Ventilation history of Nordic Seas overflows during the last (de)glacial period revealed by species-specific benthic foraminiferal 14C dates

Formation of deep water in the high-latitude North Atlantic is important for the global meridional ocean circulation, and its variability in the past may have played an important role in regional and global climate change. Here we study ocean circulation associated with the last (de)glacial period, using water-column radiocarbon age reconstructions in the Faroe-Shetland Channel, southeastern Norwegian Sea, and from the Iceland Basin, central North Atlantic. The presence of tephra layer Faroe Marine Ash Zone II, dated to similar to 26.7 ka, enables us to determine that the middepth (1179m water depth) and shallow subsurface reservoir ages were similar to 1500 and 1100 C-14 years, respectively, older during the late glacial period compared to modern, suggesting substantial suppression of the overturning circulation in the Nordic Seas. During the late Last Glacial Maximum and the onset of deglaciation (similar to 20-18 ka), Nordic Seas overflow was weak but active. During the early deglaciation (similar to 17.5-14.5 ka), our data reveal large differences between C-14 ventilation ages that are derived from dating different benthic foraminiferal species: Pyrgo and other miliolid species yield ventilation ages >6000 C-14 years, while all other species reveal ventilation ages <2000 C-14 years. These data either suggest subcentennial, regional, circulation changes or that miliolid-based C-14 ages are biased due to taphonomic or vital processes. Implications of each interpretation are discussed. Regardless of this "enigma," the onset of the Bolling-Allerod interstadial (14.5 ka) is clearly marked by an increase in middepth Nordic Seas ventilation and the renewal of a stronger overflow.