UPLIFT OF PENINSULAR RANGES AFFECTS TERRESTRIAL CLIMATE RECORD OF ENHANCED TROPICAL MOISTURE IN LATE PLIOCENE, ANZA-BORREGO DESERT, CALIFORNIA

Evidence recovered from late Pliocene lacustrine sediments from the western Borrego Badlands, Anza-Borrego Desert, southern California shows that during Pliocene time the region experienced an enhanced precipitation regime. Fossil microfauna, particularly the presence of the ostracode species Candona patzcuaro var. mexico, suggest increased Pliocene precipitation was derived from a tropical source. Other nonmarine ostracodes assigned to the genus Limnocythere, indicate the periodic occurrence of increased regional groundwater supply, possibly forming localized spring-supported wetlands. The presence of Chara haitensis, a freshwater algae, suggests the precipitation likely followed a seasonally cyclic pattern of precipitation and evaporation, establishing ephemeral lacustrine conditions. Using available regional magnetostratigraphy (Opdyke et al., 1977; Johnson et al., 1983), the Pliocene lacustrine sediments were constrained in time to the period 2.9 - 2.4 Ma. This time segment encompasses the final positive subchron of the Gauss positive polarity chron. Five ostracode abundance peaks suggest the initiation of regional lacustrine conditions was linked to the 100,000-year portion of the Milankovitch cycle, and may represent precession enhanced eccentricity records. Subsequent to late-Pliocene time, regional lacustrine conditions cease. Elsewhere in the American southwest, cooler, wetter conditions than those experienced in Anza-Borrego were typical of the Pliocene. These cooler, wetter conditions continued well into Pleistocene time. Pre-Pliocene uplift of the Peninsular Ranges likely formed a barrier between moist Pacific air masses and the Anza-Borrego region. Milankovitch forcing, on a 100,000-year scale, was sufficient to overcome the Pliocene “rain shadow”. By about 2Ma, uplift of the Peninsular Ranges was sufficient to install a permanent “rain shadow” across south-central California, initiating modern desert conditions.