Scherer, Reed P.
Dodd, Justin P.
Ph.D. (Doctor of Philosophy)
Department of Earth, Atmosphere and Environment
This dissertation covers three major transitions in Earth's climate system throughout theCenozoic (the past 65 million years of Earth's history). The progressive breakup of the super continent Pangea and the ultimate change in ocean circulation (and redistribution of heat) as a result of tectonic events dating back to the Mesozoic, primed Earth's climate system for cooling and ultimately bipolar glaciation. Climate transitions in three distinct environments were studied using absolute diatom abundance, diatom assemblages, and geochemical tracers of diatoms (i.e., biogenic silica weight%). Two geographic regions are the focus of this dissertation work. Region one is the Ross Embayment of West Antarctica where a range of subglacial environments were studied. The material analyzed was collected as part of the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project, and decades of archived Caltech core samples. Fossils and biomarkers from these subglacial sites of Paleogene age, provide constraints for a paleotopography model, a marine and terrestrial history of the West Antarctic basin, and highlight the wealth of information available for study in these sediment archives. Paleotopography models are built using observations from decades of research spanning a wide variety of disciplines focused on understanding the Eocene-Oligocene Transition. This transition is inferred from a 1‰ shift in oxygen isotope values, that suggests either an increase in ice volume or significant cooling for this interval.
Region two is the Indian-Atlantic Ocean Gateway south of Africa, where the two siteswere studied were collected as part of the International Ocean Discovery Program (IODP) Expedition 361, which was tasked with collecting records of the last 5 million years of Agulhas Current (major western boundary current) history. Research from the first site is focused on the Agulhas Plateau Site U1475, which seeks to understand the controls on diatom deposition in this region between 3.4 and 2.7 million years ago (Ma). This interval includes the Mid-Piacenzian Warm Period (3.2 to 3 Ma), when CO2 in the atmosphere was ~400 parts per million and temperatures were 2 to 3°C warmer than today. Additionally, this interval includes three pronounced cooling events of the late Pliocene (i.e., the M2 glacial event [~3.3 Ma] the KM2 glacial event [3.15 Ma], and the intensification of Northern Hemisphere Glaciation [~2.7 Ma]) that highlight tectonic and circulation changes. The second site in Region 2 is Cape Basin Site U1479, which seeks to understand the controls of diatom deposition in the region over the Mid-Pleistocene Transition. With a specific focus of testing inferred circulation models and evidence of frontal migration in the Indian-Atlantic Ocean Gateway.
The subglacial sites demonstrate that portions of West Antarctica were below sea-levelduring the Eocene-Oligocene Transition, which provides useful constraints for the reconstruction of the past configuration of West Antarctica. Additionally, till samples provide a wealth of paleoecological data spanning the Cenozoic history for both terrestrial and marine realms. Diatom data from U1475, infers productivity in the Southern Ocean, ocean frontal migrations, and intermediate water connections to the site over the interval studied from the late Pliocene. Additionally, it highlights the role sea ice extent and tectonic gateway closure has influenced circulation in the Southern Ocean and Atlantic Meridional Overturning Circulation. Observations, from U1479 (Cape Basin) revealed multiple controls on diatom deposition. Sea-level, atmospheric dust fluxes, and southern sourced water expansion all played a large role in the diatom productivity between 900 and 725 kyr at this site. This confirms conceptual models that suggest ocean fronts were north of their modern positions during this interval of weak overturning and sea-ice expansion. Observations at this site also suggest an inferred divide in which Leg 177, U1479, and 1087 experience an expanded Southern Ocean, while at U1475 fronts were south and stationary suggesting a contraction of the Southern Ocean and its fronts.
Each record provides insights into Cenozoic cooling events and highlights the potentialfor diatom assemblage and accumulation data for palaeoceanographic research.
Supplemental _les have been uploaded with this dissertation. The first supplementalfile was created with Microsoft Word and is the supporting information for the Coenen et al. (2020) manuscript recreated with permissions from John Wiley & Sons, Incorporated. The supporting information is made up of Text S1 to S7, Figures S5.1 to S6.1, and Tables S1.1 to S3.1. Details about the age constraints for the diatoms are discussed in Text S1, a table of total age ranges is presented Table S1.1 with their respective references Table S1.2, and distribution of diatoms from the Ross Embayment is presented in Table S3.1. A discussion about how we can use microfossils in tills is presented in Text S2. A more specific explanation of materials and methods used for this study are shown in Text S3. A more detailed discussion of biomarkers used for this study is presented in Text S4. Plates of characteristic diatoms, calcareous nannoplankton, and palynomorphs are presented in Text S5. A brief comparison between the original Eocene-Oligocene Boundary Paleotopography and the most recent model is presented in Text S6. A brief discussion on the utilization of palynomorphs is presented in Text S7.
The second supplemental file was created with Microsoft Excel and is the data for Chapters3 and 4 of this dissertation. Sheet one of the Excel Workbook is the raw data for counts/absolute diatom abundance (ADA) calculations for Site U1479. Sheet two of the workbook is a data table of the preservation index for Site U1479. Sheet three is the relative abundances of the groups defined by their paleoecology, and sheet four is the abundance counts of individual species and the calculated percentage that species represents of the total counts for Site U1479. Sheet five of the workbook is the raw data for counts/absolute diatom abundance (ADA) calculations for Site U1475. Sheet six of the workbook is a data table of the preservation index for Site U1475. Sheet seven is the relative abundances of the groups defined by their paleoecology, and sheet eight is the abundance counts of individual species and the calculated percentage that species represents of the total counts for Site U1475.
Coenen, Jason James, "Diatoms and their Response to Tectonic Gateway Changes: Case Studies from Southern Hemisphere Sites" (2021). Graduate Research Theses & Dissertations. 6938.
Northern Illinois University
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