Bomb radiocarbon in the Pacific: Annual and seasonal timescale variations

Ellen R M Druffel

doi:10.1357/002224087788326876

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.

Highlights

Examination of bomb radiocarbon levels in surface ocean dissolved inorganic carbon (DIe) (Linick, 1978, 1980; Nydal and Loveseth, 1983; Ostlund and Stuiver, 1980), shows pronounced spatial variability in the ..1.14C values within major water masses
The tropical study sites lie in the path of the South Equatorial Current, a westward flowing water mass that originates in the Peru Current (PC) along the western coast of South America
Time histories of post-bomb radiocarbon levels are reconstructed from analyses of annual coral bands from the temperate Pacific (Hawaiian Islands and Tahiti; Fig. 3b), and from the tropical Pacific Ocean (Fanning, Uva and Galapagos Islands; Fig. 3a and c)

Summary

Introduction

Examination of bomb radiocarbon levels in surface ocean dissolved inorganic carbon (DIe) (Linick, 1978, 1980; Nydal and Loveseth, 1983; Ostlund and Stuiver, 1980), shows pronounced spatial variability in the ..1.14C values within major water masses. From the GEOSECS results, Broecker and Peng (1980) noticed that surface waters in the temperate North Atlantic (20-35N) had ..1.14C values 350/00higher in September 1972, just before late fall deepening of the mixed layer, than during March 1973 when the depth of the mixed layer was 200-400 m. Evidence is presented of seasonal variation in bomb 14e levels during the early 1970's in surface waters of the mid-tropical Pacific using banded corals as the data base. As aragonite does not dissolve in saturated surface waters, nor is it believed to exchange its carbonate with other sources of carbon, the accreted aragonite provides a permanent, unaltered record of the isotopic ratios present in sea water at the time of formation

Sampling locations

Methods

Annual records of bomb radiocarbon

Seasonal records of bomb radiocarbon

Findings

Discussion