Late Pleistocene boreal molluscs in the Gulf of Cadiz: Past and current oceanographic implications

Remains of molluscs were collected from the seafloor on the north-eastern margin of the Gulf of Cadiz, between 300 and 1000 m water depth, using different sampling methods (e.g. dredging, trawling and box-coring), during several deep-sea expeditions. Samples contained a suite of species which nowadays mostly occur northwards of the English Channel, together with other widespread species. Species now locally extinct in the Gulf of Cadiz and restricted to northern latitudes, which unequivocally indicate a faunal shift, include the gastropods Buccinum undatum, Colus gracilis, Liomesus ovum and Neptunea antiqua, the bivalves Arctica islandica, Chlamys islandica, Modiolus modiolus, Mya truncata and Nuculana pernula and the scaphopod Antalis entalis. These species represent “Boreal Guests” of marked palaeoclimatic significance, some of which are reported for the first time in the Gulf of Cadiz. The boreal species collected were mostly large (>5 cm) whereas smaller boreal species were extremely scarce, probably winnowed away by strong bottom currents. The pteropod Limacina retroversa, at present restricted to water masses northwards of the Iberian Peninsula but widespread in Mediterranean sediments of the last glaciation, was also recorded. Accelerator mass spectrometry (AMS) C dates obtained from nine specimens of molluscs ranged between 26.1 and 14.6 kyr B.P., thus confirming their attribution to a last glacial assemblage. The abundance of these molluscan remains in the present Mediterranean Outflow Water pathway could be explained if this outflow was reduced in intensity or more likely shifted to a deeper level, leaving the upper slope in contact with suitable Atlantic intermediate waters. The findings of Boreal Guests in the Gulf of Cadiz document the continuity of the faunal shift which is well-known in the Mediterranean basin. Species still living in the Gulf of Cadiz and the Alboran Sea nevertheless account for 84.6% of specimens among the larger species.

We would like to thank the help of many colleagues at different stages of this work and during the sampling expeditions ANASTASYA 09/99, INDEMARES/CHICA 0610, 0211 and 0412, and CIRCASUR 2020, and as well as the captains and all the crews of the R/Vs Emma Bardán, Cornide de Saavedra and Ramón Margalef. This study was supported by the INDEMARES/CHICA project with EC contract INDEMARES-LIFE (07/NAT/E/000732); the LIFE IP PAF INTEMARES project ‘‘Integrated, Innovative and Participatory Management for N2000 network in the Marine Environment’’ (LIFE15 IPE/ES/000012); and the 18-ESMARES2-CIRCA project “Monitoring and assessment of circalitoral and bathyal benthic habitats” from the C.N. Instituto Español de Oceanografía (IEO-CSIC), under the framework of the tasks assigned to the IEO-CSIC by the Ministerio de Transición Ecológica y Reto Demográfico (MITERD) of the Spanish government for the application of the Marine Strategy Framework Directive (MSFD) in Spanish waters. We really appreciate the help provided by Gonzalo Jiménez (Universidad de Granada) and Anxo Mena (Universidad de Vigo) regarding the age calibration. Funding for open access charge was provided by Universidad de Málaga/CBUA.