SIDERITE-IRON-SILICATE EQUILIBRIA IN PALEOSOLS AS AN ATMOSPHERIC CO2 PALEOBAROMETER OR PALEOPRODUCTIVITY INDEX?

A CO₂ paleobarometer based on siderite and iron-silicate equilibrium (Rye et al., 1995, Nature, v. 378, no. 6557) is one of several paleobarometers devised for reconstructing the abundance of greenhouse gases in the geological past. Although devised largely for study of Precambrian paleosols, here we test its resolution using earliest Triassic paleosols in which potentially compromising soil-forming factors are better understood than for the Precambrian. A CO₂ and CH₄ greenhouse following the Permian-Triassic extinction is indicated by degree of weathering and carbon isotopic composition of paleosols (Krull and Retallack, 2000, GSA Bull., v. 112, no. 9). Earliest Triassic paleosols of Antarctica are berthierine-bearing, but contemporaneous Australian paleosols have both berthierine and siderite. Geochemical modeling presented here of a wide range of ground waters in equilibrium with both minerals suggests atmospheric pCO₂ levels of 60-100 times PAL which is unreasonable for the maximal likely earliest Triassic greenhouse. Siderite precipitation may reflect soil productivity rather than atmospheric composition because soil pCO₂ is 10-100 times atmospheric levels, and siderite precipitation is highly dependent on pore water HCO₃^-. Siderite-iron-silicate equilibrium is unlikely to be useful for attempts to constrain Precambrian paleoatmospheric conditions using palaeosols unless their biological productivity was unrealistically low.