Anatomy of a Sub-Cambrian Paleosol in Wisconsin: Mass Fluxes of Chemical Weathering and Climatic Conditions in North America during Formation of the Cambrian Great Unconformity

摘要

A paleosol beneath the Upper Cambrian Mount Simon Sandstone in Wisconsin provides an opportunity to evaluate the characteristics of Cambrian weathering in a subtropical climate, having been located at 20°S paleolatitude 500 My ago. The 285-cm-thick paleosol resulted from advanced chemical weathering of a gabbroic protolith, recording a total mass loss of 50%. Weathering of hornblende and plagioclase produced a pedogenic assemblage of quartz, chlorite, kaolinite, goethite, and, in the lowest part of the profile, siderite. Despite the paucity of quartz in the protolith and 40% removal of SiO_(2) from the profile, quartz constitutes 11%–23% of the pedogenic mineral assemblage. Like many other Precambrian and Cambrian paleosols in the Lake Superior region, the paleosol experienced potassium metasomatism, now containing 10%–25% mixed-layer illite-vermiculite and 5%–44% potassium feldspar. Estimates of mean annual precipitation and mean annual temperature are 1777 mm y~(?1) and 20.1°C, respectively, which are consistent with a paleolatitude of 20°S. For an atmospheric CO_(2) concentration of 4000–6000 ppm at 550–500 Ma, the duration of weathering is constrained to have been between 20,000 and 100,000 y. When the effects of erosion and influence of protolith composition are considered, the degree, or maturity, of weathering for the Wisconsin paleosol and four other sub-Cambrian paleosols is comparable to that for two modern soils in subtropical and temperate climates, despite the lack of land plants in Cambrian time. Such correspondent degrees of weathering likely result from the effects of elevated levels of atmospheric CO_(2) and microbial activity on weathering in Cambrian time.