ABSTRACT: Neogene Strontium Isotope Chronostratigraphy and Geochemistry
David D. A. Hodell, Paul A. Mueller, Gregory A. Mead
The Neogene 87Sr/86Sr seawater curve is marked by a series of steep climbs and plateaus with important slope changes at 5.5 and 2.5 Ma, both of which mark the beginning of periods of steep increase in 87Sr/86Sr. Potential chronostratigraphic application is best during the latest Miocene (5.5 to 4.5 Ma) and late Pliocene-Pleistocene (2.5 Ma to present) when resolution ranges from approximately 0.25 to 0.5 Ma. Temporal resolution is poor during much of the late Miocene (10 to 5.5 Ma) and Pliocene (4.5 to 2.5 Ma). The stratigraphic application of the strontium isotope seawater curve will be demonstrated by examples from the Hawthorn Group (Miocene) of Florida, pecten-bearing tillites from Antarctica, the type section of the Miocene Pliocene boundary, and Pliocene-Pleistocene shallow-marine deposits from southwest Florida.
Using a nonequilibrium model for the modern Sr budget of the oceans, we calculated the residence time of strontium to be approximately 2.5 m.y., about half of previously accepted estimates of 4 to 5 m.y. Numerical modeling suggests increases in 87Sr/86Sr during the late Neogene could have resulted from an increase in riverine fluxes of strontium (up to 30%) and/or an increase in the average 87Sr/86Sr of river input (up to 0.0007). This implies increased chemical weathering of the continents and/or an increase in the proportion of silicate to mafic-carbonate weathering on land. During the late Neogene, the primary factors affecting chemical weathering rates were probably increased glacial activity (particularly at 2.5 Ma) and increased rates f tectonic uplift (particularly of the Himalayan-Tibetan region).
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990