Fig. 3.

Records of planktonic FB-δ¹⁵N and planktonic and benthic foraminiferal calcite δ¹⁸O over the last eight glacial cycles. FB-δ¹⁵N is plotted decreasing upward, such that N₂ fixation increases upward. FB-δ¹⁵N of O. universa (A−C, red) at MD97-2142 is similar to the FB-δ¹⁵N of G. ruber (A, blue) during the last 130 ky. Error bars indicate 1 SD of the oxidation replicates (Methods). FB-δ¹⁵N is highly correlated with the δ¹⁸O of G. ruber (B, gray) (64) and of C. wuellerstorfi (C, black). The glacial/interglacial change in the δ¹⁸O of the benthic foraminifera C. wuellerstorfi is typically considered to be mostly due to change in global ice volume (and hence sea level), with deep-water temperature change having a smaller effect. The light gray bars indicate the glacial terminations.