Non-rainfall water inputs (NRWI) are important sources of water in arid regions. The least studied NRWI is the direct sorption of atmospheric water vapor. It occurs near the surface in sandy desert soils but has not been investigated in wetter climates, where it could be important during seasonal or episodic drought, or in clay-rich soils, where it might be expected to occur more extensively. We report soil temperatures and soil water isotopic compositions indicating sorption of atmospheric water vapor deep within floodplain Vertisols in Texas, USA where annual rainfall exceeds 1000 mm. Profiles during the summer show that soil water delta 18O and delta D values above 40 cm plot to the right of the local meteoric water line (LMWL), consistent with liquid water undergoing evaporation, whereas those below 40 cm plot to the left of the LMWL (by as much as 2 parts per thousand in delta 18O space), confirming the presence of a diffused vapor component. Spatial and temporal coincidence at 100 cm during JuneJuly of 1) maximum latent heating, 2) maximum deviation to the left of the LMWL and 3) agreement between independent hydrogen and oxygen isotope mass balance calculations indicate that as much as 5 % of the soil water at this depth is condensed atmospheric vapor. The downward vapor flux is probably facilitated by the high smectite content, which makes these soils highly hydrophilic and results in deep cracking during the summer. These cracks provide air-filled conduits through which water vapor diffuses and sorbs in the cool subsurface. The downward flux of water vapor during the summer may help mitigate drought by partially replacing transpired water. Plain Language Summary: In most soils, water addition occurs exclusively through precipitation. In some soils, fog and dew are substantial water sources. In dry climates, some soils have been shown to remove water vapor directly from the air as it sticks to the surfaces of soil particles overnight. Although this water source is small in comparison to rainfall, the process of atmospheric vapor sorption helps moderate how much the land surface heats up during the day and may help sustain organisms during drought. In this study, we report observations that are consistent with an atmospheric water vapor source to clay-rich soils at deeper depths and in a wetter climate than has been previously recognized. We use isotopic compositions of water and soil temperatures as indicators of this process and provide a conceptual model in which the clay-rich soils of interest crack when dry providing conduits for vapor to penetrate to about 1 m depth where it is absorbed by the clay. This process may buffer the effects of drought.

Identifying paleosols with aquic conditions is essential for understanding the evolution and distribution of ancient wetland systems. Yet there are no guidelines for paleopedologists for writing field descriptions to target properties that indicate wetness. Here we provide field indicators for aquic conditions modified from the U.S. Soil Taxonomy based on organic materials, matrix color, and iron redoximorphic features. These field-based soil morphological properties have been correlated to numerous hydrological monitoring stations. The universal indicators presented here were developed from four of the eleven mineral soil orders and are the most conservative for identifying aquic conditions across paleosol types. If aquic conditions are not met with these indicators, others are provided for specific taxonomic orders where it is difficult to form or observe aquic indicators; for example, iron rich paleo-Oxisols. Strategies are provided to help separate paleosol properties that may be diagenetically altered from those representing environmental conditions during soil formation prior to burial. The aquic indicators were tested 102 paleosols showing that more than one-fourth were originally mis-identified. However, testing of a stratigraphic succession of two paleosol studies indicated that between 25 and 50 % failed to meet the interpretations of the original investigators. Testing also showed the difficulty of identifying aquic conditions in seasonally wet paleosols, but the indicators provide consistency in how those decisions were made. The aquic condition criteria may be applied to paleosols classified by any taxonomic scheme. This is a first approximation and will require further testing of the indicators provided here.

An investigation of terrestrial weathering reveals that the magnitude of Phanerozoic weathering was greater than that of Precambrian weathering; specifically, 36 +/- 3 % of the labile constituents, SiO2, CaO, Na2O, and K2O, were removed from the protoliths of five modern soils, 37 +/- 11 % from four Cretaceous protoliths, 24 +/- 4 % from four Cambrian protoliths, 18 +/- 2 % from nine Proterozoic protoliths, and 17 +/- 2 % from four Archean protoliths. Potassium metasomatism is a common phenomenon in paleosols, and fourteen of seventeen of the Cambrian and Precambrian paleosols record the post-weathering addition of potassium, which was corrected for in calculating mass fluxes of weathering. The greater magnitude of Phanerozoic weathering compared to Precambrian weathering is attributed to the effects of elevated concentrations of organic acids in the critical zone, which were produced by land plants that evolved in Cambrian time and eventually colonized the continents.

Living hominoids are distinguished by upright torsos and versatile locomotion. It is hypothesized that these features evolved for feeding on fruit from terminal branches in forests. To investigate the evolutionary context of hominoid adaptive origins, we analyzed multiple paleoenvironmental proxies in conjunction with hominoid fossils from the Moroto II site in Uganda. The data indicate seasonally dry woodlands with the earliest evidence of abundant C4 grasses in Africa based on a confirmed age of 21 million years ago (Ma). We demonstrate that the leaf-eating hominoid Morotopithecus consumed water-stressed vegetation, and postcrania from the site indicate ape-like locomotor adaptations. These findings suggest that the origin of hominoid locomotor versatility is associated with foraging on leaves in heterogeneous, open woodlands rather than forests.

Two sediment profiles exposed along the floodplain of the Tennessee River provide an excellent opportunity to compare radiocarbon and optically stimulated luminescence dating of fluvial sediments, and to use soil micromorphology as a tool to assess the reliability of these dating methods. The profiles occur as vertical stacks of floodplain soils, buried soils, and fluvial deposits, with the sediments of both profiles indicating an alluvial origin, but with different degrees of soil development. Micromorphological analysis showed pedogenic clay coatings are common in both profiles. These pedofeatures provide evidence of relative age of the deposits, because layered, well-developed, thick clay coatings generally take thousands of years to form. Radiocarbon results indicate that the profiles span from the late early Holocene to late Holocene. OSL dating indicates that one profile is relatively recent (<600 yrs. B.P.) while the other is of late middle Holocene age (3.2 ka B.P. to 5.3 ka B.P.). Clay coatings support the results from OSL because the relatively recent profile has very thin coatings, in contrast with thick, well-developed clay coatings in the older profile. Some of the radiocarbon ages appear to be too old owing to redeposition, but other dates are consistent with soil development and micromorphology.

The assembly of Africa's iconic C4 grassland ecosystems is central to evolutionary interpretations of many mammal lineages, including hominins. C4 grasses are thought to have become ecologically dominant in Africa only after 10 million years ago (Ma). However, paleobotanical records older than 10 Ma are sparse, limiting assessment of the timing and nature of C4 biomass expansion. This study uses a multiproxy design to document vegetation structure from nine Early Miocene mammal site complexes across eastern Africa. Results demonstrate that between ~21 and 16 Ma, C4 grasses were locally abundant, contributing to heterogeneous habitats ranging from forests to wooded grasslands. These data push back the oldest evidence of C4 grass-dominated habitats in Africa-and globally-by more than 10 million years, calling for revised paleoecological interpretations of mammalian evolution.

Five sites in the upper Tennessee River Basin with overbank deposits exposed in cut-banks and inactive point bars were examined to identify evidence of paleofloods and other Holocene fluvial activity. Sediments exposed in cut-banks and inactive point bars are not commonly used in paleoflood studies, but this investigation shows that they can provide valuable information in areas where no other records are available. Paleofloods records are present in the study area as alternating layers of granular sediments overlying buried soils. Buried soils represent periods of relative stability that allowed soil formation until interrupted by deposition of fluvial sand and silt. Sedimentological characterization was performed to determine micromorphological characteristics, particle-size distribution, magnetic susceptibility, organic matter content, and elemental composition of the sediments. Buried soils are predominantly fine-grained with high content of clays present as clay coatings and clay-size minerals, and ferromagnesian minerals as reflected in higher magnetic susceptibility values. Paleoflood deposits have coarser grain-sizes, with mineralogy dominated by quartz and feldspar, lower organic content, and lower magnetic susceptibility. Elemental composition, as determined by ‘ex situ’ portable X-Ray Fluorescence, pXRF, differs between buried soils and paleofloods, with concentrations of Al, Fe, Mn, Ti, and Rb higher in buried soils than in the paleofloods, and Si concentrations higher in paleoflood deposits, reflecting the differing mineralogical composition of the sediments. AMS radiocarbon dates on charcoal particles reveal that the investigated profiles contain information on paleofloods between 10.7 and 0.2 ka B.P. Early Holocene deposits from 10.7 to 8.2 ka B.P. are part of a coarsening-upward sequence intercalated with moderately-developed buried soils that might be a product of multiple water pulses influencing the behavior of the Tennessee River. The initial part of the middle Holocene is associated with vertical growth of the floodplain with moderately developed soils and moderate-energy overbank deposits. The middle Holocene deposits indicate periods of high rainfall interrupted by abrupt decreases in precipitation. The late Holocene from 4.7 ka B.P. to present shows the results of a more stable behavior in climate expressed in a fining-upward sequence consistent with vertical growth of the floodplain interlayered with weakly developed paleosols and no major activity since 0.2 ka B.P. The findings of this study agree with paleoenvironmental reconstructions from previous studies for the southeastern US and indicate the potential for using paleoflood deposits for paleohydrology studies and reconstructions of past fluvial activity when no other records are available.

Six Proterozoic, two Cambrian, and two Cretaceous paleosols in the Lake Superior region of midcontinental Laurentia were investigated in detail. All but the Cretaceous paleosols experienced potassium metasomatism, which resulted in the precipitation of muscovite in Proterozoic paleosols or illite and microcline in Cambrian paleosols. A comparison of the magnitude of potassium metasomatism among the paleosols is provided by depth-normalized mass flux (DNMF), where DNMF=1000×[(mass flux)/(depth of weathering)], which normalizes for different thicknesses of weathering profiles. Average DNMF values for the total addition of K2O are 0.98 ± 0.19 mol cm−3 for the Proterozoic paleosols and 1.27 ± 0.06 mol cm−3 for the Cambrian paleosols. The ages of potassium metasomatism were determined by 40Ar/39Ar isotopic dating of metasomatic muscovite in the Proterozoic McGrath, Ville Marie, and Baraboo paleosols, which yielded ages of 1742 ± 3, 1589 ± 3, and 1467 ± 11 Ma, respectively, the former being coeval with the Yavapai orogeny and the latter with the Baraboo orogeny. Metasomatic microcline in the Cambrian Trempealeau paleosol yielded a plateau age of 488.0 ± 1.0 Ma, which corresponds to the age of the Cambrian-Ordovician boundary. SiO2, CaO, and Na2O were substantially removed from the paleosols by weathering, as was K2O (before metasomatism). The average total amount of SiO2, CaO, Na2O, and K2O removed was 17.6% ± 1.9% from six Proterozoic paleosols, 28.5% ± 4.2% from two Cambrian paleosols, 36.5% ± 8.6% from two Cretaceous paleosols, and 34.2% ± 2.7% from five modern soils. The greater magnitude of weathering in the Phanerozoic weathering profiles compared with the Proterozoic ones, despite lower levels of CO2 in the Phanerozoic atmosphere, may reflect the emergence of land plants at ∼500 Ma and their profound effects on weathering.

During the Late Paleozoic Ice Age, the fault-bounded equatorial Cumberland Basin of Nova Scotia experienced rapid subsidence, accumulating kilometer-thick fluvial sedimentary units derived from two highlands to the northwest and southeast. Major variations are recorded in the paleosols exposed at the Joggins Fossil Cliffs, ranging from oxidized and well-drained paleosols with recognizable vertic features to highly reduced organic-rich paleosols. These different soil lithologies suggest alternating conditions between well-drained floodplain environments and water saturation associated with overall poor soil development. Although halokinetic subsidence of the Cumberland Basin is known to have been operative during deposition of these units, previous research favored glacioeustatic processes as the primary forcing mechanism of sedimentation. A total of 474 fluvial aggradational cycles were identified within a kilometer-thick interval and show a fluctuating accommodation history with a very abrupt nature. The series of fluvial aggradational cycles was used to develop threshold autoregressive models based on 1) their thickness, 2) their paleosol thickness, 3) their sandstone content, and 4) their paleosol-to-sandstone ratio. For each model, results suggest no evidence of statistically significant cyclicity, contradicting the hypothesis that fluvial sedimentation was mainly driven by glacio-eustatic cyclothems. Additionally, a total of 7 lithologies were recognized through 1,655 beds. Evaluation of 8 spherical semivariograms suggests no evidence for cyclicity in the frequency, order, or distribution of the data based on lithologies, although some covariance was found at distances between 550 and 750 m suggesting similar processes controlling sedimentation in the lower and upper Joggins Formation. The Cumberland Basin is known to have been rapidly subsiding, mainly because of ductile deformation of salt deposits in the deeper basinal units. Our results suggest that Joggins records tectonically induced ponding of a part of the sedimentary basin, allowing more extensive preservation of abundant coal and organic-rich units, as well as still-standing fossil forests exposed along the cliffs. These new results suggest that tectonic subsidence of the Cumberland Basin during the Late Paleozoic Ice Age was a more important driver of fluvial sedimentation than previously thought. This novel application of the TAR methodology provides a mathematical description of the sediment accumulation history of terrestrial basins when applied to conformable sedimentary successions, along with the means of linking paleosol development to climatic processes.

Earth's changing climates, landscapes, and atmospheres are recorded in paleosols, which form in the Earth's critical zone by interactions between the lithosphere/pedosphere, biosphere, atmosphere and hydrosphere. Weathering during much of the Precambrian Eon was dominated by very high pCO(2) (10x to >20x present atmospheric level, PAL) leading to acidic chemical weathering, with additional and very poorly constrained weathering influences of primitive biota. The Great Oxidation Event at 2.0-2.2 Ga was marked by a major increase in pO(2), which was still very low compared to modern conditions. Towards the end of the Precambrian (Neoproterozoic) at least two major Snowball Earth glaciations occurred, punctuated by rapid warming, which intensified weathering processes, leading to releases of nutrients to oceans and the Cambrian Explosion and diversification of life. By the early Paleozoic the first nonvascular land plants evolved; these were small in stature, lacked deep root systems, were spore-reproducing, and were limited to wet soil environments. They were followed by the arrival of invertebrate terrestrial soil organisms. By the Middle to Late Devonian, trees with deep-penetrating root systems evolved that accelerated weathering and soil formation through the release of organic acids, which enhanced clay production. Coincident with afforestation, a significant drop in pCO(2) (at or below PAL) and concomitant rise in pO(2) (for a time exceeding PAL) culminated at the end of the Paleozoic Era with widespread Carboniferous coal swamps. Paleosols record the end-Permian mass extinction and the Cretaceous-Paleogene mass extinction and complement the marine records of these events. The Paleocene-Eocene Thermal Maximum (PETM), a transient 200 kyr warming spike attributed to release of methane hydrates, is considered the closest ancient analog to modern climate change. Evolution of angiosperms (flowering plants) in the Cretaceous, and C-4 grasses in the Miocene, record increasing diversification of land plant strategies and ability to occupy all known major terrestrial ecological niches.

The Archean atmosphere is thought to have been devoid of oxygen but, instead, containing high concentrations of greenhouse gases, such as CO2 and possibly CH4, that were required to keep the Earth surface environments warm enough for maintenance of liquid water under faint-young-Sun conditions. Earlier studies have suggested that the CO2 concentrations must have been as high as over 1000 times present atmospheric levels (PAL) to overcome the effects of weaker solar luminosity. However, more recent studies of Precambrian paleosols imply existence of at least tenfold-lower, but still high pCO(2) levels. The appearance of minute amounts of atmospheric O-2 at the transition of Archean and Proterozoic is well documented, however, the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated. Different paleoclimate proxies of the similar to 2.45 Ga Kuksha paleoweathering crust suggest its formation in a cool, temperate climate. Paleoatmospheric pCO(2) estimates by geochemical and isotopic mass-balance models suggest remarkably low paleoatmospheric CO2 levels, with the best guess estimates of atmospheric CO2 between 1 and 10 PAL. We speculate that the Kuksha paleoweathering crust either predates, or partly overlaps with, the onset of Huronian glaciation and corresponds to a time interval of low CO2 level during the first recorded large glaciation in Earth history.