drainage due to structural deformation produce the most extensive and complex lake and dune systems (e.g., the Etosha Pan, northern Namibia; Soda Lake, Carrizo Plain, California). In these settings, the large central basin is associated with lunettes that may be more than 10 km long and as much as 50 m high. Tens of lunette ridges may mark former lake levels and channels of the desiccated drainage system. Some basins lack external drainage for hydrological reasons (low precipitation, drainage diversion, etc.). These hydrologically closed basins may also host saline lakes and lunettes (e.g., Lake Malheur, Oregon) though they are generally smaller and less complex. Shallow depressions may occur by the thousands on the surface of arid and semi-arid plains such as Brazil's Pantanal, and the Kalahari and Transvaal of southern Africa, the High Plains of Texas and New Mexico, and several parts of Australia. Although they have not been described in the literature, pans also cover large areas in China, Tibet, and Mongolia. Multiple theories have been advanced to explain the occurrence of plains pans including deflation, piping, subsidence, and animal activity. In the plains setting, pans can occur in large numbers, up to 100 per 100 km² and may cover 20% or more of the area. Individual pans vary in size by at least three orders of magnitude with maximum areas of a <10 km². Larger pans may have lunettes, though the strength of the association with pan size varies with climate and substrate. The least commonly documented occurrences of lunettes are in coastal settings, all of which occur at low latitudes (11-28 degree from the equator). The dunes most commonly occur along shallow channels and lagoons that are flooded only during spring tides or in association with storms and strong on-shore winds. Coastal lunettes have been described in deltaic environments (e.g. Senegal River, Senegal; Mitare River, Venezuela) and lagoonal settings (e.g., Laguna Madre and Copano Bay, Texas). Research into the origin and history of lunettes has been dramatically changed by new dating techniques and detailed geochemical studies. What once seemed a simple relationship between climate and landform has been demonstrated to be complex and non-uniform. The variety of settings in which lunettes form should be added to the list of the factors requiring consideration in making paleoenvironmental interpretations based on their occurrence.