The degree of chemical weathering in soils increases with mean annual precipitation (P; mm) and mean annual temperature (T; ЊC). We have quantified these relationships using a database of major-element chemical analyses of 126 North American soils. The most robust relationship found was between P and the chemical index of alteration without potash (CIA-K): with. Another strong relationship was found between P and 0.0197(CIA-K) 2 P p 221.12e Rp 0.72 the molecular ratio of bases/alumina (B): with. A Mollisol-specific relationship 2 P p Ϫ259.34 ln (B) ϩ 759.05 R p 0.66 was found relating P to the molar ratio of calcium to aluminum (C) as follows: with P p Ϫ130.93 ln (C) ϩ 467.4. Relationships between weathering ratios and T are less robust, but a potentially useful one was found 2 R p 0.59 between T and the molecular ratio of potash and soda to alumina (S) where with 2 T p Ϫ18.516(S) ϩ 17.298 R p. Our data also showed that most Alfisols can be distinguished from Ultisols by a molecular weathering ratio of 0.37 bases/alumina of !0.5 or by a chemical index of alteration without potassium !80. Application of these data to a sequence of Eocene and Oligocene paleosols from central Oregon yielded refined paleoprecipitation and paleotemperature estimates consistent with those from other pedogenic and paleobotanical transfer functions for paleoclimate.

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Three main features of paleosols are useful for distinguishing them from enclosing rocks: root traces, soil horizons, and soil structures.Fossil root traces are best preserved in formerly waterlogged paleosols. In oxidized paleosols their organic matter may not be preserved, but root traces can be recognized by their irregular, tubular shape, and by their downward tapering and branching. Often root traces are crushed like a concertina, because of compaction of the surrounding paleosol during burial. The top of a paleosol may be recognized where root traces and other trace fossils are truncated by an erosional surface. Root and other trace fossils are not useful for recognizing paleosols of middle Ordovician and older age, since large land organisms of such antiquity are currently unknown.Soil horizons usually have more gradational boundaries than seen in sedimentary layering. Commonly these gradational changes are parallel to the truncated upper surface of the paleosol. Some kinds of paleosol horizons are so lithologically distinct that they have been given special names; for example, cornstone (Bk) and ganister (E); the letter symbols are equivalent horizon symbols of soil science.Compared to sedimentary layering, metamorphic foliation, and igneous crystalline textures, soil structure appears massive, hackly, and jointed. The basic units of soil structure (peds) are defined by a variety of modified (for example, iron-stained or clayey) surfaces (cutans). Peds may be granular, blocky, prismatic, columnar, or platy in shape. Concretions, nodules, nodular layers, and crystals are also part of the original soil structure of some paleosols.Complications to be considered during field recognition of paleosols include erosion of parts of the profile, overlap of horizons of different paleosols, development of paleosols on materials eroded from preexisting paleosols, and the development of paleosols under successive and different regimes of weathering.

To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1-8 Myr) and early Permian (275-290 Myr) times. But for most of the Mesozoic era (65-250 Myr), CO2 levels were high (1,000-2,000 p.p.m.v.), with transient excursions to even higher CO2 (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records, but suggest that CO2 reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.

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The depth to carbonate nodular (Bk) horizon in soils (D in cm) is correlated with mean annual precipitation (P in mm), so that Bk horizons are deep in subhumid regions, but shallow in semiarid regions. Previous quantifications of this relationship are unchanged by this new compilation of 807 soils: P ‫؍‬ 137.24 ؉ 6.45D ؉ 0.013D 2 , where R 2 ‫؍‬ 0.52, and standard error (S.E.) ‫؍‬ ؎147 mm. In most North American postglacial soils, the Bk horizon is thin and well defined, whereas in monsoonal tropical soils of Pakistan and Kenya, the Bk horizon is thick and diffuse. Data from 675 modern soils define the relationship between the thickness of soil with nodules (T in cm) and mean annual range of precipitation (M in mm difference between monthly means of wettest and driest months): M ‫؍‬ 0.79T ؉ 13.71, where R 2 ‫؍‬ 0.58, and S.E. ‫؍‬ ؎22 mm). The relationship between carbonate nodule size (S in cm) and soil age (A in ka) is quantified by 9 radiocarbon-dated soils from Las Cruces, New Mexico: A ‫؍‬ 3.92S 0.34 , where R 2 ‫؍‬ 0.57, and S.E. ‫؍‬ 1.8 k.y. These transfer functions are applied to reconstructing paleoclimatic change from Paleocene-Eocene paleosols of the North Horn Formation and Flagstaff Limestone of Axhandle Canyon, Utah. The terminal Paleocene spike of warmth recorded by fossil plants in nearby Wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in Utah.

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Ediacaran fossils are taphonomically similar to impressions of fossil plants common in quartz sandstones, and the relief of the fossils suggests that they were as resistant to compaction during burial as some kinds of Pennsylvanian tree trunks. Fossils of jellyfish are known from siderite nodules and fine-grained limestone, and even in these compaction-resistant media were more compressed during burial than were the Vendobionta. Vendobionta were constructed of materials that responded to burial compaction in a way intermediate between conifer and lycopsid logs. This comparative taphonomic study thus falsifies the concept of Vendobionta as thin soft-bodied creatures such as worms and jellyfish.Lichens, with their structural chitin, present a viable model for the observed preservational style of Vendobionta, as well as for a variety of other features that now can be reassessed from this new perspective. The diversity of Ediacaran body plans can be compared with the variety of form in fungi, algae, and lichens. The large size (ca. 1 m) of some Ediacaran fossils is reasonable for sessile photosynthetic symbioses, and much bigger than associated burrows of metazoans not preserved. Microscopic tubular structures and darkly pigmented cells in permineralized late Precambrian fossils from Namibia and China are also compatible with interpretation as lichens. The presumed marine habitat of Ediacaran fossils is not crucial to interpretation as lichens, because fungi and lichens live in the sea as well as on land.