Magnitude and origin of the anthropogenic CO₂ increase and ¹³C Suess effect in the Nordic seas since 1981

Abstract

This study evaluates the anthropogenic changes of CO 2 (ΔC ant ) and δ 13 C (Δ δ 13 C ant ) in the Nordic seas, the northern limb of the Atlantic Meridional Overturning Circulation, that took place between 1981 and 2002/2003. The changes have been determined by comparing data obtained during the Transient Tracers in the Ocean, North Atlantic Study (TTO‐NAS) with data obtained during the Nordic seas surveys of R/V Knorr in 2002 and R/V G.O. Sars in 2003 using an extended multilinear regression approach. The estimated Δ δ 13 C ant and ΔC ant and their relationship to each other and to water mass distribution suggest that the Polar Water entering the Nordic seas from the north is undersaturated with respect to the present atmospheric anthropogenic CO 2 levels and promotes a local uptake of C ant within the Nordic seas. In contrast, the Atlantic Water entering from the south appears equilibrated. It carries with it anthropogenic carbon which will be sequestered at depth as the water overturns. This preequilibration leaves no room for further uptake of C ant in the parts of the Nordic seas dominated by Atlantic Water. The upper ocean p CO 2 in these regions appears to have increased at a greater rate than the atmospheric p CO 2 over the last 2 decades; this is reconcilable with a large lateral advective supply of C ant .