GEOSECS Atlantic 32Si profiles

Details GEOSECS Atlantic 32Si profiles GEOSECS Atlantic 32Si profiles

Oct 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987e%26psl..85..329s&link_type=abstract

Earth and Planetary Science Letters, Volume 85, Issue 4, p. 329-342.

Computer Science

12

Scientific paper

Measurements of five cosmogenic 32Si 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 for 32Si. 32Si measurements were made by milking and counting the daughter activity, 32P. The net 32P activities range from 0.7 to 6.8 cph; typical errors in measurements of the 32P activities are 20-30%.
The 32Si 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 of 32Si 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 32Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of 32Si (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 activity 32Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic 32Si 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 32Si specific activities.
The 32Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1-1.4) × 10-2 dpm 32Si/cm2, which is consistent with the expected fallout of cosmogenic 32Si. However, the 32Si 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 32Si 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 32Si dissolved from sinking particulates.

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