The Oligocene marine microfossil record: Age assessments using strontium isotopes

A strontium isotope record for the latest Eocene to earliest Miocene has been produced for two well‐preserved, deep‐sea sequences from the Atlantic, (Deep Sea Drilling Project sites 516F and 563) which have magnetostratigraphic age control and four sequences from the Pacific (sites 77B, 292, and 593) and Atlantic (site 529) which lack magnetostratigraphy. The composite87Sr/86Sr age record of sites 516F and 563 forms a comprehensive, composite standard for the Oligocene against which other sequences can be correlated. Unlike previously published87Sr/86Sr Oligocene sea water curves which have suggested that the sea water isotopic composition changed monotonically during the latest Paleogene, we note a distinct change in slope between the early Oligocene (36 to 27 Ma) and the late Oligocene to earliest Miocene (27 to ∼20 Ma). The slope of the early Oligocene segment is less steep than that of the late Oligocene to earliest Miocene. We have fit two linear regression curves to the Oligocene to early Miocene seawater δ87Sr data of sites 516F and 563, which we use to establish Sr isotope‐age relationships for the Oligocene. The δ87Sr age equations were used to evaluate the synchroneity of specific planktonic and calcareous nannofossil datum levels used to correlate Oligocene sections. Although some datum levels appear to be synchronous between the Atlantic and Pacific, our results show that others are diachronous along longitudinal gradients. This is most evident in several last appearance datums that occurred earlier in the western equatorial Pacific than in the eastern equatorial Pacific. This diachroneity extends into the Atlantic where last occurrences were later than in the eastern equatorial Pacific.