Variations in the strontium isotopic ratio of seawater during the Miocene: Stratigraphic and geochemical implications

Abstract

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87 Sr/86 Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid‐Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87 Sr/86 Sr 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 87 Sr/86 Sr 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 ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of 87 Sr/86 Sr growth slowed and 87 Sr/86 Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87 Sr/86 Sr resumed and continued until ∼9 Ma, when the rate of 87 Sr/86 Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high‐order function, and the first derivative (d 87 Sr/86 Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87 Sr/86 Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan‐Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87 Sr/86 Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87 Sr/86 Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO 2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO 2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO 2 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 removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ 13 C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ 18 O values), which did not begin until ∼14.5 Ma.