Fe-oxide and Fe-sulfide trace minerals in sediments and sedimentary rocks provide proxy records of biogeochemical processes, record past variations in the geomagnetic field, and can serve as proxies for climatic variations. An important class of these Fe-oxides is produced by bacteria. Magnetic particles produced by magnetotactic bacteria have been proposed as a primary recorder of the geomagnetic field in many terrestrial marine sediments, and have also been suggested to represent fossil evidence of life on the planet Mars. To better understand their distribution and preservation in the sediment column, and their relationship to other biochemical processes, we present rock-magnetic data that document the occurrence and abundance of fossil biogenic magnetite (magnetofossils) in marine sediments from the Blake/Bahama Outer Ridge. Magnetic hysteresis and low-temperature magnetism both indicate that the occurrence of magnetofossils is closely linked to the depth of the modern Fe-redox boundary within the sediment column, and that a fraction of the magnetic minerals in the sediment column above the Fe-redox boundary are in the form of intact and relatively unaltered chains of nanophase magnetite crystals. Below the Fe-redox boundary the abundance of these magnetofossils is markedly decreased. The important conclusions of this work are to demonstrate that nondestructive rock-magnetic methods can be used to successfully document the occurrence and relative abundance of magnetofossils in geologic materials.