GEOSECS Pacific and Indian Ocean 32 Si profiles

• Main Authors: Somayajulu, B. L. K., Rengarajan, R., Lal, D., Craig, H.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier Science 1991
• Subjects: QE Geology; Indian; Pacific; South Atlantic Ocean
• Online Access: http://repository.ias.ac.in/49769/; http://www.sciencedirect.com/science/article/pii/0012821X9190055M

Results of measurements of twelve 32 Si 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 SiO 2 range from ~1 to 25 g. The volumes of water from which dissolved silicon was extracted range from 200 to 9×10 5 kg. The net 32 P activities range from 0.7 to 3.8 cph. It is possible to measure accurately 32 Si ( 32 P) activities as low as 2×10 −2 dpm from 25 g SiO 2 with the present techniques. The 32 Si concentrations in water range from 0.1 dpm/10 6 kg seawater to 178 dpm/10 6 kg seawater. The overall pattern of 32 Si 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 32 Si/SiO 2 ratios vary from ~1 in deeper waters ( > 1000 m) to 81.5 dpm/kg SiO 2 in the surface waters. Three depth variation patterns are observed in the Pacific and Indian oceans: (i) a low ratio, ~10 dpm/kg SiO 2 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 SiO 2 ) to a low value ( ~5 dpm/kg SiO 2 ) 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 32 Si and Si in the north Pacific show a rather similar latitudinal variation, which is a coincidence. The column inventories of 32 Si 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 32 Si. A similar feature was observed in the south Atlantic Ocean. Now that the 32 Si measurements are available for the three major oceans, the oceanic budget of 32 Si can be estimated. There are ~ 920×10 14 dpm of 32 Si in the major oceans which corresponds to a global-average-production rate of ≥4.3×10 −4 atoms 32 Si/cm 2 s. Furthermore, since the atmospheric fall out of 32 Si has been determined, we can obtain a fairly accurate half-life of 32 Si. 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 32 Si/SiO 2 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 32 Si and 14 C data. The values ofw range from 1.3 to 15 m/yr and ofK from 0.5 to 8 cm 2 /s, except for the Somali basin where we obtain:w=53m/yr and K=16cm 2 /s. The 32 Si-based values are about 3 times higher than those based on 14 C.