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Date of Graduation
Bachelor of Science (BS)
Department of Geology and Environmental Science
Kristen St. John
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 sub-units. Quartz-grains are also generally well-rounded. Differences between the gray and black layers within the zebra interval were limited; gray bands tended to have greater abundances of quartz and muscovite, slightly higher Ti abundances, and lower abundances of Fe, Co, Ni, K, P, As, Sr, and Mn than the black layers.
The sedimentological observations are perplexing. Rounded quartz suggests reworking in a shallow water setting that has wave or tide action, so this, along with the tilted and cross-banding support the micropaleontological interpretation of an estuarine depositional setting for the zebra interval. The compositional differences (abundance of quartz and muscovite) may indicate semi-regular differences in transport energy or provenance that could relate to variations in input due to seasonal changes, storms or tides. However, the duration of a black-gray couplet is unknown, making it difficult to infer a specific depositional control. Complicating this further is the lack of graded bedding or other distinct changes in grain size between the gray and black layers, which would be expected if shallow water currents produced the tilted and cross-bedded layers. However, these inconsistencies may be artifacts of the sample processing – the samples were semi-lithified and therefore difficult to disaggregate. Additional grain size work that does not require disaggregation should be conducted. The dominance of the clay mineral smectite in subunit 1/5 suggests transport from the Kara and/or Laptev Sea regions.
An alternative interpretation for the origin of the black and gray layers is that the color changes could be the result of post-depositional semi-regular changes in oxidation of the bottom waters. In an estuary this might occur due to seasonal overturn, and could be sea-ice influenced However, this is difficult to reconcile with the tilted and cross-bedded nature of the black and gray layers.
At a broader scale, the distinct shift in clay mineralogy and drop in sand abundance from subunit 1/5 (“zebra interval”) to overlying subunit 1/4 may indicate a change in circulation from a strong transpolar drift current to an increase in terrigenous transport to the central Arctic by the Beaufort Gyre. This puzzling time of paleoceanographic and paleoclimatic transition will be investigated further in upcoming IODP Expedition 377.
Hojnacki, Victoria, "Multi-proxy characterization of ACEX subunit 1/5 (the “zebra” interval) to better understand sediment deposition at this critical age boundary and paleoceanographic transition" (2019). Senior Honors Projects, 2010-2019. 681.