Bomb radiocarbon in the Pacific: Annual and seasonal timescale variations
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Bomb radiocarbon in the Pacific: Annual and seasonal timescale variations

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Abstract

Banded corals are used as proxy recorders of bomb radiocarbon in the surface of the Pacific Ocean. Bomb radiocarbon levels appeared to still be rising in the tropical Pacific by 1982, in contrast to temperate locations that peaked in the early 1970's. This is representative of the geostrophic transport of bomb-laden waters from higher latitudes toward the equator. The seasonal radiocarbon signal at Canton Island (3S, 172W) during the early 1970's was twice the amplitude of that at Fanning Island (4N, 159W), and the radiocarbon minima at these locations were offset by several months. The phase lag is caused primarily by the seasonally variant transequatorial Ekman transport, which funnels more upwelled, 14C-poor water from summer to winter hemisphere. The seasonal variation of Δ14C at Canton is larger because the peak input of 14C-poor South Equatorial Current water is coincident with the period of greatest transequatorial Ekman transport to this region. Whereas, at the Fanning site, these inputs reach their maxima during opposing seasons, which causes a damping of the seasonal Δ14C signal. A time-stepped, multi-box model calculation is made to describe bomb radiocarbon distributions in the tropical Pacific, and hence to determine the influence of the South Equatorial Current flow on the chemistry of the central equatorial Pacific. The annual model results show that radiocarbon is influenced to a minor extent by this lateral flow, in agreement with previous studies. However, the seasonal version of the model reveals that the South Equatorial Current flow varies by a factor of 2–3 in order to explain the seasonal variations in bomb radiocarbon. Meridional geostrophic convergence and transequatorial Ekman transport from summer to winter hemisphere alone are not sufficient for defining the observed seasonal signals.

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