Submarine Weathering of Basalt

The reaction of CO2 dissolved in seawater with Ca and Mg silicate minerals in basalt can result in the formation of calcium and magnesium carbonates within the basalt (Staudigal et al., 1989). This is an additional process that could affect atmospheric CO2, because oceanic and atmospheric CO2 are in exchange contact. Some carbon cycle models include this process as a major control on CO2 (Brady and Gislason, 1997; Sleep and Zahnle, 2001; Wallmann, 2001). The idea is simply that if atmospheric CO 2 rises, the concentration in the ocean rises, and there is enhanced uptake by submarine basalt weathering. In this way the process serves as a negative feedback for stabilizing atmospheric CO2, and, in combination with silicate weathering on the continents, this would lead to greater dampening of CO2 fluctuations over time. This process would have provided a more important feedback during the Precambrian (Sleep and Zahnle, 2001) because of a thermally enhanced mantle-crust-ocean-atmosphere carbon cycle in the early earth. Furthermore, Brady and Gislason (1997) have suggested that increases in deep sea temperature, due to the greenhouse effect of elevated CO2, could lead to increased submarine basalt weathering and thereby act as an additional negative feedback.

The impact of the process of submarine weathering on atmospheric CO2 over the Phanerozoic has been suggested to be minor to negligible (Berner, 1991; Berner and Kothavala, 2001). Reasons for this conclusion are as follows: (1) If the process is driven by changes in basaltic seafloor creation (spreading) rate, as is assumed in all models treating this phenomenon, then increased CO2 uptake by faster seafloor spreading should be counterbalanced by faster CO2 emission due to thermal degassing at mid-oceanic rises and subduction zones. (2) Caldeira (1995) demonstrated that basalt weathering to CaCO3 cannot respond appreciably to changes in atmospheric and oceanic CO2 because the pH of the interstitial water in contact with the basalt is buffered to a high value, and basalt dissolution is thereby greatly impeded. (3) Caldeira (1995) also showed that the thermal feedback mechanism of Brady and Gislason (1997) is untenable based on thermal gradients within oceanic crust. (4) Based on strontium isotope analyses, Alt and Teagle (1999) stated that 70100% of the calcium in CaCO3 found in altered submarine basalts is derived from seawater, not from the basaltic minerals. This means that most of the basalt is not actually being weathered but is simply providing a place for the interstitial precipitation of Ca++ and HCO3- from sea-water. The neutral-to-alkaline interstitial seawater environment, due to hydrolysis of the basalt (Caldeira, 1995), may serve to raise pH and induce CaCO3 precipitation. (5) Most Ca release from basalt is not due to dissolution by CO2 (as carbonic acid) but by exchange with dissolved Mg in seawater (Alt and Teagle, 1999). (6) Assuming as a maximum that 30% of Ca is derived from basalt, and using the total formation rate of

CaCO 3 in submarine basalts given by Alt and Teagle, insertion of this degree of submarine weathering in the GEOCARB model (Berner and Kothavala, 2001) results in a very minor change in calculated CO2 levels.

During the distant geologic past, submarine basalt weathering may have been more important than at present. In the present oceans more carbonate is deposited in deep sea sediments than in submarine basalts, and the carbonates later join in contributing to CO2 degassing during subduction, but with basaltic carbonate being a lesser contributor. During the period before 150 Ma when there is little evidence for deep sea carbonate deposition (see chapters 3 and 4), it is possible that the subduction of basaltic carbonate could have been a more important factor in global degassing and in changes of atmospheric CO2 level.

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  • abdullah
    What are the process that change the submarine weathering?
    2 years ago

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