Oceanic radiocarbon: Separation of the natural and bomb components

Abstract

An improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean. The improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica. This method is applied to radiocarbon measurements made on samples collected during the Geochemical Ocean Sections Study (GEOSECS), Transient Tracers in the Ocean (TTO) and South Atlantic Ventilation Experiment (SAVE) expeditions. On the basis of this new separation we provide not only an estimate of the global inventory of bomb 14C at the time of the GEOSECS survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean. We also document the evolution of the bomb 14C inventory and penetration along thermocline isopycnals in the North Atlantic Ocean between the times of the GEOSECS (1972–1973) and TTO (1980–1982) surveys and in the South Atlantic Ocean between the times of the GEOSECS (1973) and SAVE (1987–1989) surveys. In addition, we show that the bomb tritium to bomb 14C ratio (expressed in the tritium unit (TU) 81 units/100‰) for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline. This contrast offers long‐term potential as an indicator of inter‐hemispheric transport of upper ocean waters.

Developments in Radiocarbon Technologies: From the Libby Counter to Compound-Specific AMS Analyses

“…One very obvious question is: how much 14 C has been taken up by the oceans during the bomb tests? Using modeling simulations, Peacock (2004) discusses a discrepancy between assessments from the 1990s (Broecker et al 1995;Hesshaimer et al 1994), arriving at a number between the 2 earlier estimates for the mid1970s, in the order of about 2.5 × 10 28 atoms, and 3.2-3.5 × 10 28 atoms (±10%) for the mid-1990s. These results were confirmed by Naegler and Levin (2006) using a different model, starting from a global bomb 14 C production estimate between 1945 and 1980 of 6.0-6.3 × 10 28 atoms.…”

Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models

“…Simulations with the four models that participated in the first phase of OCMIP, all suggest that the Southern Ocean is a region of large uptake of anthropogenic CO 2 , with the models giving widely different views of the regional distribution of this uptake (Orr et al, 2001). Evaluation of the performance of these models is based on their comparison with data-based estimates for anthropogenic CO 2 (Gruber et al, 1996;Sabine et al, 1999) and bomb C-14 (Broecker et al, 1995). However, these data-based estimates may be subject to large systematic errors in some regions such as the Southern Ocean.…”

Evaluation and intercomparison of three-dimensional global marine carbon cycle models

“…Atmospheric A14C (the fractionation-corrected permil departure from a standard 14C/t2C ratio) is set to 0 permil and gas exchange is parameterized usin If the same formulation as for CO, In th model, a simplification is made by assuming that the C/12C ratio is conservative with respec ,mixmg b (Fiadiero, 1982, Toggweiler et al, 1989a) These simulations will be run until the deep ocean comes into equilibrium, which typically takes several thousand years Natural radiocarbon is used most effectively as a diagnostic of deep ocean circulation because of the limited number of observations made before the influence of nuclear bomb tests on the radiocarbon distribution 3 3.2 Bomb radiocarbon Beginning with the atmospheric nuclear weapons tests during the 195Os, the 14C content of the atmosphere and upper ocean increased sharply As shown in Figure 8, oceanographers have been able to use the bomb 14C signal in the ocean as a primary data set with which to evaluate ocean circulation models on decadal time scales. Here the observed surface A14C values from the eastern Pacific GEOSECS stations and the WOCE P17 line are compared with the global ocean circulation model of Toggweiler and Samuels (1995a,b) Interpretation of the bomb signal in surface waters for one time period is difficult if the natural signal is not removed (Broecker er al, 1985, Broecker et al, 1995 But taking the difference between two time periods easily isolates the bomb signal In doing so, we see that the model difference between 1973 and 1993 is much larger than the GEOSECS to WOCE difference, implying that the model is removing t4C from c-17 the surface ocean (presumably downward) much faster than is actually occurring This might suggest, for example, that this model would overestimate the uptake of anthropogenic CO, and the delivery of nutrients to the euphotic zone, particularly in the high latitudes. We will simulate bomb radiocarbon in all four of our models following OCMIP guidelines, which are similar to the formulations of Toggweiler et al (1989b) The initial conditions will be the natural radiocarbon distribution simulated by each model, and the atmospheric history of radiocarbon from 176.5 to the present will be specified This extended time period is simulated so that the dilution of atmospheric and oceanic radiocarbon with fossil fuel CO;?…”