Daily variability of dissolved inorganic radiocarbon at three sites in the surface ocean

Ellen R.M Druffel,Sheila Griffin

doi:10.1016/j.marchem.2008.04.004

Abstract

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during cruises to the central North Pacific, the Sargasso Sea and the Southern Ocean. The ranges of Δ 14C measurements for each cruise (11–30‰) were larger than the total uncertainty (7.8‰, 2-sigma) of the measurements. The variability is attributed to changes in the upper water mass that took place at each site over a two to four week period. These results indicate that variability of surface Δ 14C values is larger than the analytical precision, because of patchiness that exists in the DIC Δ 14C signature of the surface ocean. This additional variability can affect estimates of geochemical parameters such as the air–sea CO 2 exchange rate using radiocarbon.

Highlights

Bomb radiocarbon (14C) was produced in the late 1950s and early 1960s by thermonuclear weapons testing in the stratosphere and caused 14C levels in tropospheric CO2 to nearly double by 1964 (Nydal and Lovseth, 1983)
Maximum Δ14C values measured in surface water dissolved inorganic carbon (DIC) were attained in the 1970s, indicating that the turnover time of CO2 in the atmosphere with respect to transfer to the surface ocean is ∼10 years (Druffel and Suess, 1983)
We report daily surface DIC Δ14C values obtained for cruises to the NCP and SS sites, and a site in the Southern Ocean

Summary

Introduction

Bomb radiocarbon (14C) was produced in the late 1950s and early 1960s by thermonuclear weapons testing in the stratosphere and caused 14C levels in tropospheric CO2 to nearly double by 1964 (Nydal and Lovseth, 1983). Maximum Δ14C values measured in surface water dissolved inorganic carbon (DIC) were attained in the 1970s, indicating that the turnover time of CO2 in the atmosphere with respect to transfer to the surface ocean is ∼10 years (Druffel and Suess, 1983). Our results highlight the fact that the surface ocean Δ14C signature varies by a larger amount than previously indicated by uncertainties assigned to the individual values (3–4‰). This is relevant because surface radiocarbon values are used to calculate such quantities as air–sea CO2 exchange rate and bomb 14C inventory in the ocean, and additional error in the radiocarbon can impart larger error into these biogeochemical parameters

Methods

Results and discussion

Sargasso Sea

Southern Ocean

Implications for surface ocean variability in DIC Δ14C and δ13C