EARLY DIAGENETIC PROCESSES IN DEEP LABRADOR SEA SEDIMENTS - REACTIVE AND NONREACTIVE IRON AND PHOSPHORUS

A sequential extraction procedure was applied to separate the oxides and lithogenous phases of iron and manganese and the organic and inorganic phosphorus phases in four box cores and one piston core from the slopes and rises of the Labrador Sea. Sedimentation rate, rather than the location in the basin, appears as a master variable of the diagenetic transformations of Fe, Mn, and P. High sedimentation rate, characteristic of two of the box cores, led to the creation of zones near the redox boundary of partial reprecipitation of dissolved Fe, Mn, and P released in the deeper reducing portions of the sediments. In contrast, surficial sediments from box cores with 10 times lower sedimentation rate only have sufficient reductive capacity to remobilize Mn hydroxides while leaving the Fe oxyhydroxides intact. Under these conditions, there is evidence for a redistribution of reactive inorganic P leading to the crystallization of carbonate fluorapatite in the top 30 cm of the cores. Gradual transformation of buried orthophosphate to authigenic apatite under suboxic conditions is also observed in the top 400 cm of the high sedimentation rate piston core. As in the box cores, the reaction is complete after several thousand years of burial and occurs in sediments characterized by low CaCO3 content. Depending upon the sedimentation rates, the carbonate fluorapatite crystallization may be superimposed on the changes in detrital sedimentary fluxes accompanying the onset of deglaciation. A proxy indicator of paleo-redox conditions, and thus of biodegradable organic matter accumulation at the sediment-water interface, is given by the ratio of iron oxides to the reactive inorganic P (solid orthophosphate plus authigenic apatite).