Geochemistry of basal dolomitic and Fe-Mn sediments from the Tyrrhenian Sea

In the Tyrrhenian Sea (Western Mediterranean), unusual reddish, soft to lithified, dolomitic sediments up to 45 m thick overlie igneous crust at the base of thick Pliocene-Quaternary deep-sea sediment successions in the Marsili (Site 650) and Vavilov (Site 651) basins. These sediments also overlie the Gortani Ridge, a basaltic Seamount near the base of the Sardinian continental margin (Site 655). At both basinal sites (650, 651), the lowest sediments are dolomitic, with manganese oxide (MnO) segregations. Whole-rock X-ray diffraction indicates abundant dolomite and quartz, with subordinate calcite, illite (authigenic), feldspar and minor kaolinite, chlorite, and anhydrite. Chemical analyses show strong enrichment in magnesium oxide (MgO) and MnO relative to shale or deep-sea clay. Mg and Mn correlate positively and exhibit decreasing concentrations up the succession in the Marsili Basin (Site 650). The following scenario is proposed: peridotites were exposed on the seafloor in the Vavilov Basin (Site 651) and then eroded, depositing talc in local fine-grained dolomitic sediments within the igneous basement. After local magmatism ended, the igneous basement at each site subsided rapidly (about 800 m/m.y.) and was blanketed with calcareous and clay-rich oozes. During early diagenesis (from isotopic evidence; McKenzie et al., this volume) tepid fluids, of modified seawater composition, reacted with and dolomitized the overlying deep-sea sediments. At Site 651 additional Mg may have been extracted from asthenosphere peridotite cored at shallow depths (about 100 m). One can hypothesize that fluids rich in Mg and Mn were flushed from the igneous basement, triggered by extensional faulting and local tilting during subsidence of the basement, and that these fluids then dolomitized the base of the overlying sediment succession. Late tectonic movements in the Vavilov Basin (Site 651) fractured already lithified dolomitic sediments and more reducing (? hydrothermal) fluids locally remobilized Fe and Mn and corroded dolomite crystals.

Concentrations of Fe, Mn, Cu, Zn, Co, Ni, Pb, Cd, B, and SiO2 in interstitial waters from bottom sediments of the Atlantis II and Discovery Deeps, Red Sea rift zone

Chemical analyzes show that interstitial waters from ore-bearing bottom sediments of the Atlantis II and Discovery Deeps are enriched in Fe, Mn, Cu, Ni, Co, Zn, Pb, and Cd compared to sea water. Enrichment factors of these trace elements in the interstitial waters of the Atlantis II Deep relative to the sea water vary within the following ranges: for Fe from 100 to 7000, for Mn from 19047 to 32738, for Zn from 500 to 1600, for Pb from 78333 to 190000, for Cu from 107 to 654. Comparison of average weighted concentrations of Fe, Mn, Zn, Pb, Cu, Ni in the bottom sediments and the interstitial waters of the Atlantis II Deep indicates common regularities and good relationship in distribution of these elements along sediment cores. Differences in concentrations and distribution of the studied trace elements in the interstitial waters of the Atlantis II and Discovery Deeps result from different chemical compositions of hydrothermal fluids entering these deeps.

Chemical composition of Fe-Mn ores from the North Pacific seamounts

Ore crusts from the Mid-Pacific Seamounts were studied by scanning electron microscopy and by atomic-absorption and chemical analysis. Characteristic ultramicroscopic structures of ore material of these crusts are globular, fibrous, conchoidal and cellular. Non-ore components are represented by fragments of bedrocks, zeolites, biogenic carbonates, and apatite. Contents of ore elements are: Fe 5.53-15.82%; Mn 14.92-23.45%; Co 0.32-0.82%; Ni 0.22-0.70%; Cu 0.02-0.12%, Mn/Fe ratio varies from 1.02 to 3.39. In general elevated contents of Co (>0.55%) in Fe-Mn crusts correspond to elevated (>1) Mn/Fe ratios.

Annotated record and spectrochemical analyses of Mn nodules from Georgia Strait, British Columbia obtained during CNAV Laymore in 1970

A study of the distribution, dispersal and composition of surficial sediments in the Strait of Georgia, B.C., has resulted in the understanding of basic sedimentologic conditions within this area. The Strait of Georgia is: a long, narrow, semi-enclosed basin with a restricted circulation and a single, main, sediment source. The Fraser. River supplies practically all the sediment now being deposited in the Strait of Georgia, the bulk of it during the spring and summer freshet. This river is building a delta into the Strait from the east side near the south end. Ridges of Pleistocene deposits within the Strait and Pleistocene material around the margins, like bedrock exposures, provide local sources of sediment of only minor importance. Rivers and streams other than the Fraser contribute insignificant quantities of sediment to the Strait. Sandy sediments are concentrated in the vicinity of the delta, and in the area to the south and southeast. Mean grain size decreases from the delta toward the northwest along the axis of the Strait, and basinwards from the margins. Silts and clays are deposited in deep water west and north of the delta front, and in deep basins northwest of the delta. Poorly sorted sediments containing a gravel component are located near tidal passes, on the Vancouver Island shelf area, on ridge tops within the Strait, and with sandy sediments at the southeastern end of the study area. The Pleistocene ridges are areas of non-deposition, having at most a thin veneer of modern mud on their crests and upper flanks. The southeastern end of the study area contains a thick wedge of shandy sediment which appears to be part of an earlier delta of the Fraser River. Evidence suggests that it is now a site of active submarine erosion. Sediments throughout the Strait are compositionally extremely similar, with-Pleistocene deposits of the Fraser River drainage basin providing the principal, heterogeneous source. Gravels and coarse sands are composed primarily of lithic fragments, dominantly of dioritic to granodloritlc composition. Sand fractions exhibit increasing simplicity of mineralogy with decreasing grain-size. Quartz, felspar, amphibole and fine-grained lithic fragments are the dominant constituents of the finer sand grades. Coarse and medium silt fractions have compositions similar to the fine sands. Fine silts show an increase in abundance of phyllosilicate material, a feature even more evident in the clay-size fractions on Montmorillonite, illite, chlorite, quartz and feldspar are the main minerals in the coarse clay fraction, with minor mixed-layer clays and kaolinite. The fine clay fraction is dominated by montmorillonite, with lesser amounts of illite and chlorite. The sediments have high base-exchange capacities, related to a considerable content of montmorillonite. Magnesium is present in exchange positions in greater quantity in Georgia Strait sediments than in sediments from the Fraser River, indicating a preferential uptake of this element in the marine environment. Manganese nodules collected from two localities in the Strait imply slow sediment accumulation rates at these sites. Sedimentation rates on and close to the delta, and in the deep basins to the northwest, are high.