Paired isotope records of carbonate and organic matter from the Middle Ordovician of Argentina: Intrabasinal variation and effects of the marine chemocline

We investigate the expression of the Middle Darriwilian isotope carbon excursion (MDICE) across marine shelf environments in the Argentine Precordillera. Previous work identified the MDICE in the Las Chacritas Formation in Argentina, but did not recognize the expression of the MDICE in time-equivalent strata of the deeper-water Las Aguaditas Formation (Albanesi et al., 2013; Palaeogeography, Palaeoclimatology, Palaeoecology, v. 398, p. 48–66). Recent biostratigraphic investigations of these units have, for the first time, provided the opportunity for high-resolution correlation, which suggest that the MDICE, or at least the initiation of the MDICE, should be observed in both the Las Chacritas and Las Aguaditas formations. Here we present new paired carbon isotope data of carbonate and organic carbon from the Las Chacritas and Las Aguaditas formations. We identify a 2‰ positive shift in the isotopic composition of marine carbonate in the Las Chacritas Formation, whereas values abruptly fall to < −1‰ in equivalent strata of the Las Aguaditas Formation. This is the first record of divergence from the globally recognized MDICE event. There are also small, yet distinct differences in the isotopic composition of marine organic matter between these two sections. We suggest that the divergent C-isotope trends of carbonate in the Las Chacritas and Las Aguaditas formations represent deposition in fundamentally different parts of the water column — above and below the marine chemocline, respectively — during the MDICE interval. This interpretation is consistent with data from the Las Aguaditas Formation that shows elevated Mn and Fe concentrations in carbonate phases with little evidence for recrystallization, and with a growing consensus for regionally anoxic conditions for the Middle Ordovician. Our data indicate that redox gradients can play a critical role in the behavior of marine carbon isotope excursions.