Changes in calcareous nannofossils across the Toarcian and Early Aaienian from the Lusitanian Basin (Portugal)

摘要

This study provides a comprehensive and detailed account of calcareous nannoplankton assemblage changes across the Toarcian and early Aalenian in the strategically located Lusitanian Basin in Portugal. During the Toarcian, this basin was a seaway connecting the NW Tethys and Mediterranean water masses, so the nannoplankton community in the seaway would be expected to respond to influences from both water bodies. Accurate biostratigraphic control from two ammonite-calibrated and continuous sections (Raba?al and Cabo Mondego) were merged with an intermediate section (Brenha). This resulted in a complete 286-m-thick composite section of Toarcian and lower Aalenian sediments. Qualitative and quantitative analyses of calcareous nannofossil assemblages were performed, along with factor analysis and stable carbon and oxygen isotopes analyses from brachiopods shells. Three factors were extracted from the factor analysis, and they provide good illustrations of temperature, NW European water mass incursions, and primary productivity. Three major ecological events also were recognized from this work. During the early Toarcian, a hot and humid transgressive period where calcareous nannoplankton thrived, the nannofossil assemblages document water mixing between NW European and Mediterranean water masses. Across the middle and part of the late Toarcian, as climatic conditions stabilized, water masses became more stratified, and primary productivity decreased, as testified by δ~(13)C data and total nannofossil abundances. From part of the late Toarcian ammonite speciosum Zone upwards, the environment again became meso-eutrophic as the connection between northern and southern water masses decreased, and high fertility taxa such as Schizosphaerella thrived within the shallow photic zone. Moreover, perturbations in water paleotemperatures seem to be linked to Lotharingius and Watznaueria radiation in the middle Toarcian and early Aalenian.