Check out our New Publishers' Select for Free Articles

Aquatic Geochemistry (v.11, #4)

CO₂ Air–Sea Exchange due to Calcium Carbonate and Organic Matter Storage, and its Implications for the Global Carbon Cycle by Abraham Lerman; Fred T. Mackenzie (pp. 345-390).

Release of CO₂ from surface ocean water owing to precipitation of CaCO₃ and the imbalance between biological production of organic matter and its respiration, and their net removal from surface water to sedimentary storage was studied by means of a quotient θ = (CO₂ flux to the atmosphere)/(CaCO₃ precipitated). θ depends not only on water temperature and atmospheric CO₂ concentration but also on the CaCO₃ and organic carbon masses formed. In CO₂ generation by CaCO₃ precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric CO₂ concentration (195–375 ppmv), and increasing CaCO₃ precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic carbon counteracts the CO₂ production by carbonate precipitation and it results in lower CO₂ emissions from the surface layer. When atmospheric CO₂ increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO₂ transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric CO₂. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon from land, the calculated CO₂ flux to the atmosphere is a function of the CaCO₃ and C_org net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO₂ flux near the Last Glacial Maximum is 17 to 7×10¹² mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time it is 38–42×10¹² mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic emissions of CO₂ to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total alkalinity owing to the CaCO₃ storage and consequent emission of CO₂. These results are in agreement with the conclusions of a number of other investigators. As the CO₂ uptake in mineral weathering is a major flux in the global carbon cycle, the CO₂ weathering pathway that originates in the CO₂ produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large imbalances of the atmosphere and land organic reservoir at 10²–10⁴-year time scales.

Keywords: carbon cycle; CO₂ air–sea flux; CO₂ production; euphotic zone; sedimentary storage; pre-industrial time; Last Glacial Maximum; cycle imbalances; carbon pathways

The Geochemistry of Supraglacial Streams of Canada Glacier, Taylor Valley (Antarctica), and their Evolution into Proglacial Waters by Sarah K. Fortner; Martyn Tranter; Andrew Fountain; W. Berry Lyons; Kathleen A. Welch (pp. 391-412).

We have investigated the geochemistry of supraglacial streams on the Canada Glacier, Taylor Valley, Antarctica during the 2001–2002 austral summer. Canada Glacier supraglacial streams represent the link between primary precipitation (i.e. glacier snow) and proglacial Lake Hoare. Canada Glacier supraglacial stream geochemistry is intermediate between glacier snow and proglacial stream geochemistry with average concentrations of 49.1 μeq L⁻¹ Ca2+, 19.9 μeq L⁻¹ SO₄²⁻, and 34.3 μeq L⁻¹ HCO₃⁻. Predominant west to east winds lead to a redistribution of readily soluble salts onto the glacier surface, which is reflected in the geochemistry of the supraglacial streams. Western Canada Glacier supraglacial streams have average SO₄²⁻:HCO₃⁻ equivalent ratios of 1.0, while eastern supraglacial streams average 0.5, suggesting more sulfate salts reach and dissolve in the western supraglacial streams. A graph of HCO₃⁻ versus Ca2+ for western and eastern supraglacial streams had slopes of 0.87 and 0.72, respectively with R2 values of 0.84 and 0.83. Low concentrations of reactive silicate (> 10 μmol L⁻¹) in the supraglacial streams suggested that little to no silicate weathering occurred on the glacier surface with the exception of cryoconite holes (1000 μmol L⁻¹). Therefore, the major geochemical weathering process occurring in the supraglacial streams is believed to be calcite dissolution. Proglacial stream, Anderson Creek, contains higher concentrations of major ions than supraglacial streams containing 5 times the Ca²⁺ and 10 times the SO₄²⁻. Canada Glacier proglacial streams also contain higher concentrations (16.6–30.6 μeq L⁻¹) of reactive silicate than supraglacial streams. This suggests that the controls on glacier meltwater geochemistry switch from calcite and gypsum dissolution to both salt dissolution and silicate mineral weathering as the glacier meltwater evolves. Our chemical mass balance calculations indicate that of the total discharge into Lake Hoare, the final recipient of Canada Glacier meltwater, 81.9% is from direct glacier runoff and 19.1% is from proglacial Andersen Creek. Although during a typical, low melt ablation season Andersen Creek contributes over 40% of the water added to Lake Hoare, its overall chemical importance is diluted by the direct inputs from Canada Glacier during high flow years. Decadal warming events, such as the 2001–2002 austral summer produce supraglacial streams that are a major source of water to Lake Hoare.

Phase Transformations and Proton Promoted Dissolution of Hydrous Manganite (γ-MnOOH) by Madeleine Ramstedt; Staffan Sjöberg (pp. 413-431).

The objective of this study was to describe the proton promoted disproportion of synthetic manganite (γ-MnOOH) and to characterise the resulting phase transformations. The solution and remaining solid phase after disproportionation was analysed by techniques including atomic absorbance spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). In suspensions with pH between 5 and 7, −log[H⁺] was monitored for 17 months and equilibrium constants were determined at 9, 12 and 17 months of reaction time for the following reaction (25 °C, 0.1 M (Na)NO₃): $$2gamma hbox{-MnOOH} + 2hbox{H}^{+} ightleftharpoonshbox{MnO}_{2} + hbox{Mn}^{2+} +2hbox{H}_{2}hbox{O}$$ The formed MnO₂ ages with time and the equilibrium constant for a metastable phase (ramsdellite or nsutite) as well as the most stable phase, pyrolusite (β-MnO₂), was determined. Furthermore, combined pH and pe (Eh) measurements were performed to study the equilibrium; $$gamma hbox{-MnOOH(s)} ightleftharpoonsetahbox{-MnO}_{2}(hbox{s}) + hbox{H}^{+} + hbox{e}^{-}$$ Real-time AFM measurements of the dissolution showed shrinkage of the length of the manganite needles with time (2 hours). After 1 week SEM images showed that this decreased length also was followed by a reduced thickness of the manganite needles. From the SEM images the morphology of the formed Mn(IV) oxides was studied. At pH 2.6, pyrolusite (β-MnO₂) and MnCl₂ were found in the XRD patterns. Throughout the pH range there were indications of ramsdellite (MnO_1.97) in the XRD patterns, which coincided with the existence of a fraction of needle shaped crystals with smaller dimensions (compared to manganite) in the SEM images. These observations together with the long term dissolution experiments suggest that the dissolution of manganite initially forms a ramsdellite or nsutite phase that over time rearranges to form pyrolusite.

Keywords: manganite; gamma-MnOOH; disproportionation; dissolution; pyrolusite; MnO₂AFM; XRD; SEM; phase transformation

Hyperfiltration of Nacl Solutions Using a Simulated Clay/Sand Mixture at Low Compaction Pressures by Rosanna Saindon; T. M. Whitworth (pp. 433-444).

It is widely recognized that clays and shales can demonstrate membrane properties. When a hydraulic head differential exists across a membrane-functioning clay-rich barrier, some of the solute is rejected by the membrane. This process is known as hyperfiltration. Some shallow geologic environments, including aquitards bounding shallow perched aquifers and unconfined aquifers, some river and stream beds, and some lake bottoms contain clay–soil mixes. Many engineering structures such as landfill liners, mixed soil augered barriers, and retention pond liners also consist of soil–clay mixes. No previous testing has been performed to investigate the likelihood that hyperfiltration may occur in such mixed soils. Therefore, we performed five experiments using different mixes of Na-bentonite and glass beads (100, 50, 25, 12 and 0% clay) to determine if any of these mixes exhibited membrane properties and to investigate what effect clay content had upon the membrane properties of the soil. Each mixture was compacted to 345 kPa and the sample mixtures were 0.58–0.97 mm thick. All the experiments used an approximately 35 ppm Cl⁻ solution under an average 103 kPa hydraulic head. Experimental results show that all the simulated clay–sand mixtures do exhibit measurable membrane properties under these conditions. Values of the calculated reflection coefficient ranged from a low of 0.03 for 12% bentonite to 0.19 for 100% bentonite. Solute rejection ranged from 5.2% for 12% clay to a high of over 30% for the 100% clay. The 100% glass bead sample exhibited no membrane properties.

Keywords: clay; bentonite; hyperfiltration; reverse osmosis; engineered barriers; aquitard

Contents to Volume 11 (pp. 445-446).

Sections

Personal tools

Aquatic Geochemistry (v.11, #4)

Sections

Personal tools

Local resources

Aquatic Geochemistry (v.11, #4)