GEOSECS Atlantic <sup>32</sup>Si profiles

Somayajulu, BLK, Rengarajan R, Lal D, Weiss RF, Craig H.  1987.  GEOSECS Atlantic 32Si profiles. Earth and Planetary Science Letters. 85:329-342.

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Measurements of five cosmogenic32Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO2 extracted range from 2 to 54 g; the amounts of water from which SiO2 was extracted range between 540 kg and 270, 000 kg. In additon, SiO2 recovered from four surface particulate composites (64°N to 61°S) were also analyzed for32Si.32Si measurements were made by milking and counting the daughter activity, 32P. The net32P activities range from 0.7 to 6.8 cph; typical errors in measurements of the32P activities are 20–30%.The32Si concentrations vary from 0.6 dpm/106 kg of water in the North Atlantic surface waters to 235 dpm/106 kg at 400 m depth in the circumpolar waters. The vertical profiles of32Si 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 and32Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of32Si (32Si/SiO2 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 activity32Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic32Si 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 high32Si specific activities.The32Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10−2 dpm32Si/cm2, which is consistent with the expected fallout of cosmogenic32Si. However, the32Si 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 excess32Si 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 of32Si dissolved from sinking particulates.