Medieval Climate In the Mojave Desert Suggests Regional Lake Rise Between Extended Droughts

The Medieval climate period is well-known for extended “megadroughts” in part of the southwestern United States but records of drought are difficult to extend to the warm deserts. However, unusually wet periods can be identified in these deserts by lake deposits. Ephemeral lakes in the Mojave Desert are created by persistent stream flow from high mountains, and thus are governed by precipitation during winter storms. We have identified two lakes by shoreline deposits dated at ca. 1270 A.D. Watersheds are entirely within the desert (Silurian Lake) and head in the adjacent Transverse Ranges (Cronese Lake) and therefore record desert-wide wet conditions at this time. Climate reconstructions from tree-rings in the Transverse Ranges and from bristlecone pine records along the northern margin of the Mojave Desert support this timing for increased precipitation. Other southern California records such as coastal marshes and woodrat middens do not help resolve the timing. Climate records in the Sierra Nevada are consistent with the 1270 A.D. wet event, with permissible lake highstands between megadroughts at Mono (Rush delta stand), Carson, and Walker lakes between ca. 1215 and 1295 A.D. Tree-ring reconstructions for flow on major Sierra streams show moderately good correspondence as well. Although each climate record is sensitive to different hydrologic variables, which creates ambiguity in the spatial extent of the ca. 1270 A.D. wet event, there is converging evidence that the event was regional. If the wet event was governed by the same climate forcings as the droughts, it may be a useful event to track in the warm deserts of the US and Mexico to better understand how large a region was affected by both wet and dry periods. A regional wet event between two megadroughts provides a natural example for studying geomorphic and ecosystem responses to pronounced climate changes.