HR: 0830h
AN: OS31B-0209    [PDF]
TI: Variability of Precipitation in Southern California During the Late Quaternary: Inferences From Clay Mineral Assemblages of the Santa Barbara Basin (Images Core MD02-2503)
AU: * Robert, C
EM: robert@com.univ-mrs.fr
AF: PBDS, Centre d'Oceanologie de Marseille, Luminy Universite de la Mediterranee, Marseille, 13009 France
AU: Tourneur, F
EM: tourneurfred@voila.fr
AF: PBDS, Universite de Lille 1, Villeneuve d'Ascq, 59655 France
AU: Chamley, H
EM: Herve.Chamley@univ-lille1.fr
AF: Sciences de la Terre, Universite de Lille 1, Villeneuve d'Ascq, 59655 France
AB: Clay mineral assemblages have been investigated in a late Quaternary sequence cored in the deepest (569m) part of the semi-enclosed Santa Barbara Basin, off Southern California. Core MD02-2503 consists of 45.27m of silty clay, nannofossil silty clay, and nannofossil clay, ranging from dark olive and dark olive grey to dark grey and very dark grey. The sediment is mostly laminated from top to 15.50m and from 18.00m to 21.00m, and massive to slightly bioturbated in other parts with rare laminated intervals below 32.00m. The distribution of laminated intervals in MD 02-2503 and ODP Site 893 nearby is very comparable and suggests an age of approximately 32ka at the bottom of the core. Smectite dominates the clay assemblages and is associated with abundant illite, kaolinite and chlorite being present in low contents. Smectite is especially abundant in the terrigenous load of the Santa Clara river, which drains the semi-arid continental interior of the Transverse Range. Illite is more abundant in the terrigenous load of the Ventura river and short intermittent rivers, as well as in the suspended load of the Santa Ynez river Northwest of the Santa Barbara Basin. The Santa Clara river is the largest source of terrigenous sediment of Southern California and by far the major contributor to the sedimentation of the Santa Barbara Basin, especially during intervals of intense precipitation. The content of smectite in Core MD 02-2503 is used as a tracer of the terrigenous discharges of the Santa Clara river, and related regional precipitation. Their variations are better expressed through relative abundances of smectite and illite (S/I index). Maximum values of the S/I index express intervals of intense precipitation and river discharge, whereas low values of the S/I index express more arid conditions. The S/I index increased from 28ka to 26.5ka and after 21.5ka, suggesting that precipitation intensified in Southern California during an interval marked by a warming of Pacific surface waters, increased lake level in Death Valley and the Mojave area, and glacier advances in the Sierra Nevada. Peak values of the S/I index interpreted as short intervals of maximum precipitation between 19.5ka and 12.5-13ka occurred during the Tioga Glaciation of the Sierra Nevada, and finished in coincidence with the end of Mojave lacustrine episodes and with vegetational changes (decline of pine and stabilization of oak) in the Santa Barbara area. A further interval of intermittently high S/I and intensified precipitation from 10ka to 6.5ka coincides with lacustrine development and oscillations in the California hinterland. Low values of the S/I index after 6.5ka, suggest little precipitation in Southern California as lakes dessicated in the continental interior. A further decrease of S/I and inferred precipitation at 2.8ka to 3.0ka marked the onset of modern conditions in the Santa Barbara area. Major increases of precipitation in the Santa Barbara area coincide with important climatic events: increase of West Pacific surface temperature at 26-28ka, peak of the last glaciation at 21ka, early Holocene moisture and formation of Mediterranean sapropel S1 from 10ka to 6.5ka. The decrease of precipitation after 6.5ka coincides with the development of aridity in the thirties of latitude worldwide.
DE: 1600 GLOBAL CHANGE (New category)
DE: 3022 Marine sediments--processes and transport
DE: 4267 Paleoceanography
DE: 4863 Sedimentation
DE: 4885 Weathering
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting