Enhanced juvenile mortality of orbitolinids (larger benthic foraminifera) in the late early Aptian of the Langshan Formation, Tibet: a possible link to OAE1a?

While much is known about the effects of Cretaceous Ocean Anoxic Events (OAE) on open oceanic benthic and planktic foraminifera only scarce data are available concerning the influence on neritic assemblages. The Lower to mid-Cretaceous Langshan Formation of the Lhasa Block, Tibet is well known for its richness in Orbitolinidae. The studied Tibetan section comprising uppermost lower–upper Aptian shallow-water carbonates is punctuated by an orbitolinid assemblage dominated by juvenile specimens, including isolated megalospheric embryonic apparatuses, or forms with just a few postembryonic chambers, thus documenting an enhanced juvenile mortality. Because of the dominant species present, the assemblage is assigned to the late early Aptian Praeorbitolina cormyi orbitolinid biozone that falls into the period of the OAE1a (Selli Event) or its immediate aftermath period. This apparently stressed assemblage and other observations, namely the occurrence of orbitolinid twin embryos as reported in the literature, are discussed as possibly causally linked to the associated environmental perturbations.