BAJOCIAN SEDIMENTARY CRISIS IN WESTERN THETHYS: A GEOCHEMICAL SIGHT

A carbonate productivity crisis, starting in Early Bajocian (Middle Jurassic) is recorded in the Western Thethys and it coincides with the onset of biosiliceous sedimentation in several basins and is preceded by a d13C positive excursion in carbonates which is of regional extent (Bartolini et al. 1996; Bartolini & Cecca 1999).

High CO2 level in the atmosphere is assumed for this period, linked with the birth of the Pacific Plate and also a major pulse of subduction related magmatism (Bartolini and Larson 2001). Opening of the Liguro-Piemontese ocean may have cause rearrangement of oceanic current patterns of circulation. These global conditions are supposed to have lead to eutrophication and induced an ecological crisis of the main carbonate factories. Recently, Muttoni et al. (2005) on the basis of paleolatitude data, explained the switch from carbonates to radiolarites throughout the Alpine-Mediterranean region, during the Middle-Late Jurassic, as the result of plate motion towards near-equatorial upwelling zone of high biosiliceous productivity.

In order to test if the early Bajocian carbon perturbation and carbonate crisis is linked to eutrophication conditions outside a contexte of low latitude high biosiliceous production, we investigated trough a geochemical multi-proxies study a section from the Subbetic realm (South Spain), caracterized by calcareous-marly alternation. We compared the data with a section of Umbria-Marche area (Italy), caracterised by increased radiolarian production. Geochemical data are associated with paleontological observations.

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