G. N. Baturin

Phosphorus Cycle in the Ocean

The behavior of phosphorus is considered at major stages of the sedimentary cycle beginning with supply sources for its delivery into the ocean to precipitation and its sedimentation, localization and subsequent diagenetic redistribution in bottom sediments. River runoff represents the main phosphorus source in the ocean. It delivers annually about 1.5 Mt of dissolved phosphorus and more than 20 Mt of suspended phosphorus into the ocean. Up to 80% of the suspended phosphorus incorporated in the lithogenic material precipitates within submarine continental margins. Phosphorus dissolved in seawater repeatedly participates in biogeochemical processes owing to its assimilation by phytoplankton that annually consumes from 1.5 to 2.5 Gt of phosphorus. Dissolved phosphorus is incorporated in organic remains and precipitates from seawater by a biogenic mechanism, too. Only a part of phosphorus settled onto the bottom is buried in sediments. Due to reducing diagenetic processes, up to 30–40% of the primarily precipitated phosphorus diffuses from the upper layer of sediments into bottom water. Diffusion flux into the ocean significantly exceeds the supply of dissolved phosphorus from river runoff. The absolute mass phosphorus dispersed in sediments is several orders of magnitude greater than the mass concentrated in phosphorite deposits. However, the majority of phosphorite formation epochs coincide with the intensification of total phosphorus accumulation in marine sediments in conditions of humid climate, intense chemical weathering of rocks on continents, and considerable expansion of the oceanic shelf area.

Issue of the relationship between primary productivity of organic carbon in ocean and phosphate accumulation (Holocene-Late Jurassic)

By analogy with the present-day ocean, the primary productivity of paleoceans can be reconstructed using calculations based on the content of organic carbon in sediments and their accumulation rates. Results of calculations based on literature data show that the primary productivity of organic carbon, the mass of phosphorus involved in the process, and the content of phosphorus in oceanic waters were relatively stable in the Mesozoic and Late Mesozoic. Prior to precipitation on the seafloor together with the biogenic detritus, the dissolved phosphorus could repeatedly be involved in the biogeochemical cycle. Therefore, only less than 0.1% of phosphorus is retained in bottom sediments. The bulk phosphorus accumulation rate in oceanic sediments is partly consistent with the calculated primary productivity. Some epochs of phosphate accumulation also coincide with maximums of primary productivity and minimums of the fossilization coefficient of organic carbon. The latter fact can testify to episodes of the acceleration of organic matter mineralization and the release of phosphorus from sediments, leading to increase in the phosphorus reserve in paleoceans and phosphate accumulation in some places.

The Paragenesis of Organic Matter, Phosphorus, and Uranium in Marine Sediments

Using some uranium deposits and recent U-bearing sediments as examples, it is shown that all U-bearing rocks are characterized by an association of organic matter and calcium phosphate, irrespective of the quantitative relationship between these components. A considerable proportion of these components was delivered into sediments with remains of marine planktonic and nektonic organisms. Along with organic matter, calcium phosphate played a significant role in uranium concentration. This is related to a high sorption ability of the calcium phosphate. Uranium accumulated during diagenesis as a result of diffusion exchange between bottom and interstitial waters. The combination of anoxic bottom environment with high bioproductivity in upper aerated waters, a typical phenomenon in oceanic upwelling zones, is the most favorable factor of uranium concentration in the sedimentary process. This determines the stable paragenetic association of organic matter, phosphorus, and uranium in marine sediments, such as black shales and organogenic phosphate deposits.

Uranium in Phosphorites

The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 102ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P2O5ratio in phosphorites ranges from less than unity to 24 · 10–4(average 3.2 · 10–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P2O5) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components.

Mineral resources of the ocean

Major data concerning the history of investigation, distribution, mineral and chemical composition, and formation processes of mineral resources of the ocean, namely ferromanganese nodules, ore crusts, phosphorites, and hydrothermal mineral formations, including ore-bearing and metalliferous sediments, massive sulfides, and hydrothermal ferromanganese crusts are reviewed. The problem of the scale of mineral accumulations in the ocean and their quality, along with prospects of their future recovery, is discussed.

Rare earth elements in phosphate-ferromanganese crusts on Pacific seamounts

Based on publications devoted to the composition of P-rich ferromanganese crusts on Pacific seamounts, relationships between the REE distribution in the crusts and the contents of phosphates and Fe-Mn hydroxides therein are considered. It is shown that REEs in the crusts are related to all three mineral phases and their contents are variable. In general, the REEs show weak correlations with P, Mn, and Fe in different varieties of ore crust. Average REE contents are comparable in samples with the maximal and minimal phosphorus contents, suggesting irregularity of REE distribution in the phosphates and ferromanganese phases. This fact is consistent with data on the presence of natural REE minerals in the phosphates.

The Composition of Phosphatized Bones in Recent Sediments

Mineral and chemical compositions of bone phosphate were studied in two samples from the outer Namibian shelf sediments composed of fish skull fragments and whale ribbon. Fossilization of bones is accompanied by the accumulation of lithogenic components, iron, sulfur, rare earth and other trace elements (Ni, Cu, Co, Cd, Mo, La, Ce, Th, U, and others), whereas the organic and mineral carbon content decreases. The evolution of bone phosphate during fossilization consists in transition from primary hydroxylapatite to a gel-type material, which subsequently becomes globular and crystallizes as fluorcarbonate-apatite crystallites. Additionally, some authigenic minerals, including both relatively widespread minerals (pyrite, uraninite, and coffinite) and rare minerals (graphite and calcium and germanium oxides) are formed in the bones. A considerable proportion of uranium in bones consists of uranium minerals, which also contain rare earth elements.

Structure and Composition of Ferromanganese-Phosphate Nodules from the Black Sea

Ferruginate shells and tubular worm burrows from the oxygenated zone of the Black Sea (Kalamit Bay and Danube River mouth) are studied using transmission and scanning electron microscopy combined with analyses of elemental composition. Iron and manganese hydroxide nodules considered here are enriched in phosphorus. They contain variable amounts of terrigenous and biogenic material derived from host sediments. The hydroxides are mainly characterized by colloform structure, whereas globular and crystalline structures are less common. The dominating iron phase is represented by ferroxyhite and protoferroxyhite, whereas the manganese phase is composed of Fe-free vernadite. Relative to sediments, concentrations of Mn, As, and Mo increase 12–18 times, while concentrations of Fe, P, Ni, and Co increase 5–7 times during the nodule formation.

Manganese and Molybdenum in Phosphorites from the Ocean

The behavior of molybdenum and manganese is studied in phosphorite samples from shelves, seamounts, and islands of the ocean. In shelf phosphorites, molybdenum and manganese contents are 2–128 and 12–1915 ppm, respectively, while the Mo/Mn ratio ranges from 0.004 to 4.5. Phosphorites from oceanic seamounts impregnated with ferromanganese oxyhydroxides contain 0.84–14.5 ppm of Mo and 0.1–17% of Mn. The Mo/Mn ratio ranges within 0.0008–0.004. Phosphate-bearing ferromanganese crusts overlying the seamount phosphorites contain 54–798 ppm of Mo and 10–20% of Mn; Mo/Mn ratio varies within 0.002–0.005. Corresponding values for most island phosphorites are 0.44–11.2 ppm, 27–287 ppm, and 0.008–0.20, respectively. Phosphorites from reduced environments are characterized by a relative enrichment in Mo and depletion in Mn, whereas the Mo/Mn ratio reaches maximum values. The ratio decreases with transition to suboxic and oxic conditions. Molybdenum content in recent shelf sediments is commonly higher than that in authigenic phosphorites from these sediments. Recent phosphorite nodules from the Namibian shelf become depleted in Mo and Mn during their lithification, but Pliocene–Pleistocene nodules of similar composition and origin from the same region are enriched in Mo and characterized by a variable Mn content. The higher Mo content in phosphate-bearing ferromanganese crusts is a result of coprecipitation of Mo and Mn from seawater. Nonweathered phosphorites on continents and phosphorites from oceanic shelves are largely enriched in Mo with the Mo/Mn ratio ranging from 0.01 to 1.0. This is an evidence of their formation in reducing conditions.

Uranium and Thorium in Phosphatic Bone Debris from the Ocean Bottom

Uranium and thorium content, as well as their distribution patterns are studied in biogenic phosphates from the Atlantic and Indian oceans. The material studied is represented by differently lithified fish remains (bones, scales, teeth) and marine mammal bones (ribs, vertebras, earbones) collected from both reduced shelf sediments and oxidized pelagic ones. The U content in the material varies from 0.7 to 700 ppm, and the Th content ranges from less than 0.5 to 14 ppm. The U/Th ratio varies from 0.16 to 400. Contents of both elements increase with the lithification of biogenic phosphates. The U concentration is more intense on shelves, whereas the thorium concentration increases in pelagic areas. A partial positive correlation of U and Th with Fe but a negative correlation of U with organic carbon are noted. The latter corresponds to increasing lithification of biogenic phosphates. Calcium phosphate, which is transformed from hydroxyapatite to fluorcarbonate-apatite serves as the main carrier of U, while transformed organic matter is a minor agent. Thorium is mainly bound with Fe.