The near-surface distribution and processes controlling the distribution of
hydrogen peroxide were examined in the South and central Atlantic Ocean du
ring a transect from Uruguay to Barbados in May and June 1996. Four kinds o
f held experiments were conducted during the cruise including diel observat
ions, dark decay experiments, photochemical production experiments, and hyd
rogen peroxide-enrichment experiments. Significant diel variations (similar
to 25 nM) of hydrogen peroxide were observed, with surface-water concentra
tions 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 coas
tal seawater. The experimental results indicate that: (1) the decay reactio
n is a second-order reaction over all, first-order with both the concentrat
ion of hydrogen peroxide and the concentration of colloidal material; (2) s
eawater in the study area could restore its ambient levels of hydrogen pero
xide in about 4d after external perturbations.
A total of 20 vertical profiles were obtained at 11 stations that can be cl
assified as: surface maximum, surface mixed, and sub-surface maximum. Gener
ally, the concentration of H2O2 decreased with depth to less than 5 nM belo
w 200 m. Hydrogen peroxide also was determined in some water samples from b
elow 200 m, which revealed a slight increase of hydrogen peroxide with dept
h. In the surface waters of the open ocean, hydrogen peroxide increased wit
h latitude from about 24 nM in the south (33.8 degreesS) to about 80 nM in
the north (8.9 degreesN). This latitudinal variation of hydrogen peroxide c
orrelated with model-calculated solar irradiance, satellite-measured wet de
position, depth of the mixed layer, and possibly total organic carbon. The
water-column hydrogen peroxide inventory varied from 1.5 to 6.3 x 10(-3) mo
l/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. (C) 2001 Elsevie
r Science Ltd. All rights reserved.