Gary P Klinkhammer

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Abstract Pore water profiles of total-CO 2 , pH, PO 3− 4 , NO − 3 plus NO − 2 , SO 2− 4 , S 2− , Fe 2+ and Mn 2+ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized ( O 2 > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO 2 concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.

The distribution of manganese in the Pacific Ocean

Abstract This report presents the vertical distributions of “total dissolvable manganese” at 13 stations in the Pacific. Manganese concentrations vary with depth in a unique way. There is commonly a maximum concentration at the surface and a minimum at the top of the thermocline. At stations where the oxygen in the oxygen minimum falls below 100 μmol/kg, there is a manganese maximum in the oxygen minimum. At stations where the oxygen minimum is so intense that nitrate reduction occurs, especially high manganese concentrations are observed in the zone of nitrate reduction. This maximum in the O 2 minimum is believed to originate either as reduction of Mn oxides in nearshore sediments and subsequent advection or by seawater equilibrium with a metastable oxide similar to hausmannite (Mn 3 O 4 ). Surface water values range from about 3 to about 0.3 nmol/kg. The surface concentrations show a general correlation with 210 Pb, suggesting that the major source of manganese to the surface waters of the Pacific is desorption from atmospheric particulates. The residence time of Mn in surface waters is 5 to 25 years.

Early diagenesis in sediments from the eastern equatorial Pacific, II. Pore water metal results

Concentrations of Mn, Fe, Ni and Cu were measured in pore waters extracted from pelagic sediments at two sites in the eastern equatorial Pacific and at two locations in the eastern equatorial Atlantic. The results from these four sites suggest that the pore water profiles of these metals are controlled by processes involving the major oxidants (O2, NO3−, Mn and Fe oxides). Dissolved copper concentrations in the top 2 cm of sediment are 10 times the bottom water value. This shallow regeneration is an important factor in the oceanic budget of copper. Manganese, nickel and iron concentrations near the interface are indistinguishable from bottom water values. At depth in the sediment, Mn oxides are reduced and manganese diffuses upwards to be reoxidized and trapped within the sediment column. The highest nickel concentrations in the Mn reduction zone are 40 times that of bottom water. This nickel gradient drives a flux into the Mn oxidation zone which is large enough to influence the nickel concentration of the bulk sediment. Dissolved Mn concentrations range from 10 nmol/kg in the oxygenated zone to 100 μmol/kg in the reduction zone.

Early diagenesis in sediments from the eastern equatorial Pacific, I. Pore water nutrient and carbonate results

Interstitial waters were extracted from cores at three locations in the eastern equatorial Pacific and analyzed for nutrients, dissolved carbonate species, Mn and Fe. From the depth variation in pore water chemistry, we infer that organic matter oxidation reactions occur with depth in the following sequence: O2 reduction, NO3− and MnO2 reduction, and then ferric iron reduction. From NO3− results we infer that O2 is largely or totally consumed within the top few centimeters of sediment. NO3− is completely reduced at a sediment depth of 20 cm at a site near the crest of the East Pacific Rise, but is preserved at levels of 20–30 μmol/kg at 40 cm depth at a Guatemala Basin site. We have calculated the alkalinity for pore water samples assuming ions diffuse according to relative ionic diffusion coefficients, that the stoichiometry of organic matter oxidation reactions is that of “Redfield” organic matter, and that the pore waters are saturated throughout with respect to CaCO3. The measured alkalinity increase is only about half of the predicted value. The difference is probably a result of either enhanced mixing of the pore water in the top few centimeters of sediments by biological or physical processes, or the occurrence of an inorganic reaction which consumes alkalinity. At depths of oxygen and nitrate reduction in the sediments, the ion concentration product of CaCO3 is the same, within the analytical error, as the solubility product of Ingle et al. [34] at 1 atm and 4°C. This result indicates CaCO3 resaturation on pressure change during coring. Where pore water Mn concentrations become measurable, the ion concentration product increases, indicating either supersaturation with respect to calcite or that another phase is controlling the carbonate solubility.

Manganese in seawater and the marine manganese balance

Abstract Surface water samples from the Sargasso Sea, the western North Atlantic, and the northeast Pacific all have manganese concentrations of about 0.1 ppb. Sargasso Sea and northeast Pacific deep water samples have about one-fourth those concentrations; concentrations in the North Atlantic western boundary current appear intermediate. The data suggest that manganese is removed from seawater more rapidly than the general oceanic turnover time of about 1,500 years. Downward transport of manganese in downwelling surface water and falling biogenic debris can supply only a few per cent of the total amount accumulating in pelagic sediments, although it can supply all the manganese accumulating in ferromanganese crusts and nodules. A Sargasso Sea and a northeast Pacific profile show no indication of a deep water manganese maximum, suggesting that submarine volcanism and hydrothermal input is not a major source of manganese to either deep ocean water or normal pelagic sediments. Because the sedimentary manganese is supplied neither by dissolved manganese in seawater nor by hydrothermal inputs, we conclude that the ‘excess’ manganese in pelagic sediments comes from terrigenous particles.

Helium-3 and manganese at the 21°N East Pacific Rise hydrothermal site

Abstract Water samples collected at the 21°N hydrothermal site on the East Pacific Rise crest, including Deep-Tow and hydrocast samples collected in 1977 and three hot vent water samples collected recently with the submersible “Alvin”, contain significant additions of 3 He, 4 He, and Mn. Although the vent water collections were at least 50-fold diluted with ambient seawater, they are up to 53 times enriched in 3 He and 7.4 times enriched in 4 He relative to saturated seawater, with concentrations of total dissolvable manganese (TDM) up to 310 μg/kg. 3 He and 4 He covary in the vent samples, with 3 He/ 4 He about 8 times the atmospheric ratio, reflecting a mantle helium source. In contrast to the helium isotopes the Mn/ 3 He ratio in the vent samples is variable, ranging from 4.3 × 10 4 up to 1.0 × 10 5 g/cm 3 . Profiles of 3 He/ 4 He and TDM in the water column at 21°N show a sharp maximum of δ( 3 He) = 47%and TDM= 0.69 μg/kg , much higher than the average values of 34% and 0.2 μg/kg for the deep water in this region. This spike in 3 He and Mn occurs at 2400 m depth, 200 m above the level of the 21°N vents, and 100 m higher than any local bathymetry, evidence for upward transport of the hydrothermal discharge via rising plumes of hot vent water. Two of the 21°N Deep-Tow samples associated with small ( ⩽0.010°C ) temperature anomalies had δ( 3 He) = 38%and TDM= 0.28 and 0.58 μg/kg , also slightly elevated relative to background. The Deep-Tow and hydrocast samples have lower Mn/ 3 He ratios than average vent samples due to Mn removal by scavenging. Comparison of vent samples and water column measurements at 21°N indicate that the pure vent water could be detected using 3 He and Mn even when diluted ∼10 5 times with seawater, confirming that these two tracers are extremely sensitive indicators of submarine hydrothermal activity.

Hydrothermal manganese in the Galapagos Rift

HYDROTHERMAL emanations originating at mid-ocean ridges have been thought1–5 to provide a substantial source of manganese to the ocean but the evidence supporting this hypothesis has been indirect. Anomalous manganese concentrations have been measured in naturally occurring systems where seawater is in direct contact with lava flows6–8. Laboratory studies have shown that seawater tends to leach manganese from basalts at elevated temperatures and pressures9–11. Anomalously high manganese accumulation rates have also been determined for sediments adjacent to active ridge systems, most notably the East Pacific Rise12–14. No measurements of manganese concentrations in seawater near mid-ocean ridges, or in hydrothermal fluids emanating from these ridges, have yet been made, however. We report here the results of the first such direct measurements, which show that manganese is being injected into the deep sea by hydrothermal circulation of seawater through newly-formed oceanic crust.

Manganese and copper fluxes from continental margin sediments

Total dissolvable Cu and Mn have been measured in seawaters collected from the continental shelf of the eastern Bering Sea. Copper concentrations of 4 nmole kg−1 inshore of a hydrographie front over the 100 m isobath. Manganese concentrations also were low over the shelf break, 10 nmole kg−1 inshore of the hydrographic front. Depth distributions of Mn at all continental shelf stations showed gradients into the sediments, with concentrations typically >20 nmole kg−1 in a bottom layer extending about 30 m off the bottom. Benthic Cu and Mn fluxes are indicated by cross-shelf pore water profiles that show interfacial concentrations more than an order of magnitude greater than in bottom water. These data and the results of a model of metal transport across the shelf suggest that Cu and Mn fluxes, estimated at 2 and 18 nmole cm−2y−1, respectively, from continental shelf sediments may be one “source” of these metals to the deep sea.

Trace metal distributions in the Hudson River estuary

Concentations of reactive PO 4 , particulate Al and dissolved and particulate Cd, Fe, Zn, Ni, Cu and Mn were measured in the Hudson River estuary. Soluble trace metal concentrations ranged from 1 to 7 parts 10 −9 Cu; 1 to 60 parts 10 −9 Mn; 0·8 to 11 parts 10 −9 Ni; 3 to 33 parts 10 −9 Zn; 0·1 to 0·5 parts 10 −9 Cd and 5 to 96 parts 10 −9 Fe. The particulate levels were 0·2 to 6 parts 10 −9 Cu; 0·2 to 40 parts 10 −9 Mn; 0·1 to 8 parts 10 −9 Ni; 0·04 to 14 parts 10 −9 Zn; 0·005 to 0·3 parts 10 −9 Cd and 7 to 2200 parts 10 −9 Fe. With the exception of iron, the discharge of these metals with Manhattan/New Jersey sewage was comparable to the riverine flux. Anthropogenic Zn, Ni and Mn inputs were reflected in dissolved concentration maxima in New York Harbor. Mass-balance calculations demonstrate that the anthropogenic fraction of these metals left the Harbor in the dissolved form. Anthropogenic Cd, Fe and Cu were associated with solids and these metals were deposited in Harbor sediments.