HOLOCENE LAKES AND LATE QUATERNARY CLIMATE VARIABILITY IN THE CHIHUAHUAN DESERT, MEXICO

Beach ridge and lacustrine deposits from Laguna El Fresnal and Laguna Santa María in the Chihuahuan Desert, Mexico reveal Holocene lake highstands and glacial to interglacial changes in lacustrine sedimentation. Situated about 70 km south of the US-Mexico border, these currently dry basins preserve Holocene lake level variations that are attributed to millennial-scale changes in winter storm frequency, and reduced temperatures and evaporation rates over the southwestern U.S. and northern Mexico. Constructional beach ridges dated at ca. 230 and 495 cal yr B.P. (Little Ice Age), 4220 to 4795 cal yr B.P. (early Neoglacial), 6980 to 7585 cal yr B.P. (mid Holocene), and 9255 and 9430 cal yr B.P. (early Holocene) provide the first detailed Holocene lake-level chronology for northern Mexico. In addition, these results show unambiguous evidence for wetter than present conditions during the mid Holocene, a time that is widely considered to have been relatively dry in this region. Distinct pluvial episodes during the early Holocene and probably during the late Pleistocene are marked by two shorelines above an intrabasin sill. At these times, conditions were wet enough to form one large lake, pluvial Lake Palomas, which encompassed all of the sub-basins. We estimate that the largest spatial extent of this combined lake covered approximately 7030 km ² during maximum pluvial conditions.

An ~17m-long core from Laguna El Fresnal reveals glacial to interglacial changes in lacustrine sedimentology for the last ca. 70,000 calendar years. Colder than modern climate conditions coupled with low-energy lacustrine deposition before ca. 30,000 cal yr B.P. are marked by low total organic carbon and a gradual increase in clay-dominated sediments. Maximum pluvial conditions ca. 21,000 cal yr B.P. are characterized by peaks in sand-size sediment, high magnetic susceptibility (MS), and low organic productivity within El Fresnal basin. The transition to relatively drier, warmer climate ca. 10,500 cal yr B.P. is shown by a marked decrease in MS, increased organic and inorganic carbon concentrations, and pulses of fine sand. These glacial to interglacial changes in lacustrine sedimentology are synchronous with other regional paleolimnological records.