Evolution of strontium isotopes in seawater during the middle Miocene: Newresults from ODP site 758A

The Sr-87/Sr-86 ratio of seawater has increased significantly over the last 40 Ma. This increase was marked by very high rate of increase in Sr-87/Sr- 86 and almost zero growth rate. The middle Miocene is one such period when considerable changes in the slope of Sr-87/Sr-86 curve have been reported. This study tests and improves on previously published strontium isotopic re cords for this period. We report 34 high precision strontium isotopic analyses of well preserved p lanktonic foraminifera from Ocean Drilling Program (ODP) site 758A in the e astern Indian Ocean. This site was selected because of the high quality mag netostratigraphy, suitable sedimentation rate (0.3 to 1.5 cm/ka) and excell ent preservation of calcareous microfossils in sediments. Samples studied r epresent the period from 15.2 Ma to 10.3 Ma with approximately one sample p er 100-200 ka. The results show that the seawater Sr-87/Sr-86 ratio during this period inc reased from 0.70881 to 0.70892. The rate of increase in Sr-87/Sr-86 during 15.2 Ma to 14.3 was similar to 31x10(-6) / m.y., whereas significantly from 14.3 to 12.8 Ma it was about a factor of two higher (similar to 59x10(-6) / m.y.) High rate of growth in seawater Sr-87/Sr-86 during this time interv al indicates exceptionally high rates of continental weathering. However, d uring 12.8 to 10.3 Ma the growth in Sr-87/Sr-86 of seawater remained almost constant with an average value of 0.708914. This constant value may be due to decrease in weathering rates of continents as a result of extensive gla ciation. It is suggested that the tectonic uplift and denudation of Himalay as coupled with climatic changes have resulted in the variations of seawate r strontium isotopes during this period. The inflection points in the curve may be used as event marker for stratigraphic correlation of marine sedime nts.