Cadmium regeneration within the North Atlantic

Abstract

Cadmium (Cd) incorporated into the benthic microfossils has been widely used in reconstructing the past water circulation in the North Atlantic. This requires a major control by conservative mixing over regeneration process on the seawater cadmium distribution in the North Atlantic. Through coupling the recently reported Cd data at depths below 300 m with quantitative water mass analysis along the GA03 transect, we tested two models for cadmium cycling within the North Atlantic: conservative mixing alone and conservative mixing plus regeneration which is termed as regenerative mixing. The results show that the regenerative mixing model reproduces the observations (slope = 0.99 and R2 = 0.97) much better than the conservative mixing model (slope = 0.99 and R2 = 0.88). The regenerative mixing model was applied to estimate the amount of dissolved cadmium regenerated "within" the North Atlantic. This regionally regenerated cadmium contributes to ∼10% and >50% of the total cadmium in the North Atlantic at depths >1000 m and 300-1000 m, respectively, indicating that the microfossil-reconstructed seawater Cd can be attributed to the mixing from the South Atlantic to the North Atlantic with high accuracy for depths below 1000 m, while within 300-1000 m cadmium is more controlled by the regeneration process than the mixing process, complicating the attribution of the microfossil-reconstructed seawater Cd to the changes in water mass geometry within this 300-1000 m depth range. A regionally and vertically constant Cd/PO43- regeneration ratio ∼262 pM/μmol/kg was derived from the regenerative mixing model which is comparable with ratios estimated in the recent studies for the North Atlantic.