A global U-Pb and δ18O zircon database shows temporal changes in the magmatic record related to changes in the degree of crustal reworking. The δ18O composition of bulk sediment remains relatively constant through geologic time, with a mean value of 14.9‰. In contrast, the δ18O values in magmatic zircons vary from relatively low values averaging ∼6‰ in the Archean to increasingly higher and scattered values defining a series of peaks and troughs in post-Archean data. The degree of crustal reworking increases at times of supercontinent assembly. Therefore we attribute the pattern of post-Archean δ18O values recorded by magmatic zircons to a significant increase in the incorporation of high δ18O sediment in response to enhanced crustal thickening and reworking associated with the onset of collisional tectonics, especially during formation of supercontinents.