The “Northern guests” and other palaeoclimatic ostracod proxies in the late Quaternary of the Basque Basin (S Bay of Biscay)

The analysis of the ostracod assemblages from three cores obtained from the middle and outer shelves of the Basque Basin, allows us to evidence the influence of different water masses on the Basque shelf during late Quaternary (MIS 3 — the present). During pre-Holocene, the dominance of Krithe spp. and Cytherella lata marks the input of cold (< 5 °C) waters coming from deeper areas of the southern Bay of Biscay. During the Holocene, in the western outer Basque shelf the dominance of Costa edwardsii and Palmoconcha guttata evidences the presence of warm waters (11–17 °C). However, the abundance of Krithe spp., C. lata and Cytherella alvearium in the eastern outer shelf shows the input of colder waters by upwelling currents from deeper areas of the southern Bay of Biscay and their restricted influence to the eastern margin. In the middle shelf, despite the affectation of warm waters (dominance of C. edwardsii) during the early Holocene (c. 8.8–6.8 ka cal BP), the values of δ18Obull, Neogloboquadrina pachyderma sin. and “Northern guests” Acanthocythereis dunelmensis and “Trachyleberis” sp. percentages point to the entrance of colder polar surface and bottom waters during the mid-Holocene (c. 6.8–4.3 ka cal BP), which came from the west of the study area as upwelling currents. During the late Holocene (c. 4.3 ka cal BP to the present) these circumpolar waters retreated from the middle shelf and the bottom waters suffered a relative warming. Besides, some “cold” climatic events, such as Heinrich event HE 3 (c. 31.3 ka cal BP), have been characterized in the Basque shelf by the presence of “Northern guests” A. dunelmensis, Cytheropteron testudo and “Trachyleberis” sp., which defines the occasional input of cold (0–8 °C) bottom waters coming from the circumpolar area of the North Atlantic Ocean into the Basque shelf during late Quaternary.