EARLY SEDIMENT DIAGENESIS ON THE BLAKE BAHAMA OUTER RIDGE, NORTH-ATLANTIC OCEAN, AND ITS EFFECTS ON SEDIMENT MAGNETISM/

Sediment magnetic and geochemical studies of a suite of deep-sea sedim ent cores from the Blake/Bahama Outer Ridge (BBOR), North Atlantic Oce an, have identified two current redox boundaries in surficial (Holocen e), carbonate-rich sediments over much of the BBOR. The upper Mn+4/Mn2 redox boundary is associated with a spike in the concentration of so lid-phase Mn (as MnO2); the lower Fe+3/Fe+2 redox boundary is associat ed with a spike in the concentration of solid-phase Fe (as goethite, a lpha FeOOH). Over much of the BBOR, high sediment magnetic intensities occur in surficial, carbonate-rich sediments associated with these re dox boundaries and lower intensities occur in deeper (late Pleistocene ) carbonate-poor sediments. This relationship is opposite to that expe cted if sediment magnetism simply reflects the elastic (noncarbonate) sediment fraction. The surficial, high sediment magnetic intensities a re due primarily to the following two factors: (1) magnetic mineral au thigenesis associated with early diagenesis and (2) the presence of ab undant <0.1 mu m magnetite crystals interpreted to be bacterial magnet osomes. Magnetosomes are almost absent in the late Pleistocene low-car bonate sediments owing, most likely, to local Pleistocene environmenta l conditions (high elastic flux, low organic flux) which did not favor their growth. The sediment natural remanent magnetization is strongly correlated with the sediment elastic fraction and is relatively unaff ected by early diagenesis and the presence of abundant bacterial magne tite. If this is typical, bacterial magnetite may be more abundant in nature but less important to sediment paleomagnetic records, than prev iously thought.