The Aptian GSSP (lower Cretaecous) at Cau, Se Spain

The Aptian Working Group (AWG) of the International Commission on Stratigraphy (ICS), proposes the cored section at Cau (Spain) as the GSSP for the base of the Aptian. New research on ammonite biostratigraphy resulted in a revision of the chronostratigraphic age of magnetic Chron M0r, which is now considered as late Barremian. In search for another primary marker, the AWG proposes to move the base of the Aptian upwards into magnetic Chron C34n and for the first time in Mesozoic stratigraphy, select a chemostratigraphic event as the primary marker. A prominent “negative spike” in the organic carbon and carbonate carbon isotope records is widely recognised in land and oceanic sites across different palaeolatitudes and both marine and terrestrial settings. The distinctive negative spike in the carbon isotope records is selected as the new marker for defining the base of the Aptian. Two potential sites were chosen for the final vote as candidate Global Boundary Stratotype Section (GSSP): Cismon (Italy) and Cau (Spain). The majority of AWG members favoured the Cau core because of its expanded hemipelagic shelf succession. Additional markers include biostratigraphic events of calcareous nannofossils, planktonic foraminifera, ammonites, and palynomorphs, along with lithological (sharp decrease in CaCO3 contents above the boundary), and geochemical (e.g., variations in Os isotopes). Due to the correspondence of the proposed GSSP and the onset of major global environmental and biotic perturbances associated with the onset of the Oceanic Anoxic Event 1a, considered to be associated with a prominent hyperthermal, an increasing number or different geochemical and biotic records show notable changes across this boundary. These may serve as additional secondary markers after ongoing and future research has established their correlation potential. This communication presents a review of the Report ratified by the AWG in April 2026 to be submmited to the ISC for consideration as the new definition of the base of the Aptian Stage and the Cau core as the GSSP.