Carbon isotope evidence for the timing of the Cretaceous–Palaeogene macrobenthic colonisation at the Agost section (southeast Spain)

The Cretaceous–Palaeogene (K–T) boundary section of Agost (southeast Spain), one of the most complete sections existing, contains a well-developed trace-fossil assemblage, composed of dark- and light-filled trace fossils, registered in the uppermost Cretaceous (Maastrichtian, Plummerita hantkeninoides Biozone) sediments. The trace-fossil assemblage reveals that significant macrobenthic colonisation occurred around the K–T boundary interval. However, difficulties in recognising trace fossils affecting the K–T boundary layer, as well as in biostratigraphically characterising these trace fossils hindered a conclusive interpretation on the timing of this macrobenthic colonisation. With the aim of this characterisation, we determined the δ13C of the carbonate fraction of dark and light filling materials from the trace fossils, since the carbon isotope compositions of uppermost Cretaceous and lowermost Palaeogene materials are clearly different. The data compiled provide evidence for a significant distinction between light-filled trace fossils, with δ13C values higher than 1.6‰ PeeDee Belemnite (PDB), and dark-filled trace fossils, with δ13C values between 0.21 and 1.2‰ PDB, most of these being lower than 0.7‰ PDB. The comparison of these values with those from the uppermost Maastrichtian to the lowermost Danian sediments reveals that the macrobenthic colonisation occurred in different phases across the K–T boundary interval.