Early Jurassic carbon cycle perturbations recorded in terrestrial sediments of the Sichuan Basin, China

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ∼183 Ma) is marked in the sedimentary record by a sharp negative carbon isotope excursion, thought to be causally linked to the emplacement of the Karoo-Ferrar Large Igneous Province and the associated release of ¹²C-enriched carbon. The T-OAE coincided with global climate and environmental changes, as well as biotic events, indicating substantial modifications in ecosystems. Recent studies have focused on the evidence of geological responses to the T-OAE in Early Jurassic terrestrial basins in China, particularly the Sichuan Basin. Nevertheless, debate remains over the identification of this event, owing to inadequate age-constraints of many sections, and a lack of robust correlations of the carbon isotope records. Moreover, the long-term evolution of the terrestrial carbon isotope record through the Early Jurassic, and its correlation to marine records, is still not firmly established. In this paper, we present new carbon isotope analyses of carbonate (δ¹³C_carb) from lacustrine carbonates and terrestrial organic matter (δ¹³C_org) from bulk rocks within the Ma’anshan and Da’anzhai members of the Ziliujing Formation from the Dacao ‘D’ (DCD) section in the eastern Sichuan Basin. Palynological-palynofacies analysis reveals a predominance of Classopollis pollen together with marker taxa such as Ischyosporites variegatus, Contignisporites problematicus, in the palynological assemblage, indicating a Pliensbachian–Toarcian age. A negative carbon isotope excursion (NCIE) is recorded in the organic carbon isotope data at the topmost part of the Pliensbachian Ma’anshan Member, which can be correlated to the Pliensbachian–Toarcian Boundary Event. This is followed, in the Toarcian Da’anzhai Member, by a major NCIE recorded in both organic matter and carbonate carbon isotope data which can be correlated to the T-OAE NCIE. A long-term carbon isotope record spanning the Sinemurian to Toarcian in Sichuan terrestrial sediments is also been reconstructed and its correlation with coeval marine records is proposed. A broader review of δ¹³C data from Chinese terrestrial basins spanning the Pliensbachian–Toarcian highlights a distinct ¹³C-depleted signature in the Sichuan Basin compared to basins at higher latitudes. Changes in latitudinal gradients and organic matters in the lake sediments were likely important factors influencing the amplitudes of the T-OAE NCIE and the carbon isotope values in terrestrial sedimentary records.