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The Sogno Core represents one of the deepest pelagic records of the Toarcian Oceanic Anoxic Event (T-OAE) in the Alpine-Mediterranean Tethys. New sedimentological, elemental, Rock-Eval, and biomarker data are presented here, with the aim of reconstructing the depositional conditions that characterized the sedimentation of this succession during the broad positive carbon-isotope excursion of the T-OAE, and its negative isotopic anomaly (Jenkyns Event). Higher Mn xs concentrations, inversely correlated with the δ18O curve, were observed starting slightly below the onset level of the Jenkyns Event up to the lowermost part of the negative carbon-isotope anomaly, reflecting a combination of a gradual warming and a progressive increase in fresh waters delivered by runoff. Increased weathering intensity was accompanied by enhanced detrital input with higher concentrations in lithogenic elements, and by a relative increase in palaeoproductivity, consistently with higher Si xs concentrations and changes in calcareous nannoplankton assemblages. The enhanced fresh-water input favored water-column stratification, less efficient deep-water circulation and oxygen depletion. Except for two discrete black shales characterized by anoxic pore waters, redox conditions at Sogno turned at maximum suboxic, as indicated by rare to absent bioturbation, limited enrichments in redox-sensitive elements, and molecular biomarkers. The Sogno Core record shows that dominant oxic–suboxic conditions were present also at bathyal depths in the Alpine-Mediterranean Tethys during the Jenkyns Event, thereby contrasting with the anoxic–euxinic environments present both in the shallower water basins and sub-basins of epicontinental northern Europe and, locally, in the relatively deep-water areas of the Alpine Tethys. • In the Lombardy Basin suboxic to oxic conditions prevailed during the Toarcian Oceanic Anoxic Event. • Water stratification caused by enhanced runoff and warmer conditions favoured the onset of bottom-water suboxia. • Reducing conditions became generally more severe around the lowest point in the δ13C trough that defines the Jenkyns Event. • The Alpine-Mediterranean Tethys was mainly characterized by suboxic conditions during the Toarcian Oceanic Anoxic Event. [ABSTRACT FROM AUTHOR]

The Mesozoic Era featured emplacement of a number of Large Igneous Provinces (LIPs), formed by the outpouring of millions of cubic kilometres of basaltic magma. The radiometric ages of several Mesozoic LIPs coincide with the dates of Oceanic Anoxic Events (OAEs). As a result of these coincidences, a causal link has been suggested, but never conclusively proven. This study explores the use of mercury as a possible direct link between the Karoo–Ferrar LIP and the coeval Toarcian OAE (T-OAE). Mercury is emitted to the atmosphere as a trace constituent of volcanic gas, and may be distributed globally before being deposited in sediments. Modern marine deposits show a strong linear correlation between mercury and organic-matter content. Results presented here indicate departures from such a simple linear relationship in sediments deposited during the T-OAE, and also during the Pliensbachian–Toarcian transition (an event that saw elevated benthic extinctions and carbon-cycle perturbations prior to the T-OAE). A number of depositional settings illustrate an increased mercury concentration in sediments that record one or both events, suggesting a rise in the depositional flux of this element. Complications to this relationship may arise from very organic-rich sediments potentially overprinting any Hg/TOC signal, whereas environments preserving negligible organic matter may leave no record of mercury deposition. However, the global distribution of coevally elevated Hg-rich levels suggests enhanced atmospheric mercury availability during the Early Toarcian, potentially aided by the apparent affinity of Hg for terrestrial organic matter, although the relative importance of aquatic vs terrestrial fixation of Hg in governing these enrichments remains uncertain. A perturbation in atmospheric Hg is most easily explained by enhanced volcanic output. It is suggested that extrusive igneous activity caused increased mercury flux to the Early Toarcian sedimentary realm, supporting the potential of this element as a proxy for ancient volcanism. This interpretation is consistent with a relationship between LIP formation and a perturbed carbon cycle during the Pliensbachian–Toarcian transition and T-OAE. The recording of these two distinct Hg excursions may also indicate that the Karoo–Ferrar LIP released volatiles in temporally distinct episodes, due either to multiple phases of magmatic emplacement or sporadic release of thermogenic gaseous products from intrusion of igneous material into volatile-rich lithologies. [ABSTRACT FROM AUTHOR]

Abstract: Deposits with unusually high Mn contents sampled at Monte Mangart in the Julian Alps include organic-rich marlstone and black shale with interbedded manganoan and siliceous limestone, which were deposited during the early Toarcian Oceanic Anoxic Event. Mn enrichment during that period has been related to global sea-level change coincident with increasing subsidence rate. The formation of Fe–Mn nodules, marking a hardground at the base of the Monte Mangart section, seems to be triggered by release of Mn from remote hydrothermal vents into a region of relatively elevated submarine topography where oxidizing conditions prevailed. However, very high Mn contents in carbonate phases above the hardground imply an additional diagenetic source of this element in the lower part of this section. The whole stratigraphic sequence (ca 30m) displays a transition from Mn-rich (up to 8.8%) sediments, in the lower part, to Mn-poor (less than 1.8%) sediments in the middle and upper parts. The drastic decrease in Mn content''s up-section is accompanied by a clear decrease in the mean size of pyrite framboids, indicating more intense anoxia/euxinia in the water column. In the presence of Mn2+, conditions of high alkalinity induced precipitation of Mn carbonates during early diagenetic processes. Negative δ13Ccarb values coincident with high Mn contents indicate involvement of organic matter in the mineralization process. The striking similarity of Ce/Ce* and Mn profiles demonstrates that, consistent with redox-chemistry of Mn and Ce under anoxic conditions, Ce3+ and Mn2+ were mobilized and released into pore water where precipitation of Mn carbonates occurred. [Copyright &y& Elsevier]