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Latitudinally different responses of Tethyan shoal-water carbonate systems to the Early Aptian oceanic anoxic event (OAE 1a)

Ocean-wide anoxic events represent intensively investigated anomalies in the global carbon cycle. Most previous research has focussed on hemipelagic and pelagic settings and on the relationship between black-shale deposition and carbon-isotope excursions. The study of ocean-wide anoxic events and coeval shallow-water settings is now increasingly seen as an interesting complementary approach, but one that is not without problems. Whereas platform drowning characterizes the Early Aptian of the northern Tethyan margin, Lithocodium–Bacinella-rich facies and ongoing shoal-water sedimentation at the southern Tethyan margin (Oman) bears important information on potential causes of carbon-cycle perturbations. The present paper seeks to test the supra-regional relevance of the Oman data by investigating coeval central Tethyan limestones. Three Lower Aptian shoal-water sections in Istria (Croatia), deposited on the isolated Adriatic Carbonate Platform, are investigated applying chemostratigraphy (carbon and strontium) and detailed sedimentological analysis. The focus is on peritidal to lagoonal facies characterized by mass occurrences of Lithocodium–Bacinella, an enigmatic microencruster community. Lithocodium–Bacinella facies occurs predominantly in layers ranging from one to several centimetres in stratigraphic thickness, with several layers merging to metre-thick packages. Growth fabrics within the layers include oncoidal morphotypes, lumps, interconnected patches and columns, layers and rare nodular to massive bindstone facies. These growth patterns show a remarkable regional extent and consistency over study sites distributed several kilometres apart. This widespread distribution suggests that specific Lithocodium–Bacinella morphotypes might serve as regional stratigraphic markers. The high-resolution carbon-isotope chemostratigraphy presented here is based on pristine rudist shells and matrix micrite samples and calibrated against strontium-isotope data obtained from screened rudist low-Mg calcite. The chemostratigraphic data are consistent with existing biostratigraphic data and place the studied strata at the onset of Early Aptian oceanic anoxic event 1a. Moreover, results indicate the near-coeval nature of Lithocodium–Bacinella bloom facies in Istria and Oman. The outcomes of this study point to latitudinally different responses of Tethyan shoal-water carbonate systems (platform drowning versus Lithocodium–Bacinella blooms) to the ocean-wide anoxic event 1a.

black shales; carbon isotopes; carbonate platforms; Early Aptian; Lithocodium–Bacinella bloom periods; OAE 1a; Tethys

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