Rise of the Glass Animals (and Protozoans): Tracking the Paleozoic Transformation of the Silica Cycle by Biomineralizers

Phanerozoic oceans have witnessed the transformation of the marine silica cycle from a system primarily controlled by abiotic precipitation reactions to one in which silica biomineralization (by diatoms) is the largest silica sink. In theory, secular decreases of the concentration of dissolved silica ([dSi]) in seawater across this time should reflect the timing and magnitude of these changes. Prevailing hypotheses favor a 2-step decrease in [dSi], with an initial decrease coinciding with the appearance of siliceous sponges and radiolarians in the Cambrian and a second decrease resulting from the Cenozoic expansion of diatoms. Measurements of stable Si isotope ratios (δ30Si) in sponge spicules and other biogenic silica (bSi) fossils are emerging as a powerful new tool to test hypotheses about the trajectory of seawater [dSi] over Phanerozoic time. The Si isotopic fractionation between ambient dSi and sponge spicule bSi is strongly dependent on [dSi], while the analogous fractionation in diatom frustules and radiolarian tests are not, making their δ30Si values good proxies for seawater dSi δ30Si values. Isotopic offsets between δ30Si values of sponge spicules relative to coeval diatom frustules or radiolarian tests could therefore be used as a proxy for [dSi] in deep time. In this talk, I will show spicule and radiolarian δ30Si data analyzed in situ in rocks via secondary ion mass spectrometry representing three snapshots in time (Cambrian, Ordovician/Silurian, and Triassic/Jurassic). These δ30Si data suggest that seawater [dSi] has been relatively low for the past ca. 500 Myr, consistent with the idea that the first silica biomineralizers—siliceous sponges and radiolarians—were responsible for the bulk of the biological transformation of the marine silica cycle.

Speaker: Lizzy Trower (University of Colorado, Boulder)

Talks are held in the STScI John N. Bahcall Auditorium. Light lunch (provided) starts at 12pm; talk starts at 12:30pm.

Planets, Life, and the Universe Lecture Series presentations are also webcast live. Webcasts can be viewed at the STScI webcast site during the scheduled presentation, and can be found afterward in the STScI webcast archive.

STScI is located in the Muller Building on the Johns Hopkins University Homewood campus. View a JHU map and directions.