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Diagenesis and Sr isotopic evolution of seawater using data from DSDP 590B and 575 |
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| Authors: | Frank M Richter and Donald J DePaolo |
| Institution: | 1Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637 U.S.A. 2Department of Earth and Space Science and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024 U.S.A. |
| Abstract: | New Sr isotopic and concentration analyses of bulk carbonate and porewaters from a carbonate-rich Miocene section sampled at DSDP Site 575 are reported and used, along with our earlier data from a section of similar age at DSDP Site 590B, in a numerical model to determine for each site the rate of Sr exchange during diagenesis and the degree to which this amount of exchange has shifted the87Sr86Sr of the bulk carbonate from the ratio each increment of sediment had when first deposited. Despite the fact that the two sites have very different water depth (4536 m at 575 vs. 1299 m at 590B), different sedimentation rate (  10 m/My at 575 vs. 40 m/My at 590B), and significantly different porewater Sr2+ gradients, we find that the rate of Sr exchange as a function of sediment age is almost indistinguishable between the two sites. The rate of Sr exchange determined at the two sites is such that the resulting shift in bulk carbonate87Sr86Sr due to diagenesis is small compared to the total range of87Sr86Sr values measured, but large compared to the analytical uncertainty of the individual isotopic ratio measurements. By taking this shift into account we reconstruct the original87Sr86Sr of each increment of carbonate sampled, which when plotted as a function of age becomes our best estimate of the Sr isotopic evolution of seawater. Because Sr is very well mixed in the ocean, at any given time there is a single worldwide value of seawater87Sr86Sr. Therefore, if we are quantitatively accounting for the effect Sr exchange, we should find the same seawater evolution curve regardless of what DSDP Site is used. When we compare the observed bulk carbonate87Sr86Sr vs. age at the two sites they are seen to differ by amounts that are sometimes large compared to the analytical uncertainties of the measurements. However, when these data are corrected for the post-depositional Sr isotopic shifts predicted by our diagenetic model, we find almost perfect agreement. This agreement suggests that we have made a realistic determination of the rate of Sr exchange and its consequences in terms of shifting the87Sr86Sr of the bulk carbonate, and more importantly, that Sr isotopes can be used to correlate marine sediments with an accuracy comparable to the very small analytical uncertainties of modern isotopic measurements. |
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