Rudists and carbonate platforms in the Aptian: a case study on biotic interactions with ocean chemistry and climate

Abstract

AbstractVariations in Aptian carbonate platform growth and associated turnover among rudists are reviewed and the results synthesized with evidence for climate change to yield an explanatory model. Extensive platform growth throughout the Atlantic/Tethys/low‐palaeolatitude Pacific seamount belt in the earliest Aptian was accompanied by prolific rudist diversification, especially among the predominantly aragonitic caprinids occupying platform margins. It was ubiquitously interrupted in the mid‐Early Aptian, in tandem with major perturbations of the global carbon cycle that culminated in Oceanic Anoxic Event 1a, although the causal linkages remain contentious. Platform growth terminated along most of the northern Tethyan margin and maybe also in the New World. Meanwhile, Lithocodium/Bacinella or similar microbial encrustations became widespread in lower palaeolatitudes. Recovery of Tethyan platforms in the late Early Aptian was limited to lower palaeolatitudes. Caprinid‐rich platform margin facies again prevailed in central and southern Tethyan areas, but gave way to those dominated by rudists with a thickened calcitic outer shell layer (polyconitids and requieniids) in marly successions around Iberia. The end of the Early Aptian saw the Lazarus‐style disappearance of caprinids, and renewed Tethyan platform growth in the Late Aptian was dominated by calcite‐rich rudists, although rudists remained scarce in the New World until the terminal Aptian. This study postulates that sea water acidification influenced both the mid‐Early Aptian platform debacle and the subsequent late Early Aptian geographical restriction of platform recovery, although in contrasting atmospheric regimes. At first, it was forced by the increasing atmospheric levels of volcanically derived CO2, with mitigation in low latitudes from thermal expulsion of aqueous CO2 (here termed the ‘kettle effect’) due to greenhouse warming. However, subsequent cooling due to drawdown of atmospheric CO2 by organic carbon burial could have sustained acidification of platform waters in higher latitudes, by reducing the protective kettle effect there. Caprinid susceptibility to such assaults may have been as much due to exposure on their preferred outer platform habitats as to their mineralogy. The increased calcite/aragonite ratio among rudists through the Aptian resulted largely from this taxonomic turnover.