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

Abstract

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 ~26.7 ka, enables us to determine that the middepth (1179 m water depth) and shallow subsurface reservoir ages were ~1500 and 1100 14C 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 (~20–18 ka), Nordic Seas overflow was weak but active. During the early deglaciation (~17.5–14.5 ka), our data reveal large differences between 14C ventilation ages that are derived from dating different benthic foraminiferal species: Pyrgo and other miliolid species yield ventilation ages >6000 14C years, while all other species reveal ventilation ages <2000 14C years. These data either suggest subcentennial, regional, circulation changes or that miliolid‐based 14C ages are biased due to taphonomic or vital processes. Implications of each interpretation are discussed. Regardless of this “enigma,” the onset of the Bølling‐Allerød interstadial (14.5 ka) is clearly marked by an increase in middepth Nordic Seas ventilation and the renewal of a stronger overflow.

Nordic Seas polynyas and their role in preconditioning marine productivity during the Last Glacial Maximum

“…No local reservoir age (Δ R = 0) was applied. We are aware of large uncertainties in reservoir ages in the Nordic Seas during the LGM and early deglaciation 9 , 32 and the consequence of younger ages when applying local reservoir ages between Δ R = 0 to 400–500. Here, we elected to use Δ R = 0 to enable us to make a direct comparison to other key records from the North Atlantic and Nordic Seas.…”

Extreme lowering of deglacial seawater radiocarbon recorded by both epifaunal and infaunal benthic foraminifera in a wood-dated sediment core

“…Calcium carbonate overgrowth (via the conversion of CaCO 3 to CaSO 4 or gypsum) was observed in Santa Barbara Basin sediments (Magana et al, 2010), but would not influence the 14 C content of the microfossil. What's more, extreme 14 C depletions of mixed benthic foraminifera from this and other sites were found to be biased by Pyrgo spp., which are inexplicably depleted in 14 C (Ezat et al, 2017). Other work suggests younger-than-expected 14 C ages from the precipitation of carbonate onto foraminifera tests after core recovery .…”

“…The inconsistency of the available 14 C records is compounded by assumptions about the reliability of the foraminifera archive as a recorder of seawater DIC 14 C. For example, an important assumption when using planktic foraminifera is that the depth of calcification does not vary based on modern observations (e.g., Field, 2004). The use of benthic foraminifera seemingly circumvents this problem, and those that live at the sediment-water interface ("epifaunal") have been demonstrated to record seawater carbon chemistry (Keigwin, 2002;Roach et al, 2013).…”

Synchronous Changes in Sediment Transport and Provenance at the Iceland‐Faroe Ridge Linked to Millennial Climate Variability From 55 to 6 ka BP

“…Highest log (Ti/K) occur within the FMAZ layers and the lowest ratio is recorded during HSs. (g) In order to compare our XRF data with nearby core FM11‐FI‐19PC (northeast of the Faroe Islands) the XRF (Ti/K) ratios of this core are converted to a logarithmic scale and plotted against the age model of Ezat et al () and Hoff et al (), which is based on AMS 14 C dates, tephra stratigraphy, and linkages of proxy records to NGRIP. The log (Ti/K) record of FM11‐FI‐19PC resembles those of core 905 and 909, although the onset of D‐O 8 and D‐O 12 is abrupt in 905, followed by gradual decreases toward GIs.…”