Evidence for Low Dissolved Silica in mid-Mesozoic Oceans

The geologic history of dissolved silica concentration in the ocean (DSi) is central to understanding the evolution of silica biomineralization, the interactions between the global carbon and silicon cycles, and their combined role controlling global climate over geologic time. However, the silica cycle in the geologic past is under-constrained, especially during major mass extinction events that impacted biosilicifiers and were associated with dramatic climate change. We measured the silicon isotope ratios (δ³⁰Si) of 76 sponge spicules from the Panthalassic Ocean spanning the Triassic–Jurassic boundary (ca. 201 Ma) to constrain DSi concentrations during the mid-Mesozoic. Spicule measurements have mean δ³⁰Si values of –0.25‰ ± 0.99‰. Our data, combined with constraints on seawater δ³⁰Si from coeval radiolarians, suggest that mid-Mesozoic DSi was between 20–100 µM, a similar range to the modern ocean. Our results support increasing evidence that by the Mesozoic DSi had already decreased by orders of magnitude relative to the Precambrian. These results imply that radiolarians and sponges were drawing down DSi prior to diatom ecological dominance. Increasing sponge δ³⁰Si values across the Triassic–Jurassic boundary, coupled with modeling evidence and previous palaeoecological observations, support that warming, increased weathering, and Si delivery before the end-Triassic extinction may have facilitated sponge expansion during the extinction recovery interval.