Proliferation of Chondrodonta as a proxy of environmental instability at the onset of OAE1a: Insights from shallow‐water limestones of the Apulia Carbonate Platform

Abstract

Chondrodonta is an opportunistic, oyster-like bivalve, common in shallowwater carbonates of the Cretaceous Tethyan Realm. Despite its high abundance and widespread geographic distribution, the precise relationship between the early Aptian proliferation and environmental perturbations resulting from the Oceanic Anoxic Event 1a (OAE1a), has not been investigated. Stratigraphic and geochemical analyses of the lower Aptian Chondrodonta bedsets within the inner platform limestones of the Apulia Carbonate Platform (Gargano Promontory, southern Italy) are conducted to assess the environmental controls on the Chondrodonta proliferation and its timing and causal relationship to OAE1a. Chondrodonta occurs with sparse to common individuals within requieniid rudist floatstone-rudstones, forms monospecific biostromes during the early phase of stressed environmental conditions and then rapidly disappears at the peak of OAE1a. It proliferates in dysoxic seawater with relatively increased trophic sources, which correlate to increasing nutrient levels in the nearby pelagic realm. Chondrodontarich beds are associated worldwide with the onset of OAE1a and occur in a transitional context between a stable and a strongly stressed environment, where the opportunistic behaviour of Chondrodonta is rather efficient. Increasing nutrient load and unstable environmental conditions right below the peak of OAE1a created an environmental 'window' favourable for Chondrodonta to proliferate, outplaying the less tolerant benthos (for example, rudists). The occurrence, duration and position of the environmental window were controlled by local palaeogeographic and hydrodynamic settings (i.e. low energy, decreased seawater oxygenation and circulation). Further increase in inhospitable conditions, leading to OAE1a, constituted an upper threshold for Chondrodonta and allowed mesotrophic taxa like Bacinella-Lithocodium and orbitolinids to dominate the benthic 3191

Aptian Oceanic Anoxic Event 1a in the shallow, carbonate‐dominated intrashelf Kazhdumi Basin, Zagros Mountains

“…That the δ 13 C fluctuations in the Dariyan Formation were not the result of such diagenesis is suggested by the lack of characteristic vadose zone products, including lack of karstic and/or palaeosol features. Nor did the δ 13 C fluctuations result from facies changes, as individual units show no positive δ 13 C versus δ 18 O covariance (Del Viscio et al ., 2021). Also, none of the sequence boundaries is associated with unconformities, which in shallow‐marine settings are commonly associated with negative δ 13 C shifts immediately beneath them (e.g.…”

“…In shallow‐marine carbonate platforms, rudist–coral associations were replaced by Lithocodium–Bacinella encrusters in the central and southern Tethys, whereas the northern Tethyan platforms drowned, and extensive hardgrounds and condensed sequences were formed (e.g. Bover‐Arnal et al ., 2009; Huck et al ., 2010; Skelton & Gili, 2012; Del Viscio et al ., 2021). The coincidence of Bacinella blooms with the OAE 1a has been challenged based on the biostratigraphic and sequence stratigraphic data (e.g.…”

Stratigraphy and tectonic evolution of a portion of the Simbruini-Ernici Mountains (Central Apennines - Italy): review and new data from detailed geological mapping

“…Shift in oceanic chemistry (mainly observed by C-and O-stratigraphic isotope curves) coincident with changes in trophic resources and sea-water acidification (i.e., Parente et al, 2008) were testified by the proliferation and, subsequent, demise of taxa like the bivalves Chondrodonta in the sedimentary record. These can be related to the effects of the oceanic anoxic events of the early Aptian ("Selli Event'" or OAE1a -Erba, 1994;Graziano et al, 2013) and Cenomanian-Turonian (OAE2, represented by the basinal "Bonarelli Level" - Jenkyns, 2010;Bottini and Erba, 2018), respectively pre-dated by the flowering of C. glabra and C. joannae (Posenato et al, 2018(Posenato et al, , 2020Del Viscio et al, 2021). Chondrodonta accumulations are therefore proved as a useful field tool for identifying and mapping OAEs-related stratigraphic boundaries (CCG/CMS and IBX/RDT).…”