Chromium behavior in the ocean: Global versus regional processes

Abstract

Total chromium (TCr) has been analyzed by atomic absorption spectrophotometry in seven vertical profiles from the North Atlantic Ocean, three from the Mediterranean Sea, and four from the South and North Pacific Ocean. Chromium (III) data were also obtained for a few of these by isotope dilution mass spectrometry. On the large scale, TCr increases from 3 nmol/kg to 5 nmol/kg between the North Atlantic and the South Pacific deep waters. This distribution, and a slight depletion in surface waters (∼0.5 nmol/kg), suggests a biogeochemical control of Cr. While it is not possible to assess which phase dominates the Cr control, if it is assumed that this is silica, the Cr/Si ratio would be of the order of 0.023 × 10−3 mol/mol. In addition, a decrease of TCr content of 1 to 2 nmol/kg occurs from 21°S to 13°N in the deep Pacific waters, along the East Pacific Rise. This feature, as well as the shape of vertical profiles, suggests that scavenging of Cr also occurs in deep waters. A subtraction flux in the deep Pacific Ocean is estimated as 16 to 20 × 103 nmol/cm/y. It can be partially explained by scavenging Mn and Fe from undersea hydrothermal activity. In the Atlantic subsurface waters, a small minimum of TCr (with concentrations of about 2.5 nmol/kg) is observed in the eastern basin, at depths ranging between 100 and 500 m. This minimum seems to be shallower than a Cr(III) maximum (about 1 nmol/kg) observed at the level of maximum apparent oxygen utilization. It is suggested that the TCr minimum results from advection along isopycnal horizons of slightly depleted surface waters from the relatively productive isopycnal outcrop areas near 40°‐50°N. In the Mediterranean sea, variations of the TCr content are weak (25%): The slightly Cr‐depleted input water from the Atlantic seems to get mixed as the water moves eastward at the surface.