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The Aptian La Florida carbonate platform in north Spain was affected by global environmental changes related to the early Aptian Oceanic Anoxic Event (OAE 1a). This event was characterized by intensified greenhouse conditions that produced an increase in oceanic and atmospheric temperature, accumulation of organic-rich deposits, major perturbations in the C cycle and an increase in continental weathering, erosion and runoff rates.
La Florida platform exhibits a continuous succession of shallow-water platform carbonates that includes a unit of early Aptian open marine, organic-rich marls, which are thought to represent the local expression of the OAE1a. Facies analysis and diagenetic studies of the limestones below the organic-rich marls lead us to distinguish two stages of carbonate production separated by an unconformity. This is observed as solution cavities and brecciated fabrics on top of the first platform stage. The cavities are draped by a thin Fe-crust and subsequently filled with deposits of the second carbonate stage.
Although in both carbonate stages deposition took place in shallow water conditions, they show a marked compositional change in the type of carbonate production. The first stage was characterized by photozoan skeletal assemblages developed in oligotrophic conditions, whereas the second stage was composed of heterozoan skeletal associations typical of mesotrophic conditions. In addition, the diagenetic studies let us deduce hydrological perturbations that accompanied the anoxic event by means of the identification of widespread early meteoric diagenesis in the two stages of carbonate production that preceded the platform drowning. δ18O and δ13C chemostratigraphic profiles throughout the succession reveal prominent negative excursions for the marly interval that represents the drowning of the platform. These excursions may be associated to important global changes in climate and Corg burial rates.
We suggest that the compositional changes that preceded the isotopic excursions document conditions of environmental stress caused by changing nutrient levels and a more active paleohydrological cycle linked to the anoxic event. This is consistent with both, a widespread regional siliciclastic input during this time and the diagenetic evidences of inflow of fresh-water lenses into the carbonate platform. Under extreme greenhouse conditions the more active hydrological cycle increased the rate of mean precipitation and continental runoff, increasing the riverine influence that may have produced: 1) a perturbation in nutrient supply/source to the platform that induced the suppression of the photozoan-dominated carbonate production, 2) changes in salinity and sea-surface temperature, and 3) distribution of early meteoric diagenetic features favoured by the freshwater input into the platform in the states that preceded the OAE.
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