Multi-proxy characterization of ACEX subunit 1/5 (the “zebra” interval) to better understand sediment deposition at this critical age boundary and paleoceanographic transition

• Main Author: Hojnacki, Victoria
• Other Authors: Kristen St. John
• Format: Thesis
• Language: unknown
• Published: JMU Scholarly Commons 2019
• Subjects: geology; paleoclimate; paleoceanography; Arctic; Lomonosov Ridge
• Online Access: https://commons.lib.jmu.edu/honors201019/681; https://commons.lib.jmu.edu/context/honors201019/article/1739/viewcontent/hojnacvm.pdf

Sediment cores recovered from the Lomonosov Ridge on IODP Expedition 302, the Arctic Coring Expedition (ACEX), provided the first major insights into long-term Cenozoic history of climate and ocean conditions in the central Arctic. However, the ACEX record is hampered by a major hiatus or severely condensed interval (depending on age-model interpretations) at 198.7 mcd separating the middle Eocene and Miocene records. Lithologic subunit 1/5 lies above this depth horizon, and is informally called the “zebra interval” because of distinctive stripes - black and gray tilted and cross-banded silty-clay layers, up to 3 cm thick that characterize the lower ~2.5 m of the subunit. Prior studies provide micropaleontological evidence for a brackish, shallow water depositional environment below and across the hiatus, continuing into subunit 1/5; and the cross-banding suggests a high-energy shallow water setting conducive to reworking and erosion. In addition, prior studies place the hiatus and subunit 1/5 within an overall up-section transition from warmer to cooler temperatures with more persistent ice-rafting, and from restricted to more open water circulation. Given the paleoclimatic and paleoceanographic differences above and below this interval and its unique lithologic pattern, this study aimed to better characterize the sedimentology of the black and gray couplets. Twenty-four samples from the black and gray layers and eleven samples from above and below the “zebra interval” were analyzed to characterize the XRF elemental composition, XRD bulk and clay mineral composition, grain size, and coarse sand composition and abundance. Overall, the zebra interval had less Fe and more variability in grain size than the stratigraphic intervals above and below it. In addition, the zebra interval showed upcore shifts from smectite to illite-dominated clays and greater abundance of terrigenous coarse sand. The sand-sized grains are quartz-dominated, showing very little change in composition throughout all three of the lithologic ...