Biological fractionation of silicon isotopes in Southern Ocean surface waters

The fractionation of silicon isotopes by diatoms during silicification (i.e., opaline cell wall formation) provides a new tool for paleoceanographic studies of the silicon cycle. Here we examine the natural variations of the 30Si:28Si ratio in silicic acid (Si (OH)4) and biogenic silica (bSiO2) in surface waters of the Antarctic Circumpolar Current (ACC) along 170°W. The results provide direct evidence of biologically mediated fractionation of silicon isotopes, with an enrichment factor (epsilon) of between -1.1 and -1.9‰, depending on the model employed. Comparison of the mass flux of bSiO2 captured in sediment traps deployed in the study area with the silicon isotopic composition of that material establishes a direct linkage between diatom dynamics in surface waters and the isotopic signature of exported particles. We calculated Si(OH)4:NO3 - utilization ratios from variations in silicon and nitrogen isotopic ratios in surface waters and sediment traps that agree well with direct observations of 4:1 in this region. This work supports the use of variations in silicon isotopes in sedimentary opal as a proxy for relative Si(OH)4 utilization in surface waters.