The oxygen isotope composition (delta O-18) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid delta O-18 on the rocks' isotopic composition. A new delta O-18 record in marine iron oxides covering the past similar to 2000 million years shows a similar secular rise. Iron oxide precipitation experiments reveal a weakly temperature-dependent iron oxide-water oxygen isotope fractionation, suggesting that increasing seawater d(18)O over time was the primary cause of the long-term rise in delta O-18 values of marine precipitates. The O-18 enrichment may have been driven by an increase in terrestrial sediment cover, a change in the proportion of high-and low-temperature crustal alteration, or a combination of these and other factors.