A comparison of several models of carbon turnover in the ocean with respect to their distributions of transit time and age, and responses to atmospheric CO₂ and ¹⁴C

Abstract

Five models of carbon depth distribution in the world ocean are compared with respect to carbon transit-time distribution, age distribution, and integrated responses to histories of fossil CO₂ and weapons ¹⁴C. Two models represent the ocean as two well-mixed layers and differ only in the relative sizes of these compartments. The remaining three models consider 19 well-mixed layers and differ in the patterns of carbon transfer among the layers. All five models are required to be compatible with a preindustrial depth profile of natural ¹⁴C, which is used to fix the values of transfer coefficients. Within this constraint the five models exhibit significantly different responses to fossil CO₂ and weapons ¹⁴C regimes. Of the estimated 134 × 10¹⁵ g of fossil carbon produced in the period 1860–1975, the models estimate a net uptake range of 24 × 10¹⁵ to 66 × 10¹⁵ gC by the ocean. For weapons ¹⁴C the range is 6.0 × 10² to 1.0 × 10³ kg ¹⁴C. Each of these models shows shifts in the implied role of the terrestrial biota between net carbon source and sink, with the latter role dominating in the last years of the simulated period. The largest estimate of source strength for the five models was 0.74 × 10¹⁵ gC yr^-1, in contrast to the fossil CO₂ production rate of ~5 × 10¹⁵ gC yr^-1 in 1975. The five models were also used to predict atmospheric CO₂ levels during the period 1975–2300 for a logistic source that would ultimately release 5000 × 10¹⁵ gC.