Changes in Seawater 87Sr/86Sr Curve during Early Miocene to Early Late Miocene: Implications of ODP Site 758A Record

A seawater ⁸⁷Sr/⁸⁶Sr record is generated for early Miocene to early late Miocene based on measurements of 114 samples of cleaned planktonic foraminifera from Ocean Drilling Program site 758 (Hole 758A) in the eastern Indian Ocean. Age model for this Site is based on nannofossil events and interpolated numerical ages are compared with Sr Isotopically-Derived ages, There is a fairly good agreement between the two stratigraphic tools (<±1.5 Ma). ⁸⁷Sr/⁸⁶Sr ratio during ∼23.7 Ma to ∼8.1 Ma increased from 0.70828 to 0.70895, with variations in the rate of ⁸⁷Sr/⁸⁶Sr increase. Seventh-Order polynomial fit curve shows the ⁸⁷Sr/⁸⁶Sr increase rates of 44 ×10^-6m .y.^-1 for -23.7 to ∼19.2 Ma, 41 × 10^-6 m.y.^-1 for -18.2 to - 10.8 Ma, and 25 × 10^-6 m.y.^-1 for ∼10.8 to ∼8.1 Ma. These rates are broadly consistent with the growth rates of ⁸⁷Sr/⁸⁶Sr reported from other DSDP and ODP sites with some minor differences.

A composite Sr isotope curve constructed for the 24-8 Ma is broken into three linear segments with a different linear regression equation. The overall rate of ⁸⁷Sr/⁸⁶Sr increase for ∼24-8 Ma at site 758A (40 x 10^-6 m.y.^-1) is in agreement with the rate of ⁸⁷Sr/⁸⁶increase of the composite curve (42 x 10^-6 m.y.^-1).