OLIGOCENE-MIOCENE STRONTIUM ISOTOPES - STRATIGRAPHIC REVISIONS AND CORRELATIONS TO AN INFERRED GLACIOEUSTATIC RECORD

This study tests and improves on previously published early and middle Miocene Sr-87/Sr-86 marine correlations, presents Sr isotopic age cor relations for this interval using the new timescale of Cande and Kent [1992], and evaluates Sr isotopic changes against an inferred glacioeu static 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 mos t of this interval for independent age estimates, very good foraminife ral preservation, and excellent core recovery. Comparisons of new Sr-8 7/S-86 data from Hole 747A with previously published data from Deep Se a Drilling Project (DSDP) Sites 608 [Miller et al., 1991a] and 588 [Ho dell et al., 1991] yield the following results: (1) confirmation and r efinement of the early Miocene Sr isotope changes, (2) improved defini tion 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 M iocene 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 comb ined records from DSDP Site 522 [Miller et al., 1988] and ODP Hole 747 A. 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 at tributed to problems in the age assignments for Hole 588A for the inte rval approximately 14-11 Ma and Site 747 for the interval 11-8 Ma. Bec ause Hole 747A results provide a better chronology than Site 588 for m ost of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference secti on for Miocene Sr-87/Sr-86 variations from ca. 23 to 11 Ma. Using Sr-8 7/Sr-86 data from Sites 522, 608, and 747A, we relate late Eocene to e arly Miocene inflections in the Sr-87/Sr-86 isotope record to oxygen i sotope increases and decreases inferred to represent glacioeustatic ev ents. The decreases (deglaciations) observed in the deltaO-18 record a pparently lead the Sr-87/Sr-86 inflections by 1 to 1.5 m.y.