Oligocene-Miocene Strontium Isotopes - Stratigraphic Revisions and Correlations to an Inferred Glacioeustatic Record

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This study tests and improves on previously published early and middle Miocene Sr-87/Sr-86 marine correlations, presents Sr isotopic age correlations for this interval using the new timescale of Cande and Kent [1992], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene Sr-87/Sr-86 isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new Sr-87/S-86 data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991a] and 588 [Hodell et al., 1991] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of Sr-87/Sr-86 near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as +/-0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval approximately 14-11 Ma and Site 747 for the interval 11-8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene Sr-87/Sr-86 variations from ca. 23 to 11 Ma. Using Sr-87/Sr-86 data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the Sr-87/Sr-86 isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the deltaO-18 record apparently lead the Sr-87/Sr-86 inflections by 1 to 1.5 m.y.


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