Weathering rind

Colman, S.M., and Pierce, K.L., 1992, Varied records of early Wisconsinan alpine glaciation in the western United States derived from weathering-rind thickness, in The Last Interglacial-Glacial Transition in North America, Clark, P.U. and Lea, P.D., eds.. Geological Society of America Special Paper no 270, pp. 269-278. 

Cation leaching (Dorn and Krinsley, 1991) from varnish provides materials for redeposition within the weathering rind - thus case hardening the outer shell of rock (Haberland, 1975)... 

Thickness Thickness increases over time The thickest varnishes grow in wetter micropositions that foster erosion of the underlying weathering rind or on spalled fracture faces. Thus, although varnish does thicken over time if all other formation factors are held constant, the thinnest varnishes can be the oldest at any given site... 

Tratebas, A., Cerveny, N. Dorn, R.I. (2004). The effects of fire on rock art Microscopic evidence reveals the importance of weathering rinds. Physical Geography 25, 313-333. 

Paleosols. Paleosols formed before 2.2 Ga tend to contain iron silicates, rather than siderite-iron carbonate. Rye et al. (1995) used carbonate-silicate mineral equilibria in the 2.75 Ga Mount Roe paleosol to estimate the partitioning of COa between soil and air in the late Archaean. Their calculations suggest a maximum partial pressure of 10 14 (0.04) atmospheres COa in the late Archaean atmosphere, significantly lower than the estimate of Lowe and Tice (2004) based on nahcolite. A lower limit for atmospheric COa comes from the study of siderite-clay mineral equilibria for weathering rinds on clasts in river gravels from the 3.2 Ga Moodies Group of the Barberton greenstone belt, in south Africa (Hessler et al., 2004). In this study the minimum partial pressure for atmospheric COa at 3.2 Ga was calculated to be 2.5 X 10 3 bars at 25°C. 

The data presented above can be used to set some limits on C02 levels over geological time. The 2.75 Ga paleosol evidence sets an upper limit of 0.04 bars on COa levels whereas the weathering rind study of Hassler et al. (2004) may set a lower limit of 0.0025 bars for 3.2 Ga. Calculations by Mel nik (1982) show that C02 levels were not greater than 0.1-0.15 bars, for otherwise BIFs would be present as iron carbonates rather than oxides. The nahco-lite study of Lowe and Tice (2004) lies within this range, although this result must be regarded as a maximum, as does the estimate of Zahnle and Sleep (2002) based purely on a mass balance between volcanic COa and subducted, carbonated ocean crust (Fig. 5.15). 

There are several important caveats to the use of this observational model. There is, for example, a considerable range of undersaturation, down to 50% in the case of the quartz, where surface defects are not especially reactive, and crystals appear smooth (Brandy et al., 1986). The formation of weathering rinds and the build-up of clays and other secondary... 

Sak, P.B., Fisher, D.M., Gardner, T.W., Murphy, C., Brantley, S., 2004. Rates of weathering rind formation on Costa Rican basalt. Geochim. Cosmochim. Acta 68, 1453-1472. 

The lichen prefer rock surfaces that face north because they are warmed by solar radiation, as demonstrated by Miotke (1979a) in Taylor Valley. His results indicated that the temperature at a depth of 10 cm in marble and dolerite fluctuated daily and approached +10°C in the afternoon, provided that the sun was shining. Liquid water that forms by melting of snow can infiltrate the underlying rock where it supports the metabolic activity of the endolithic organisms (Friedmann 1978) and promotes chemical weathering (Miotke 1980). When the weathering rind flakes off, the endolithic lichen are exposed at the surface where they cannot survive. 

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