Deep Sea Drilling Project Site 503 was cored, using the Hydraulic Piston Corer, at the location of Site 83 on the north flank of the Galapagos Rise. Two holes were cored to a total depth of 235 m and a section was recovered which represents approximately the past 8 m.y. The upper 100 m (Holocene through mid-early Pliocene) is virtually complete whereas below 100 m (to late Miocene) about 70% of the section was recovered. The sediment is a uniformly siliceous calcareous ooze with carbonate and color cycles throughout. These cycles have periodicities from 20,000 to 40,000 years per cycle. Clay content remains fairly constant at low percentages to 226 m but then abruptly increases to greater than 25%. This increase is thought to be the result of hydrothermal activity or weat ering of the igneous basement or both. The section is highly undercompacted, which is thought to result from a significant percentage of diatoms.

Aboard ship we were able to identify most magnetostratigraphic boundaries above the Gauss-Gilbert. Sedimentation rates are about 4 cm/k.y. from lower Pliocene through mid-Pliocene but decrease to 1.5 to 2.5 cm/k.y. for the mid-Pliocene through Quaternary. Calculated bulk accumulation rates steadily decrease from late Miocene to late Quaternary with a distinct event of low accumulation rates in mid-Pliocene. This event may be a reflection of the emergence of the Isthmus of Panama.

The high accumulation rate and high quality of these cores provide the most detailed stratigraphic record of the past 8 m.y. available from the equatorial Pacific.

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