Abstract
The stable carbon isotopic compositions of carbonate sediments (delta (super 13) C (sub carb) ) have been widely used to reconstruct the global carbon cycle and as a stratigraphic correlation tool. This work investigates a delta (super 13) C (sub carb) 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 (Neuquen Basin, Argentina). The high-resolution record of delta (super 13) C (sub carb) values of these strata from outcrops at various locations in the basin differ from those in the coeval pelagic strata of the Tethys. The delta (super 13) C (sub carb) profile in the Puerta Curaco area shows an overall increasing trend from average values of -7 ppm (early Tithonian) to approximately 0 ppm (early Valanginian). The negative delta (super 13) C (sub carb) values occur in a nearly equal 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 delta (super 13) C (sub carb) values suggests that the major trends in the delta (super 13) C (sub carb) 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 delta (super 13) C (sub carb) values while the opposite occurs in carbonate-rich regressive hemicycles. A trend from "normal" Tithonian surface-water values (avg. +2 ppm) in the southern shelf to increasingly negative delta (super 13) C (sub carb) 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 delta (super 13) C (sub carb) profile in the Tethyan realm does not record this episode of high burial of organic carbon occurred in the Neuquen basin during the Tithonian and Berriasian but display a slightly decreasing trend between +2.5 and +1.5 ppm.
