Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (delta Si-30(dSi)) and biogenic silica (delta Si-30(bSi)) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More delta Si-30(bSi) data in today's ocean are required to validate and improve the understanding of paleo records. Here, we present the first delta Si-30(bSi) data (together with delta Si-30(dSi)) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The delta Si-30(dSi) signatures in the surface waters were higher (+1.7 parts per thousand to +3.0 parts per thousand) than delta Si-30(bSi) (+1.0 parts per thousand to +2 parts per thousand) with offsets between diatoms and seawater (Delta Si-30) ranging from -0.4 parts per thousand to -1.0 parts per thousand. In contrast, delta Si-30(dSi) and delta Si-30(bSi) signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in delta Si-30(dSi). A strong relationship between delta Si-30(bSi) and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling delta Si-30(bSi). A comparison of delta Si-30(bSi) samples from the water column and from underlying core-top sediments (delta Si-30(bSi_sed.)) in the central upwelling region off Peru (10 degrees S and 15 degrees S) showed good agreement (delta Si-30(bSi_sed.) = +0.9 parts per thousand to +1.7 parts per thousand), although we observed small differences in delta Si-30(bSi) depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using delta Si-30(bSi) data as a paleo proxy for the reconstruction of dSi utilization in the region.