Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Hull, Pincelli

Abstract

Silicon is the second most abundant element in Earth’s crust and yet its seawater concentrations are far below saturation for any silicate mineral due to the activity of silicifiers, or organisms that make their shells, tests or spicules out of amorphous silica. On geologic timescales, the marine silica cycle is inextricably tied to the carbon cycle and global climate via the silicate weathering feedback and other pathways, and the concentration of silica in the ocean can vary the strength of these feedbacks. Silicifiers, primarily consisting of sponges, diatoms (phytoplankton) and radiolarians (zooplankton), are in turn affected by silica availability, and their morphology and abundance in the rock record can provide clues to silica cycle dynamics in the deep past. The goal of this dissertation is to explore the ecological and geochemical effects of pelagic silicifiers throughout the Phanerozoic, and to advance tools for interpreting their fossil record. In Chapter 2 I investigate radiolarian morphology and taxonomic assemblage across the Paleocene-Eocene Thermal Maximum (PETM), a hyperthermal event that likely produced a Si pulse to the global ocean from enhanced continental weathering. Radiolarian silicification—the amount of silica used in constructing a test—appears to decline over the Cenozoic, purportedly due to decreasing [Siseawater]. To see if morphological change could be observed on shorter timescales, I measured size in radiolarians from a New Zealand locality (Mead Stream) and analyzed taxonomic assemblage data from additional sites, focusing on silicification. I found little evidence for a uniform morphological response to the PETM. In Mead Stream radiolarians I observed an assemblage-level size increase coeval with the PETM, extending beyond its recovery. I investigate oligotrophy, seawater silica concentration, and redeposition as potential drivers. The rise of diatoms is one of the biggest shifts in Cenozoic marine ecosystems. In Chapter 3, I reconsider this apparent rise in light of secular changes in sedimentation rate and temperature through the Cenozoic. I compiled records of deep-sea sedimentation rates and bottom-water temperatures and used a diagenetic model to show that, due to these factors alone, biogenic silica preservation likely improved substantially toward the present. From this I generated a taphonomic null hypothesis of the diatom fossil record, which shows a late Cenozoic (~5-20 Ma) increase in the relative abundance of diatoms comparable to empirical records, suggesting the observed late Neogene ‘rise of diatoms’ may be driven by changing preservation potential. Futhermore, because the relationship of preservation to temperature and sedimentation rate is non-linear with regard to solubility, diatoms are disproportionately affected by the changes in preservation relative to radiolarians, skewing the timeline of an apparent transition from radiolarians to diatoms. These results suggest diatoms may have been geochemically and ecologically significant earlier than traditionally thought, and offer a way to reconcile their fossil record with Si isotope evidence of an earlier [Siseawater] drawdown. In Chapter 4 I consider the carbon cycle ramifications of these results. Mass balance dictates that if the proportion of biogenic silica buried as silica (opal-A) was less in the Paleogene than it is today then Si must have been removed from the ocean in some other form to maintain steady state. Authigenic clays are one of the few other forms in which the excess silica could have been buried. Unlike opal-A, however, their formation consumes bicarbonate and dissolved Si and releases CO2. Using a carbon-silica cycle box model and a Monte Carlo approach to account for input variation, I estimated the effect on atmospheric CO2 over the Cenozoic if the diffusive silica flux from Chapter 3 was incorporated into authigenic clays. The potential impact is substantial, capable in timing and magnitude of accounting for the steep decline in CO2 and temperature in the last 15 Myrs. This result adds another hypothesis to the ongoing debate over the drivers (and stabilizers) of Cenozoic cooling. In Chapter 5 I return to questions of radiolarian morphology, this time in the Paleozoic. Unlike Cenozoic radiolarians, which retain their transparent opal-A composition, Paleozoic radiolarians are recrystallized, rendering transmitted-light microscopy futile for measuring silicification. Using scanning electron microscopy I measured test size and wall thickness on exceptionally preserved Paleozoic radiolarians from five localities. When compared to Cenozoic data, the Paleozoic radiolarians examined thus far show a significantly higher average size and wall thickness but are all within the Cenozoic ranges. This work demonstrates it is possible to measure wall thickness on Paleozoic radiolarian fossils, opening the door to its application as a rough proxy for the relative timing of an early Paleozoic decline in seawater dissolved silica concentrations.

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