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 (δ³⁰Si_dSi) and biogenic silica (δ³⁰Si_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 δ³⁰Si_bSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ³⁰Si_bSi data (together with δ³⁰Si_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 δ³⁰Si_dSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ³⁰Si_bSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ³⁰Si) ranging from −0.4‰ to −1.0‰. In contrast, δ³⁰Si_dSi and δ³⁰Si_bSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ³⁰Si_dSi. A strong relationship between δ³⁰Si_bSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ³⁰Si_bSi. A comparison of δ³⁰Si_bSi samples from the water column and from underlying core-top sediments (δ³⁰Si_{bSi_sed.}) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ³⁰Si_{bSi_sed.} = +0.9‰ to +1.7‰), although we observed small differences in δ³⁰Si_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 δ³⁰Si_bSi data as a paleo proxy for the reconstruction of dSi utilization in the region.