<p><strong>Abstract.</strong> Short-term hypoxia in epeiric water masses is a common phenomenon of modern marine environments and causes mass mortality in coastal marine ecosystems. Here, we test the hypothesis that during the Early Aptian, platform-top hypoxia temporarily established in some of the vast epeiric seas of the Central Tethys and caused, combined with other stressors, significant changes in reefal ecosystems. Potentially interesting target examples include time intervals

more »

... ed by the demise of Lower Aptian rudist-coral communities and the establishment of microencruster facies as previously described from the Central and Southern Tethys and from the proto-North Atlantic domain. These considerations are relevant as previous work has predominantly focused on Early Aptian basinal anoxia in the context of the Oceanic Anoxic Event (OAE) 1a, whereas the potential expansion of the oxygen minimum zone in coeval shallow water environments is underexplored. Well known patterns in the δ13C record during OAE 1a allow for a sufficiently time-resolved correlation with previously studied locations. This paper presents and critically discusses the outcome of a multi-proxy study (e.g., REE, U isotopes and redox sensitive trace elements) applied to Lower Aptian shallow water carbonates today exposed in the Kanfanar Quarry in Istria, Croatia. These rocks were deposited on an extensive, isolated high in the Central Tethys surrounded by hemi-pelagic basins. Remarkably, during chemostratigraphic segment C2, the depletion of redox sensitive trace elements As, V, and Mo in platform carbonates, deposited in normal marine oxic waters, record the first occurrence of basinal, organic rich sediment deposition in which these elements are enriched. During the C3 segment, seawater oxygen depletion establishes on the platform top as indicated by the patterns in Ce/Ce* and uranium isotopes. Shifts in redox sensitive proxies coincide with the expansion of microencruster facies. Segment C4 witnesses the return to normal marine reefal faunas on the platform top and is characterized by patterns in redox sensitive proxies typical of normal marine dissolved oxygen levels. It remains unclear, if platform-top hypoxia resulted from the expansion and upwelling of basinal, oxygen-depleted water masses, or if spatially isolated, shallow hypoxic water bodies formed on the platform. Data shown here are relevant as they shed light on the driving mechanisms that control poorly understood faunal patterns during OAE1a in the neritic realm and provide evidence on the intricate relation between basinal and platform-top water masses.</p>