HR: 1400h
AN: PP43B-05    [Abstracts]
TI: Composition and Origin of Non-Annual Laminae From the Gulf of California: Its Possible Relation with ENSO Events
AU: * Perez-Cruz, L
EM: perezcruz@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Laboratorio de Paleomagnetismo y Paleoambientes, Instituto de Geofisca, UNAM, Ciudad Universitaria, Coyoacan, Mexico, 04510, Mexico
AU: Fucugauchi, J U
EM: juf@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Laboratorio de Paleomagnetismo y Paleoambientes, Instituto de Geofisca, UNAM, Ciudad Universitaria, Coyoacan, Mexico, 04510, Mexico
AB: Alfonso Basin sediments spanning the last 8000yr are characterized by laminated structure. In contrast to central Gulf basins, laminae are non-annual. Sediment rate is about 0.3 mm/yr, whith average 11.2yr for the dark-light laminae. Microfossils, X-ray dispersive energy, and magnetic mineral analyses have been conducted on 13 light and 13 dark laminae to investigate its genesis. Radiolarian abundance and diversity in light laminae are higher than in dark. Paleo-productivity inferred on radiolarians per/gram is low, in comparison with basins in the Gulf. Radiolarian taxa in light laminae (Acrosphaera murrayana, Euchitonia triangulum and Phorticium pylonium (group)) suggest origin related with local upwelling conditions, promoting high productivity events. These conditions could be induced by wind stress mainly during windy seasons. Dark laminae contain high relative abundance of tropical radiolarian assemblage dominated by Tetrapyle octacantha and Stylochlamidium venustum. T. octacantha and S. venustum are associated with the subtropical plankton assemblage that is important during ENSO. S. venustum have showed an extraordinary contribution (60-95%) of total radiolarians during El Nino events. Dark laminae contain more terrigenous material, dominated by silicoaluminates. Almost all elements (Si, Al, Fe, Mg, K, S) analyzed, except Ca, are always higher in the dark laminae. Ca is higher in the light laminae and mostly represents biogenic input. Si correlate with typical terrigenous elements, which can be related to the siliceous tuffs. Magnetic hysteresis loops, IRM acquisition and back-field IRM demagnetization curves were measured in dark and light laminae, to investigate magnetic mineralogy, domain state and relative contents. In dark laminae hysteresis loops show saturation at low fields and high saturation magnetization. Contribution of SP material, also suggested from frequency-dependent susceptibility, is apparent from data-point distribution above mixing SD-MD curves. Changes in radiolarian taxa, chemical composition and, magnetic minerals reflect penetration of warm equatorial waters and terrigenous input, we suggest that El Nino events are reflected in the dark laminae.
DE: 1512 Environmental magnetism
DE: 1637 Regional climate change
DE: 4522 ENSO (4922)
DE: 4922 El Nino (4522)
DE: 4944 Micropaleontology (0459, 3030)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Joint Assembly