Abstract: The marine geochemistry of phosphorus links the burial rate of organic carbon in marine sediments to the oxygen content of the atmosphere and may serve as a major component of the system that controls atmospheric oxygen. The return flux of phosphate from marine sediments to seawater is an important part of the marine phosphorus cycle. This paper examines the relationship between the return flux of phosphate and the oxidation state of marine sediments, a necessary preliminary step in defining the efficacy of the oxygen control mechanism. The diffusive return flux of phosphate from marine sediments to overlying bottom waters was calculated for 193 published pore water phosphate profiles that met a number of stringent criteria for sediment core and pore water recovery and processing. The phosphate return fluxes, scaled to carbon regeneration fluxes, are significantly greater from highly reduced sediments than from highly oxidized sediments. In highly reduced sediments the return phosphate flux from carbon regeneration is frequently augmented by a large phosphate flux released during the reductive dissolution of ferric (oxy)hydroxides. in highly oxidized sediments the return phosphate flux can be somewhat less than the flux to be expected from carbon regeneration. The missing phosphate is probably adsorbed on ferric (oxy)hydroxides in these sediments. The strong coupling between the oxidation state of marine sediments and the return phosphate flux to seawater suggests that the marine phosphate cycle is indeed an important part of the system that stabilizes atmospheric O 2 . The total preagricultural return flux of P from marine sediments was ca. 12 × 10 11 mol P/yr. This rate is more than an order of magnitude larger than the riverine flux of total dissolved phosphorus to the oceans, ca. 0.3 × 10 11 mol P/yr. We estimate that the total continental preagricultural flux of reactive P that ultimately appears dissolved in ocean water was ca. 3.5 × 10 11 mol P/yr. The large flux of continental P to seawater, via direct input of riverine dissolved inorganic and organic P and via the diagenetic return flux from reactive continental particulate P deposited in marine sediments, indicates that the marine residence time of phosphate with respect to terrigenous inputs is ca. 10,000 years. This figure depends quite heavily on the fraction of terrigenous, particulate-phase phosphate that is released to seawater during diagenesis. Variations in this fraction can significantly affect the marine residence time of phosphate and the relative proportion of detrital versus authigenic phosphate phases in marine sediments.

Coupled, one-dimensional transport equations for CO 2 and O 2 in pre-Middle Ordovician soils have been developed. The value of the ratio ΔP O 2 /ΔP CO 2 , where ΔP O 2 is the difference between the O 2 pressure in the atmosphere and at any level in a particular soil profile, and where ΔP CO 2 is the difference between the CO 2 pressure in the atmosphere and at the same level in the same soil profile, depends on the relative importance of O 2 and CO 2 transport by diffusion and transport by advection in soils. The transport equations were solved in closed form for a particularly simple soil model. Numerical solutions were obtained for more complex soil profiles, and the sensitivity of the P O 2 profile in soils to changes in a variety of parameters was explored. The results indicate that free oxygen has been present in the atmosphere at least since early Proterozoic time. The range of atmospheric oxygen pressure permitted by the available paleosol data is large. Our currently preferred range of the oxygen pressure in the atmosphere between ca. 2.5 and 1.8 Ga is between 5 × 10 −4 and 1 × 10 −3 atm.

Abstract The isotopic composition of strontium in sulfate minerals from the Fukazawa and Kosaka ore deposits has been measured in order to evaluate the importance of seawater in the development of the Kuroko deposits. The 87 Sr/ 86 Sr values in samples of anhydrite and gypsum from the sekkoko (gypsum) units in both deposits fall in a narrow range (0.7082-0.7087) whose upper limit approaches that estimated for Miocene seawater. The 87 Sr/ 86 Sr values of the analyzed barites are generally slightly lower than those of the anhydrite and gypsum from the sekkoko and cover a wider range (0.7069-0.7079). None of the ratios are higher than that estimated for Miocene seawater. Coarsely crystalline barite specimens from the siliceous ore zones have 87 Sr/ 86 Sr values which are indistinguishable from those of fine-grained barites in the strata-bound sulfide ores. Anhydrite was probably deposited in shallow convection cells established in the immediate vicinity of rhyolite intrusives. The mixing of a heated, seawater-dominant hydrothermal solution which had acquired a limited amount of isotopically nonradiogenic strontium from the Miocene volcanics with relatively fresh seawater within the porous, poorly consolidated tuff-aceous sediments near the seawater-sediment interface appears to be the most reasonable mechanism of sekkoko formation. The quantity of barite in individual orebodies greatly exceeds that which could be formed from the barium contained in the white rhyolite domes associated with the ore deposits. The measured depletion of strontium and cations in the volcanics underlying the ore horizon, and the alteration of the Paleozoic basement 450 m below the Kuroko orebodies in the Kosaka area strongly suggest that the ore solutions penetrated beneath the Miocene volcanic cover to leach these cations from the basement metamorphics. The 87 Sr/ 86 Sr data for barite are consistent with a model in which seawater is progressively modified through interaction with the Miocene volcanics, the Paleozoic basement, deep granitic plutons, and possibly through the addition of magmatic fluids associated with the granitic plutons. The most appealing model for the formation of the Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately above the vent exit in the overlying seawater. However, before a full understanding of the mechanism-of sulfide and barite precipitation is possible, much more must be learned about the nature of the vent, the flow rate of hydrothermal solutions, the mixing properties of the ore solution with seawater, and the thermal gradients in the upper part of the feeder channel.

Abstract The partitioning of strontium between anhydrite and mixtures of seawater with NaCl solutions has been studied experimentally. Anhydrite was precipitated by heating mixtures of seawater and NaCl solutions to temperatures between 150° and 250°C. It was found that the partition coefficient of Sr in this system depends less on the NaCl concentration, temperature, and precipitation rate than on the degree of supersaturation of the solutions with respect to anhydrite and/or the morphology of the precipitated anhydrite crystals. Acicular anhydrite was precipitated from solutions with a relatively high degree of supersaturation; rectangular anhydrite was precipitated from solutions with low degrees of supersaturation. The partition coefficient of Sr between solutions and rectangular anhydrite at 150°, 200°, and 250°C is 0.35 ± 0.05, 0.24 ± 0.05, and 0.27 ± 0.05, respectively.

Abstract In the Kuroko deposits of Japan anhydrite is very abundant in sekko ore, which underlies the strata-bound sulfide ores. Anhydrite in sekko ore is usually nodular. The diameters of the anhydrite nodules vary with stratigraphic position and range from a few millimeters to several meters. The pyroclastic rocks in sekko ore horizons have undergone intense hydrothermal alteration. Pyroclastic rocks containing large amounts of anhydrite are altered to Mg chlorite. Pyroclastic rocks containing sericite and sericite-montmorillonite mixed layer clay minerals generally contain small amounts of anhydrite. The strontium content of the sekko anhydrite ranges from about 200 to 2,000 ppm and tends to increase both stratigraphically upward and according to nodule size. The filling temperature of fluid inclusions in the anhydrites ranges from ca. 240° to 340°C. The mode of occurrence, texture, Sr content, nature of the contained fluid inclusions, and isotopic composition of Sr, S, and O in anhydrite together with the mineralogy of the sekko ore suggest that anhydrite deposition was probably related to subsurface mixing of fluids.