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James Gleason, University of Michigan (United States)
Debbie Thomas, University of Texas (United States)
Ted Moore, University of Michigan (United States)
Joel Blum, University of Michigan (United States)
Robert Owen, University of Michigan (United States)
Brian Haley, Institute for Marine Sciences (Germany)
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Nd-Sr radiogenic isotope analysis of well-preserved, early-middle Eocene ichthyoliths (fossil fish debris) at Lomonosov Ridge (IODP Arctic Coring Expedition 302) confirms that connections between the Arctic Basin and surrounding oceans were extremely limited from 55 to 45 Ma. Epsilon Neodymium of pre-cleaned ichthyoliths varies from -5.7 to -7.8, suggesting significant REE inputs from Arctic rivers draining the Siberian platform. Seawater exchange with neighboring oceans (e.g., North Pacific, Tethyan, North Atlantic) is not necessary to explain Nd isotope and REE data, although some exchange is permissible from these data.
Strontium isotope ratios of pre-cleaned ichthyoliths (87Sr/86Sr = 0.7079 to 0.7087) are consistently more radiogenic than Eocene seawater, indicating brackish or fresh water environments at the surface of the Arctic Ocean between 55 and 45 Ma.
Reduced organic-rich coatings on ichthyoliths (and bulk detritus) are more radiogenic (87Sr/86Sr > 0.7090) than pre-cleaned ichthyoliths, indicating different Sr sources for the organic component. 87Sr/86Sr variations may record changes in river inputs of Sr to the Eocene Arctic Basin, or possibly a shifting halocline, consistent with a dynamic Arctic hydrologic cycle superimposed on eustatic-tectonic controls of seawater exchange with the world ocean. Sr mixing calculations are permissible of salinities in the 5 to 20 per mil range, lower than estimates based on O isotopes from Eocene fish bone carbonate (16 to 26 per mil). Low salinity waters during the ∼55 Ma PETM do not correspond to large excursions in 87Sr/86Sr ratios, nor is there a shift in 87Sr/86Sr during the ∼48.5 Ma Azolla salinity minimum. 87Sr/86Sr and Nd isotope data reinforce evidence for low salinity surface waters of dominantly Asian continental provenance within the photic zone over much of the early-middle Eocene at this site, stabilizing a poorly ventilated, anoxic water column that saw only intermittent exchange with the world ocean before the first appearance of sea ice at ∼46 Ma.
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