Climate and environmental response to the break-up of Pangea during the Early Jurassic (Hettangian-Pliensbachian): the Dorset coast (UK) revisited

Abstract

Between the end-Triassic mass extinction and the Toarcian Oceanic Anoxic Event, the Early Jurassic witnessed important changes in carbon burial, palaeogeography and paleoceanography, which were linked to the initial breakup of Pangaea. In order to better understand the climate and environmental impact of this phase of major tectonic rearrangement we revisited a key section along the Dorset coast (UK) and used a multi-proxy geochemical approach for its analysis. Carbon isotopes (δ13C) measured on organic matter were corrected for the influence of changes in the type of organic matter. The thus obtained δ13C-HI index and an additional δ13Ccarb record measured on bulk carbonate indicate a suite of carbon-isotope excursions (CIEs) including the negative main CIE (Triassic-Jurassic boundary to early Sinemurian) the negative Sinemurian-Pliensbachian boundary CIE and the positive margaritatus Zone (late Pliensbachian) CIE. Based on elevated hydrogen index values and enrichments in Mo, U, V, total organic carbon (TOC) and pyrite, the Hettangian-Sinemurian interval was interpreted to have been characterised by high productivity conditions and strong oxygen depletion of the basin. The chemical alteration index (CIA), clay mineralogy, and δ18O data provide independent evidence for increased continental runoff triggered by humid climate conditions on the adjacent continent during this time period. The strongest oxygen-depleted conditions were inferred for the lowermost Hettangian interval, coincident with the onset of the main negative CIE. A further less strongly oxygen-depleted interval was recorded through the negative CIE near the Sinemurian-Pliensbachian boundary (jamesoni and ibex Zones). This interval was characterised by drier conditions and possibly cooler water temperatures (e.g. CIA, δ18Ocarb and kaolinite/illite ratio). Based on low CIA values and kaolinite/illite ratios, the Pliensbachian climate is considered to have been generally drier, superimposed by more humid episodes restricted to the davoei and margaritatus Zones. The long-term trend to drier conditions is explained by: (1) diminished atmospheric CO2 contents due to a reduction in emissions of volcanogenic greenhouse gases, and/or; (2) changes in the palaeogeography and current patterns related to the break-up of Pangaea and more specifically to the opening of the Hispanic corridor.