Evolution of Strontium Isotopes in Seawater During the Middle Miocene: New Results from ODP Site 758A

Thc ⁸⁷Sr/⁸⁶Sr ratio of seawater has increased significantly over the last 40 Ma. This increase was marked by very high rate of increase in ⁸⁷Sr/⁸⁶Sr and almost zero growth rate. The middle Miocene is one such period when considerable changes in the slope of ⁸⁷Sr/⁸⁶Sr curve have been reported. This study tests and improves on previously published strontium isotopic records for this period.

We report 34 high precision strontium isotopic analyses of well preserved planktonic foraminifera from Ocean Drilling Program (ODP) site 758A in the eastern Indian Ocean. This site was selected because of the high quality magnetostratigraphy, suitable sedimentation rate (0.3 to 1.5 cm/ka) and excellent preservation of calcareous microfossils in sediments. Samples studied represent the period from 15.2 Ma to 10.3 Ma with approximately one sample per 100-200 ka.

The results show that the seawater ⁸⁷Sr/⁸⁶Sr ratio during this period increased from 0.70881 to 0.70892. The rare of increase in ⁸⁷Sr/⁸⁶Sr during 15.2 Ma to 14.3 was -31 x 10^-6/m.y., whereas significantly from 14.3 to 12.8 Ma it was about a factor of two higher (-59x10^-6/ m.y.). High rate of growth in seawater ⁸⁷Sr/⁸⁶Sr during this time interval indicates exceptionally high rates of continental weathering. However, during 12.8 to 10.3 Ma the growth in ⁸⁷Sr/⁸⁶Sr of seawater remained almost constant with an average value of 0.708914. This constant value may he due to decrease in weathering rates of continents as a result ofextensive glaciation. It is suggested that the tectonicuplift and denudation of Himalayas coupled with climatic changes have resulted in the variations of seawater strontium isotopes during this period. The inflection points in the curve may be used as event marker for stratigraphic corretation of marine sediments.