LATE HOLOCENE CYCLES OF SEDIMENTATION AND ENVIRONMENTAL CHANGE IN WESTERN MEXICO: THE RECORD FROM SANTA MARIA DEL ORO, NAYARIT, PRELIMINARY RESULTS

Preliminary results of a multiproxy paleoenvironmental research project in late Holocene lake sediments are presented. Santa María del Oro is a crater lake 60 m deep, in Western tropical Mexico (21.3° N, 104.5° W, 760 m asl). Four parallel cores, between 4 and 9 m long, were recovered from a shallower area (< 12 m in depth). Sediments are characterized by alternated laminations (few millimeters to 2 cm) of brown silt, sand, peat, reddish silt, or ochre, carbonate-rich silt. The sediment 14-C dated at 8 m depth, yielded an age of ca. 2,500 yrs. B.P. Given this date, it is possible that each set of laminations represent annual sedimentation cycles. The record is a potential high-resolution archive of environmental and climatic variability for western Mexico for late Holocene. To investigate the controlling factors in the sedimentation of the sequence, several analyses are currently under progress: computerized axial tomography (CAT), total organic/inorganic carbon (TOC/TIC), x-ray diffraction, diatom, pollen, ostracodes, and mineral magnetic properties analyses. It is considered that this multiproxy approach will allow to characterize the laminae and to identify the authigenic and allogenic processes controlling sedimentation in this specific area of the lake. One of our aims is to assess the existence of sedimentation cycles using magnetic properties in finding the frequency and the probable origin of the changes sedimentation mechanisms. High-resolution CAT images, scanned and converted to a 255 gray-scale, help in characterizing the sediments and in finding periodicities in the sedimentary processes. Preliminary results show that the main differences among each type of layer are in TIC and TOC content, in variations in the concentration of magnetic minerals and their paramagnetic contribution, and in the particle-size distribution. Initial magnetic measurements of susceptibility vs. high temperature show that partially oxidized magnetite and Ti-magnetites are the main magnetic phases identified in most layers. Pollen is scarce in most of the sequence, while diatoms and ostracodes show nearly monospecific “blooms”.