The chemostratigraphy and environmental significance of the Marlstone and Junction Bed (Beacon Limestone, Toarcian, Lower Jurassic, Dorset, UK)

Abstract

Two fallen blocks of the Marlstone and stratigraphically overlying Junction Bed sampled on the beach below Doghouse Cliff in Dorset, UK (Wessex Basin) have been examined for carbon and oxygen isotopes of bulk carbonate as well as for strontium, carbon and oxygen isotopes and Mg:Ca ratios in the contained belemnites. The sequence, which contains most of the Toarcian zones and subzones within a metre or less of grey to yellow to pink, red and brown fossil-rich nodular limestone, is extremely condensed and lithologically similar to pelagic red limestones of the Tethyan Jurassic that are locally mineralized with Fe-Mn oxyhydroxides (e.g., Rosso Ammonitico). Strontium-isotope ratios of the contained belemnites are compatible with existing reference curves and both blocks show a rise to more radiogenic values post-dating the Pliensbachian–Toarcian boundary. The high degree of correlation between the relatively negative carbon and oxygen isotopes of the bulk carbonate is compatible with significant diagenetic overprint, and contrasts with higher carbon-isotope values in coeval condensed coccolith-rich limestones elsewhere. Evidence for the characteristic signature of the Toarcian Oceanic Anoxic Event, as represented by organic-rich sediment, is absent, possibly owing to a stratigraphic gap. Both blocks exhibit abrupt carbon-isotope shifts to lower values, one of which could represent the limbs of an incompletely recorded negative excursion associated with the Toarcian Oceanic Anoxic Event. That the Toarcian Oceanic Anoxic Event was also a significant hyperthermal is illustrated in both blocks by a drop in oxygen-isotope values and rise in Mg:Ca ratios of belemnites close to the base of the Junction Bed in the lowest part of the serpentinum zone.

Carbon- and oxygen-isotope signature of the Toarcian Oceanic Anoxic Event: insights from two Tethyan pelagic sequences (Gajum and Sogno Cores – Lombardy Basin, northern Italy)

“…Both strontium-isotope ( 87 Sr/ 86 Sr) and osmium-isotope ( 187 Os/ 188 Os) anomalies have also been documented as evidence of accelerated continental weathering in response to a global climate change (Jenkyns 2003, Jenkyns 2010, Cohen et al 2004, Ullmann et al 2013, Percival et al 2016, Them et al 2017, Jenkyns and Macfarlane 2021 with extraordinary warmth (Dera et al 2011, Korte and Hesselbo 2011) associated with a major marine transgression (Hallam 1981, Haq et al 1987. Available records suggest that, in addition to anoxia, these extreme conditions triggered a biocalcification crisis (Erba 2004, Mattioli et al 2004, Tremolada et al 2005, Casellato and Erba 2015, Erba et al 2019a, enhanced primary productivity (Erba 2004, Jenkyns 2010, and ocean acidification (Erba 2004, Trecalli et al 2012, Casellato and Erba 2015, Posenato et al 2018, Müller et al 2020, Ettinger et al 2021.…”

A new subsurface record of the Pliensbachian–Toarcian, Lower Jurassic, of Yorkshire

“…In the case of the Laurasian Sea, the source of organic carbon, as well as okenone (French et al 2014), was perhaps the extremely shallow seafloor of platforms and highs (e.g. Market Weighton; Wright 2022), where a form of dynamic bypassing resulted in stratigraphic condensation on the platform and the concentration of (carbonaceous) sediment into the subsiding basins (Hallam and Sellwood 1976;Jenkyns and Macfarlane 2022;and see fig. 8 in Wignall et al 2005).…”

“…Dark argillaceous mudstone is the dominant lithology. During the Early Jurassic, even though the seafloor of the epeiric sea was relatively level, differential subsidence resulted in areas in which a thicker and more carbonaceous Lower Jurassic succession accumulated (Hallam and Sellwood 1976;Ziegler 1981Ziegler , 1990Howarth 1992;Adrichem Boogaert andKouwe 1993-1997;Jenkyns and Clayton 1997;Caswell and Coe 2012;Jenkyns and Macfarlane 2022). The Lower Jurassic sediments of North Yorkshire accumulated in a small basin, the Cleveland Basin (Fig.…”

Suboxic conditions prevailed during the Toarcian Oceanic Anoxic Event in the Alpine-Mediterranean Tethys: The Sogno Core pelagic record (Lombardy Basin, northern Italy)

“…Anoxic conditions were accompanied by enhanced continental weathering (Jenkyns, 2003(Jenkyns, , 2010Cohen et al, 2004;Ullmann et al, 2013;Percival et al, 2016;Them et al, 2017;Jenkyns and Macfarlane, 2021), increased nutrient input into the oceans (Cohen et al, 2004;Jenkyns, 2010;Percival et al, 2016;Izumi et al, 2018;Kemp et al, 2020), extraordinary warmth (Bailey et al, 2003;Dera et al, 2011;Korte and Hesselbo, 2011;Gómez et al, 2016;Ruebsam et al, 2020b) and major marine transgression (Hallam, 1981;Haq et al, 1987;Hardenbol et al, 1998). These processes would have also enhanced primary productivity (Erba, 2004;Jenkyns, 2010), triggered a biocalcification crisis (Erba, 2004;Mattioli et al, 2004;Tremolada et al, 2005;Casellato and Erba, 2015;Erba et al, 2019a;Reolid et al, 2020) and ocean acidification (Erba, 2004;Trecalli et al, 2012;Casellato and Erba, 2015;Posenato et al, 2018;Müller et al, 2020a;Ettinger et al, 2021).…”