CONTROLS ON THE SR-87 SR-86 COMPOSITION OF SEAWATER FROM THE MIDDLE EOCENE TO OLIGOCENE - HOLE-689B, MAUD RISE, ANTARCTICA/

A Sr-87/Sr-86 isotope curve of the middle Eocene to Oligocene was prod uced from analysis of foraminifera in Ocean Drilling Program Hole 689B , Maud Rise, near the coast of Antarctica. Sediments from the hole are well preserved with no evidence of diagenetic alteration. The sequenc e is nearly complete from 46.3 to 24.8 Ma, with an average sampling in terval of 166 kyr. Excellent magnetostratigraphy in Hole 689B allows c alibration to the geomagnetic polarity time scale of Cande and Kent (1 992). Marine strontium isotopic ratios were nearly stable from 46.3 to 35.5 Ma, averaging near 0.70773, after which they began to increase. A slow increase began after 40.4 Ma, rising at a rate of only about 8x 10(-6)/m.y. from base values of 0.707707. From 35.5 Ma to 24.8 Ma the average slope increased to 40x10(-6)/m.y. The slope remained constant at least until 24.8 Ma, when the record becomes discontinuous owing to unconformities. We evaluate several possible controls on the marine s trontium isotope curve that could have led to the observed growth in S r-87/Sr-86 ratios near the Eocene/Oligocene boundary. Three mechanisms are considered, including the onset of Antarctic glaciation, increase d mountain building in the Himalayan-Tibetan region, and decreased hyd rothermal activity. None of the mechanisms alone seems to adequately e xplain the increased Sr-87/Sr-86 ratios during the Oligocene. Glaciati on as a weathering agent was too episodic and probably began too late to explain the upturn in marine Sr-87/Sr-86 ratios. There is evidence that uplift in the Himalayan-Tibetan region began in the Miocene, much too late to control Oligocene strontium isotope ratios. Lastly, hydro thermal flux changes since the Eocene were apparently not great enough alone to account for the rise in marine Sr-87/Sr-86 ratios. We sugges t that a combination of causes, such as decreased hydrothermal activit y perhaps followed by increased glaciation and mountain building, migh t best explain the growth of the marine Sr-87/Sr-86 curve during the O ligocene.