AUTHOR=Grasse Patricia , Haynert Kristin , Doering Kristin , Geilert Sonja , Jones Janice L. , Brzezinski Mark A. , Frank Martin 

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

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.697400

DOI=10.3389/fmars.2021.697400

ISSN=2296-7745

ABSTRACT=<p>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 (δ<sup>30</sup>Si<sub><italic>dSi</italic></sub>) and biogenic silica (δ<sup>30</sup>Si<sub><italic>bSi</italic></sub>) 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 δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> data (together with δ<sup>30</sup>Si<sub><italic>dSi</italic></sub>) 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 δ<sup>30</sup>Si<sub><italic>dSi</italic></sub> signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ<sup>30</sup>Si) ranging from −0.4‰ to −1.0‰. In contrast, δ<sup>30</sup>Si<sub><italic>dSi</italic></sub> and δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ<sup>30</sup>Si<sub><italic>dSi</italic></sub>. A strong relationship between δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ<sup>30</sup>Si<sub><italic>bSi</italic></sub>. A comparison of δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> samples from the water column and from underlying core-top sediments (δ<sup>30</sup>Si<sub><italic>bSi_</italic></sub><sub><italic>sed.</italic></sub>) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ<sup>30</sup>Si<sub><italic>bSi_</italic></sub><sub><italic>sed.</italic></sub> = +0.9‰ to +1.7‰), although we observed small differences in δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> 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 δ<sup>30</sup>Si<sub><italic>bSi</italic></sub> data as a paleo proxy for the reconstruction of dSi utilization in the region.</p>