Reverse weathering

There is a bicarbonate imbalance between weathering and marine CaCOs precipitation of 3.4 - 2.4 x 10 = 1.0 x 10 eq (Note that eq and mol are interchangeable for HCO3 because it is singly charged.) 

The results of measurements of major ions in sediment porewaters collected by in situ filtration. Values are presented as the difference in the concentration between bottom water and porewater samples. Stations are from the continental margin and deep sea in the South Atlantic (A) on the mid-Atlantic Ridge between Rio de Janeiro and Capetown at c.2000 m (B) from the continental rise off Rio de Janeiro at 2000 m (C) from the continental slope off Rio de Janeiro at c. 1000 m and (D) from the Venezuela Basin east of Barbados at c.3000 m. Note that the concentrations of both K and Mg + decrease with depth in almost all cases. From Sa/les, (1979). 

Although the porewater measurements have become somewhat discredited because of this failure to achieve a balance between predicted reaction rates and sedimentary concentrations, it may be that Sayles s conclusion about the importance of these reactions in near-shore and river-mouth sediments (where the bulk of particulate material from rivers is deposited) is correct. One could assess this possibility by comparing Mg/Al and K/Al quotients in these sediments with those measured in the particulate material of the rivers. It would require a lot of highly accurate determinations (to within a few percent) and an excellent year-round measure of the Mg/Al quotient in the river particulate material to compare with the sediment measurements. Probably for this reason, this research has not been attempted. 

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This is a weathering reaction. Sillen argued that Goldschmidt s reaction could also go the other direction. The reverse reaction would be called reverse weathering. 

A proponent of "reverse weathering" suggested that gibbsite, kaolinite, and quartz exist in equilibrium according to the following equation. In equilibrium expressions for these reactions, water will appear as the activity, rather than concentration. The activity can be approximated by the mole fraction of water. What is the activity of water if this equilibrium is maintained Could this equilibrium exist in seawater, where the mole fraction of water is about 0.98 AG values (kj/mol) gibbsite — 2320.4 kaolinite — 3700.7 quartz —805.0 water —228.4. 

Michalopoulos, P. and Aller, R. C. (1995). Rapid clay mineral formation in Amazon Delta sediments reverse weathering and oceanic elemental cycles. Science 270, 614-617. 

Over longer time scales, clay minerals can undergo more extensive reactions. For example, fossilization of fecal pellets in contact with a mixture of clay minerals and iron oxides produces an iron- and potassium-rich, mixed-layer clay called glauconite. This mineral is a common component of continental shelf sediments. Another example of an authigenic reaction is called reverse weathering. In this process, clay minerals react with seawater or porewater via the following general scheme ... 

Direct evidence supporting the occurrence of reverse weathering has proven difficult to obtain for two reasons. First, the same kinds of clay minerals produced by this process are also transported to the ocean as part of the suspended load in river runoff. Second, the rate of reverse weathering is so slow that laboratory studies of this process are difficult to conduct. 

Reverse weathering is thought to occur during halmyrolysis, diagenesis, and catagenesis as clay minerals react with the major ions, bicarbonate, and DSi in seawater. The nature of these reactions is not well known. Their general form is thought to be... 

The need for invoking high rates of cation removal via reverse weathering was eliminated in the late 1970s when marine chemists realized that hydrothermal uptake and... 

Reverse weathering Chemical reactions that are theorized to occur in the sediments. In these reactions, seawater is thought to react with clay minerals and bicarbonate producing secondary clays and consuming alkalinity and some cations. This process is approximately the reverse of chemical weathering on land that produces clay minerals. 

Inputs and outputs of calcium, magnesium, and carbon can be balanced for the modern ocean. This balance necessitates reverse weathering reactions in the ocean system in which CO2 consumed in weathering on land is released to the ocean-atmosphere system during the formation of minerals in the ocean. If this were not the case, regardless of juvenile emissions of CO2 to the ocean-atmosphere system, atmospheric CO2 concentrations could be reduced to vanishingly small values in less than 5000 years. 

Mackenzie F.T., Ristvet B.L., Thorstenson D.C., Lerman A. and Leeper, R.H. (1981) Reverse weathering and chemical mass balance in a coastal environment. In River Inputs to Ocean Systems (eds. J.M. Martin, J.D. Burton and D. Eisma). pp. 152-187. UNEP-UNESCO, Switzerland. 

Ristvet B. (1978) Reverse weathering reactions within recent nearshore marine sediments, Kaneohe Bay, Oahu. Test Directorate Field Command, Kirtland AFB, New Mexico, 314 pp. 

Von Damm and Edmond (1984) utilized the lakes of the Ethiopian and northern Kenya rift zones to examine reverse weathering (the formation of authigenic clay minerals), because here evaporative concentration had not proceeded to the extent that salt precipitation interfered with a mass balance approach. They found that —60% of an alkalinity deficit could be accounted for by processes other than carbonate precipitation, and concluded that solute magnesium was lost as rapidly to clay as solute calcium was to carbonate. This situation, particularly in volcanic terrain, was also initially recognized at saline Lake Abert, Oregon, by Jones and VanDenburgh (1966). 

The Importance of Aluminum and the Rebirth of Reverse Weathering" APPENDIX A... 

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