METHODS

Samples for mineralogy were analyzed at ~1.5-m intervals from 150 to 225 mcd and 1-m intervals for the lower part of the studied section. This corresponds to an age resolution of ~25 k.y. in the upper interval and ~35 k.y. in the lower interval. Samples were freeze-dried and weighed then wet sieved at 63 µm. The >63-µm and <63-µm fractions were then dried and weighed. The <63-µm sediments were then wet sieved at 20 µm with the aid of a sonic dismembrator, and the <20-µm fraction was split into 2- to 20-µm and <2-µm size fractions by means of centrifugation (relative centrifugal force = 145 for 2 min, 43 s). Sediments were saturated with MgCl₂ to reduce d-spacing variability caused by cation differences; they were then rinsed with warm deionized water, dried, and weighed. The <2-µm fraction was spiked with a 10% talc internal standard. Samples were homogenized by grinding in a mortar and pestle under acetone; they were then suspended in deionized water, mixed on a vortex mixer, and drawn onto 0.45-µm silver filters under vacuum. The X-ray diffraction analysis was run from 2° to 30°2 at 40 kV and 40 mA at 2°2/min using CuK radiation. Peak areas for smectite, illite, kaolinite, chlorite, quartz, plagioclase, and calcite were determined using MacDiff software. The relative proportions of kaolinite and chlorite were determined by the relative proportions of the kaolinite (002) and chlorite (004) peak areas. Mineral peak areas were normalized to the internal standard peak areas. Normalized peak areas were then recalculated on a carbonate-free basis using the calcite peak area for correction.