SILICON ISOTOPE ANALYSES OF EARLY PALEOZOIC SILICEOUS MICROFOSSILS INDICATE THAT SPONGES TRANSFORMED THE MARINE SILICA CYCLE DURING THE CAMBRIAN

The Ediacaran-Cambrian Transition is a dynamic interval in Earth’s history marked by extraordinary global geobiological change, notably including the radiation of complex life. The evolution of silica-biomineralizing organisms during this time led to a transformation from the abiotically controlled silica cycle of the Precambrian to the biologically dominated cycle in our oceans today. The concentration of dissolved silica ([dSi]) of Precambrian seawater is hypothesized to have been much higher than modern values (up to 2.2 mM), because it was primarily controlled by the solubilities of amorphous silica and clay minerals rather than by Si uptake by silica biomineralizers. The appearance and expansion of the first silica biomineralizers (sponges and radiolarians) is hypothesized to have drawn down [dSi] below the solubility of amorphous silica, with a secondcrease to modern [dSi] brought on by the Cenozoic radiation of diatoms. But when and how quickly did the bulk of this biological takeover occur, and which organisms were responsible?

To answer this long-standing geobiological question, we are developing a SIMS silicon isotope (δ³⁰Si) record spanning the early Paleozoic Era. We measured δ³⁰Si values of in situ and epoxy mounted Cambrian and Ordovician siliceous spicules and radiolarians from the Niutitang and Qingxi Formations of South China, Bonanza King Formation of the southern Great Basin, USA, and the Cow Head Group of Western Newfoundland, Canada via secondary isotope mass spectrometry (SIMS). We observe low spicule δ³⁰Si values (averaging -1.01 to -0.89‰) in the early Cambrian followed by a ~1‰ increase in spicule δ³⁰Si over the next ~30 Myr, indicating that silica biomineralization emerged and gained ecological significance in the Cambrian, not the Neoproterozoic. Furthermore, the spicule δ³⁰Si data support a significant decrease in seawater [dSi] at the beginning of the Paleozoic Era, significantly earlier and of greater magnitude than previously hypothesized. Our results indicate that sponges and radiolarians transformed the marine silica cycle long before the Cenozoic radiation of diatoms.