The use of molybdenum isotopes (delta Mo-98) as a paleo-redox proxy requires quantifications of fluxes and isotopic fractionations in each molybdenum sink. However, no systematic tests on delta Mo-98 variations in different oxic sediments have been made due to limitations of analytical precision and sample number in pioneering studies. Here, we present the delta Mo-98 data of ferromanganese nodules sampled at various global basins, which exhibit large variations from -0.95 parts per thousand to -0.51 parts per thousand with appreciable negative correlations between delta Mo-98 and manganese-iron ratios. Diagenetic nodules, experiencing dissolution and re-precipitation of manganese oxides in marine sediments, have lower delta Mo-98 by similar to 0.3 parts per thousand and higher molybdenum contents relative to hydrogenetic nodules. We suggest that changes in relative proportions of manganese and iron oxides and early diagenesis of marine sediments likely influence overall molybdenum isotope fractionations in oxic sinks. Hence, evolution of ancient marine manganese and iron cycles can influence sedimentary delta Mo-98 signatures, especially in low-oxygen Precambrian oceans.