Philip N. Froelich

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.

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.

Arsenic, barium, germanium, tin, dimethylsulfide and nutrient biogeochemistry in Charlotte Harbor, Florida, a phosphorus-enriched estuary

Concentrations of dissolved nutrients (NO3, PO4, Si), germanium species, arsenic species, tin, barium, dimethylsulfide and related parameters were measured along the salinity gradient in Charlotte Harbor. Phosphate enrichment from the phosphate industry on the Peace River promotes a productive diatom bloom near the river mouth where NO3 and Si are completely consumed. Inorganic germanium is completely depleted in this bloom by uptake into biogenic opal. The GeSi ratio taken up by diatoms is about 0·7 × 10−6, the same as that provided by the river flux, confirming that siliceous organisms incorporate germanium as an accidental trace replacement for silica. Monomethylgermanium and dimethylgermanium concentrations are undetectable in the Peace River, and increase linearly with increasing salinity to the seawater end of the bay, suggesting that these organogermanium species behave conservatively in estuaries, and are neither produced nor consumed during estuarine biogenic opal formation or dissolution. Inorganic arsenic displays slight removal in the bloom. Monomethylarsenic is produced both in the bloom and in mid-estuary, while dimethylarsenic is conservative in the bloom but produced in mid-estuary. The total production of methylarsenicals within the bay approximately balances the removal of inorganic arsenic, suggesting that most biological arsenic uptake in the estuary is biomethylated and released to the water column. Dimethylsulfide increases with increasing salinity in the estuary and shows evidence of removal, probably both by degassing and by microbial consumption. An input of DMS is observed in the central estuary. The behavior of total dissolvable tin shows no biological activity in the bloom or in mid-estuary, but does display a low-salinity input signal that parallels dissolved organic material, perhaps suggesting an association between tin and DOM. Barium displays dramatic input behavior at mid-salinities, probably due to slow release from clays deposited in the harbor after catastrophic phosphate slime spills into the Peace River.

Phosphorus accumulation rates in metalliferous sediments on the East Pacific Rise

Abstract Based on phosphorus, iron and manganese analyses in 16 cores (5 dated) from the crest and flanks of the East Pacific Rise and the Bauer Deep we estimate that phosphorus is being deposited about 20 times faster in metalliferous sediments near the rise crest than in adjacent flank deposits, and about 40 times faster on the crest than in the Bauer Deep. Almost all of the phosphorus on the rise crest is contained in poorly crystallized hydrothermal iron oxyhydroxides, supporting Berners (1973) proposal of phosphate sorption by these phases. The phosphate is probably derived from seawater, but some hydrothermal contribution cannot be excluded at this time. Flux estimates indicate that metalliferous sedimentation could remove 15–40% of the pre-agricultural river input of dissolved phosphate.

Chemical evidence for advection of hydrothermal solutions in the sediments of the Galapagos Mounds Hydrothermal Field

Pore water profiles of Ca, Mg, F, PO4−3 and Mn in the Galapagos Mounds Hydrothermal Field are believed to reflect, in part, upwelling of hydrothermal solutions through the sediments. Concentration-depth profiles in a low heat flow area just north of the Mounds Field display diagenetic changes typical of those found in pore waters underlying highly productive surface waters, consistent with the inference of no water flow or very slow downwelling (w < 5 cm/yr) of bottom water through these sediments. Rates of upward advection calculated from Mounds Field pore water profiles of Ca, Mg, and F profiles agree well with each other, averaging about 1 cm/yr in the pelagic sediments near the mounds and 15–30 cm/yr within the hydrothermal mounds themselves. The upward advection also modifies the shape of PO4−3 and Mn profiles. Advection rates inferred from the pore water data are generally in reasonable agreement with those made from heat flow data. The higher Ca and lower Mg, F, PO4−3 and Mn concentrations in Mounds Field pore waters (compared with those of the low heat flow area) suggest chemical exchange between the solution and basalt prior to upwelling. Li+, K+, Rb+, Sr++ and SO4− concentrations are indistinguishable from bottom water. This suggests very high effective water/rock ratios during the reactions which produced the upwelling solutions, perhaps due to extensive prior alteration of basalt adjacent to the flow path of water through the crust Inferred reaction temperatures are between 70–150°C.

Hydrothermal germanium over the southern East pacific rise.

Germanium enrichment in the oceanic water column above the southern axis of the East Pacific Rise results from hydrothermal solutions emanating from hot springs along the rise crest. This plume signature provides a new oceanic tracer of reactions between seawater and sea floor basalts during hydrothermal alteration. In contrast to the sharp plumes of 3He and manganese, the germanium plume is broad and diffuse, suggesting the existence of pervasive venting of low-temperature solutions off the ridge axis.

Measurement of lithium isotope ratios by quadrupole-ICP-MS: application to seawater and natural carbonates

We present an improved method for lithium isotope ratio (7Li/6Li) determinations with low total lithium consumption ( 99.98%), high isotope ratio precision (<±0.8‰, 2σ), and low blanks (1.0 ± 0.5 pg). We refine a single step ion chromatographic method to quantitatively recover and separate lithium from all matrix elements using small volume resin (2 ml/3.4 meq AG 50W-X8) and low volume elution (6 ml, 0.5 N HCl) with low procedural blanks (<500 fg/ml). We optimize the procedure for analyses of natural carbonates (foraminifera) containing 1 to 2 ppm lithium. This lithium separation method is applicable to other natural samples (e.g. seawater, pore-waters, mineral grains) by appropriate scaling. Isotope ratio measurements are made by a single collector Quadrupole ICP-MS (Agilent 7500cs) using cool plasma (600 W), soft extraction, peak jumping, and pulse detection mode with sample-standard bracketing. The precision is better than ±0.8‰ (2σ) for L-SVEC lithium standards and better than ±1.5‰ (2σ) for natural samples. We report a high matrix tolerance limit for sodium (∼0.6 mol/mol, Li/Na) and calcium (<20 µmol/mol, Li/Ca) for our Quadrupole ICP-MS method. Our seawater δ7Li value (30.75 ± 0.41‰, 2σ, n = 10) is the same as that reported by other workers (∼31.0 ± 0.5‰). Species-specific and bulk sample δ7Li analyses of two size fractions of core-top foraminifera yield values similar to modern seawater.

Systematic absorbance errors with technicon autoanalyzer II colorimeters

Abstract Technicon AutoAnalyzer II colorimeters have spectrophotometric flowcells with curved optical ends. Light passing through these flowcells is refracted severely at the non-perpendicular ends. Curved ends therefore cause differences in apparent absorbance measured at the phototube which are related to differences in the refractive indices of the solutions in the cells. This can cause significant error in the colorimetric analysis of solutions with different refractive indices (caused, for example, by variations in salinity). This effect is responsible for a systematic error which, for typical AutoAnalyzer determinations of phosphate in seawater or estuarine waters, can approximate 0.2 μM.

Geochemical behavior of inorganic germanium in an unperturbed estuary

Abstract Eleven monthly estuarine profiles of dissolved inorganic germanium (Gei) and silica (Si) in a natural, pristine river/bay system demonstrate that Ge-removal and -input parallel the seasonal silica cycle, reflecting Ge-uptake by and -dissolution from diatoms. The Ge/Si atom ratio of the river is 0.6 ± 0.15 × 10−6, which is near the average value for continental granites and for uncontaminated, remote, natural rivers (0.7 ± 0.3 × 10−6). The Ge Si ratio escaping this estuary to the ocean is 0.8 × 10−6, reflecting some estuarine enhancement of the fluvial Ge-flux, probably due to release of Gei from fluvial particulates. Nevertheless, the post-estuarine Ge Si ratio is not significantly different from the continental crustal ratio but is very different from the ratio in sea-floor hot springs and mid-ocean ridge hydrothermal plumes (4 ± 2 × 10−6) and in oceanic basalts (2.6 × 10−6). Thus natural estuarine processes do not obscure the contrasting Ge Si signatures entering the ocean from dissolution of continental and sea-floor silicates.

Deep silicate content as evidence of renewal processes in the Venezuela Basin, Caribbean Sea

Abstract Average dissolved silicate concentrations in waters deeper than 1400 m of the Venezuela Basin (Eastern Caribbean Sea) increase from 27.7 μ m in the north to greater than 29.0 μ m in the south. Standard deviations of these values (a measure of temporal heterogeneity) decline from 1.8 in the north to 1.0 in the south. These gradients result from sporadic inflow of silicate-poor North Atlantic Deep Water over the Jungfern Sill at the north end of the basin. This inflow lowers the silicate concentrations and increases temporal variability in the north. Mixing of this inflow water southward across the basin causes the observed north-to-south gradients in mean silicate concentrations and in standard deviations about these means.