Black shale deposition during Toarcian super-greenhouse driven by sea level

Abstract

Abstract. One of the most elusive aspects of the Toarcian oceanic anoxic event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the end of the overarching positive trend in the carbon isotopes that characterises the T-OAE. In the present study, we have attempted to establish a sequence stratigraphic framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE (defined by the positive carbon isotope trend) due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by > 10 % Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the atmosphere-ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales during the serpentinum and bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the oxygen availability in the water column whose nature has still to be determined through cyclostratigraphic analysis.

Multiproxy geochemical analysis of a Panthalassic margin record of the early Toarcian oceanic anoxic event (Toyora area, Japan)

“…Evidence from other European and Arctic basins also indicates strongly transgressive conditions through the early Toarcian (e.g. Hallam, 1997Hallam, , 2001Nikitenko and Mickey, 2004;Hesselbo, 2008;Hermoso et al, 2013). Localised sea-level changes in the Tabe Basin could have driven the observed changes, with the position of the basin in the Early Jurassic within a convergent island arc setting supporting the likely importance of tectonic controls on basin development (e.g.…”

Eruptive history of the Karoo lava flows and their impact on early Jurassic environmental change

“…The end Pliensbachian and Early Toarcian have also witnessed major carbon cycle perturbations. A sharp negative carbon isotope excursion (CIE) is observed during the Early Toarcian in marine and continental organic matter, in carbonates of the Western Tethyan ocean [e.g., Hesselbo et al, 2000Hesselbo et al, , 2007Jenkyns et al, 2001;Gomez et al, 2008;Hermoso et al, 2009Hermoso et al, , 2012Hermoso et al, , 2013Suan et al, 2010;, in polar regions [Suan et al, 2011], in the Panthalassa [Canada -Caruthers et al, 2011;Japan-Kemp and Izumi, 2014;Peru-Guex et al, 2012], and in the Neuquén Basin in Argentina [Al-Suwaidi et al, 2010]. It coincides with an oceanic anoxic event (Toarcian oceanic anoxic event) evidenced by black shale in European epeiric seas and western Tethyan margins, a major marine transgression and a phase of global warming reaching an acme during the Falciferum biozone [McArthur et al, 2000;Bailey et al, 2003;Gomez et al, 2008;Suan et al, 2008;Dera et al, 2009;Suan et al, 2010;Krencker et al, 2014].…”

Calibrating the magnitude of the Toarcian carbon cycle perturbation

“…The negative CIE and the following positive CIE rebound seen in the upper half of the falciferum zone in several successions [Jenkyns, 1988 are often considered as better time markers for the T-OAE than the organic-rich strata. Indeed, if one considers the lower Toarcian negative CIE as reflecting global and synchronous changes in carbon cycling, the lower Toarcian organic-rich strata, as with many organic-rich deposits associated with other OAEs, appear to be slightly to highly diachronous Hermoso et al, 2013;Suan et al, 2013;van de Schootbrugge et al, 2013]. In NW Europe and N Siberia, the organic-rich strata occur stratigraphically both within and well above the CIE (up to the top of the bifrons ammonite zone) [Röhl et al, 2001;Suan et al, 2011;Hermoso et al, 2013], whereas in S Europe, when present, they are generally bound to strata recording the CIE [Sabatino et al, 2009;Kafousia et al, 2011].…”

“…Indeed, if one considers the lower Toarcian negative CIE as reflecting global and synchronous changes in carbon cycling, the lower Toarcian organic-rich strata, as with many organic-rich deposits associated with other OAEs, appear to be slightly to highly diachronous Hermoso et al, 2013;Suan et al, 2013;van de Schootbrugge et al, 2013]. In NW Europe and N Siberia, the organic-rich strata occur stratigraphically both within and well above the CIE (up to the top of the bifrons ammonite zone) [Röhl et al, 2001;Suan et al, 2011;Hermoso et al, 2013], whereas in S Europe, when present, they are generally bound to strata recording the CIE [Sabatino et al, 2009;Kafousia et al, 2011]. In most known successions, however, strata recording the negative CIE exhibit dramatic changes in geochemical, sedimentological, and micropaleontological composition [Hesselbo et al, 2007;Mattioli et al, 2009;Hermoso et al, 2012Hermoso et al, , 2013Kemp and Izumi, 2014], suggesting that the carbon cycle perturbation was intimately linked with profound climatic and oceanographic disturbances.…”