Browsing by Author "Duarte, Luis V."
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- The palynological response to the Toarcian Oceanic Anoxic Event (Early Jurassic) at Peniche, Lusitanian Basin, western PortugalPublication . Correia, Mariana F.; Riding, James B.; Duarte, Luis V.; Fernandes, Paulo; Pereira, ZeliaThe uppermost Pliensbachian and lower Toarcian (Lower Jurassic) succession exposed at Peniche in the central part of the Lusitanian Basin, western Portugal, was examined for palynomorphs. The 45 samples span the Emaciaticeras emaciatum, Dactylioceras polyrnorphum and Hildaites levisoni ammonite biozones (ABs), and the succession includes the Global boundary Stratotype Section and Point (GSSP) for the Toarcian Stage and the Toarcian Oceanic Anoxic Event (T-OAE). A low diversity dinoflagellate cyst flora, typical of the Sub-Boreal Realm, was recovered from the Emaciaticeras emaciatum and Dactylioceras polymotphum ABs. The dominant element is the cold water species Luehndea spinosa, which is an index for the Pliensbachian to earliest Toarcian, and is thought to have migrated from the more northerly Boreal Realm. Prior to the T-OAE, dinoflagellates thrived in the Lusitanian Basin, except during a brief warm period in the earliest Toarcian. Despite the latter, the recovery from this event was relatively rapid and was characterised by a return to relatively cool temperatures. The Hildaites levisoni AB at Peniche represents the T-OAE and the overlying strata, and is characterised by a profound reduction in dinoflagellate cyst relative abundances. This dinoflagellate cyst 'blackout', and the associated rise of prasinophytes, reflects significant environmental stress, such as marine anoxia, elevated temperatures and reduced salinity, with the former two probably being most important. The low proportions of dinoflagellate cysts following the T-OAE indicates a protracted recovery phase from the bottom and water column anoxia developed throughout the Lusitanian Basin.
- The palynology of the lower and middle Toarcian (Lower Jurassic) in the northern Lusitanian Basin, western PortugalPublication . Correia, Vania F.; Riding, James B.; Fernandes, Paulo; Duarte, Luis V.; Pereira, ZeliaThe lower and middle Toarcian (Lower Jurassic) successions of the northern Lusitanian Basin in western Portugal were examined for palynomorphs. Two localities, the Maria Pares and the Vale das Fontes sections, were sam-pled. The sections span the Dactylioceras polymorphum, Hildaites leyisoni and Hildoceras bifrons ammonite biozones. The samples produced relatively low diversity dinoflagellate cyst floras which are typical of those from coeval European successions; the most abundant species is Luehndea spinosa. The other forms encountered were Mancodinium semitabulatum, Mendicodinium microscabratum, M. spinosum subsp. spinosum, Mendicodinium sp., Nannoceratopsis ambonis, N. gracilis and N. senex. Dinoflagellate cysts typically dominate throughout the Dactylioceras polymorphum ammonite biozone; their abundance significantly decreased in the overlying Hildaites levisoni and Hildoceras bifrons ammonite biozones. The low diversity Luehndea-Nannoceratopsis dinoflagellate cyst flora of the northern Lusitanian Basin is characteristic of the Sub-Boreal region of Europe. This is a transitional region, intercalated between the Boreal and Tethyan realms. The Toarcian Oceanic Anoxic Event (T-OAE) in the northern Lusitanian Basin is characterised by a sudden decline in palynomorph abundance and diversity, including the virtual absence of acritarchs and dinoflagellate cysts. Following the T-OAE, Mancodinium semitabulatum and Mendicodinium spp. were the only dinoflagellate cysts recorded. This 'blackout' of dinoflagellate cysts during the T-OAE, and their partial recovery following the event, shows that dinoflagellate populations were responding to a major palaeoenvironmental change. (C) 2016 Elsevier B.V. All rights reserved.