Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels

•High-resolution Mo and U isotope records in Mediterranean sapropels S5 and S7.•Consistent temporal patterns and large changes on relatively short timescales.•These changes are controlled by local basin redox and hydrography.•Increasing authigenic δ238U during early sapropel stages in non-sulfidic anoxia.•Combined δ98Mo-δ238U patterns allow detailed reconstructions of developing anoxia.