GEOSECS Atlantic32Si profiles

Abstract Measurements of five cosmogenic 32 Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO 2 extracted range from 2 to 54 g; the amounts of water from which SiO 2 was extracted range between 540 kg and 270, 000 kg. In additon, SiO 2 recovered from four surface particulate composites (64°N to 61°S) were also analyzed for 32 Si. 32 Si measurements were made by milking and counting the daughter activity, 32 P. The net 32 P activities range from 0.7 to 6.8 cph; typical errors in measurements of the 32 P activities are 20–30%. The 32 Si concentrations vary from 0.6 dpm/10 6 kg of water in the North Atlantic surface waters to 235 dpm/10 6 kg at 400 m depth in the circumpolar waters. The vertical profiles of 32 Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and 32 Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of 32 Si ( 32 Si/SiO 2 ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2–4 times, than the bottom water values. These data are consistent with generation of the highest specific activity 32 Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic 32 Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high 32 Si specific activities. The 32 Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10 −2 dpm 32 Si/cm 2 , which is consistent with the expected fallout of cosmogenic 32 Si. However, the 32 Si column inventories south of 40°S are higher by a factor of ∼ 5–7, whereas the corresponding Si inventories increase by only a factor of 3. This excess 32 Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of 32 Si dissolved from sinking particulates.