Master of Science
Analysis of low-Mg calcite brachiopod shells for δ18O and δ13C signatures provide quantitative independent geochemical data for paleoenvironmental interpretations. We analyzed 59 brachiopod shells from lower Silurian carbonate rocks of Anticosti Island, the Hudson Bay Lowlands and Niagara regions for spaciotemporal isotopic trends for the early Silurian. Screening tests using scanning electron microscopy, cathodoluminescence and trace element analysis demonstrate exceptional preservation, suggesting retention of primary isotopic values.
δ13C and δ18O values of shells range from 0.2‰ to 2.9‰ and -6.9‰ to -4.3‰ (VPDB), respectively. Temporally, increases in δ13C and δ18O values occurred in response to the Late Aeronian glaciation. Paleogeographically, δ18O values were not significantly different between regions while δ13C was tied to open ocean vs epeiric sea settings. We argue for ocean δ18O value of ~ -1.5‰ (VSMOW), producing paleotemperatures of 30 °C to 36 ⁰C, consistent with the proposition that ocean δ18O has remained constant over geologic time.
Summary for Lay Audience
During the Silurian Period (443–419 million years ago), Earth was vastly different compared to the modern day — the supercontinent Gondwana stretched from the South Pole to the equator and there was no major landmass in the temperate and polar zones of the northern hemisphere; atmospheric CO2 levels and related overall global temperature were much higher and than today. The Silurian had a relatively stable climate compared to the earlier Ordovician Period which underwent a drastic shift from a greenhouse climate to a major glaciation. During the Silurian, the greenhouse climate was punctuated by minor, regional glaciations, with three cooling episodes recognized within the first 13 million years of the Silurian. The objective of my study is to study early Silurian brachiopod fossil shells and the geochemical information their shells reveal about ancient climate. Brachiopods are marine shelled organisms that construct a shell out of the mineral calcite that appear superficially similar, but not related, to clams. Their calcitic shells lock in the C and O isotope ratios of the seawater in which they live. The isotopic values of fossil shells, therefore, can be used as a proxy to infer paleoenvironmental conditions, such as water temperature. This study was carried out in the following steps. 1) Early Silurian brachiopod shells were screened using techniques to ensure that they are pristinely preserved because alteration of the shell material can distort the isotopic signatures, leading to erroneous isotopic and interpretations; 2) Shells that passed the screening were sampled using micro-drilling procedures to produce powdered material, which was analyzed for isotopic composition for a spaciotemporal gradient of approximately 5 million years and from ~22° to 10° south of the palaeoequator; and 3) the oxygen isotopic data were used to reconstruct sea water temperature and other paleoenvironmental factors. The results of this study revealed that, during the early Silurian, stable O isotope values of contemporaneous shells did not vary significantly from higher to lower tropics, whereas C isotopes varied significantly, due to increased ocean connectivity. Temporally, however, two sub-million-year scale isotopic excursions were detected, corresponding to episodes of global oceanic cooling that punctuated a generally greenhouse climate.
Siewnarine, Jordan N., "Stable Carbon and Oxygen Isotopes of Early Silurian Pentameride Brachiopods from Québec, Ontario, and Nunavut, and Paleoenvironmental Implications" (2022). Electronic Thesis and Dissertation Repository. 8605.