The distribution of hydrogen peroxide in the southern and central Atlantic ocean

Abstract

Abstract The near-surface distribution and processes controlling the distribution of hydrogen peroxide were examined in the South and central Atlantic Ocean during a transect from Uruguay to Barbados in May and June 1996. Four kinds of field experiments were conducted during the cruise including diel observations, dark decay experiments, photochemical production experiments, and hydrogen peroxide-enrichment experiments. Significant diel variations (∼25 nM) of hydrogen peroxide were observed, with surface-water concentrations increasing during the day and decreasing at night. With a dark decay half-life of 5.5 days and a net rate of photochemical production of 8.3 nM/h at local noon, it appears that both decay rate and photo-production rate of hydrogen peroxide are much smaller in oligotrophic seawater than in coastal seawater. The experimental results indicate that: (1) the decay reaction is a second-order reaction over all, first-order with both the concentration of hydrogen peroxide and the concentration of colloidal material; (2) seawater in the study area could restore its ambient levels of hydrogen peroxide in about 4 d after external perturbations. A total of 20 vertical profiles were obtained at 11 stations that can be classified as: surface maximum, surface mixed, and sub-surface maximum. Generally, the concentration of H 2 O 2 decreased with depth to less than 5 nM below 200 m. Hydrogen peroxide also was determined in some water samples from below 200 m, which revealed a slight increase of hydrogen peroxide with depth. In the surface waters of the open ocean, hydrogen peroxide increased with latitude from about 24 nM in the south (33.8°S) to about 80 nM in the north (8.9°N). This latitudinal variation of hydrogen peroxide correlated with model-calculated solar irradiance, satellite-measured wet deposition, depth of the mixed layer, and possibly total organic carbon. The water-column hydrogen peroxide inventory varied from 1.5 to 6.3×10 −3 mol/m 2 . Although the greatest shallow water concentrations were observed at stations in the Amazon plume, these stations showed a dramatic decrease in hydrogen peroxide with depth and integrated-water-column hydrogen peroxide was not significantly higher than at open ocean stations.