H. Herbert Veeh

Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite. The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of Kolodny and Kaplan (1970) that the major mechanism of 234U238U fractionation is displacement of 234U atoms into sites where they are more ‘oxidizable’ than the 238U parent. Age estimates based on 234U(IV) and 230Th contents are internally consistent and range from late Pleistocene to Recent. The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times.

Metal accumulation rates in the Southeast Pacific and the origin of metalliferous sediments

Abstract Metal accumulation rates over the East Pacific Rise and Bauer Deep rule out normal authigenic precipitation of iron and manganese as the major mechanism of enrichment to the metalliferous sediments. A hydrothermal source located along the East Pacific Rise is compatible with the transition metal and aluminum accumulation rates. For the Bauer Deep the accumulation rate data suggest either that metal-bearing phases are being transported from the rise to the protected basins of the deep or that a second hydrothermal source exists within the Bauer Deep. A major portion of the minor elements being deposited in the Bauer Deep could result from authigenic precipitation, thus accounting for their distinctive chemical composition.

Sedimentation dynamics and redox iron-cycling : controlling factors for the apatite-glauconite association on the East Australian continental margin.

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Oxygen isotopes of phosphate and the origin of island apatite deposits

In an attempt to test the hypothesis that insular phosphorites on high islands (e.g., Nauru) formed by chemical reactions between seabird guano and carbonate, the oxygen-isotope compositions of phosphate ( δ18Op ) in modern guano and in the apatite component of phosphorites of various ages and locations have been investigated. Guano from several species of seabird appears to have δ18Op values that are in equilibrium with the ambient seawater temperature at each site. However, recent apatites closely associated with modern guano can have δ18Op values that are as much as 2‰ higher than their inferred guano phosphate source. This shift in δ18Op values could be the result of isotopic re-equilibration with percolating meteoric waters which have been enriched by evaporative processes in the upper part of the soil profile. Bacterial degradation of the guano in the soil zone is a possible mechanism for resetting the δ18Op values in this environment. Ancient apatites on high islands such as Nauru, Ocean Island and Makatea have δ18Op values that are significantly lower than the δ18Op values of recently formed apatites. The results indicate that ancient apatite deposits could not have formed by processes similar to those forming recent apatites without unreasonably high palaeotemperatures and/or equilibration with water with much lower δ18O values, such as meteoric water in a karstic terrain at the time of maximum infiltration. If the δ18Op values in apatite are subject to modification during subaerial weathering and diagenesis, perhaps aided by the metabolic activity of micro-organisms, the application of δ18Op signatures of insular phosphorites to palaeoenvironmental reconstructions is limited to post-depositional history and provides few clues to original setting.

Concordant 230Th and 231Pa ages of marine phosphorites

The reliability of uranium-series age determinations of marine phosphorites has been tested by simultaneous determination of 230 Th/ 234 U and 231 Pa/ 235 U. The measured 230 Th/ 234 U and 231 Pa/ 235 U activity ratios in representative phosphorite samples from the continental margins of Eastern Australia and Peru are consistent with the theoretically expected development with time of these isotope ratios on a concordia diagram and hence support the assumption that the initial 230 Th and 231 Pa contents are insignificant in comparison with the uranium contents, and that the phosphorites have remained a chemically closed system with respect to uranium, thorium and protactinium. Alpha-recoil-induced leakage of 234 U from the phosphorites appears to be too slow to cause any serious errors in the 230 Th ages, and hence can be neglected. These results demonstrate that reliable uranium-series age determinations can be obtained for marine phosphorites on the basis of a simple, closed system model.

Mixing and evaporation processes in an inverse estuary inferred from δ2H and δ18O

Abstract We have measured δ2H and δ18O in Spencer Gulf, South Australia, an inverse estuary with a salinity gradient from 36‰ near its entrance to about 45‰ at its head. We show that a simple evaporation model of seawater under ambient conditions, aided by its long residence time in Spencer Gulf, can account for the major features of the non-linear distribution pattern of δ2H with respect to salinity, at least in the restricted part of the gulf. In the more exposed part of the gulf, the δ/S pattern appears to be governed primarily by mixing processes between inflowing shelf water and outflowing high salinity gulf water. These data provide direct support for the oceanographic model of Spencer Gulf previously proposed by other workers. Although the observed δ/S relationship here is non-linear and hence in notable contrast to the linear δ/S relationship in the Red Sea, the slopes of δ2H vs. δ18O are comparable, indicating that the isotopic enrichments in both marginal seas are governed by similar climatic conditions with evaporation exceeding precipitation.

Are Phosphorites Reliable Indicators of Upwelling

Most models of phosphorite genesis involve upwelling as an essential element, if only to provide a mechanism for continuous nutrient supply to ocean surface water and hence sustain a high flux of phosphorus to the sediment via organic matter. Although recent studies have confirmed a close link between upwelling and phosphorite formation on the continental margins of Namibia and Peru-Chile, the simplified assumption that phosphorites in the sedimentary record invariably are indicative of upwelling can be challenged on several grounds: (a) some major phosphorite deposits in the geologic record cannot be adequately explained in terms of reasonable upwelling models; (b) formation of phosphorites in modern times, as demonstrated by uranium-series age determinations and other methods, is not confined to major upwelling centers, but also occurs on the continental margin of Eastern Australia, an area of only moderate seasonal upwelling, and quite limited organic productivity. Apatite in phosphorites off Eastern Australia is most likely formed during post-mortem alteration of organic phosphorus originally present in bacterial cells. Populations of transiently motile, facultatively chemolithotropic bacteria may proliferate within the upper continental slope sediments during times when the environment became quite restrictive, such as when nutrient supply was drastically limited. Additional clues for differentiating phosphorites might be found in the rare earth element pattern of the “Eastern Boundary Current”-type; i.e., absence of the Ce-anomaly. Phosphorites or phosphatic sediments lacking these features do not reliably indicate upwelling and should not be used indiscriminately in the reconstruction of former upwelling centers.

The behaviour of uranium and radium in an inverse estuary

Spencer Gulf in South Australia is an inverse estuary, where an excess of evaporation over precipitation and the absence of major rivers lead to a well defined horizontal salinity gradient from 36‰ near its entrance to about 46‰ at its head. The excess of salt is exported annually from the gulf as a gravity current moving along the sea floor and across the shelf. Water balance is maintained by an inflow of less saline shelf water at the surface. The inverted salinity gradient which persists throughout the year can be used to advantage in the study of processes which govern the cycling of radionuclides of the uranium and thorium decay series in the nearshore marine environment. Radionuclides investigated include 234U, 238U226Ra and 228Ra. The results obtained indicate that uranium isotopes in Spencer Gulf have a typical marine signature, but that uranium is being removed from the water column at a rate of about 0.2 μg cm−2 y−1, based on conservation of salt. The 234U/238U ratio of uranium in the predominantly carbonate sediments of Spencer Gulf are consistent with uranium input from seawater, and the measured accumulation rate of authigenic uranium in the sediments supports the model removal rate, indicating th at Spencer Gulf is a net sink for uranium. A systematic increase in 228Ra/226Ra with salinity, coupled with a general increase in 226Ra above that predicted from evaporation of open ocean sea water demonstrate that both radium isotopes enter the gulf. In order to maintain the excess radium in the water column, diffusive fluxes of 0.3 dpm cm−2 y−1 and 0.05 dpm cm−2 y−1 for 228Ra and 226Ra, respectively, from the seafloor would be required. The standing crop of 232Th in the surface mixed layer of the sediment as a potential source for 228Ra via decay and recoil processes is sufficient to support the excess 228Ra in the water column, but there appears to be insufficient 230Th in the sediment to generate the required 226Ra flux, suggesting an external source for some of the excess 226Ra, such as periodic surface runoff after rainfall events and/or submarine discharge of groundwater from granitic basement rocks.

Uranium-series ages of corals and coexisting phosphate deposits on Pelsaert Reef Complex, Houtman-Abrolhos Islands, Western Australia

Abstract An unusually well-defined association between guano-derived phosphate rock (apatite) and coral carbonates on Pelsaert Island has provided an opportunity to test uranium-series dating methods as applied to insular phosphorites. The phosphate deposit, which is bracketed by late Pleistocene and Holocene corals with 230 Th 234 U ages of 120,000 and 4700 yr B.P., respectively, has 230 Th 234 U ages ranging from 85,000 to 112,000 yr B.P. The mutually consistent results suggest that phosphate deposition commenced soon after the peak of the last interglaciation and has been largely controlled by sea-level fluctuations and probably other factors associated with late Quaternary climate in this area.

Uranium-series isotopic studies of marine phosphorites and associated sediments from the East Australian continental margin

The uranium-series disequilibria of phosphorites, sediments, and coexisting solitary corals from the outer continental shelf and upper slope off eastern Australia have been determined in an attempt to relate the genesis of phosphorites in this area to an absolute time scale. The234U/238U,230Th/234U, and231Pa/235U activity ratios in the phosphorites are internally consistent, and indicate that the apatite component of the phosphorites has remained a closed system with respect to U, Th, and Pa, except for very minor losses of234U. These losses do not affect the230Th/234U-derived ages, and comprise only 1–2% of the total234U activity, significantly less than the losses observed in phosphorite nodules from off Peru-Chile. A morphological subdivision of the phosphorites into either non-ferruginous or ferruginous is supported by the isotopic data. Earthy, friable non-ferruginous phosphatic nodules range in age from 2.5 to > 250 kyr; Holocene ages were obtained from five different locations in water depths ranging from 365 to 450 m. In contrast, the uranium-series isotopes in well-indurated ferruginous nodules occurring in the same general area, but in shallower ( < 350 m) water, are in radioactive equilibrium, indicating that their ages exceed 800 kyr. Solitary corals (Caryophyllia planilamellata Dennant 1906) are associated with non-ferruginous nodules on the upper slope, and commonly contain internal phosphatic molds. Several corals and their internal molds from one location were dated by uranium-series methods; the coral ages ranged from 17 to 20 kyr, and were typically 10–15 kyr older than their respective phosphatic internal molds. These results are consistent with the sedimentological observations, and provide independent evidence that geologically meaningful ages can be obtained from marine phosphorites. Phosphate deposition off eastern Australia appears to have been largely continuous throughout the late Quaternary, rather than being restricted to times of high sea-level, as reported for phosphorites off Peru-Chile.