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Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica-depleted Paleozoic oceans

Trower, Elizabeth J. and Strauss, Justin V. and Sperling, Erik A. and Fischer, Woodward W. (2021) Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica-depleted Paleozoic oceans. Geobiology, 19 (5). pp. 460-472. ISSN 1472-4677. doi:10.1111/gbi.12449.

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The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ³⁰Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ³⁰Si and δ¹⁸O of sponge spicules and radiolarians from Ordovician–Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

Item Type:Article
Related URLs:
URLURL TypeDescription code for each silica cycle model scenario analysis maps, SIMS data, and EMP maps
Trower, Elizabeth J.0000-0001-9898-5589
Strauss, Justin V.0000-0003-3298-3227
Sperling, Erik A.0000-0001-9590-371X
Fischer, Woodward W.0000-0002-8836-3054
Additional Information:© 2021 John Wiley & Sons Ltd. Issue Online: 07 August 2021; Version of Record online: 17 May 2021; Manuscript accepted: 26 April 2021; Manuscript received: 22 January 2021. The authors thank Yunbin Guan (Caltech), Aaron Bell (University of Colorado Boulder), and Eric Ellison (University of Colorado Boulder) for assistance with SIMS, EMP, and Raman microspectroscopy analyses, respectively, and Michael Melchin (St. Francis Xavier University) and Tiffani Fraser (Yukon Geological Survey) for assistance studying the Peel River section. The authors thank Johanna Marin-Carbonne and three anonymous reviewers for their constructive comments. E. J. T acknowledges support from the Agouron Institute Geobiology Postdoctoral Fellowship. J. V. S. and E. A. S. were supported by National Science Foundation (NSF) grants EAR-1624131 and EAR-1922966, respectively, along with additional support from the Yukon Geological Survey. W. W. F. was supported by the Simons Foundation Collaboration on the Origins of Life, the GPS Discovery Fund, and the Caltech Center for Evolutionary Sciences. Data Availability Statement: MATLAB code for each silica cycle model scenario is archived at: SIMS analysis maps, SIMS data, and EMP maps are archived at:
Funding AgencyGrant Number
Agouron InstituteUNSPECIFIED
Yukon Geological SurveyUNSPECIFIED
Simons FoundationUNSPECIFIED
Caltech Division of Geological and Planetary SciencesUNSPECIFIED
Caltech Center for Evolutionary SciencesUNSPECIFIED
Subject Keywords:Paleozoic; silica cycle; siliceous sponges; silicon isotopes
Issue or Number:5
Record Number:CaltechAUTHORS:20210519-141321546
Persistent URL:
Official Citation:Trower, EJ, Strauss, JV, Sperling, EA, Fischer, WW. Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica-depleted Paleozoic oceans. Geobiology. 2021; 19: 460–472.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109195
Deposited By: George Porter
Deposited On:24 May 2021 15:10
Last Modified:16 Aug 2021 17:54

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