Variability of the northeast Atlantic sea surface Δ14C and marine reservoir age and the North Atlantic Oscillation (NAO)

We compiled new 14C analyses of mollusc shells (bivalves and gastropods) of known age from the collection of the Muséum National d’Histoire Naturelle de Paris and previously published data to investigate changes in the sea surface Δ14C and reservoir age in the northeast Atlantic sector (NEA) between 1823 and 1952 AD. The mollusc shells are mainly located off the Atlantic margin between 45°N and 60°N downstream of the North Atlantic Current (NAC). We show that the temporal variability of the NEA Δ14C is independent of the mollusc species, depth habitat, diet and latitudinal distribution. The quasi-null difference between mollusc Δ14C and the marine model indicate that the mollusc Δ14C reflects the Δ14C values of open marine conditions. Between 1823 and 1850 AD, the pre-anthropogenic mean of Δ14C is −45 ± 5‰, corresponding to a reservoir age of 380 ± 60 years and a ΔR value of −7 ± 50 years, in agreement with previous estimates. The Δ14C values show a significant long-term decrease of ∼12‰ from 1823 to 1952 AD attributed to changes in 14C production between 1823 and 1900 AD and the Suess effect between 1900 and 1952 AD.Between 1885 and 1950 AD, Δ14C fluctuations of ∼10‰ up to 18‰ occurred in the northeast Atlantic, corresponding to reservoir age variations of ∼90 years up to 170 years. These fluctuations are very similar to changes of Δ14C in the southern Norwegian Sea. Spectral analyses of the NEA Δ14C exhibit quasi-periodic cycles of about 7.4 years, almost equivalent to the quasi-periodic cycles of the winter index of the North Atlantic Oscillation (NAO) with a period around 6.5 years. We find that changes of NEA Δ14C cannot be attributed to changes in river runoff or the precipitation/evaporation budget. The Δ14C lows (or high reservoir ages) correspond to the more intense phase of the winter NAO, with a time lag of ∼1–3 years. Such a time lag may reflect the eastward transit time of upstream changes originating in the Labrador Sea, then entrained within the NAC along with the subpolar gyre into the northeast Atlantic sector. During these events, we estimate that about 70% of the total transport in the NAC was due to water originating in the Labrador Sea.