Are Diatoms the Main Biogenic Silica Substrate for Authigenic Products in Marine Deltaic Sediments?

Silicon isotopes (δ³⁰Si) in sediment biogenic silica (bSi) are widely used as a paleoproxy for marine silicic acid usage by pelagic diatoms. However, there is a lack of understanding regarding signals from other reactive Si pools in sediments. In marine deltaic systems, this oversight misses valuable information on early diagenetic products and biases sedimentary bSi δ³⁰Si to only represent the minority fractions which have not been altered by diagenesis. In Mississippi River plume sediments, we quantified δ³⁰Si values for operationally defined reactive Si pools using a multi-step digestion approach and traditional alkaline-only digestion. bSi δ³⁰Si consistently showed differences between those derived from the alkaline-only vs. multi-step digestion protocols, with the latter being isotopically heavier. Using this offset and the mass differences per gram of sediment between these pools, we can define the δ³⁰Si for bSi which has been modified by authigenesis. Overall, authigenically altered bSi was isotopically heavier (2.14 ‰) than unaltered bSi (1.77 ‰). While recent publications have demonstrated the important contributions of sponge spicules and Rhizaria tests for sediment bSi pools (so called “dark bSi”), the isotopically heavy altered bSi suggests that exported water-column diatoms are the most likely substrate for formation of authigenic products in these sediments; this independently supports electron microscopy data from previous regional studies showing sediment diatom frustule microarchitecture develops mineral coatings. These data demonstrate both a direct (burial of frustules) and indirect (precipitation substrate for authigenic projects) role for diatoms in sequestering elements within coastal deltaic systems.