Mössbauer spectroscopic investigations of deep-sea sediments from the Peru Basin revealed a prominent downcore increase of the bulk sediment Fe(II)/Fe(III) ratio across the well-known tan-green color transition. This increase occurs along a continuous gradient extending over a 3 cm zone at the two deep-sea sites that we report. Remarkably constant values of the Fe(II)/Fe(III) ratios above (11 ± 2%/89 ± 2%) and below (37 ± 2%/63 ± 2%) the iron redox transition-zone delimit the gradient clearly. A minor peak of diagenetic iron enrichment is manifest at the redox front. This Fe enrichment has accumulated in a redox cycle and amounts to 16 ± 4% of the total Fe at the peak maximum. The data imply that 26 ± 4% of the detrital iron has experienced in situ redox transition from Fe(III) to Fe(II) within the iron redox transition-zone.