The figure above is copied from the data of Martinson et al., 1987. The values for the Y axis are not actual δ18O values. They are standardized scores, computed by subtracting the average from each δ18O, and dividing by the standard deviation.

standardized score

standard deviation.

"Real" δ18O measurements of foraminifera are seldom negative. Ordinarily, they are more positive and less posative, with more positive (larger) values being typical of glacials. Example ODP 847