Sediments underneath the high productivity areas of the coastal upwelling and equatorial divergence zones in the Pacific and Indian oceans are enriched in barium, and on this basis the barium content of the sediments has long been suggested as a potential palaeoproductivity indicator. Analyses of sediments from the Peru margin corroborate the pattern of enhanced barium accumulation in areas of high primary productivity in the deep ocean, but also show that this pattern cannot be extended to shallow-water deposits. Sediments from the Peru shelf lack any barium enrichment, whereas this element is significantly enriched in slope and basinal deposits in water columns deeper than 2000 m. This depth effect is further illustrated at Site 682 located in the rapidly subsiding Lima Basin. The Quaternary sequence deposited at water depths >3000 m is enriched in organic carbon, opal and biogenic barium. The Miocene sediments, although deposited under highly productive waters associated with a coastal upwelling centre, do not show an enrichment in the barium record corresponding to the very high organic carbon and opal levels; a pattern that is consistent with deposition in shallow basins. In addition to the effect of water depth on the barium distribution, overprinting of primary signals can be observed in sediments from upwelling areas undergoing strong anoxic diagenesis. Microcrystalline barite is partly dissolved in intervals depleted in interstitial sulphate as shown for sediments from the Peru margin. Dissolution of barite in sulphate depleted intervals of deep water sections leads to high barium concentrations near the termination of the sulphate reducing zone, where downward sulphate and upward barium diffusion foster local barite precipitation in diagenetic fronts. The barium distribution in sedimentary oxic and suboxic environments at deep water depositional sites has a high potential as a palaeoproductivity indicator; however, barium accumulation as a proxy for ocean fertility should be used with caution in areas of varying water depths and in anoxic diagenetic environments where sulphate depletion undersaturates the interstitial waters with respect to barite.

The observed distribution of dissolved magnesium in the pore water of rapidly accumulating sediments shows significant deviations from the seawater value. We have shown that deviations during early diagenesis can be explained by reactions occurring at the surface of sediment particles. In anoxic pore water environments with high levels of dissolved total carbon dioxide the formation of Mg+2-CO;' complexes significantly reduces the concentration of the free Mg+' ion. This decrease in the Mg+* activity results in desorption of magnesium from the solid surfaces due to the re-equilibration of the adsorbed Mg+' with the dissolved species. The effect of increasing carbonate complexation of Mg+2 in anoxic environments is initially compensated by the loss of sulfate, which is also a strong Mg+* l&and. Therefore, significant changes in free Mg+2 concentration and thus in the magnesium desorption from solid surfaces by ligand competition for Mgt2 are more pronounced in sulfatedepleted systems undergoing methanogenesis. Such conditions are characteristic of most continental margin sediments. Another consequence of the decomposition of organic matter in hemipelagic sediments is the accumulation of high levels of ammonium ions which also displace Mg+2 from sediment-particle surfaces by ion exchange. These equilibria in the pore water-sediment systems can be described by empirical parameters, which were experimentally obtained. A computer model was used to determine the equilibrium conditions for solid-solution reactions as a function of changes in the pore-water composition in organicrich hemipelagic environments. This model includes complex formation, competition for Mg+' between dissolved ligands and exchange sites, and Mg*'/NH: exchange reactions. The relative proportion of desorbed and displaced Mg+' from the solid surface depends on the characteristics of the sediment and on the ZCOz:NH: regenerative ratio in the pore waters. In sediments from Bransfield Strait, the Gulf of California, and the Peru margin, both release mechanisms for Mg"-l&and competition and ion exchange with ammonium-were evaluated as part of the complex reaction system in order to explain the observed maxima in the dissolved magnesium profiles. Overlying the Mg+' maxima, the Bransfield Strait and Gulf of California pore waters show minima in the dissolved magnesium concentration, concurrent with a measured increase in the cation exchange capacity (CEC) of the sediments. By including the observed CEC changes in the multi-component model we show that the negative anomaly in the dissolved Mg+' profiles is a consequence of changes in the CEC of sediments during the very early stages of anoxic diagenesis.

Sediments from four sites drilled during Ocean Drilling Program (ODP) Leg 112 exhibit chemical variability that distinguishes diatomaceous muds in shallow water (Site 680) from those muds of deeper water (Sites 682, 685, and 688). Differences in the concentrations of elements precipitated or mobilized during epigenesis and diagenesis, in particular barium abundance, show promise as indicators for depositional environment. The distribution of barium is related to detrital components in Site 680 and related to epigenetic and diagenetic processes in deep-water sites (Sites 682, 685, and 688), making it a potential indicator for water depth in siliceous, organic carbon-rich sediments. Bromine abundance relative to organic carbon may be inversely related to the age of the sections. We document that bromine (in solids) and bromide (in dissolved) phases do not behave conservatively, but that they are instead related to organic matter remineralization. On the basis of differences in the abundances of detrital and diagenetic components, we can statistically discriminate between the shelf and slope deposits of the Peru forearc basins. A significant deficit in detrital accumulation is apparent in upper Miocene through Pliocene sediments at Sites 682 and 685 on the lower slope, as compared to a steady, if erratic, increase of detrital influx during the Pleistocene. Our data on the concentration of elements in the detrital component on the lower-slope sediments further suggest that substantial sources of clastic material deposited on the lower slope have not yet been identified, because published estimates of river runoff and clastic sedimentation on the upper slope and shelf fall short by at least an order of magnitude.

The barium distribution in sediments and pore fluids from five sites drilled in the Japan Sea have been used to illustrate the geochemical behavior of this element as it pertains paleoproductivity reconstructions, diagenetic remobilization, and barite precipitation in authigenic fronts. Sites where sulfate is depleted in the pore fluids also show high concentrations of dissolved barium, reflecting dissolution of biogenic barite. The high rate of sedimentation at Sites 798 and 799 results in a rapid sulfate depletion, which in turn leads to barite dissolution and reprecipitation in diagenetic fronts. The dissolved barium distribution at these sites has been used to quantify the rate of barite dissolution; we estimate a first-order rate constant for barite dissolution to be 2 × lO^/s at Site 799 and 2 × 10" 7 /s at Site 798. Authigenic barite has been documented in sediments from Site 799 at 323 meters below seafloor by scanning electron microscopy and X-ray fluorescence analysis. These results indicate barite precipitation in a diagenetic front near the zone of sulfate depletion by upward migration of dissolved barium and downward diffusion of sulfate. Barite precipitation has also been inferred at Sites 796 and 798 based on sedimentary and dissolved barium distributions. Sulfate is not depleted in the pore fluids of Site 794. The lack of diagenetic remobilization of biogenic barium at this site preserves the high barium signal associated with the high-productivity sequences deposited during the late Miocene to Pliocene. Significantly, the organic carbon distribution does not indicate high accumulation rates during the periods of high opal and barium deposition. Instead, higher organic carbon accumulations are recorded in the Quaternary and middle Miocene sequences; intervals that are also characterized by deposition of siliciclastic turbidites. The presence of a terrestrial component in the organic carbon record renders barium a more useful indicator than organic carbon for paleoproductivity reconstructions in this marginal sea.

Shipboard and shore-based analyses of -230 interstitial-water samples gathered at six sites during ODP Legs 127 and 128 indicate that diagenetic reactions within sediment columns throughout the Japan Sea record the influence of bacterial degradation of organic matter, transformation of amorphous biogenic opaline silica to opal-CT, and formation and alteration of carbonate phases, as well as alteration of crystalline basement and volcanic ash.Organic matter oxidation not only affects the distribution of oxidants (SO 4 , Mn, and CO 2 ) and dissolved metabolites (alkalinity, NH 4 , and PO 4 ), but also controls the distribution of many authigenic solid phases. Site 794, located deep in the Yamato Basin, is the only site where sulfate is not completely consumed, whereas Sites 798 and 799 record the total consumption of sulfate at shallow depths and concomitant extreme generation of alkalinity, NH 4 , and PO 4 . Authigenic carbonates, phosphates, and barite also record the importance of bacterial activity. The diagenetic opal-A/opal-CT transition is a notable chemical, diagenetic, and stratigraphic horizon that occurs more shallowly in the basinal sites because of their greater geothermal gradient. Due to its effect on the porosity and permeability of the involved lithologies, the siliceous phase transformation effectively restricts diffusion between reactions occurring in the uppermost sediment column from those involved in deep basement alteration. Processes that influence dissolved Ca, Mg, Sr, and alkalinity tend to be preferentially dominant at either basinal (Site 794, 795, and 797) or ridge (all other) sites, due to the distinctly different depths and sedimentation rates of the two depositional regimes. At basinal sites, where sediment accumulation is relatively slow and occurs below the CCD, basement alteration is the dominant influence on dissolved Ca and Mg profiles. At ridge sites, where accumulation is relatively fast and occurs above the CCD, diagenetic reactions such as calcite dissolution, inorganic carbonate precipitation, dolomitization, and siderite/ankerite formation control Ca and Mg behavior. Basement alteration reactions are responsible for large overall decreases in Mg, K, Rb, Li, B, δ l8 0, δD, 87 Sr/ 86 Sr, and Na with depth, as well as for increases in Ca and Sr. Alteration of ash in the sediment column also influences these downhole profiles. Site 795, in the Japan Basin, preserves the greatest diagenetic signal of basement alteration. Gradients resulting from basement alteration document the importance of diffusive chemical exchange between the endogenic and exogenic reservoirs.