MONTANEZ, ISABEL, University of California-Davis, Davis, CA
ABSTRACT: Evolution of Strontium and Carbon Isotope Composition of Cambrian Oceans: Potential for Tectonic, Paleoceanographic, and Biogeochemical Events
Systematic variations in the Sr (87Sr/86Sr) and C (13C) isotopic composition of seawater, reconstructed from the isotopic composition of ancient carbonates, provide unprecedented quantitative constraints on crustal and surficial processes throughout Earth's history. In addition, these systematic isotope variations enhance chronostratigraphic resolution for intra- and interbasinal correlations in intervals plagued by a paucity of age-diagnostic biostratigraphic markers. This seminar will present a set of high-resolution seawater Sr and C isotope curves for the Cambrian recently developed by Montanez and colleagues (Montanez, Osleger, Banner, Mack, and Musgrove, GSA Today, May 2000). These curves are defined in continuous exposures of marine carbonates in the Great Basin and southern Canadian Rockies, and they are used to better constrain primary variations in ocean chemistry during this time period.
The Cambrian was a time of dramatic change in Earth systems with large-scale continental reorganization associated with the amalgamation of Gondwana and anomalously fast (>30 cm/yr) rotation (up to 90) and latitudinal drift of continents driven by an inertial interchange true polar wander event. Major changes in oceanic circulation, geochemistry, and primary productivity, as well as enhanced rates of continental weathering and organic carbon burial brought on by this tectonic forcing appear to be recorded in the Sr and C isotope seawater curves. The Sr curve documents a rapid rate of increase through this period that is comparable to that recorded by the late Cenozoic seawater Sr proxy record of uplift and attendant weathering of the Himalaya-Tibetan Plateau. The Cambrian rise in Sr values is interpreted to record Pan African-Brasiliano orogenesis, and reaches 87Sr/86Sr values that are higher than any other time in Earth history. Numerous superimposed smaller scale oscillations may constrain the timing of individual shorter term tectonic events. The C isotope curve for the same time interval reveals several previously unrecognized short-term fluctuations of up to 4o/oo. A sharp shift in d13 Ccarb and d13C kerogen values near the Early-Middle Cambrian boundary indicates major paleoceanographic and climatic change associated with a trilobite mass extinction event. Used in concert, this set of high-resolution 87Sr/86Sr and d13C records provide complimentary quantitative constraints for the chronology of Cambrian tectonic, paleoceanographic, paleoecologic, and biogeochemical events.
AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures