HIGH-RESOLUTION DATING OF DEEP-SEA CLAYS USING SR ISOTOPES IN FOSSIL FISH TEETH

Strontium isotopic compositions of ichthyoliths (microscopic fish rema ins) in deep-sea clays recovered from the North Pacific Ocean (ODP hol es 885A, 886B, and 886C) are used to provide stratigraphic age control within these otherwise undatable sediments. Age control within the de ep-sea days is crucial for determining changes in sedimentation rates, and for calculating fluxes of chemical and mineral components to the sediments. The Sr isotopic ages are in excellent agreement with indepe ndent age datums from above (diatom ooze), below (basalt basement) and within (Cretaceous-Tertiary boundary) the clay deposit. The Sr-87/Sr- 86 ratios of fish teeth from the top of the pelagic clay unit (0.70898 9), indicate an Late Miocene age (5.8 Ma), as do radiolarian and diato m biostratigraphic ages in the overlying diatom ooze. The Sr-87/Sr-86 ratio (0.707887) is consistent with a Cretaceous-Tertiary boundary age , as identified by anomalously high iridium, shocked quartz, and speru les in Hole 886C. The Sr-87/Sr-86 ratios of pretreated fish teeth from the base of the clay unit are similar to Late Cretaceous seawater (0. 707779-0.707519), consistent with radiometric ages from the underlying basalt of 81 Ma, Calculation of sedimentation rates based on Sr isoto pic ages from Hole 886C indicate an average sedimentation rate of 17.7 m/Myr in Unit II (diatom ooze), 0.55 m/Myr in Unit IIIa (pelagic clay ), and 0.68 m/Myr in Unit IIIb (distal hydrothermal precipitates). The Sr isotopic ages indicate a period of greatly reduced sedimentation ( or possible hiatus) between about 35 and 65 Ma (Eocene-Paleocene), wit h a linear sedimentation rate of only 0.04 m/Myr The calculated sedime ntation rates are generally inversely proportional to cobalt accumulat ion rates and ichthyolith abundances. However, discrepancies between S r isotope ages and cobalt accumulation ages of 10-15 Myr are evident, particularly in the middle of the clay unit IIIa (Oligocene-Paleocene) .