H. Elderfield

Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments

Abstract Analyses have been made of REE contents of a well-characterized suite of deep-sea (> 4000 m.) principally todorokite-bearing ferromanganese nodules and associated sediments from the Pacific Ocean. REE in nodules and their sediments are closely related: nodules with the largest positive Ce anomalies are found on sediments with the smallest negative Ce anomalies; in contrast, nodules with the highest contents of other rare earths (3 + REE) are found on sediments with the lowest 3 + REE contents and vice versa. 143 Nd 144 Nd ratios in the nodules (∼0.51244) point to an original seawater source but an identical ratio for sediments in combination with the REE patterns suggests that diagenetic reactions may transfer elements into the nodules. Analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to REE contents of nodules and sediments is negligible. Inter-element relationships and leaching tests suggest that REE contents are controlled by a P-rich phase with a REE pattern similar to that for biogenous apatite and an Fe-rich phase with a pattern the mirror image of that for sea water. It is proposed that 3 + REE concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate. Processes which favour the enrichment of transition metals in equatorial Pacific nodules favour the depletion of 3 + REE in nodules and enrichment of 3 + REE in associated sediments. In contrast, Ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions.

Strontium isotope stratigraphy

The 87Sr/86Sr ratio of sea water has changed significantly with time in response to the input of varying proportions of Sr derived from continental crust and upper mantle sources, as moderated and buffered by carbonate recycling. Because of these fluctuations, the marine Sr isotope record can be used for stratigraphic correlation and absolute dating purposes. Recent work has been reviewed and results for 220 samples collated in δ87Sr units. They show four main periods of change over the past 100 million years within which variations during the Holocene to Late Miocene, Middle Miocene to Middle Eocene and Late Cretaceous (Maastrichtian and Campanian) are the most promising for Sr isotope stratigraphy with age uncertainties of better than ±1 myr. Equations describing the Sr isotope-age relationships are presented. Application of the method using carefully selected carbonate samples is now feasible; its extension to unfossiliferous sediments appears promising but it remains necessary to establish that specific authigenic phases retain the Sr isotopic compositions of contemporaneous sea waters.

On the biophilic nature of iodine in seawater

Abstract Vertical profiles of concentrations of iodate- and total-iodine have been measured at thirty stations in the Pacific, Atlantic and Antarctic Oceans. The salinity-normalised iodine profiles are indicative of both iodine removal and iodate reduction in the euphotic zone. Thus, surface waters appear to be depleted in iodate-iodine (by 0.03−0.22 μM) but less so in total-iodine (by ) when compared with the near-constant iodine concentrations (∼0.46 μM) at depth. Graphs of specific total-iodine versus specific phosphate fit a linear model well and lie within a narrow envelope for all stations, suggesting a direct coupling of iodine and nutrients during assimilation/regeneration. The I/C atom ratio calculated from these hydrographic data (1.0 × 10 −4 ) agrees well with contemporary plankton compositions ( I/C= 1.4 (±0.8) × 10 −4 ). Similar graphs involving specific iodate also fit a linear model well. However, their gradients vary from station to station leading to a variability in I/C interconversion ratio, analogous to the variability of Redfield nutrient ratios for coastal waters. This variation is attributed to changes in both productivity and nitrate availability. Pacific deep waters contain anomalously high total-iodine concentrations which may reflect regional differences of I/P ratio in surface waters or else diffusion of iodine from bottom sediments.

Chromium speciation in sea water

Abstract Thermodynamic considerations suggest that the ions Cr(H2O)4(OH)2+ and CrO42− are the major forms of trivalent and hexavalent chromium in sea water. Cr (VI) should predominate in water in solubility equilibrium with the atmosphere but analysis shows that theoretically unstable Cr (III) species are often dominant.

Negative cerium anomalies in the rare earth element patterns of oceanic ferromanganese nodules

Abstract Ferromanganese nodules from the Bauer Basin of the south equatorial Pacific are unlike virtually all oceanic nodules so far analyzed in showing negative Ce anomalies in their REE abundance patterns. In comparison with similarly Cu-Ni-enriched nodules from the north equatorial Pacific they are depleted in REE by 50–80% and are heavy REE enriched relative to intermediate REE. The REE patterns can be accounted for by the input of hydrothermal iron oxyhydroxides and associated REE to the Bauer Basin and the transfer of the REE to the nodules because of diagenetic reactions in the sediment. The excess iron input also is reflected in lower Cu/Ni ratios in the nodules as compared with nodules from the north equatorial zone, apparently because of the larger proportions of a residual Fe phase in the nodules relative to todorokite. Cerium anomalies of the Bauer Basin nodules range from −0.17 to −0.29 as compared with +0.33 to +0.07 in the north equatorial Pacific but show a parallel sensitivity of Ce anomaly to Mn/Fe ratios of the nodules. Nodules with the more positive anomalies within each group have the smallest Mn/Fe ratios and have been subjected to the greatest seawater influence whereas nodules with the more negative anomalies have the largest Mn/Fe ratios and have been subjected to the greatest diagenetic influence.

Rare earth element zonation in Pacific ferromanganese nodules

Abstract The lower surfaces of ferromanganese nodules from the north equatorial Pacific Ocean, which are enriched in Mn, Cu and Ni, and the upper surfaces, which are enriched in Fe, P and Co, have been analyzed for La, Ce, Nd, Sm, Eu, Gd, Dy, Er and Yb. The REE contents are lower and the Ce anomaly is smaller in the lower surfaces than in the upper surfaces. The magnitude of the Ce anomaly increases with decreasing Mn Fe ratio, indicative of a seawater origin. The zonal distribution of the other REE supports the conclusion derived previously from inter-nodule and nodule/sediment relationships that diagenetic fixation of rare earths in sediments affects their enrichment by nodular iron oxyhydroxides.

Strontium isotope geochemistry of Icelandic geothermal systems and implications for sea water chemistry

Abstract Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. 87 Sr 86 Sr ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising 87 Sr 86 Sr ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water.

Manganese fluxes to the oceans

Application of a simple model describing regional variations in the contents of manganese and associated minor metals in deep-sea sediments suggests that solid manganese phases are being removed from the <0.5 μm fraction of seawater at ∼1–7 · 1012g yr−1 in excess of the rate of stream-supplied manganese. This flux is consistent with: (1) the relative rates of sediment accumulation in the Atlantic and Pacific Oceans; (2) the contrast between the oceanic residence time of manganese calculated from stream-supply data (14 · 103 yr) and from the flux of manganese precipitating in marine sediments or as manganese nodules (0.38–2.4 · 103 yr); (3) the surplus mass of manganese revealed by geochemical balance calculations (22.9 · 102g). On this basis excess manganese is accumulating in deep-sea sediments at 0.2–2.0 · 10−6 g cm−2 yr−1. Manganese supplied to the upper layers of marine sediments by diagenesis has been evaluated with the aid of vertical advection—diffusion—reaction models. The calculated diagenetic flux of manganese at the sediment surface in a near-shore environment is in agreement with the known accretion rate of manganese deposits (1.7 · 10−2 g cm−2 10−3 yr−1) and the regionally variable flux over the area assessed is consistent with the presence or absence of manganese nodules at or near the water-sediment interface. The diagenetic flux at the surface of deep-sea sediments has been calculated at 0.7 · 10−4 g cm−2 10−3 yr−1 when the upper, oxic, zone of the sediment is ∼20 cm thick. A limiting factor on the in situ production flux of dissolved manganese in deep-sea sediments appears to be the availability of reducing agents for manganese dissolution rather than the rate of downward transport of manganese-rich sediment to a reaction boundary where dissolution takes place. Various estimates of the rate of upward-migrating manganese suggest that manganese precipitates in the oxic zone with a rate constant of ∼10−7 sec−1 with the result that diagenetic processes cannot supply the flux of excess manganese through more than ∼0.25 m of oxic sediment. However, estimates of the flux of manganese to the oceans by submarine volcanic processes (0.79–1.1 · 1012g yr−1) are similar to the surplus mass of manganese detected by geochemical balance calculations (0.7 · 1012g yr−1). If submarine hydrothermal solutions provide only 10% of this excess then their computed discharge rate (39 g cm−2 yr−1) and residence time in the upper layer of oceanic crust (130,000 yr) agree well with these parameters for continental thermal springs.

87Sr86Sr and 18O16O ratios, interstitial water chemistry and diagenesis in deep-sea carbonate sediments of the Ontong Java Plateau

Abstract Interstitial waters and sediments from DSDP sites 288 and 289 contain information on the chemistry and diagenesis of carbonate in deep-sea sediments and on the role of volcanic matter alteration processes. Sr Ca ratios are species dependent in unaltered foraminifera from site 289 and atom ratios (1.2–1.6 × 10−3) exceed those predicted by distribution coefficent data (~0.4 × 10−3). During diagenesis Sr Ca ratios of carbonates decrease and reach the theoretical distribution at a depth which is identical to the depth of Sr isotopic equilibration, where 87 Sr 86 Sr ratios of interstitial waters and carbonates converge. Mg Ca ratios in the carbonates do not increase with depth as found in some other DSDP sites, possibly because of diagenetic re-equilibration with interstitial waters showing decreasing Mg2+/Ca2+ ratios with depth due to Ca input and Mg removal by alteration of volcanic matter. Interstitial 18 O 16 O ratios increase with depth at site 289 to δ18O = 0.67%. (SMOW), reflecting carbonate recrystallization at elevated temperatures (

The determination of zirconium and hafnium in natural waters by isotope dilution mass spectrometry

= 20°C), the first recorded evidence of this effect in interstitial waters. Interstitial Sr2+ concentrations reach high levels, up to 1 mM, chiefly because of carbonate recrystallization. However, 87 Sr 86 Sr ratios decrease from 0.7092 to less than 0.7078, lower than for contemporaneous sea water, showing that there is a volcanic input of strontium at depth. This volcanic component is recorded in the Sr isotopic composition of recrystallized calcites. Isotopic compositions of the unrecrystallized calcites suggests that the rate of increase of the 87 Sr 86 Sr ratio of sea water with time has been faster since 3 my ago than in the preceding 13 my.