For 30 million years, beginning 530 million years ago, the oxygen levels of the ocean dropped steadily, four different sets of chemical measurements suggested. (sciencemag.org)
An important aspect of this sampling strategy is linking chemical measurements to physical measurements. (esrl.noaa.gov)
The cores were also sampled for chemical measurements in Dartmouth's Ice Core Laboratory to determine the age of each ice layer. (sciencedaily.com)