Registro de la composición isotópica del Jurásico inferior de la Cuenca Vascocantábricametodología analítica y relación con los depósitos orgánicos.

Resumen

The Pliensbachian-lower Toarcian hemipelagic carbonates outcropping in the Reinosa area of northern Spain provide the opportunity to establish a high-resolution, biostratigraphically calibrated stable isotope stratigraphy. Using optical, cathodoluminescence and chemical criteria, it has been possible to select very well preserved samples of belemnite rostra that retain the original marine isotope composition of the Lower Jurassic seawater from northern Spain. Nearly 200 samples from three composite sections (Camino, Santiurde de Reinosa and San Andrés) have been analysed for their 8,3C, 8,hO, and minor and trace-element composition (Ca, Mg, Sr, Mn and Fe) to both constrain the effects of any possible post-depositional diagenetic alteration, and provide a record of the geochemical changes on seawater composition accompanying black shale deposition. The C-isotope record is marked by five positive excursions within the jamesoni, ibex, margaritatus and serpentinus zones. These 8,JC positive excursions area accompanied by an enrichment in the organic content ot the sediments. The O-isotope record is marked by two minimum during the margaritatus and serpentinus Zones and two maximum during the lower part of the margaritatus Zone and the spinatum Zone respectively. The near-coincidence in timing between deposition of black shales, 8' C maximum and 8ihO minimum suggests that palaeoenvironmental changes, related to palaeotemperature and/or palaeosalinity variations, occurred during black shale deposition.