HR: 0800h
AN: PP51D-0630    [Abstracts]
TI: Composition and Origin of Laminae in Holocene Sediments From the Southern Gulf of California
AU: * Perez-Cruz, L
EM: perezcruz@geofisica.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circuito Exterior de Ciudad Universitaria, Coyoacan, Mexico, D.F 04510 Mexico
AU: Urrutia-Fucugauchi, J
EM: juf@geofisica.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circuito Exterior de Ciudad Universitaria, Coyoacan, Mexico, D.F 04510 Mexico
AB: The sediment sequence of Alfonso Basin in the southern Gulf of California spanning the last 8000 years is characterized by a laminated structure. In contrast to sediments from basins in the central part of the Gulf, the Alfonso Basin laminae are non-annual sediments. To characterize the sediments and investigate on its genesis, X-ray, scanning electron microscopy, microfossil, geochemical and magnetic mineral analyses have been conducted on dark and light laminae. The laminated sediments consist of alternating millimeter to sub-millimeters bands of different shades of olive gray. On average, dark laminae are thicker than light laminae, resulting in a dominant dark color of the sequence. Sediment rate is about 0.3 +/- 0.04 mm/yr, which results in an average 11.2 years for the dark-light laminae. Light laminae contain more radiolarian microfossils (up to 45 percent) and less terrigenous and organic material; they predominantly contain quartz and calcite minerals. The dark laminae contain more terrigenous material, dominated by clay and quartz. In general, almost all chemical elements (Si, Al, Fe, Mg, K, S) analyzed, except Ca, are always higher in the dark laminae and show positive correlation to organic carbon. Ca is higher in the light laminae and mostly represents biogenic input and shows a negative correlation to organic carbon. Si correlate with typical terrigenous elements (Al, Fe, K and Mg), this suggests that most of Si has terigenous origin, which can be related to the volcanic rocks, particularly siliceous tuffs, surround Bay of La Paz. Smectite and illite are present in dark and light laminae in similar proportions. In dark laminae magnetic hysteresis loops show saturation at low fields and relatively high saturation magnetization values, which suggest contribution of very fine-grained superparamagnetic minerals. The laminated slope sediments of the Alfonso Basin are an example of nearshore depositational system dominated by sedimentation of terrigenous. Basin receives an episodic pulse of biogenic material which results in light laminae.
DE: 3030 Micropaleontology (0459, 4944)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005