29-6 Si Isotope Analyses of Cambrian-Ordovician Sponge Spicules and Radiolarians Indicate Decreased dSi Concentrations in Early Paleozoic Oceans

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts (Posters)

Poster Booth No.: 193

Presenting Author:

Nicole Mizrahi

Authors:

Mizrahi, Nicole¹, Tang, Qing², Karim, Talia³, Xiao, Shuhai⁴, Cantine, Marjorie⁵, Fischer, Woodward W.⁶, Simpson, Carl⁷, Trower, Lizzy⁸

(1) University of Colorado Boulder (Department of Geological Sciences), Boulder, CO, USA, (2) Nanjing University (School of Earth Sciences and Engineering), Nanjing, China, (3) University of Colorado Museum of Natural History, Boulder, CO, USA, (4) Virginia Tech (Department of Geosciences and Global Change Center), BLACKSBURG, VA, USA, (5) University of Washington (Department of Earth and Space Sciences), Seattle, WA, USA, (6) California Institute of Technology (Division of Geological & Planetary Sciences), Pasadena, CA, USA, (7) University of Colorado Boulder (Department of Geological Sciences), Boulder, CO, USA; University of Colorado Museum of Natural History, Boulder, CO, USA, (8) University of Colorado Boulder (Department of Geological Sciences), Boulder, CO, USA,

Abstract:

The evolution of silica-biomineralizing organisms at the beginning of the Paleozoic Era 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 thought 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 of silica biomineralizers. It has long been hypothesized that the rise of the earliest silica biomineralizers (sponges and radiolarians) at the beginning of the Paleozoic Era initiated a slight decrease in [dSi], followed by a second much larger decrease to modern [dSi] brought on by the radiation of diatoms in the Cenozoic Era. However, our understanding of the pace of the ecological expansion of silica biomineralizers and the trajectory of [dSi] during this biological takeover remains limited.

Studies of modern silica biomineralizing organisms have shown that the Si isotope fractionation between sponge spicule bSi and ambient seawater dSi is strongly dependent on [dSi]. This concentration-dependent relationship is not observed in diatoms or radiolarians. Therefore, the Si isotope ratios (δ³⁰Si values) of diatoms and radiolarians can be used to infer δ³⁰Si values of past seawater dSi. Paired measurements of the δ³⁰Si values of coeval sponge spicules and either diatoms or radiolarians can then be used to constrain past seawater [dSi], because their difference (Δ³⁰Si) is sensitive to [dSi]. We present secondary ion mass spectrometry (SIMS) δ³⁰Si and δ¹⁸O analyses of Cambrian-Ordovician sponge spicules and radiolarians from South China and Laurentia. We observe a decrease in Δ³⁰Si which is consistent with a significant decrease in seawater [dSi] in the early Paleozoic Era. We integrate the δ³⁰Si record with a mathematical isotope mass balance model of the marine silica cycle tracking steady-state [dSi] and δ³⁰Si_dSi values of surface and deep ocean reservoirs for the simultaneous expansions of sponges and radiolarians. Our analysis determines that sponges and radiolarians transformed the marine silica cycle long before the Cenozoic radiation of diatoms.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10162

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