ASTRONOMIC TIMESCALE FOR THE PLIOCENE ATLANTIC DELTA-O-18 AND DUST FLUX RECORDS OF OCEAN DRILLING PROGRAM SITE-659

High-resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the deltaO-18 record indicates that the 41-kyr period of Earth's orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earth's climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y. The tuned ages for the magnetic boundaries Gauss/Gilbert and Top Cochiti are about 6-8% older than the ages of the conventional timescale. A major phase of Pliocene northern hemisphere ice growth occurred between 3.15 Ma and 2.5 Ma. This was marked by a gradual increase in glacial Atlantic deltaO-18 values of 1 parts per thousand and an increase in amplitude variations by up to 1.5 parts per thousand much larger than in the Pacific deepwater record (site 846). The first maxima occured in cold stages G6-96 between 2.7 Ma and 2.45 Ma. Prior to 3 Ma, the isotope record is characterized by predominantly low amplitude fluctuations (< 0.7 parts per thousand). When obliquity forcing was at its minimum between 4.15 and 3.6 Ma and during the Kaena interval, deltaO-18 amplitude fluctuations were minimal. From 4.9 to 4.3 Ma, the deltaO-18 values decreased by about 0.5 parts per thousand, reaching a long-term minimum at 4.15 Ma, suggesting higher deepwater temperatures or a deglaciation. Deepwater cooling and/or an increase in ice volume is indicated by a series of short-term deltaO-18 fluctuations between 3.8 and 3.6 Ma.