The Jurassic to Cretaceous transition in the Torre de’ Busi section (Southern Alps): Integrated stratigraphy, geochemistry and microfossil changes

During the Late Jurassic–earliest Cretaceous, major palaeoceanographic and biotic changes affected pelagic ecosystems of the Tethys Ocean, culminating in the onset of widespread pelagic carbonate sedimentation. This study investigates the Tithonian–Berriasian interval using the fully pelagic and stratigraphically complete Torre de’ Busi section (western Tethys, Lombardian Basin), tightly constrained by integrated nannofossil and calpionellid biostratigraphy, and magnetostratigraphy. High-resolution analyses of stable carbon and oxygen isotopes, carbonate content, magnetic susceptibility, gamma-ray spectroscopy data, and quantitative microfossil assemblages provide a multiproxy reconstruction of environmental and biotic evolution across the Jurassic-Cretaceous transition. Results document a progressive increase in carbonate content from radiolarian-rich cherts of the Radiolariti Formation to the calcilutites of the Maiolica Formation, accompanied by a marked decline in terrigenous input. This lithological transition coincides with major changes in planktonic communities, including the decline and final disappearance of Saccocoma, the diversification and proliferation of calpionellids, and the expansion of heavily calcified calcareous nannoplankton, particularly nannoconids. A major turnover in microfossil assemblages occurred in the middle late Tithonian (base of nannofossil Zone NJT17; base of calpionellid Subzone Crassicollaria intermedia), synchronous with the onset of Maiolica deposition. Carbon isotope data show a long-term Tithonian decrease in δ13C values, reflecting a reorganization of the global carbon cycle, whereas minimum δ13C values do not coincide with abrupt biotic changes. The synchronous increase in pelagic carbonate production and calcifying plankton abundance indicates supra-regional controls related to seawater chemistry, climate aridity, and reduced nutrient supply, highlighting the sensitivity of Tethyan pelagic systems to climate-driven oceanographic change.