GEOSECS Pacific and Indian Ocean 32Si profiles

Somayajulu, B. L. K. ; Rengarajan, R. ; Lal, D. ; Craig, H. (1991) GEOSECS Pacific and Indian Ocean 32Si profiles Earth and Planetary Science Letters, 107 (1). pp. 197-216. ISSN 0012-821X

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Results of measurements of twelve 32Si vertical profiles, nine from the Pacific Ocean at latitudes 45° N-58°S, and three from the Indian Ocean between the Equator and 38°S are presented. The amounts of in-situ extracted SiO2 range from ~1 to 25 g. The volumes of water from which dissolved silicon was extracted range from 200 to 9×105 kg. The net 32P activities range from 0.7 to 3.8 cph. It is possible to measure accurately 32Si (32P) activities as low as 2×10−2 dpm from 25 g SiO2 with the present techniques. The32Si concentrations in water range from 0.1 dpm/106 kg seawater to 178 dpm/106 kg seawater. The overall pattern of 32Si increase with depth in the oceans resembles that of Si but the two differ appreciably; the enrichment of the former is controlled by its relatively short half-life. The 32Si/SiO2 ratios vary from ~1 in deeper waters ( > 1000 m) to 81.5 dpm/kg SiO2 in the surface waters. Three depth variation patterns are observed in the Pacific and Indian oceans: (i) a low ratio, ~10 dpm/kg SiO2 varying within a factor of two between the surface and ~5000 m depth; (ii) a monotonic increase from a high surface value ( > 10 dpm/kg SiO2) to a low value ( ~5 dpm/kg SiO2) at ~3000 m beyond which the value either remains constant or increases slightly with depth; and (iii) in half of the cases, the ratios increase with depth in the depth interval 1-3 km. The above patterns do not show any latitudinal variation. The column inventories of 32Si and Si in the north Pacific show a rather similar latitudinal variation, which is a coincidence. The column inventories of 32Si show a pronounced peak at ~35°N/S in the Pacific and Indian oceans that must be attributed to an increased tropospheric fallout in the spring injections in mid-latitudes. This is also expected in view of the short half-life of 32Si. A similar feature was observed in the south Atlantic Ocean. Now that the 32Si measurements are available for the three major oceans, the oceanic budget of 32Si can be estimated. There are ~ 920×1014 dpm of 32Si in the major oceans which corresponds to a global-average-production rate of ≥4.3×10−4 atoms 32Si/cm2 s. Furthermore, since the atmospheric fall out of 32Si has been determined, we can obtain a fairly accurate half-life of 32Si. The geochemical half-life is estimated to be ≥120 yrs, in good agreement with the best value of 140 yrs based on other recent estimates. The lower limit reflects on the present uncertainty in the size and lifetime of the transient silicon pool at the water-sediment interface. From the observed decrease in the water column-averaged 32Si/SiO2 ratios from the Circumpolar Atlantic waters to their Pacific counterparts, we deduce an apparent velocity of 55 km/yr for the Circumpolar current. Using a one-dimensional model, the vertical advection velocity (w) and turbulent diffusion coefficient (K) have been deduced using both the 32Si and 14C data. The values ofw range from 1.3 to 15 m/yr and ofK from 0.5 to 8 cm2/s, except for the Somali basin where we obtain:w=53m/yr and K=16cm2/s. The 32Si-based values are about 3 times higher than those based on 14C.

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