Dated Marine Carbonates Aid in Reconstructions of Deep- Time Climate Change

Troy Rasbury¹, W. Bruce Ward², N. Gary Hemming³, Isabel P. Montanez⁴, and G. Michael Grammer^5
1Geosciences, Stony Brook University, Stony Brook, NY
²Earthworks LLC, Sandy Hook, CT
³SEES, Queens College CUNY, Flushing, NY
⁴Geology, UC Davis, Davis, CA
⁵Geosciences, University of Western Michigan, Kalamazoo, MI

U-Pb dating of carbonates remains one of the only direct methods of dating sedimentary strata with minerals that formed in situ and approximately syndepositionally. Carbonates also archive proxies of ocean-atmospheric conditions. New ages from macroscopic calcite-replaced aragonite marine cements with good preservation of primary textures and compositions continue to demonstrate that marine cement ages can date stratigraphy, and provide Sr chemostratigraphy. The relative timing of these cements may be constrained by careful cement stratigraphy. Although all the Paleozoic samples analyzed have undergone diagenesis, the best preserved cements appear to have retained U quantitatively based on stratigraphically reasonable ages and uniform U concentrations within samples. There is a trend of decreasing U from middle Pennsylvanian values of 5.5 ppm to upper Permian values of 4.1 ppm. Calcite-replaced NeoProterozoic aragonite fan cements contain ~2.5 ppm U, while unaltered lower Holocene marine aragonite cements contain 6.4 ppm U, and concentrations of 4 ppm have been reported for Recent aragonite ooids.

Could these values reflect a global evolution of U in seawater or local variations? U is primarily soluble in the oxidized state. Its residence time is controlled by precipitation of authigenic minerals, by interaction with ocean floor materials including basalts, and by interaction with organic matter in suboxic to anoxic sediments. Work is needed on Neogene samples to understand local versus global controls on U concentrations in carbonates and the relationship to ocean chemistry. With this insight, stratigraphically detailed sampling of U in ancient marine cements may allow the reconstruction of changes in oxygen levels that can be linked to other aspects of ocean chemistry recorded in the same carbonates to give a holistic picture of global change and its drivers.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas