Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications

Abstract

International audienceThe Early Toarcian is marked by a global perturbation of the carbon cycle and major marine biological changes. These coincide with a general decrease in calcium carbonate production and an increase in organic carbon burial, and culminate in the so-called Toarcian Oceanic Anoxic Event. It is believed that the environmental crisis was triggered by the activity of the Karoo-Ferrar large igneous province. In order to further document the Early Toarcian palaeoenvironmental perturbations, carbon isotope, total organic matter, calcareous nannofossils and phosphorus content of the Amellago section in the High Atlas rift basin of Morocco were investigated. This section is extremely expanded compared to the well-studied European sections. Its position along the northern margin of the Gondwana continent is of critical importance because it enables an assessment of changes of river nutrient input into the western Tethyan realm. The carbon isotope curve shows two negative excursions of equal thickness and amplitude, at the Pliensbachian-Toarcian boundary and at the transition from the Polymorphum to the Levisoni Zone. This confirms the supra-regional nature of these shifts and highlights the possible condensation of the first "boundary" shift in European sections. Phosphorus content is used to trace palaeo-nutrient changes and shows that the two negative carbon isotope shifts are associated with increased nutrient levels, confirming that these episodes are related to enhanced continental weathering, probably due to elevated greenhouse gases in the atmosphere. In the High Atlas Basin, the increase in nutrient levels at the Pliensbachian-Toarcian boundary is moreover likely to be the main factor responsible for the coeval demise of the Saharan carbonate platform. A middle Toarcian event, centered on the boundary between the Bifrons and Gradata Zones, characterized by a positive carbon isotope excursion and nutrient level rise, is documented in the Amellago section

Palaeoclimatic oscillations in the Pliensbachian (Early Jurassic) of the Asturian Basin (Northern Spain)

“…Finally, the Early Toarcian is characterized by a prominent δ 13 C positive excursion that has been detected in all the sections considered herein, as well as in some South American (Al-Suwaidi et al, 2010) and Northern African (Bodin et al, 2010) sections. This positive CIE is interrupted by a negative excursion of approximately 1 ‰ δ 13 C bulk located around the Tenuicostatum−Serpentinum zonal boundary.…”

Astronomical calibration of the Toarcian Stage: Implications for sequence stratigraphy and duration of the early Toarcian OAE

“…4). In contrast, the Polymorphum Mediterranean equivalent Zone in the Amellago section (Morocco) is ~65 m thick (Bodin et al, 2010). Also, a duration of 0.8 Myr for the Polymorphum Zone was inferred from the Peniche section, Portugal , although a problem in their cyclostratigraphic interpretation is likely (Kemp et al, 2011; see below, Section 5.1.2).…”

New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

“…positive δ 13 C excursion is not clearly recorded neither in the High Atlas of Morocco (Bodin et al, 2010) nor in Canada (Caruthers et al, 2011), indicating a probably regional, but not global, extent of this CIE.…”