Factors controlling the silicon isotope composition of dissolved silicate and biogenic silica in the Peruvian Upwelling

Abstract:

The stable silicon (Si) isotope composition (δ³⁰Si) of dissolved silicic acid (Si(OH)₄) and of biogenic silica (bSiO₂) reflects changes in nutrient utilization and biogeochemical cycling in the present and past ocean. Here we present dissolved and biogenic Si isotope signatures at 9 shelf stations in the upwelling area off Peru obtained during RV Meteor cruise M93 in February 2013 in the frame of the German SFB 754 project to better understand the processes influencing the Si isotope distributions. Primary productivity in the upwelling area of Peru is dominated by diatoms, which preferentially incorporate the lighter ²⁸Si when building their frustules thus leaving seawater enriched in ³⁰Si. Previous studies have suggested that the fractionation factors accompanying this process are species dependent ranging from -0.5‰ to -2.1‰ and that the dissolved Si isotope signature off Peru is strongly influenced by water mass mixing resulting in less fractionated surface water signatures during strong upwelling. To disentangle these processes, information on the Si isotope composition of dissolved and particulate Si is compared with that of the corresponding diatom assemblages. Si isotope signatures of particulate samples are consistently lower than the surrounding seawater ranging from 1‰ to 2‰ (±0.2 2sd). The highest fractionation factors between bSiO₂ and Si(OH)₄ (Δ_{bSiO2-SI(OH)4}) in surface waters are found at the lowest Si(OH)₄ concentrations with an average fractionation factor of -0.9‰, which agrees well with previously reported values. Diatom species effects do not seem to exert a major influence on the isotope signature of the biogenic silica, which instead appears to be mainly controlled by fractionation during biological uptake and water mass mixing.