The Strontium Isotope Geochemistry of the Cenozoic Ocean

Author

Jennifer Hess, University of Rhode Island

Date of Award

1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Michael Bender

Abstract

A detailed record of the 87Sr/86Sr variations in seawater over the last 100 Ma has been determined by measuring the 87Sr/86Sr of well-preserved and well-dated fossil foraminifera from twelve Deep Sea Drilling Project cores of overlapping ages. Diagenetic alteration of the sample foraminifera was evaluated in three ways: by Sr/Ca ratios, by pore water chemistry, and by visual examination under the scanning electron microscope. Age assessments were made on the basis of magnetobiostratigraphy.

Foraminiferal ⁸⁷Sr/⁸⁶Sr measurements show an almost continuous increase in the seawater ⁸⁷Sr/⁸⁶Sr from 0.70738 at 100 Ma to 0.70920 today. The rise has not been linear. Long-term features of the Sr isotope-age curve show that the seawater ⁸⁷Sr/⁸⁶Sr increased rapidly and nearly linearly between 100 and 70 Ma. After 70 Ma, the ratio increases across the Cretaceous/Tertiary boundary and then decreases to a value of about 0.70775 at 45 Ma. A second, sharper rise in the seawater ⁸⁷Sr/⁸⁶Sr occurs between the late Eocene and the early middle Miocene, followed by a period between 2 and 14 Ma of a more gradual increase in the ⁸⁷Sr/⁸⁶Sr.

Superimposed on the long-term changes are smaller, short-term fluctuations including (i) a plateau of seawater ⁸⁷Sr/⁸⁶Sr at 12 Ma; (ii) periods of relatively rapid rise of the ⁸⁷Sr/⁸⁶Sr ratio beginning at 40, 22, 16 and 1 Ma (Eocene-Oligocene boundary, early Miocene, early middle Miocene, and the late Pliocene-Pleistocene, respectively); and (iii) a sharp increase in the ⁸⁷Sr/⁸⁶Sr of 1 x 10^-4 forming a spike-like peak at the Cretaceous-Tertiary Boundary.

The sharp increase in the ⁸⁷Sr/⁸⁶Sr at the K-T boundary implies a sudden addition of radiogenic ⁸⁷Sr. An obvious source is the impact of a meteorite. Iridium anomalies, contemporaneous with mass biological extinctions at the boundary, have already suggested that the collision of a bolide was responsible for the catastrophic termination of the Mesozoic era. However, mass balance calculations show that neither the Sr content of a bolide alone nor Sr resulting from the vaporization of continental crust during the impact of a bolide on land are sufficient to explain the 87Sr/86Sr increase at the K-T boundary.

Unlike previously published ⁸⁷Sr/⁸⁶Sr seawater curves which have suggested that the sea water isotopic composition changed monotonically during the latest Paleogene, our data show 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. Two linear regression curves were fit to the Oligocene to early Miocene seawater ⁸⁷Sr/⁸⁶Sr data, and used to establish Sr isotope-age relationships for the Oligocene. The ⁸⁷Sr/⁸⁶Sr-age equations were used to evaluate the synchroneity of specific planktonic foraminiferal and calcareous nannofossil datum levels commonly used to correlate Oligocene sections.

Although some datum levels appear to be synchronous between the Atlantic and Pacific, 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.

⁸⁷Sr/⁸⁶Sr were also measured in discharging geothermal waters from three ridge flank hydrothermal systems representing a range of half-spreading rates and tectonic environments - the Galapagos Mounds Hydrothermal Field, on 0.7 Myr old crust; the west flank of the East Pacific Rise at 19°S, on 4.6 Myr old crust; and the Mariana Trough on ~3 Myr old crust. At each site, the hydrothermal nature of the discharging solutions was confirmed by supporting heat flow and chemical evidence. Results show that the ⁸⁷Sr/⁸⁶Sr of these ridge flank hydrothermal solutions are virtually identical to that of modern seawater, indicating that Sr isotope exchange on the ridge flanks occurs too slowly to be quantitatively important in the oceanic mass balance of Sr isotopes.

Recommended Citation

Hess, Jennifer, "The Strontium Isotope Geochemistry of the Cenozoic Ocean" (1991). Open Access Dissertations. Paper 1350.
https://digitalcommons.uri.edu/oa_diss/1350