Sources and biological fractionation of Silicon isotopes in the Eastern Equatorial Pacific

Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4 degrees N and 3 degrees S latitude at 110 degrees W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of a estimated by applying a steady-state isotope fractionation model to data from all stations between 4 degrees N and 3 degrees S was -0.77 +/- 0.12 parts per thousand (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of a was -1.08 +/- 0.27 parts per thousand (std. err.) similar to the value established in culture studies (-1.1 parts per thousand). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of a was significantly more positive, -0.61 +/- 10.16 parts per thousand (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of -1.1 parts per thousand for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of a can only be constrained to be between -0.6 and -1.1 parts per thousand. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant delta Si-30 of +1.32 +/- 0.05 parts per thousand. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9-1.0 parts per thousand) and it is slightly less positive than new measures of the delta Si-30 of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)(4) > 180 mu M, delta Si-30 = +1.46 +/- 10.12 parts per thousand (SD, n = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing delta Si-30(OH)(4) with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water delta Si-30(OH)(4) within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea. (c) 2008 Elsevier Ltd. All rights reserved.