Graduate Thesis Or Dissertation
 

Exploring the Magnetic Properties of Specific Grain-Sizes to Understand Marine Depositional Systems: A Case Study from the Eirik Ridge South of Greenland over the Last Glacial Cycle

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/xs55mk24x

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  • Disentangling sediment source from sediment transport is a fundamental marine geologic challenge critical to the interpretation of any sedimentary record. The Eirik Ridge, a sediment drift south of Greenland, receives terrigenous sediment primarily from subglacial erosion of south Greenland’s Precambrian bedrock and Paleogene volcanics that outcrop in east Greenland and Iceland. These sediments are transported to the Eirik Ridge by the Deep Western Boundary Current (DWBC), a deep-water system that in general deepens over the drift during interglacial periods and shoals during glacial periods. Prior work on terrestrial sediments show that sources can be discriminated using magnetic and geochemical properties, providing constraints on the origin and as a result glacial processes within the region. Coarse-grained, near-stochiometric magnetites are associated with sediments sourced from Greenland, while fine-grained titanomagnetites are indicative of sediments from Iceland and the Nordic Basalt Province (NBP) with the magnetic properties of the silt fraction distinct between the two sources. To explore this further and the differences between bulk and grain-size specific magnetic properties, samples were taken from two sites drilled on the Eirik Ridge during Integrated Ocean Drilling Program (IODP) Expedition 303. Site U1305 (3,459 meters water depth (mwd)) contains expanded sediment sections when the DWBC is in a deep-interglacial mode, while Site U1306 (2,273 mwd) captures expanded sediments sections when the DWBC is shoaled, a dominantly glacial configuration. Chronology for both cores were updated using previously published radiocarbon dates, oxygen isotope records, and relative geomagnetic paleointensity. Both cores were sampled at ~1 kyr resolution over the last ~150 kyrs. Bulk sediment samples were separated into clay, three silt-size fractions, and a sand fraction to characterize the variability in sediment texture and their magnetic properties. Magnetic properties at both sites are strongly particle size dependent with ferrimagnetic minerals concentrated within the silt-size fractions, resulting in a bulk magnetic record largely dominated by silts. Hysteresis parameters and low temperature magnetic susceptibility profiles suggest that Greenland and Iceland/NBP are important sediment sources to the drift at different times. In Site U1305, the sGIS magnetic signature, captured by coarse-grained magnetite and indicated by the hysteresis values in the silt fraction to dominate the sediment during marine isotope stage (MIS) 5e, however, ceases to be a significant component later in MIS 5. Coeval with the sGIS signal disappearance at Site U1305 (at the end of MIS 5e) is a shift in silt deposition, indicating a change in how sediments are delivered to the site and suggesting a re-organization of the DWBC. In contrast, sediments at Site U1306 are condensed and likely winnowed during peak MIS 5e as a result of deepening of the DWBC. When sedimentation resumes, after MIS 5e, coarse-grained magnetite with a Greenlandic signature is detected suggesting continued deterioration of the sGIS after MIS 5e that is not detected at U1305. The absence of an sGIS signal during MIS 5a at both sites suggests prolonged re-growth of the sGIS without substantial ablation during MIS 5a, prior to return of ablation signatures at both sites during MIS 3 and a presumably larger sGIS. This work demonstrates that multiple sites, along an Eirik Ridge depth-transect provide a more complete record of source variations and inferred changes in sGIS subglacial erosion than obtainable from any single site.
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  • National Science Foundation
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  • Ongoing Research
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  • 2020-01-04 to 2021-02-04

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