Intermediate steps of southwards faunal shifts testified by last glacial submerged thanatocoenoses in the Atlantic ocean

Abstract

Several southwards faunal shifts (SFS) of “boreal” species (BGs: Boreal Guests of authors) are known to have occurred in the colder periods of the E. Atlantic Pleistocene. The latest invasion of BGs took place during the last glacial (75,000–10,000 yr B.P.) and is recorded all over the Mediterranean by a number of submerged thanatocoenoses containing sublittoral to bathyal faunas. On the other hand, these SFS are quite poorly documented within the Atlantic Ocean. Pre-Modern Quaternary thanatocoenoses have been discovered at many sites along the eastern Atlantic margin between ca. 35° and 48°N. These assemblages contain typical BGs ( Modiolus modiolus, Chlamys islandica, Pseudamussium septemradiatum, Arctica islandica, Trichotropis borealis, Buccinum undatum) and/or scleractinian corals ( Lophelia pertusa, Madrepora oculata, Caryophyllia smithi). Samples of representative species were radiocarbon dated showing four age clusters. Cluster 1 (ca. 27,000 yr B.P.) is relative to the pleniglacial and is documented by C. islandica assemblages in the Bay of Biscay and by coral thanatocoenoses in the Ibero-Moroccan Gulf; cluster 2 (ca. 12,000–13,500 yr B.P.) represents latest glacial faunas containing BG-assemblages of milder-climatic significance ( M. modiolus) and corals dredged in the Ibero-Moroccan Gulf; cluster 3 (7500–10,000 yr B.P.) may represent retreating time-delayed and/or postglacial acclimatization of mild-BGs ( P. septemradiatum) in the Ibero-Moroccan Gulf; finally, cluster 4 is relative to quite recent assemblages (late Holocene) containing potential BGs ( B. humphreysianum) the paleoclimatic significance of which has to be revised. Our results show that intermediate steps (missing links) of SFS during the Quaternary between the NE Atlantic BG-reservoir and the Mediterranean are more frequent than previously supposed calling for their careful survey in order to shed light on expansion and contraction of cold faunas during the last glacial.