Palaeoenvironmental implications of the deepwater ostracod (Crustacea, Ostracoda) fauna in the Norwegian Basin during the Middle Pleistocene (MIS 10–15)

This paper presents the Pleistocene Ostracoda (Crustacea) record of Calypso piston core MD992277, recovered from the Norwegian Basin at 2,800 m water depth along the Jan-Mayen Ridge. The core was part of a previous study that empirically tested the deep-sea glacial disturbance hypothesis as a cause of the present-day, unusually low Norwegian Sea diversity conducted by Jöst et al. (2024). Here, we provide a comprehensive taxonomic overview of the MD992277 species record used in the previous study, complemented by palaeoecological, palaeoclimatic, and palaeobiogeographic interpretations. A total of 15,391 fossil specimens were identified from 432 sediment samples spanning ~620–360 ka. Two genera accounted for more than 80% of all specimens: Krithe Brady, Crosskey & Robertson, 1874 (N = 9,749; ~60%) and Cytheropteron Sars, 1866 (N = 3,150; ~20%). Although both Krithe and Cytheropteron occurred throughout the studied interval of MIS 15–10, species-specific climatic preferences were observed. Echinocythereis echinata (Sars, 1866), absent from the modern Nordic seas (i.e., Norwegian Sea & Greenland Sea), was present during MIS 12 and MIS 11 (~455–380 ka), suggesting a temporal southward migration. The occurrence of Rosaliella svalbardensis Yasuhara, Sztybor, Rasmussen, Okahashi, Sato & Tanaka, 2018, a species considered endemic to cold-seep environments around Svalbard, may tentatively indicate a palaeo-cold seep near the drilling locality. By comparison of the stratigraphic distribution of our Norwegian Sea assemblages to records from the North Atlantic, our data revealed that Argilloecia Sars, 1866, a key genus during climatic transitions, occurred during cooling phases in the Norwegian Sea, whereas its temporal distribution in the North Atlantic is restricted to warming phases. This difference in temporal distribution of Argilloecia between North Atlantic proper and the Norwegian Sea may reflect regional differences in environmental stability and water mass dynamics. We found shallow-marine, allochthonous taxa in several sediment layers, indicating downslope transportation processes along the Jan-Mayen Ridge. We provide census data with scanning electron microscope (SEM) images of selected specimens for most species. Species-specific climatic preferences (glacial, interglacial, transitional), and stratigraphic distribution are discussed with focus on environmental and faunal changes across the Mid-Brunhes Event, a significant climatic event of the Pleistocene.