MIOCENE ISOTOPE REFERENCE SECTION, DEEP SEA DRILLING PROJECT SITE 608: AN EVALUATION OF ISOTOPE AND BIOSTRATIGRAPHIC RESOLUTION

We developed an isotope (Sr-87/Sr-86, delta O-18) reference section for the uppermost Oligocene to lower upper Miocene (ca. 25-8 Ma) at Site 608 in the northeastern North Atlantic. This site contains the least ambiguous magnetostratigraphic record of Miocene polarity changes available, providing direct correlations to the Geomagnetic Polarity Time Scale (GPTS). We integrate biostratigraphic, magnetostratigraphic, Sr isotope, and stable isotope data to provide a reference section for Miocene isotope fluctuations. The direct correlation of isotopes and biostratigraphy to the Geomagnetic Polarity Time Scale (GPTS) provides relatively precise age estimates. We use these age estimates to evaluate the timing of first and last occurrences of planktonic foraminifera, and conclude that many of these are synchronous within a 0.5 m.y. resolution between subtropical Site 563 (33 degrees N) and high-latitude Site 608 (43 degrees N). In addition, we use this chronology to estimate the ages of previously established Miocene oxygen isotope Zones Mi1 through Mi7 and to compare the Sr isotope record at Site 608 with previously published Sr-87/Sr-86 records. We approximate latest Oligocene to early late Miocene (25-8 Ma) Sr isotope changes with two linear regressions. The rate of increase of Sr-87/Sr-86 was high from the latest Oligocene (similar to 25 Ma) to earliest middle Miocene (similar to 15 Ma), with an estimated rate of 0.000059/m.y. Our ability to reproduce Sr isotope measurements is +/- 0.000030 or better, yielding a stratigraphic resolution of as good as +/- 0.5 m.y. for this interval. The rate of change was much lower from about 15 to 8 Ma (on average, 0.000013/m.y.), yielding Sr isotope stratigraphic resolution of worse than +/- 2.3 m.y. The causes of the late Eocene to Miocene Sr-87/Sr-86 increases are not known. We speculate that a moderate Sr-87/Sr-86 increase (0.000030/m.y) which occurred during the late Eocene-latest Oligocene can be explained by intermittent glaciations and deglaciations of the Antarctic continent. These pulse-like changes in the input of glacial weathering products yield what appears to be a monotonic, linear increase. The increase in the frequency of glaciations during the latest Oligocene-early Miocene can explain the higher rate of change of Sr-87/Sr-86 at this time. We speculate that by the middle Miocene, the development of a permanent east Antarctica ice sheet resulted in decreased input of glacial weathering products and a lower rate of Sr-87/Sr-86 change.