Organic matter production and preservation in the Lusitanian Basin (Portugal) and Pliensbachian climatic hot snaps

Abstract

The Pliensbachian (Early Jurassic) hemipelagic carbonate series of the Lusitanian Basin (Portugal) are of major importance as this basin is located at a key position between the Tethyan and Boreal (North European) realms. One of the most conspicuous features of this basin is the organic-rich nature of most of the Margaritatus Chronozone hemipelagic deposits, with the occurrence of several well-defined regional black shales. The analysis of selected redox sensitive elements (V, Cr, Co, Ni, Mo, and U) from the worldwide reference section of Peniche indicates that in this basin most of the organic-rich facies of the Ibex–Margaritatus chronozones were deposited under an oxic–dysoxic regime, while the rare centimetre thick well-defined black shales were deposited and preserved under suboxic–anoxic, intermittently euxinic conditions. Based on the integration of sedimentological, biostratigraphical, geochemical, and organic petrographic evidence we suggest that these basinwide black shales, corresponding to widespread mucilage and microbial outbreaks, were driven by extreme climate warming coupled with high oceanic productivity and intermittently stratified epeiric seas. Regionally, these “hot snaps” may have promoted the observed rapid but short-lived expansion of Tethyan ammonites into Boreal domains or led to decrease benthic diversity. Ensuing cooling during the Margaritatus Chronozone was accompanied by southwards influx of northwards ammonite fauna or radiation of cyst forming dinoflagellates.Worldwide preservation of organic matter during the Late Pliensbachian seems to have resulted in decreased atmospheric CO2 levels through geological storage of carbon, triggering and/or amplifying the Spinatum Chronozone icehouse event. Ultimately, this cooling event may have led to the occurrence of permafrost and/or methane gas hydrates in locations easily disturbed by the subsequent Early Toarcian warming, or/and volcanic activity driven by crustal unloading due to deglaciation.