Radiocarbon Reservoir Age of High Latitude North Atlantic Surface Water During the Past 20, 000 Years Inferred From Deep-Sea Corals and Foraminifera

The Δ14C value (reservoir age) of North Atlantic surface water is expected to vary according to the rate of North Atlantic Deep Water (NADW) formation and the air-sea exchange rate. Previous attempts to estimate the Δ14C of North Atlantic surface waters indicate potentially large changes but these prior studies involve many unsupported assumptions. Here we present our estimations of surface water reservoir ages during the Younger Dyras (YD) and Bolling-Allerod (BOA) via the use of corals collected from shallow seamounts or shallow ridges in the high latitude North Atlantic; sites which are rapidly ventilated by North Atlantic surface waters on an annual (e.g. deep winter mixing) time scale. As well as reservoir age estimations during the glacial via using varved sediment record from Cariaco Basin (Hughen et al., 2004) and deep-sea core from Iceland Sea (Voelker et al., 2000). In this study, paired 230Th/234U/238U and radiocarbon dates were obtained from D. cristagalli from Orphan Knoll (50°26'N, 46°22'W; 1600 meters water depth), northwestern North Atlantic to estimate North Atlantic surface water reservoir age (maximum) during the last deglaciation. We numerically estimate the reservoir age by 'back-tracking' the computed Δ14Cwater to the intersection with the Δ14Catm record computed from our radiocarbon calibration curve (Fairbanks et al., 2005; http://www.radiocarbon.LDEO.columbia.edu). During the BOA (samples date from 13.5 ka to 14.0 ka) deglacial climate interval, the computed reservoir age was 400 years. It indicates that NADW at that time was as strong as its modern condition. The reservoir age increased to 600 to 700 years preceding the onset of the YD, indicating a more sluggish mode of NADW. Both deep-sea core PS 2644 (Voelker et al., 2000) and Cariaco basin varved sediment record (Hughen et al., 2004) obtained their chronologies by correlating with GISP2 ice core. We retain the original published GISP2 (wiggle matched) tie points for these cores but converted the Cariaco chronology, and by correlation the GISP2 chronology, to calendar years using our radiocarbon calibration program. We found that the glacial high latitude North Atlantic Ocean surface reservoir age were approximately 1000 years, which is twice modern values. Our results add one more proxy of the production and ventilation of NADW during the last glacial and deglaciation to compare to a rapidly growing array of deepwater proxy records.