Changes in bivalve assemblages at the onset of the OAE2 event in the Peri-Tethyan area (Bohemian Cretaceous Basin)

Abstract

The Ocean Anoxic Event (OAE2) at the Cenomanian-Turonian boundary presents a unique record of gradual global warming and its effects on benthic organisms. The present research considers a palaeoecological, systematical and statistical evaluation of bivalve assemblages from the Pecínov Member in the Bohemian Cretaceous Basin. The well-exposed succession of Cenomanian through lower Turonian strata contains one of the most complex records of the OAE2 in central Europe. In total, 392 specimens have been studied and assigned to 24 genera within 21 families and 9 orders. The bivalve association prior to the positive shift of the δ13Corg (peak “a” sensu global carbon curve; lower part of the Metoicoceras geslinianum Zone) is strongly dominated by infaunal suspension feeders, followed by semi-infaunal suspension feeders. The near-absence of free-living epifaunal bivalves may be linked to a very fine, muddy substrate and water turbidity, a limiting factor for many epifaunal species. Infaunal deposit feeders are also present but comparatively rare, indicating a nutrient-rich environment with energy levels sufficiently high to keep organic matter in suspension. The Cucullaea glabra – Pseudoptera anomala assemblage of this level is considered to have formed during normal, shallow (15–20 m, supported by abundant material of Pinna) marine conditions. The association inhabiting the area above this level (upper part of the M. geslinianum Zone, S. gracile/E. septemseriatum Subzone) is characterized by the Panopea gurgitis – Rhynchostreon suborbiculatum assemblage suggesting considerably shallower depths and higher energy. Clusters of Modiolus and small oysters characterize the upper parts directly prior to the prominent positive δ13Corg peak “b” (the major peak of OAE2 CIE; ? base/lower part of the N. juddii Zone). No benthic fauna occurs above this level.