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Deciphering the Late Jurassic paleoenvironment through Re-Os isotope geochemistry of the Agardhfjellet Formation, Svalbard




Connors, Marisa Leigh, author
Hannah, Judith, advisor
Stein, Holly, advisor
Borch, Thomas, committee member
Hammer, Øyvind, committee member

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Accurate interpretations of environmental change throughout Earth's history rely on robust correlations of sedimentary systems. The Late Jurassic has been difficult to correlate regionally and globally due to sparse radiometric ages and lack of cosmopolitan fossils. The rhenium-osmium (Re-Os) geochronometer provides an excellent platform to approach this problem. Re-Os geochemistry provides a way to directly date organic-rich shales which can then be used to: 1) place numerical ages on boreal fossil zones and Geologic Time Scale stage boundaries, 2) correlate with regional or global units, and 3) enhance the understanding of oceanic anoxic events (OAEs) and climactic shifts when paired with additional chemical and lithological data. The Agardhfjellet Formation of Svalbard, Norway has multiple intervals of organic-rich mudrocks which are ideal for Re-Os geochemistry. Presented here are Re-Os ages which confirm placement of the Agardhfjellet Formation within the Late Jurassic to Early Cretaceous (157.9 ± 2.9 Ma to 141 ± 20 Ma). We provide an age immediately above the Kimmeridgian-Oxfordian boundary, within the Amoebocera subkitchini zone, at 153.2 ± 5.0 Ma in agreement with previous work. Furthermore we present evidence that: (1) the Agardhfjellet Formation was deposited in fluctuating anoxia conditions; (2) increasing initial 187Os/188Os (0.401±0.007 to 0.577±0.054) coupled with a decrease in δ13Corg (-25.26‰ to -29.63‰) through the Late Jurassic signifies a changing climate represented by an increase continental weathering with a warming climate and/or an increase in continental freeboard.


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