A shell‐derived time history of bomb 14C on Georges Bank and its Labrador Sea implications

Abstract

Bomb‐produced radiocarbon has been used in the past as an important tracer of ocean circulation and as a valuable tool for calculating CO2 air‐sea exchange. However, previous studies of the ocean's time‐varying bomb 14C record have been confined exclusively to analyzing banded corals, and thus their application has been limited to the lower latitudes. The first time history of bomb 14C from the high‐latitude North Atlantic Ocean is obtained from a 54‐year‐old mollusc specimen, (Bivalvia) Arctica islandica, which was collected live from Georges Bank (41°N) in 1990. The annual growth bands of its shell were analyzed for Δ14C using accelerator mass spectrometry, producing a Δ14C time history from 1939 to 1990. The depleted condition of the Georges Bank bomb 14C signal relative to two coral‐derived North Atlantic Δ14C time histories suggests a significant deepwater source for the waters on Georges Bank. Supported by previous work linking the origin of waters on Georges Bank to the Labrador Sea, the Δ14C budget on Georges Bank is modeled as Labrador Sea water, which largely becomes confined to the shelf and partially equilibrates with the atmosphere during a 1‐year transit time from the Labrador Sea to Georges Bank. This model is also used to estimate a time history of bomb 14C for the Labrador Sea. Prebomb Δ14C values calculated for the surface Labrador Sea suggest that a greater inventory of bomb 14C has accumulated here than has previously been reported. However, the estimated prebomb average Δ14C (−70.6‰) for this period is nearly identical to the −70‰ previously calculated for the prebomb source of North Atlantic Deep Water and is in agreement with Transient Tracers in the Ocean subsurface tritium data from the central Labrador Sea. Deduced variations in the ventilation and/or 14CO2 uptake rates in the Labrador Sea correspond with observed changes in surface salinity of the Labrador Sea, suggesting a reduction in deepwater formation during the late 1960s and 1970s.