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

This session is dedicated to studies that illuminate the biotic, geobiological, sedimentological, and geochemical contexts of the evolution of early animal life in the oceans, with a particular focus on the Cambrian Period.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

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

Paper number: 29-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

  • Nicole Mizrahi Presenting Author
    University of Colorado Boulder (Department of Geological Sciences), Boulder, CO
  • Qing Tang
    Nanjing University (School of Earth Sciences and Engineering), Nanjing
  • Talia Karim
    University of Colorado Museum of Natural History, Boulder, CO
  • Shuhai Xiao
    Virginia Tech (Department of Geosciences and Global Change Center), BLACKSBURG, VA
  • Marjorie Cantine
    University of Washington (Department of Earth and Space Sciences), Seattle, WA
  • Woodward Fischer
    California Institute of Technology (Division of Geological & Planetary Sciences), Pasadena, CA
  • Carl Simpson
    University of Colorado Boulder (Department of Geological Sciences), Boulder, CO
  • Lizzy Trower
    University of Colorado Boulder (Department of Geological Sciences), Boulder, CO

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 δ30SidSi 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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414216

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