Palaeoenvironmental record of the last 70 000 yr in San Felipe Basin, Sonora desert, Mexico

Preliminary results of a palaeoenvironmental research which includes particle size analyses, diatom and rock-magnetic analyses for a 9.5 m long core from Laguna Seca de San Felipe (LSSF), Baja California, are presented. AMS ¹⁴C dating gives an extrapolated age of ca. 70 kyr for the sequence analysed. Rock-magnetic measurements (including χ, χ(fd%), ARM, SIRM, hysteresis parameters and S ratios), organic content (LOI) and particle size distribution show highly contrasting conditions between glacial and late glacial-Holocene sediments. Dry conditions prevailed between 70 000 and 45 000 yr ago. The full glacial, between 34 000 and 19 000 yr ago, is characterized by moist conditions. The late glacial-Holocene sediments display a five-fold increment in most of the magnetic parameters and in sedimentation rate. This change is interpreted as an increment in runoff waters. At ca. 7000 yr B.P., a dry period is recorded, which probably spans until 6000 yr B.P., when moist conditions return. The onset of the actual arid climate occurred around 4000 yr B.P.