VARIATIONS IN THE STRONTIUM ISOTOPIC RATIO OF SEAWATER DURING THE MIOCENE - STRATIGRAPHIC AND GEOCHEMICAL IMPLICATIONS

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining Sr-87/Sr-86 measurements of p lanktonic foraminifera from Deep Sea Drilling Project sites 289 and 58 8. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligoc ene to mid-Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the times cale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (Sr-87/Sr-86 values averaged 0.70824) which is coincident with a carbon isotopic maximum ( CM-O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in Sr-87/Sr-86 that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19 .0 and 16 Ma. Beginning at approximately 16 Ma (between carbon isotopi c maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of Sr-87/ Sr-86 growth slowed and Sr-87/Sr-86 values were near constant from 15 to 13 Ma. After 13 Ma, growth in Sr-87/Sr-86 resumed and continued unt il approximately 9 Ma, when the rate of Sr-87/Sr-86 growth decreased t o zero once again. The entire Miocene seawater curve can be described by a high-order function, and the first derivative (dSr-87/Sr-86/dt) o f this function reveals two periods of increased slope. The greatest r ate of Sr-87/Sr-86 change occurred during the early Miocene between ap proximately 20 and 16 Ma, and a smaller, but distinct, period of incre ased slope also occurred during the late Miocene between approximately 12 and 9 Ma. These periods of steepened slope coincide with major pha ses of uplift and denudation of the Himalayan-Tibetan Plateau region, supporting previous interpretations that the primary control on seawat er Sr-87/Sr-86 during the Miocene was related to the collision of Indi a and Asia. The rapid increase in Sr-87/Sr-86 values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and d issolved riverine fluxes to the oceans. In the absence of another sour ce of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling throug h a reversed greenhouse effect. The paleoclimatic record, however, ind icates a warming trend during the early Miocene, culminating in a clim atic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum we re caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the remova l of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation b etween approximately 14.5 and 13 Ma. The CRFB hypothesis helps to expl ain the significant time lag between the onset of increased rates of o rganic carbon burial in the Monterey at 17.5 Ma (as marked by increase d deltaC-13 values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in deltaO-18 values), which did not begin until approximately 14.5 Ma.