Contents and accumulation rates of the rare earth and some other elements in metalliferous sediments from DSDP Hole 92-598

The sediments recovered during DSDP Leg 92 (Site 598) include a complete 16 m.y. record of hydrothermal sedimentation along the western flank of the East Pacific Rise at 19°S. Fifty samples from this sediment column were analyzed to test the hypothesis that the REE composition of the hydrothermal component is primarily acquired via scavenging from seawater. Site 598 provides an ideal sample suite for this purpose: the sediments are lithologically "simple," primarily consisting of a mixture of hydrothermal materials and biogenous carbonates; the composition of the hydrothermal component is essentially constant through space and time; and the sediments have undergone minimal diagenetic alteration. The following observations suggest the above-stated hypothesis is true. The Ce anomaly as well as key indices of light and heavy REE behavior all show that the REE pattern of hydrothermal sediments approaches that of seawater with increasing paleodistance from the rise crest. Moreover, shale-normalized REE patterns are similar to that of seawater, varying only in absolute REE content: the REE content increases with distance from the paleo-rise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Based on significant correlative relationships between paleodistance from the rise crest and both the concentration and mass accumulation rates (MARs) of REEs and Fe, we conclude the REEs in the hydrothermal component are derived from the interaction of seawater and Fe in the hydrothermal plume.

Rare earth and other element contents and speciations in samples from Core AKO26-35, Pacific Ocean, Southwest Basin

Ferromanganese micro- and macronodules in eupelagic clays at Site AKO26-35 in the Southwest Pacific Basin were studied in order to check REE distribution during ferromanganese ore formation in non-productive zones of the Pacific Ocean. Host sediments and their labile fraction, ferromanganese micronodules (in size fractions 50-100, 100-250, 250-500, and >500 ?m) from eupelagic clays (horizons 37-10, 105-110, 165-175, and 189-190 cm), and buried ferromanganese micronodules (horizons 64-68, 158-159, and 165-166 cm) were under study. Based on partition analysis data anomalous REE enrichment in eupelagic clays from Site AKO26-35 is related to accumulation of rare earth elements in iron hydroxophosphates. Concentration of Ce generally bound with manganese oxyhydroxides is governed by oxidation of Mn and Ce in ocean surface waters. Micronodules (with Mn/Fe from 0.7 to 1.6) inherit compositional features of the labile fraction of bottom sediments. Concentrations of Ce, Co, and Th depend on micronodule sizes. Enrichment of micronodules in hydrogenic or hydrothermal matter is governed by their sizes and by a dominant source of suspended oxyhydroxide material. The study of buried ferromanganese micronodules revealed general regularities in compositional evolution of oxyhydroxide matrices of ferromanganese micro- and macronodules. Compositional variation of micro- and macronodules relative to the labile fraction of sediments in the Pacific non-productive zone dramatically differs from the pattern in bioproductive zones where micronodule compositions in coarser fractions are similar to those in associated macronodules and labile fractions of host sediments due to more intense suboxidative diagenesis.

Rare earth and other element contents in surface layer bottom sediments and ferromanganese nodules along the Transatlantic profile

Behavior of rare earth elements (REE) and Th is studied along the Transatlantic transect at 22°N. It is shown that both REE and Th contents relative to Al (the most lithogenic element) increase toward the pelagic region. The increasing trend becomes more complicated due to variations in content of biogenic calcium carbonate that acts as a diluting component in sediments. REE composition varies symmetrically relative to the Mid-Atlantic Ridge (MAR) emphasizing weak hydrothermal influence on sediments of the ridge axis, although the well-known criteria for hydrothermal contribution, such as Al/(Al+Mn+Fe) and (Fe+Mn)/Ti, do not reach critical values. Variations in REE content and composition allowed to distinguish the following five sediment zones in the transect: (I) terrigenous sediments of the Nares abyssal plain; (II) pelagic sediments of the North American Basin; (III) carbonate ooze of the MAR axis; (IV) pelagic sediments of the Canary Basin; and (V) terrigenous clay and calcareous mud of the African continental slope and slope base. Ferromanganese nodules of the hydrogenous type with extremely high Ce (up to 1801 ppm) and Th (up to 138 ppm) contents occur in pelagic sediments. It is ascertained that P, REE, and Th contents depend on Fe content in Atlantic sediments. Therefore, one can suggest that only minor amount of phosphorus is bound with bone debris. Low concentration of bone debris phosphorus is a result of relatively high sedimentation rates in the Atlantic Ocean, as compared with those in pelagic regions of the Pacific Ocean.