ABSTRACT

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

Collaborative Research: Direct Re-Os dating of Ordovician graptolite biozones:
Refining global correlations and Earth Time

Judith Hannah and J. Maletz
Colorado State University
EAR-0844213

Abstract
Absolute ages for the deposition of sedimentary rocks can be determined by radiometric dating of organic material in carbon-rich shales, using the decay of rhenium (Re) to osmium (Os). In particular, fossils of organic-walled graptolites are commonly found in sufficient abundance for direct dating by Re-Os. Historically, correlation of sedimentary units across broad geographic space has relied on comparison of fossil assemblages to establish relative age (biostratigraphy). Most ages based on radiometric dating could only bracket the time of deposition by dating older or younger units. PIs propose that Re-Os dating of graptolites will provide accurate and precise ages at a resolution equal to biostratigraphy, but with the untapped advantage of pinning absolute time.

Absolute ages for fossils in the geologic record will fix the time of geologic stage boundaries and graptolite biozones. In addition, increased resolution in the time scale greatly improves the definition of rates for a host of processes, including sedimentation, sea-level change, biologic evolution, and vertical and horizontal motion of lithospheric plates. PIs have assembled a diverse, cross-disciplinary team with expertise in Re-Os geochronology, sedimentology and stratigraphy of mud-rich systems, graptolite taxonomy and paleoecology, and tectonics. PIs will place time pins in the graptolite-rich Early and Middle Ordovician sedimentary sequence in western Newfoundland to provide solid age control in a critical interval of the Ordovician system with few currently available absolute ages. Moreover, these same graptolite assemblages are found on several other continents, opening the potential for defining temporal variations in the rate of biologic evolution in different geographic realms.

Further development of this geochronologic tool and demonstration of its applicability under varied conditions is of immediate interest to the petroleum industry. Re-Os geochemistry shows promise as a tracer to link migrated oil with its source rocks, and simultaneously define the time of migration - information highly sought by the industry to refine basin-scale models.

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