Negative δ13Ccarb values at the Jurassic-Cretaceous boundary – Vaca Muerta Formation, Neuquén Basin, Argentina

Abstract

The stable carbon isotopic compositions of carbonate sediments (δ13Ccarb) have been widely used to reconstruct the global carbon cycle and as a stratigraphic correlation tool. This work investigates a δ13Ccarb record of marine organic-rich, mixed siliciclastic‐carbonate mudstones accumulated from the Tithonian (late Jurassic) to the early Valanginian (early Cretaceous) in a retro-arc basin connected to the Proto-Pacific Ocean (Neuquén Basin, Argentina). The high-resolution record of δ13Ccarb values of these strata from outcrops at various locations in the basin differ from those in the coeval pelagic strata of the Tethys. The δ13Ccarb profile in the Puerta Curaco area shows an overall increasing trend from average values of −7‰ (early Tithonian) to approximately 0‰ (early Valanginian). The negative δ13Ccarb values occur in a ~ 400 m thick TOC-rich succession (TOC > 2%) and are arranged in 3 large-scale increasing-decreasing cycles. Although absolute values are offset, main trends are similar in other outcrops and in subsurface wells. The co-occurrence of the highest U/Th (>1.4) and the most negative δ13Ccarb values suggests that the major trends in the δ13Ccarb profile might be modulated by oscillations in oxygen content within the basin probably driven by sea-level changes. Clastic-dominated transgressive hemicycles have higher TOC and more negative δ13Ccarb values while the opposite occurs in carbonate-rich regressive hemicycles. A trend from “normal” Tithonian surface-water values (avg. +2‰) in the southern shelf to increasingly negative δ13Ccarb values towards the basin center indicates a mixing between platform-derived carbonate material unaffected by diagenesis and carbonate precipitated or altered in a dysoxic/anoxic basin. The δ13Ccarb profile in the Tethyan realm does not record this episode of high burial of organic carbon occurred in the Neuquén basin during the Tithonian and Berriasian but display a slightly decreasing trend between +2.5 and +1.5‰.