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Soil Formation
Abstract
The earliest reports on the composition of deep-sea sediments resulted
from the Challenger Expedition (1873-1876) (e.g., Tizzard et al., 1885;
Murray and Renard, 1891). Many review papers on marine sediment
composition have subsequently been published, including the ones by
Revelle (1944), El Wakeel and Riley (1961), Arrhenius (1963), Goldberg
(1963), Chester and Aston (1976), Glasby (1977), Bischoff and Piper
(1979), Baturin (1982, 1988), Notholt and Jarvis (1990), Nicholson et
al. (1997), Glenn et al. (2000), and Li (2000). The constituents of a
marine sediment are often classified according to their origin ( Table
1; after Goldberg, 1963). The detrital component is made up of
cosmogenous and lithogenous materials. Cosmic spherules contain
particles of FeNi that are formed by ablation of iron meteorites as they
pass through Earth's atmosphere, as well as fragments of silicate
chondrules ( Arrhenius, 1963). Lithogenous constituents of marine
sediments are the minerals derived from weathering of rock on land or on
the seafloor, or from the volcanic eruptions ( Goldberg, 1963; see
review in Windom (1976)). The biogenous component is made up of the
tests of planktic and benthic organisms, as well as biogenic apatite
(see review in Berger (1976)). The hydrogenous fraction of marine
sediment encompasses phases formed by inorganic precipitation from
seawater. Elderfield (1976) and Piper and Heath (1989) provide
comprehensive reviews of hydrogenous material in marine sediments.
... In the weathered samples, Ti, Zr and Nb align in a Grant's iso- chon diagram (Grant, 1986), which is used to graphically identify the most immobile elements (Fig. 5). Assuming these elements were immobile (see Section 7), mass transfer undergone by the weathering profile can be calculated using the following equation (e.g., Amundson, 2003): ...
... In the studied profile, compositional changes have been investi- gated in terms of relative loss and gain of elements (Amundson, 2003;Price and Velbel, 2003). In the absence of volume and den- sity constraints, element mass transfer was estimated using immo- bile element concentrations (see Section 5). ...
In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE + Y concentrations are typical of granitoids (299–363 ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9 wt% SO3 and 7 wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite–jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.
... To evaluate the elemental gain and loss during progressive weathering, mass balance calculations were performed. The formula is as follows (Amundson, 2003): τ=(Cj,s/Ci,s)/(Cj,p/Ci,p)-1 ...
... Weathering and soil-forming processes have been the subject of extensive research for decades. For recent reviews see Brantley (2003) and Amundson (2003). However, although extensive laboratory and field studies exist, many processes are still poorly understood. ...
The understanding of biogeochemical processes at the interface between geosphere, hydrosphere and biosphere is of paramount importance for many questions related to global climate and environmental change at very different time and spatial scales. In addition, soils are the principal resource for food production and the understanding of soil formation processes and biological interactions within soils is indispensable for the development of sustainable land-use strategies. In this contribution we present a research initiative, the multidisciplinary 'BigLink' Project, aiming at a better understanding of the links between weathering, soil formation and ecosystem evolution and how these biosphere-geosphere interactions are influenced (and themselves influence) climate and environmental change.
Copper (Cu) is a bio-essential element and a potentially toxic pollutant in the plant–soil systems. Analysis of stable Cu isotopes can be a powerful tool for tracing the biogeochemical cycling of Cu in plant–soil systems. In this review, we examined the analysis method of stable Cu isotope ratios in plants and soils, and discussed the biogeochemical processes, including redox reactions, mineral dissolution, abiotic and biotic sorption, which fractionate Cu isotopes in plant–soil systems. We also reviewed the variability of the isotopic signature in different plants and plant tissues, as well as different soil types and profiles to discuss the relationship between the biogeochemical transformation of Cu and its isotope fractionation in plant–soil systems. The collected data show that δ⁶⁵Cu values range from − 2.59 to + 1.73‰ in plant–soil systems, and ∆⁶⁵Cu values range from − 1.00 to − 0.11‰ between the plant and soil. The variation in the ∆⁶⁵Cu value between the plant and soil is mainly in response to the different uptake strategies during the acquisition of Cu from soils. Cu isotope analyses are proved to be a suitable technique during the biogeochemical transformation of Cu in plant–soil systems, especially during redox reactions. Ultimately, research challenges and future directions for Cu isotope techniques as a proxy for Cu biogeochemical cycles are also proposed. This review is beneficial for soil safety, food safety, and the sustainable development of agriculture and human health.
Graphical abstract
Mercury concentration and Hg mass gain in surface soils were evaluated for soil samples taken along a N/S transect in Alaska. Mercury and organic carbon content were determined in surface and subsurface soils from 177 sites. Mercury quantification was by cold-vapor atomic absorption. Organic soil carbon calculated as the difference between total and inorganic carbon. The median Hg concentration was significantly higher in O-horizon samples compared to A-horizon or subsurface samples and significantly higher in the A-horizon samples compared to those of the subsurface. The Hg concentration in the surface horizons (Oand A) was positively correlated with both soil organic carbon content and the subsurface Hg concentration. Concentration profiles and mass gain of Hg in the surface horizons along much of the transect are consistent with a distribution that reflects atmospheric Hg deposition. However, geogenic Hg likely accounts for the increase in Hg concentration in surface soils through the Alaska Range.
In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO2) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)N and (La/Sm)N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.
This study reports U-series activity ratios from river waters and six soil profiles across a soil chronosequence formed since the last glacial retreat, in Glen Feshie, Scotland. The overall aim is to examine the geochemical behaviour of the U-series nuclides in a boreal climate setting. The U-series data show that U is being both added to and leached out of the soils, to varying degrees. The U addition elevates the (U-234/U-238) of the bulk soils (up to 1.25), which is most pronounced in the youngest and the upper organic-rich soil horizons. The U addition appears to be linked to U adsorption, controlled by the degree of flooding by the Feshie River. The Feshie River has a high (U-234/U-238) ratio (similar to 1.7), a feature shared with most high-latitude Northern Hemisphere rivers. For the soil profiles with no significant U addition, U-series nuclide modelling suggests U leaching rates on the order of 0.5-2 x 10-5 y-1, a similar range to other Northern Hemisphere highlatitude areas affected by the last glaciation, e. g. Mackenzie Basin, Canada. These two observations suggest a link between weathering rates and riverine (U-234/U-238) for areas that have been glaciated recently. A global compilation of major rivers shows that high-latitude Northern Hemisphere rivers comprise a significant U flux to the ocean with high (U-234/U-238). Thus, past changes in this Northern Hemisphere high-latitude U flux may have played a major role in oceanic (U-234/U-238) variation over glacial-interglacial cycles.
This study investigates Cr isotope fractionation during soil formation from Archean (3.1–3.3 Ga) ultra-mafic rocks in a chromite mining area in the southern Singhbhum Craton (Orissa, India). The Cr-isotope signatures of two studied weathering profiles, range from non-fractionated mantle values to negatively fractionated values as low as d 53 Cr = À1.29 ± 0.04‰. Local surface waters are isotopically heavy relative to the soils. This supports the hypothesis that during oxidative weathering isotopically heavy Cr(VI) is leached from the soils to runoff. The impact of mining pollution is observed downstream from the mine where surface water Cr concentrations are significantly increased, accompanied by a shift to less positive d 53 Cr values relative to upstream unpolluted surface water. A microbial mat sample indicates that microbes have the potential to reduce and immobilize Cr(VI), which could be a factor in controlling the hazardous impact of Cr(VI) on health and environment. The positive Cr isotope signatures of the Brahmani estuary and coastal seawater collected from the Bay of Bengal further indicate that the positively fractionated Cr isotope signal from the catchment area is preserved during its transport to the sea. Isotopically lighter Cr(VI) downstream from the mine is probably back-reduced to Cr(III) during riverine transport leading to similar Cr-isotope values in the estuary as observed upstream from the mine.
We performed a mineral, geochemical and Cr-Sr-Pb isotope study on a laterite profile developed on ca. 540 Ma old tonalitic bedrock in Madagascar with special emphasis on the behavior of chromium during tropical weathering. The observed strong depletions of Ca, Si, and P, and enrichment of Fe and Al, in the soil, relative to bedrock and underlying saprolite, are the characteristic features pertinent to laterites. The enrichment of Fe in topsoil horizon can be correlated with enrichment of P, and the redox sensitive elements Mn and Cr, and indicates redistribution of these elements related to oxidation-reduction processes. The slight scatter of Sr-87/Sr-86, and Pb-206,Pb-207,Pb-208/Pb-204 values in the profile is one consequence of such redistribution processes. Our results are compatible with a two stage process during alteration: (1) an incipient alteration stage characterized mainly by a pervasive, effective and complete loss of sulfides accompanied by patchy alteration constrained along fissures and cracks, and (2) a main stage of soil formation (actual laterization) with loss of Na, K, Ca, and Si, and accumulation of Fe and Mn relative to the unaltered bedrock. This two stage evolution is also depicted by Cr concentrations and delta Cr-53 values along the studied profile, where the first incipient alteration stage caused a pronounced depletion of Cr particularly hosted by sulfides, and where the second alteration (laterization) led to redistribution and a small increase in Cr concentration in the uppermost portions of the soil profile relative to stage one altered saprolite. This gain in Cr is accompanied by decreasing delta Cr-53 values and can be explained by partial immobilization (possibly by adsorption/coprecipitation on/with Fe-oxy-hydroxides) of mobile Cr(III) during upward transport in the weathering profile. The negatively fractionated delta Cr-53 values measured in the weathering profile relative to the unaltered tonalitic bedrock characterized by a high temperature magmatic inventory Cr isotope signature are consistent with loss of a positively fractionated Cr(VI) pool formed during weathering. The predicted existence of a former, positively fractionated and mobile chromium pool has been experimentally constrained in circumneutral and basic leachates of powdered tonalite bedrock where delta Cr-53 of +0.21 to +0.48 parts per thousand was measured. Our results show that mobilization of chromium is effective under highly oxidative conditions, which in well drained sulfide-bearing parent bedrocks potentially lead to both, acid dissolution of sulfide-hosted Cr and redox-promoted mobilization of Cr(VI) from silicates during later stages of weathering under basic pH conditions.
To evaluate the lateritization process and supergene Ni enrichment in the tropical rainforest, a well developed laterite profile over the serpentinite in the Kolonodale area of East Sulawesi, Indonesia, has been investigated using field geology methods, mineralogical and geochemical techniques. Three lithostratigraphic horizons over the bedrock are distinguished from bottom to top: the saprolite horizon, the limonite horizon, and the ferruginous cap. In general, the profile is characterized by (1) a depth-related pH ranging from 5.56 to 8.56, with a higher value in the saprolite horizon and a lower value in the ferruginous cap, (2) a highly variable organic matter concentration from 1.11% to 4.82%, showing a increasing trend from bottom to top, (3) a progressive mineral assemblage transition from the silicate mineral dominant (mainly serpentine) to the Fe-oxyhydroxide dominant (mainly goethite), and (4) a typical laterite geochemical pattern with an increase of Fe, Al, Mn, Cr and Ti but a decrease of Mg, Ca, Na and K upward from the bedrock. The highest concentration of Ni (up to 11.53%NiO) occurs in the saprolite horizon, showing nearly 40 times richer compared to the bedrock. The mineral evolution during the lateritization process shows various paths from the primary minerals to altered minerals, which is integrally affected by the nature of the primary minerals and environmental conditions. Garnierite, as a significant ore mineral formed by the secondary precipitation processes in the study profile, is identified as a mixture of talc- and serpentine-like phases. The mass-balance calculation reveals that there are diversified elemental behaviors during the serpentinite lateritization under the rainforest conditions. In particular, Ni, as the ore-forming element in the laterite profile, is associated closely with the pH environment, organic matter concentration and mineral evolution during the lateritization process.
This paper examines the weathering processes that have combined to produce the distribution of soil-regolith (SR) thickness across the Triassic Sherwood Sandstone Group outcrop (750 km2) in Nottinghamshire, UK. Archive borehole logs (n = 282) taken across the outcrop showed that SR thickness had mean and median depths of ~1·8 and 1·5 m, respectively. Cores were taken from a forested site to depths ~3 m for geochemical analysis. At this site the SR thickness was ~1·7 m. Analysis of the loss of elements, compared to bedrock using mass balance calculations (τ) showed that all the calcite and gypsum cement had been removed to depths of >3 m. Thus the major difference between the SR and the underlying saprolite was that the former exists as loose sand as opposed to a semi-durable rock. Scanning electron microscopy (SEM) analysis of core samples suggested that the non-durable rock or saprolite had greater cementation of clay particles. We propose that the mechanism through which the clay cement (and other interlocking grain bonds) was eased apart was through freeze–thaw processes associated with the summer ‘active layer development (ALD)’ during the last glacial activity in the UK. We tested this theory by developing a Monte Carlo simulation based on a simplified version of the Stefan equation. Current Arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables. These were combined with known data for thermal conductivity, bulk density and moisture content of the Sherwood Sandstone regolith. Model predictions (n = 1000) of the distribution of SR thickness accurately reflect the observed distribution thickness from the borehole logs. This is strong evidence that freeze–thaw and ‘ALD’ processes are major factors in determining the thickness of SR across this outcrop. British Geological Survey © NERC 2012
[1] To investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U-series isotopes in regolith profiles from two hillslopes (north facing versus south facing) within the east-west trending Shale Hills catchment in Pennsylvania. This catchment is developed entirely on the Fe-rich, Silurian Rose Hill gray shale. Hillslopes exhibit a topographic asymmetry: The north-facing hillslope has an average slope gradient of ~20°, slightly steeper than the south-facing hillslope (~15°). The regolith samples display significant U-series disequilibrium resulting from shale weathering. Based on the U-series data, the rates of regolith production on the two ridgetops are indistinguishable (40 ± 22 versus 45 ± 12 m/Ma). However, when downslope positions are compared, the regolith profiles on the south-facing hillslope are characterized by faster regolith production rates (50 ± 15 to 52 ± 15 m/Ma) and shorter durations of chemical weathering (12 ± 3 to 16 ± 5 ka) than those on the north-facing hillslope (17 ± 14 to 18 ± 13 m/Ma and 39 ± 20 to 43 ± 20 ka). The south-facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry, despite lower extents of chemical depletion. These results are consistent with the influence of aspect on regolith formation at Shale Hills; we hypothesize that aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south-facing side of the catchment. The difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at Shale Hills at least ~15 ka before present. At that time, the erosion rates may also have been different from those observed today, perhaps denuding the south-facing hillslope of regolith but not quite stripping the north-facing hillslope. An analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at Shale Hills is not in geomorphologic steady state (i.e., so-called dynamic equilibrium) but rather is likely still responding to the climate shift from the Holocene periglacial to the modern, temperate conditions.
New analytical techniques have opened up the possibility of addressing rates of soil processes quantitatively. Here, we present the results of an investigation into the use of single‐grain optically stimulated luminescence (OSL) dating to derive rates of soil mixing in the top 50 cm of soil profiles from two toposequences situated in the Werrikimbe National Park in Australia. Of 500 single grains analysed from each sampled depth increment, less than 25% provided a finite age, with the rest of the grains either non‐responsive or dose‐saturated. This proportion of finite‐age grains tended to decrease with soil depth. Median ages of quartz grains increased down the soil profile, with topsoil ages of up to 500 years and subsoil ages of up to 5000 years. Few ‘younger’ grains were found deeper in the profile and few ‘older’ grains near the soil surface. These trends suggest that pedoturbation is resulting in vertical transport of grains through the profile, but that there is a distribution of transport distances, with a poor probability of large transport distances from surface to subsoil or vice versa compared with a more frequent movement of grains to and from the surface in the uppermost 10–35 cm. The calculation of a single age for each soil horizon was unachievable as each horizon contained a heterogeneous mixture of grains with varying histories of transport to and from the soil surface. Soil mixing was confirmed along both toposequences studied. However, the occurrence of minor mixing rates did not lead to a homogenization of the topsoil and adjacent horizons. We postulated that mixing velocities were mostly related to flora at our study site. Vertical soil mixing rates of 0.5 and 0.2 mm year−1 were calculated from the distribution of finite single‐grain ages.
Although regolith, the mantle of physically, chemically, and
biologically altered material overlying bedrock, covers much of Earth's
continents, the rates and mechanisms of regolith formation are not well
quantified. Without this knowledge, predictions of the availability of
soil to sustain Earth's growing population are problematic. To quantify
the influence of climate on regolith formation, a transect of study
sites has been established on the same lithology – Silurian shale
– along a climatic gradient in the northern hemisphere as part of
the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania,
USA. The climate gradient is bounded by a cold/wet end member in Wales
and a warm/wet end member in Puerto Rico; in between, mean annual
temperature (MAT) and mean annual precipitation (MAP) increase to the
south through New York, Pennsylvania, Virginia, Tennessee and Alabama.
The site in Puerto Rico does not lie on the same shale formation as the
Appalachian sites but is similar in composition. Soils and rocks were
sampled at geomorphologically similar ridgetop sites to compare and
model shale weathering along the transect. Focusing on the
low-concentration, non-nutrient element Na, we observe that the extent
and depth of Na depletion is greater where mean annual temperature (MAT)
and precipitation (MAP) are higher. Na depletion, a proxy for feldspar
weathering, is the deepest reaction documented in the augerable soil
profiles. This may therefore be the reaction that initiates the
transformation of high bulk-density bedrock to regolith of low bulk
density. Based on the shale chemistry along the transect, the
time-integrated Na release rate (QNa) increases exponentially
as a function of MAT and linearly with MAP. NY, the only site with
shale-till parent material, is characterized by a QNa that is
18 times faster than PA, an observation which is attributed to the
increased surface area of minerals due to grinding of the glacier and
kinetically limited weathering in the north. A calculated apparent
Arrhenius-type temperature dependence across the transect (excluding NY)
for the dissolution of feldspar (Na depletion) is 99 ± 15 kJ
mol‑1, a value similar to field-measured values of the
activation energy (14–109 kJ mol‑1) or
laboratory-measured values of the enthalpy of the albite reaction (79.8
kJ mol‑1). Observations from this transect document
that weathering losses of Na from Silurian shale can be understood with
models of regolith formation based on chemical and physical factors such
that weathering progresses from kinetically limited sites (Wales to AL)
to the transport-limited site in Puerto Rico. Significant advances in
our ability to predict regolith formation will be made as we apply more
quantitative models to such transect studies on shales and other rocks
types.
Soil science came into its own only in the 20th century. Before this, the study of soils was dominated by geologists, agronomists and chemists. It was Dokuchaev in 1886, who recognized soil as a physical entity with properties acquired from the impact of soil-forming factors, among which the geological substrate was only one. This vision resulted in the establishment of a new discipline, called pedology. With time, geologists began to appreciate soil in a pedological context. In fact, palaeosols in particular have been utilized to interpret the stratigraphy of metamorphic and sedimentary rocks and Quaternary deposits. Also, palaeosols have been used for correlating unconsolidated sediments, faults and neotectonics, or for the relative dating of deposits or surfaces. Weathering is a field where soil chemists have interacted with geochemists to evaluate chemical denudation and landscape evolution. Geological engineering in terms of water storage, pollutant transport, and critical load, in addition to location, design and construction of roads, is another area of interaction between soil researchers and geologists. The exploration of the planets of the solar system is a field which has assembled soil chemists and geochemists to collect, analyse and interpret data sent by space vehicles. Future interactions between geology and soil science will occur on issues such as: water in the vadose zone; risks due to Earth movements; and functions of soils in ecosystems. We predict and also welcome more communication between the two disciplines, as solutions to some of these problems are demanded by society.
Soils derived from black shale can accumulate high concentrations of elements of environmental concern, especially in regions with semiarid to arid climates. One such region is the Colorado River basin in the southwestern United States where contaminants pose a threat to agriculture, municipal water supplies, endangered aquatic species, and water-quality commitments to Mexico. Exposures of Cretaceous Mancos Shale (MS) in the upper basin are a major contributor of salinity and selenium in the Colorado River. Here, we examine the roles of geology, climate, and alluviation on contaminant cycling (emphasis on salinity and Se) during weathering of MS in a Colorado River tributary watershed. Stage I (incipient weathering) began perhaps as long ago as 20 ka when lowering of groundwater resulted in oxidation of pyrite and organic matter. This process formed gypsum and soluble organic matter that persist in the unsaturated, weathered shale today. Enrichment of Se observed in laterally persistent ferric oxide layers likely is due to selenite adsorption onto the oxides that formed during fluctuating redox conditions at the water table. Stage II weathering (pedogenesis) is marked by a significant decrease in bulk density and increase in porosity as shale disaggregates to soil. Rainfall dissolves calcite and thenardite (Na2SO4) at the surface, infiltrates to about 1 m, and precipitates gypsum during evaporation. Gypsum formation (estimated 390 kg m-2) enriches soil moisture in Na and residual SO4. Transpiration of this moisture to the surface or exposure of subsurface soil (slumping) produces more thenardite. Most Se remains in the soil as selenite adsorbed to ferric oxides, however, some oxidizes to selenate and, during wetter conditions is transported with soil moisture to depths below 3 m. Coupled with little rainfall, relatively insoluble gypsum, and the translocation of soluble Se downward, MS landscapes will be a significant nonpoint source of salinity and Se to the Colorado River well into the future. Other trace elements weathering from MS that are often of environmental concern include U and Mo, which mimic Se in their behavior; As, Co, Cr, Cu, Ni, and Pb, which show little redistribution; and Cd, Sb, V, and Zn, which accumulate in Stage I shale, but are lost to varying degrees from upper soil intervals. None of these trace elements have been reported previously as contaminants in the study area.
In their seminal paper in 1979, Bull and Schick proposed a conceptual model for the geomorphic response to Pleistocene to Holocene climate change, based on the hyperarid Nahal Yael watershed in the southern Negev Desert. In this model, the change from semiarid late Pleistocene to hyperarid early Holocene climates reduced vegetation cover, increased the yield of sediment from slopes, and accelerated aggradation of terraces and alluvial fans. The model is now over 30 yr old, and during this time, chronologic, paleoenvironmental, and hydrogeomorphic research has advanced. Here, we reevaluate the model using data acquired in Nahal Yael over the 30 yr since the original model was proposed. Recent studies indicate that the late Pleistocene climate was hyperarid, and a transition from semiarid to hyperarid climates did not occur. The revised chronology reveals a major 35-20 ka episode of accelerated late Pleistocene sediment production on slopes (with lower rates probably already at ca. 50 ka) due to increased frequency of wetting-drying cycles caused by frequent extreme storms and floods between 35 and 27 ka. Without lag time, these sediments were transported and aggraded in depositional landscape components (fluvial terraces and alluvial fans). This intensified sediment production and delivery phase is unrelated to the Pleistocene-Holocene transition. The depositional landforms were rapidly incised between 20 and 18 ka. Since and/or soon after this Last Glacial Maximum (LGM) incision, most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock. Using these new data, we propose a revision to the Bull and Schick model in this hyperarid environment. Our revision suggests that the model should include the frequent storms and floods responsible for a late Pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans. We also discuss the common use and pitfalls of using the Bull and Schick conceptual model to explain observations in diverse arid environments, usually without sufficient data on basin-specific stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.
We studied enrichment of heavy metals (V, Zn, Cr, Ni, Cu, Pb, As, Sb, and Cd) in a rural stream of the Kuji River basin in central Japan in suspended particulate matter, and associated transport flux during two rainfall events (in November 2003 and in April 2008). The concentration of heavy metals in suspended particulate matter (SPM) exhibited a distinctive temporal variation, wherein the concentrations decreased with increasing water discharge and then increased as the discharge decreased. Concentration of dissolved metal forms showed a slight increase with scatters around the flow rate peak. Enrichment factors for those metals in the SPM decreased sharply as the flow rate increased, making an obvious concave shaped curve (the November 2003 rainfall event). The metal enrichment factors under low flow conditions had a similarity to those found in atmospheric deposits at a foot of the Kuji River basin, suggesting atmospheric source would contribute to enriching the SPM with those metals in part. Mineralogical analyses and carbon content analysis (the April 2008 rainfall event) of the SPM suggests the SPM matrix became more lithological as the flow rate increased. The changes observed in the matrix are thought to be directly related to progressive changes in metal enrichment within the SPM. Concerning the transport phase of several heavy metals, a dynamic change in transport phase partitioning within a rainfall event was suggested. The present enrichment factor study and the SPM matrix characterization implied the partitioning change is due to an increase in lithologic solids during high flow conditions.
As soils come under increasing pressure to maintain a range of ecosystem services, there is interest in how soils change over time in response to factors such as change in land-use. Many studies examining long- and short-term soil change have focused on soils with relatively high mineral and fertility status. Therefore, the aims of this study are to explore regional change on a marginal sandy soil formed over the Sherwood Sandstone outcrop in Nottinghamshire, U.K. (750 km2) and to assess changes in soil fertility as a function of the natural weathering process and land use change. The study uses data from three sources to examine differences between soil fertility properties under two major land-uses through the depth of the soil/mobile regolith (~ 1.6 m) and into the saprolite. It is proposed that the differences reflect in part the result of historical change in land-use. From old maps we identify the land-use changes back to 1781. This allowed us to compare soils that have been under woodland cover at least since 1781 with those that were converted to arable use in major deforestation between 1781 and 1881. Soils now under woodland have low concentrations of base cations, an acid pH and a mean organic carbon concentration (0–15 cm) of 2.7%. In contrast soils now under arable use have large concentrations of base cations, pH close to neutral and mean organic carbon concentration (0–15 cm) of 1.7%. There is evidence in the arable soils of leaching to depth of materials from applied fertilisers and lime. These results show the rapid change in properties of soil formed in bedrock, with small concentrations of nutrients and weatherable minerals, which can result from land-use change.
We present a model of hillslope soils that couples the evolution of topography, soil thickness, and the concentration of constituent soil phases, defined as unique components of the soil with collective mass equal to the total soil mass. The model includes both sediment transport and chemical denudation. A simplified two-phase model is developed; the two phases are a chemically immobile phase, which has far lower solubility than the bulk soil and is not removed through chemical weathering (for example, zircon grains), and a chemically mobile phase that may be removed from the system through chemical weathering. Chemical denudation rates in hillslope soils can be measured using the concentration of immobile elements, but the enrichment of these immobile elements is influenced by spatial variations in chemical denudation rates and spatial variations in the chemical composition of a soil's parent material. These considerations cloud the use of elemental depletion factors and cosmogenic nuclide-based total denudation rates used to identify the relationship between physical erosion and chemical weathering if these techniques do not account for downslope sediment transport. On hillslopes where chemical denudation rates vary in space, estimates of chemical denudation using techniques that do not account for downslope sediment transport and spatial variations in chemical denudation rates may be adequate where the chemical denudation rate is a significant fraction of the total denudation rate but are inadequate in regions where chemical weathering rates are small compared to the total denudation rate. We also examine relationships between transient mechanical and chemical denudation rates. Soil particle residence times may affect chemical weathering rates, and the relationship between total landscape-lowering rates and soil particle residence times can thus be quantified.
A simplified model of regolith evolution involving aqueous alteration, volcanism, and impact gardening suggests that the weathering products in the current martian regolith were produced primarily during the period of early heavy bombardment.
Soil landscape modelling is based oil understanding the spatial distribution patterns of soil characteristics. A model relating the sod's properties to its position within the landscape is used to predict soil properties in other similar landscape positions. TO develop soil landscape models, the interaction of geographic information technology advanced statistics and soil science is needed. The focus of this work is to predict the distribution Of the different soil types in a tropical mountain forest area in southern Ecuador from relief and hydrological parameters using a classification tree model (CART) for soil regionalisation. Soils were sampled along transects from ridges towards side valley creeks using a sampling design with 24 relief units. Major soil types of the research area are Histosols associated with Stagnosols, Cambisols and Regosols. Umbrisols and Leptosols are present to a lesser degree. Stagnosols gain importance with increasing altitude and with decreasing slope angle. Umbrisols are to be found only on slopes <30 degrees. Cambisols Occurrence might be related to landslides. The CART model was established by: a data set Of 315 auger sampling points. Bedrock and relief curvature had no influence on model development. Applying the CART model to the research area Histosols and Stagnosols, were identified as dominant soil types. Model prediction left Out Cambisols and overestimated Umbrisols, but showed a realistic prediction for Histosols, Stagnosols and Leptosols.
Sustainable soils are a requirement for maintaining human civilizations (Carter and Dale 1974; Lal 1989). However, as the “most complicated biomaterial on the planet” (Young and Crawford 2004), soils represent one of the most difficult systems to understand and model with respect to chemical, physical, and biological coupling over time (Fig. 1⇓).
Figure 1.
A schematic picture of the “weathering engine” at the Earth’s surface. This weathering engine is part of the Critical Zone that extends from the vegetation canopy down through the saturated zone. The regolith-bedrock interface lowers at the weathering advance rate, ω. The rate of removal of material at the surface is the erosion rate, E . Some regolith profiles grow with time in a transient mode while others may attain steady state where ω = E . As shown, many physical, chemical, and biological processes combine to control regolith in the Critical Zone. Climatic, anthropogenic, and tectonic forcings affect these processes; the sum total of weathering processes can then be read in changes in the atmosphere, hydrosphere, and pedosphere. [Used with permission of the American Geophysical Union from Anderson et al. 2004.]
Despite the complexity of these interactions, certain patterns in soil properties and development are universally observed and have been used in soil science as a means for classification. Elemental, mineralogical, or isotopic concentrations in soils plotted versus depth beneath the land surface comprise such patterns. Soil depth profiles are often reported for solid soil materials, and, less frequently, for solutes in soil pore waters. These profiles cross a large range in spatial scales that traditionally have been studied by different disciplines. For example, shallow, biologically active horizons are commonly defined as the soil zone in agronomic studies whereas the mobile layer of the regolith is referred to as soil in geomorphological studies. …
Although long-term changes in solid-state compositions of soil chronosequences have been extensively investigated, this study presents the first detailed description of the concurrent hydrochemical evolution and contemporary weathering rates in such sequences. The most direct linkage between weathering and hydrology over 3 million years of soil development in the Merced chronosequence in Central California relates decreasing permeability and increasing hydrologic heterogeneity to the development of secondary argillic horizons and silica duripans. In a highly permeable, younger soil (40 kyr old), pore water solutes reflect seasonal to decadal-scale variations in rainfall and evapotranspiration (ET). This climate signal is strongly damped in less permeable older soils (250 to 600 kyr old) where solutes increasingly reflect weathering inputs modified by heterogeneous flow.
Field and laboratory investigations of a 2690.83Ma (207Pb/206Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO2 during weathering (τTi,j), show depletion of SiO2, Al2O3, Na2O, CaO, MgO, and MnO, and to a lesser degree of K2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth.
The soils of the Atacama Desert in northern Chile have long been known to contain large quantities of unusual salts, yet the processes that form these soils are not yet fully understood. We examined the morphology and geochemistry of soils on post-Miocene fans and stream terraces along a south-to-north (27° to 24° S) rainfall transect that spans the arid to hyperarid transition (21 to ∼2mm rain y−1). Landform ages are ⩾2 My based on cosmogenic radionuclide concentrations in surface boulders, and Ar isotopes in interbedded volcanic ash deposits near the driest site indicate a maximum age of 2.1My. A chemical mass balance analysis that explicitly accounts for atmospheric additions was used to quantify net changes in mass and volume as a function of rainfall. In the arid (21mmrainy−1) soil, total mass loss to weathering of silicate alluvium and dust (−1030kgm−2) is offset by net addition of salts (+170kgm−2). The most hyperarid soil has accumulated 830kgm−2 of atmospheric salts (including 260kg sulfate m−2 and 90kgchloridem−2), resulting in unusually high volumetric expansion (120%) for a soil of this age. The composition of both airborne particles and atmospheric deposition in passive traps indicates that the geochemistry of the driest soil reflects accumulated atmospheric influxes coupled with limited in-soil chemical transformation and loss. Long-term rates of atmospheric solute addition were derived from the ion inventories in the driest soil, divided by the landform age, and compared to measured contemporary rates. With decreasing rainfall, the soil salt inventories increase, and the retained salts are both more soluble and present at shallower depths. All soils generally exhibit vertical variation in their chemistry, suggesting slow and stochastic downward water movement, and greater climate variability over the past 2My than is reflected in recent (∼100y) rainfall averages. The geochemistry of these soils shows that the transition from arid to hyperarid rainfall levels marks a fundamental geochemical threshold: in wetter soils, the rate and character of chemical weathering results in net mass loss and associated volumetric collapse after 105 to 106 years, while continuous accumulation of atmospheric solutes in hyperarid soils over similar timescales results in dramatic volumetric expansion. The specific geochemistry of hyperarid soils is a function of atmospheric sources, and is expected to vary accordingly at other hyperarid sites. This work identifies key processes in hyperarid soil formation that are likely to be independent of location, and suggests that analogous processes may occur on Mars.
Spatial distributions of fifteen trace elements (As, Be, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Sn, Tl, U, V, and Zn) potentially harmful for the human health and the ecosystem have been studied in the soils of Catalonia. Geographic Information Systems were used together with the cartographic information available in sampling design. Descriptive statistics, box-plots, and cumulative distribution figures were used to assess the differences in trace element concentrations among soil uses. A clear enrichment in Pb and Sn (mean 24.3 and 3.31 mg·kg-1 d.w., respectively) in urban soils was observed when compared to other soil uses. Values obtained for country soils where similar to those observed in previous studies carried out in Europe. Trace element enrichment or impoverishment observed in the soils is consistent with the concentration in representative rock types of the underlying geology analysed in other areas. This fact indicates that geology is an important factor influencing trace element distribution in country topsoils of the region studied. Background and reference values, computed as upper confidence limits for a central tendency value and 95th percentile, respectively, were also provided to assist in the assessment of soil pollution in the region.
The complex weathering processes which govern the production of soil from bedrock have proven difficult to understand for many lithologies. Weathering of black shale is of particular interest because it releases organic carbon and heavy metals as solutes and therefore impacts the health of terrestrial and aquatic ecosystems. To understand black shale weathering, a geochemical survey was initiated for soils developed on shales of the Marcellus Formation at a zero-order catchment at a satellite site of the Susquehanna/Shale Hills Critical Zone Observatory located in Jackson Corner, Pennsylvania. This formation is an organic- and metal-rich, carbonaceous shale that underlies much of New York, Pennsylvania, Ohio and West Virginia. In this paper, we focus on the effects of weathering on variations of Cu isotopes in the shale. Cu concentration data for soil were normalized using Ti concentrations to document the mobility of Cu relative to bedrock. At both the ridgetop and valley floor, depletion profiles for Cu are documented in the soils. The Cu in the soils is depleted in Cu-65 (average delta Cu-65 = 0.5 parts per thousand +/- 0.2) compared to the parent material (average delta Cu-65 = 0.03 parts per thousand +/- 0.15). Consistent with loss of Cu from soils, the pore waters contain 10 ppb Cu on average and are enriched in the heavy isotope (average value delta Cu-65 =1.14 parts per thousand +/- 0.44). Rayleigh fractionation models using the concentration and isotope data of the soils are consistent with pyrite weathering and loss of Cu from the ridgetop, but downslope transport and Cu re-precipitation at the valley floor.
Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However,
the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation
into continents. Igneous differentiation followed by lower crustal delamination and chemical weathering followed by subduction
recycling are possible solutions, but the relative magnitudes of each process have never been quantitatively constrained because
of the lack of appropriate data. Here, we show that the relative contributions of these processes can be obtained by simultaneous
examination of Mg and Li (an analog for Mg) on the regional and global scales in arcs, delaminated lower crust, and river
waters. At least 20% of Mg is lost from continents by weathering, which translates into >20% of continental mass lost by weathering
(40% by delamination). Chemical weathering leaves behind a more Si-rich and Mg-poor crust, which is less dense and hence decreases
the probability of crustal recycling by subduction. Net continental growth is thus modulated by chemical weathering and likely
influenced by secular changes in weathering mechanisms.
Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature
The base of the Critical Zone includes the mantle of altered soil and rock—regolith—that changes in response to chemical, physical, and biological processes occurring at Earth's surface. These processes are recorded in the chemistry of the regolith, and this long-term record can often be deciphered. For example, on eroding ridgetops where flows are generally downward for water and upward for earth material, element concentrations vary with depth to constitute depletion, addition, depletion-enrichment, and biogenic profiles. Models can be used to explore the records of mineral dissolution, atmospheric input, coupled dissolution-precipitation, and biolifting documented in these profiles. These models enable interpretation of the effects of time, climate, rates of erosion, and human and other biotic impacts on the profile patterns. By testing quantitative models against the long-term record of information in regolith, we will learn to project changes arising from human and natural perturbations of the Critic...
In this study we analyzed the large scale spatial patterns of river pH, alkalinity, and CO2 partial pressure (PCO2) in North America and their relation to river catchment properties. The goal was to set up empirical equations which can predict these hydrochemical properties for non-monitored river stretches from geodata of e.g. terrain attributes, lithology, soils, land cover and climate.For an extensive dataset of 1120 river water sampling locations average values of river water pH, alkalinity and PCO2 were calculated. The catchment boundaries and catchment properties were calculated using GIS and different sets of geodata. The correlations between the hydrochemical properties and the catchment properties were explored using simple and multiple linear regression analysis.For each of the considered hydrochemical parameters, a multiple regression equation was fitted: for pH with the predictor's mean annual precipitation and areal proportions of carbonate rocks (r2 = 0.60); for alkalinity, in addition to these two predictors, with subsoil pH and areal proportions agricultural lands (r2 = 0.66); and for pPCO2 (i.e. the negative logarithm of PCO2) with mean air temperature, mean catchment slope gradient, and mean annual precipitation (r2 = 0.43). Based on these results, we argue that spatial patterns in river water pH and alkalinity are governed by catchment processes related to chemical rock weathering. For the PCO2, on the other hand, the spatial patterns are governed by in-river processes on which catchment properties can have an indirect effect. We conclude that our approach can be used to predict averages of these parameters for non-monitored river stretches, which in-turn allows for a better spatially explicit representation of the rivers' carbonate system at the regional to global scale, which will be needed for a refined analysis of rivers in the global carbon cycle.
Our knowledge of plant and animal growth and development is far superior to that of the evolution of soil, yet soil plays a fundamental role in natural ecosystems. To understand the complexity of soil systems we need to explore processes that lead to its formation. Research in pedogenesis has been focused on formalizing soil-forming factors and processes to ultimately model soil formation in the landscape. Early models described soil formation qualitatively and were mostly limited to a description of soil evolution in the landscape. They led to the development of qualitative models of pedogenesis based on empirical observations and later to quantitative models of pedogenesis based on empirical equations or detailed differential equations derived from fundamental physics. This review highlights the main models of pedogenesis and focuses on models and rates of pedogenic processes such as the production of soil from weathering of parent materials, and vertical and lateral movements in the soil profile. It will become clear that field and laboratory work is needed to improve and validate quantitative models of pedogenesis. In order to estimate and verify model parameters, it is therefore of importance to collect real-world data.
Soils in similar geomorphic settings in hyperarid deserts (< 50 mm yr−1) should have similar characteristics because a negative moisture balance controls their development. However, Reg soils in the hyperarid southern Negev and Namib deserts are distinctly different. Soils developed on stable alluvial surfaces with only direct input of rainfall and dust depend heavily on rainfall characteristics. Annual rainfall amount can be similar (15–30 mm), but storm duration can drastically alter Reg soil properties in deserts. The cooler fall/winter and dry hot summers of the southern Negev Desert with a predominance brief (≤ 1 day) rainstorms result in gypsic-saline soils without any calcic soil horizon. Although the Namib Desert receives only 50–60% of the southern Negev annual rainfall, its rainstorm duration is commonly 2–4 days. This improves leaching of the top soil under even lower annual rainfall amount and results in weeks-long grass cover. The long-term cumulative effect of these rare rain-grass relationships produces a calcic-gypsic-saline soil. The development of these different kinds of desert soils highlights the importance of daily to seasonal rainfall characteristics in influencing soil-moisture regime in deserts, and has important implications for the use of key desert soil properties as proxies in paleoclimatology.
1] We developed a process-oriented hillslope soil mass balance model that integrates chemical and physical processes within hillslope soils. The model explicitly factors that soil chemical weathering at any hillslope position is related to the flux of soil eroded from upslope as well as soil production from underlying bedrock. The model was merged with measurements of soil elemental chemistry and cosmogenic radionuclide-based saprolite-to-soil conversion rates along a 50 m transect of a semiarid granodiorite hillslope in the southeastern Australian highlands. Inverse modeling results in the simultaneous quantification of the rates of soil chemical weathering and soil transport as a function of hillslope position. Soil chemical weathering rates per land surface area systematically varied along the transect from losses of 0.035 kg m À2 yr À1 on the ridge to gains of 0.035 kg m À2 yr À1 at the lowest slope position. The mass loss via soil chemical weathering would have been overestimated by 40% if the impact of soil transport on soil chemistry was ignored. The chemical mobility of elements, combined with biological nutrient demand, controlled the spatial redistribution of individual elements: P and Ca were preferentially retained relative to Si, Al, and Fe within the hillslope base. The calculated soil transport rate is linearly related to the product of soil thickness and slope gradient, instead of slope alone. Soil residence time was determined by calculating the time length for a 3 dimensional box (volume = 1 m 2 surface area  soil thickness) to be entirely removed by mass flux of soil transport: 4 ka on the ridge to 0.9 ka at the hillslope base. These soil residence times, combined with soil chemical weathering rates, indicate that a 1 m 2 area of soil loses $1,800 kg via chemical weathering while passing through the upslope portion of the hillslope, but that it regains $90 kg, probably via clay precipitation and biological retention of cations, during its passage through the lower segments of the transect. This study provides a previously unrecognized linkage between physical soil transport and soil chemical weathering that have implications for hillslope evolution as well as biogeochemistry., Integration of geochemical mass balance with sediment transport to calculate rates of soil chemical weathering and transport on hillslopes, J. Geophys. Res., 112, F02013, doi:10.1029/2005JF000402.
1] Linking mineral weathering rates measured in the laboratory to those measured at the landscape scale is problematic. In laboratory studies, collections of minerals are exposed to the same weathering environment over a fixed amount of time. In natural soils, minerals enter, are mixed within, and leave the soil via erosion and dissolution/leaching over the course of soil formation. The key to correctly comparing mineral weathering studies from laboratory experiments and field soils is to consistently define time. To do so, we have used reservoir theory. Residence time of a mineral, as defined by reservoir theory, describes the time length between the moment that a mineral enters (via soil production) and leaves (via erosion and dissolution/leaching) the soil. Age of a mineral in a soil describes how long the mineral has been present in the soil. Turnover time describes the time needed to deplete a species of minerals in the soil by sediment efflux from the soil. These measures of time are found to be sensitive to not only sediment flux, which controls the mineral fluxes in and out of a soil, but also internal soil mixing that controls the probability that a mineral survives erosion. When these measures of time are combined with published data suggesting that a mineral's dissolution reaction rate decreases during the course of weathering, we find that internal soil mixing, by partially controlling the age distribution of minerals within a soil, might significantly alter the soil's mass loss rate via chemical weathering.
a b s t r a c t a r t i c l e i n f o Keywords: weathering erosion soil geochemistry hillslope processes sediment transport channel incision Hillslopes have been intensively studied by both geomorphologists and soil scientists. Whereas geomorphologists have focused on the physical soil production and transport on hillslopes, soil scientists have been concerned with the topographic variation of soil geochemical properties. We combined these differing approaches and quantified soil chemical weathering rates along a grass covered hillslope in Coastal California. The hillslope is comprised of both erosional and depositional sections. In the upper eroding section, soil production is balanced by physical erosion and chemical weathering. The hillslope then transitions to a depositional slope where soil accumulates due to a historical reduction of channel incision at the hillslope's base. Measurements of hillslope morphology and soil thickness were combined with the elemental composition of the soil and saprolite, and interpreted through a process-based model that accounts for both chemical weathering and sediment transport. Chemical weathering of the minerals as they moved downslope via sediment transport imparted spatial variation in the geochemical properties of the soil. Inverse modeling of the field and laboratory data revealed that the long-term soil chemical weathering rates peak at 5 g m − 2 yr − 1 at the downslope end of the eroding section and decrease to 1.5 g m − 2 yr − 1 within the depositional section. In the eroding section, soil chemical weathering rates appear to be primarily controlled by the rate of mineral supply via colluvial input from upslope. In the depositional slope, geochemical equilibrium between soil water and minerals appeared to limit the chemical weathering rate. Soil chemical weathering was responsible for removing 6% of the soil production in the eroding section and 5% of colluvial influx in the depositional slope. These were among the lowest weathering rates reported for actively eroding watersheds, which was attributed to the parent material with low amount of weatherable minerals and intense coating of the primary minerals by secondary clay and iron oxides. We showed that both the morphologic disequilibrium of the hillslope and the spatial heterogeneity of soil properties are due to spatial variations in the physical and chemical processes that removed mass from the soil.
The weathering and origin of an autochthonous blockfield in the northern Swedish mountains were investigated through an examination of fine matrix and clasts from two pits excavated across ridge-top sorted circles; one on a summit, the other in a saddle. At the summit, fine matrix chemical weathering is limited to the production of poorly crystallized Al- and Fe-oxyhydroxides, whereas some additional vermiculitization and gibbsite crystallization occurs in the saddle. In both locations, volumes of clay-sized matrix are low, mass balance calculations indicate only minor elemental losses and no chemically etched grains are visible under a scanning electron microscope (SEM). In addition, soil horizons are absent and chemical weathering intensity is uniformly low across both excavated sorted circles. Minor clast chemical weathering consists of Fe oxidation, which dominates in the matrix-rich circle centres, and some rind development, which increases in frequency in the clast-rich rings. The dominance of physical weathering processes and the presence of only minor chemical weathering, in both fine matrix and clasts, indicate that the blockfield is not a Neogene weathering remnant. Rather, the blockfield has a Quaternary origin, developing during interglacials, interstadials and the Holocene, primarily through subsurface weathering processes.
The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ∼2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ∼1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (105–106 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ18O and relatively high δ13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ∼1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 106 yr.
Early (>3 Gy) wetter climate conditions on Mars have been proposed, and it is thus likely that pedogenic processes have occurred there at some point in the past. Soil and rock chemistry of the Martian landing sites were evaluated to test the hypothesis that in situ aqueous alteration and downward movement of solutes have been among the processes that have transformed these portions of the Mars regolith. A geochemical mass balance shows that Martian soils at three landing sites have lost significant quantities of major rock-forming elements and have gained elements that are likely present as soluble ions. The loss of elements is interpreted to have occurred during an earlier stage(s) of weathering that may have been accompanied by the downward transport of weathering products, and the salts are interpreted to be emplaced later in a drier Mars history. Chemical differences exist among the sites, indicating regional differences in soil composition. Shallow soil profile excavations at Gusev crater are consistent with late stage downward migration of salts, implying the presence of small amounts of liquid water even in relatively recent Martian history. While the mechanisms for chemical weathering and salt additions on Mars remain unclear, the soil chemistry appears to record a decline in leaching efficiency. A deep sedimentary exposure at Endurance crater contains complex depth profiles of SO4, Cl, and Br, trends generally consistent with downward aqueous transport accompanied by drying. While no model for the origin of Martian soils can be fully constrained with the currently available data, a pedogenic origin is consistent with observed Martian geology and geochemistry, and provides a testable hypothesis that can be evaluated with present and future data from the Mars surface.
We use reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. We constructed a model accounting for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO2 and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. We then fit the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO2 derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO2 dissolved in rainwater.
K/Th determined by the Mars Odyssey Gamma Ray Spectrometer varies by a factor of 3 on Mars (3000 to 9000), but over 95% of the surface area has K/Th between 4000 and 7000. K/Th is distinctly lower than average in some areas, including west of Olympus Mons in the Amazonis Planitia, the region around Memnonia Fossae, Chryse Planitia, southeastern Arabia Terra, Syrtis Major Planum, and northwest of Apollinaris Patera. On the other hand, K/Th is distinctly higher than average in other areas, including the central part of Valles Marineris and the surrounding highlands, and in the northern part of Hellas. The generally modest variation in K/Th may be explained by inherent variations in igneous rocks and by variations in the extent of aqueous alteration.
To help interpret the polygonal patterned ground on Mars, we present recent findings about a similar form of patterned ground in a particularly cold and arid region on Earth, the Dry Valleys of Antarctica. In this region, distinct arrays of interconnected polygons, which we refer to herein simply as patterned ground, characterize many surfaces, reflecting a subsurface network of interconnected, subvertical wedges of sand that grow incrementally as sand progressively fills soil fractures. The fractures form initially as thermoelastic stresses arise during periods of rapid cooling of frozen ground, and they continue to open and close in response to thermal cycles. We describe the initiation and maturation of the patterned ground using data for the growth of sand wedges and for the evolution of crack patterns and microrelief over time scales ranging up to 106 years.
The carbon ({sup 14}C and {sup 13}C) and oxygen isotopic composition of pedogenic carbonate was determined for two soil chronosequences on limestone and granitic alluvium in the Providence Mountains area in the Mojave Desert, California. The measured {sup 14}C ages of pedogenic carbonate coating on clasts were interpreted in the light of a diffusion-reaction model developed in our recent studies. Model ages of soil formation calculated from the measured {sup 14}C ages of pedogenic carbonate are in correct relative order as determined by geomorphic evidence, and are also consistent with model ages from the measured {sup 14}C ages of soil organic matter. {sup 14}C model ages suggest that the order geomorphic surfaces we studied are of late Pleistocene age (ca. 47-17 ka) and the younger surfaces formed during the Holocene (ca. 11-4 ka). These age estimates of the geomorphic surfaces are older than the previously assigned ages based on a combination of soil development, geomorphic relationships, and several infrared-stimulated luminescence dates, but they are within a few thousand years of these other age estimates. Stable carbon isotopic composition of the soil carbonate indicates either a slight increase in C{sub 4} or CAM (crassulacean acid metabolism) plants or a decrease in plant density in this area during the Holocene. Both the carbon and oxygen isotopic composition of soil carbonates suggests that the climate in the eastern Mojave Desert has, in general, become warmer and drier during the Holocene. 57 refs., 9 figs., 5 tabs.
Our galaxy is a highly evolved entity. Not merely a ran-dom assortment of stars, like so many grains of sand on a beach, it is an elegant structure that shows both order and complexity. We know that the Milky Way is a spiral disk galaxy, similar to many others we see in the sky. This sur-prisingly beautiful shape is so common among galaxies that the universe almost seems to delight in building them. The end product is especially remarkable in the light of what is believed to be the starting point: nebulous blobs of gas. How the universe made the Milky Way from such simple beginnings is not altogether clear. The task of unraveling this mystery has been cast to astronomers, such as myself, who attempt to construct models of the Galaxy's evolution based on its present appearance. These models need to account for not only the large-scale gravitational forces involved in assembling the Gal-axy, but also the chemical composition of its primary components, the stars. It turns out that the chemistry of the stars holds clues to how the Galaxy was made and how it has changed through time. The gas blobs that evolved into the Milky Way consisted merely of hydro-gen and helium (and a smattering of lithium), the ele-ments that were created in the Big Bang. All the other elements were literally created by the stars. Unlike the medieval alchemists, the stars can actually transmute one element into another—they are prodigious chemical fac-tories. Nevertheless, even today hydrogen and helium make up about 98 percent of the normal matter in the uni-verse. It's the distribution of the elements that make up the final 2 percent that makes all the difference to studies of galactic evolution. The most recent models of our galaxy's chemical evo-lution actually need to incorporate many other observed properties as constraints. These include the density of gas in various parts of the disk, the rate at which stars are born and die, refined measures of the Sun's chemical composition, and the rate at which the elements are pro-duced by the stars, among many others. Astronomers love constraints because without them a model is little more than hand-waving conjecture. The tricky part is coming up with a successful model that incorporates as many constraints as possible. Although the development of new technologies has improved the quality of the observations and so refined the constraints on astronomers' models, we are still far from a complete understanding of our galaxy's evolution. Like our galaxy, the field itself is still evolving. Here I pro-vide an overview of how astronomers attempt to uncover our galaxy's past, and I introduce a new model that ac-counts for some of the most recent observations.
The origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas (salars), and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined δ34S and 87Sr/86Sr values of soil gypsum/anhydrite and 87Sr/86Sr values of soil calcite along three east-west trending transects. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. Results from an east-west transect located along a breach in the Coastal Cordillera show that most soils within 90 km of the coast, and below 1300 m in elevation, are influenced by marine aerosols and that soils within 50 km, and below 800 m in elevation, receive >50% of Ca and S from marine aerosols (δ34S values > 14‰ and 87Sr/86Sr values >0.7083). In areas where the Coastal Cordillera is >1200 m in elevation, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Most pedogenic salts from inland soils have δ34S values between +5.0 to +8.0‰ and 87Sr/86Sr ratios between 0.7070 and 0.7076. These values are similar to average δ 34S and 87Sr/86Sr values of salts from local streams, lakes, and salars (+5.4 ±2‰ δ34S and 0.70749 ± 0.00045 87Sr/86Sr) in the Andes and Atacama, suggesting extensive eolian reworking of salar salts onto the surrounding landscape. Ultimately, salar salts are precipitated from evaporated ground water, which has acquired its dissolved solutes from water-rock interactions (both high and low-temperature) along flowpaths from recharge areas in the Andes. Therefore, the main source for Ca and S in gypsum/anhydrite in non-coastal soils is indirect and involves bedrock alteration, not surficially on the hyperarid landscape, but in the subsurface by ground water, followed by eolian redistribution of ground-water derived salar salts to soils. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local marine and salar salt, supporting arguments for tropospheric nitrogen as the main source for soil nitrate.
Chemical weathering and the subsequent export of carbonate alkalinity (HCO3– + CO3–2) from soils to rivers account for significant amounts of terrestrially sequestered atmospheric CO2. We show here that during the past half-century, the export of this alkalinity has increased dramatically from North America's
largest river, the Mississippi. This increased export is in part the result of increased flow resulting from higher rainfall
in the Mississippi basin. Subcatchment data from the Mississippi suggest that the increase in the export of alkalinity is
also linked to amount and type of land cover. These observations have important implications for the potential management
of carbon sequestration in the United States.
The Miniature Thermal Emission Spectrometer (Mini-TES) on Spirit has studied the mineralogy and thermophysical properties at Gusev crater. Undisturbed soil spectra show evidence for minor carbonates and bound water. Rocks are olivinerich basalts with varying degrees of dust and other coatings. Dark-toned soils observed on disturbed surfaces may be derived from rocks and have derived mineralogy (+/-5 to 10%) of 45% pyroxene (20% Ca-rich pyroxene and 25% pigeonite), 40% sodic to intermediate plagioclase, and 15% olivine (forsterite 45% +/-5 to 10). Two spectrally distinct coatings are observed on rocks, a possible indicator of the interaction of water, rock, and airfall dust. Diurnal temperature data indicate particle sizes from 40 to 80 microm in hollows to approximately 0.5 to 3 mm in soils.
Reviews scientific work done on soil erosion prediction, soil formation and the effect of erosion on productivity so far as they affect agricultural practices in the US. Based on experience and some past researches some advisory bodies now recommend a soil tolerance value of 5 tons per acre per year, but some dispute continues about the value of this particular guideline.-from Author
The temperature, moisture and CO2 content of an uncultivated and an irrigated soil in the western San Joaquin Valley of California were monitored for more than 1 year in order to determine the degree to which irrigated agriculture affected the annual variations of these important ecological properties. The uncultivated soil underwent strong seasonal changes in moisture and CO2, the highest values being obtained in the late winter and early spring and the lowest occuring in the late summer and early autumn. In contrast, as adequate moisture for biological activity was available year round in the irrigated soil, the CO2 levels varied in response to variations in temperature. The organic C content of the irrigated soil was half that of the uncultivated soil (1.8 versus 3.6 kg m−2), presumably owing to an enhanced level of organic matter decomposition in the irrigated soil. Calculated annual rates of soil respiration for the two soils were nearly identical (0.25 g m−2 h−1). Based on estimates of annual biomass production in the uncultivated soil, it was estimated that the mean residence time of soil organic matter was approximately 61 years. Although this estimate indicates a relatively rapid turnover of C, it is consistent with previous work indicating rapid nutrient cycling in annual grasslands.
The origin of the mound microrelief known by such names as “hogwallows,” “pimple mounds” or “mima mounds” has long been a mystery; although many solutions have been proposed, none has yet received widespread acceptance.
During the course of the soil surveys of eastern Merced and Stanislaus Counties of California, where mound microrelief is widespread, the phenomenon was studied over a period of eight years. All known theories were evaluated and compared with the evidence which could be observed in the field. The evidence includes the nature of the mounds such as shape, size, internal structure, distribution of gravel, the burrows of ants and rodents; the distribution of the microrelief with respect to soil types and the pattern of occurrence of mounds under various conditions.
The theories of origin which were tested included erosional theories based on wind action and water action, fluviatile, submarine and sub‐lacustrine currents; eruptional theories suggesting gas‐vents, sand boils and hydrostatic pressure; and biological theories based upon the activity of ants, ground squirrels and pocket gophers.
It is concluded that the evidence points clearly to a biological explanation, and that the pocket gopher is responsible for Mima mound microrelief.
Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10−17 mol m−2 s−1 for biotite. Faster weathering rates of 1.8 to 3.6×10−16 mol m−2 s−1 are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.
Soils play an important role in the carbon cycle as the nutrition of
photosynthesized biomass. Nitrogen fixed by microbes from air is a
limiting nutrient for ecosystems within the first flush of ecological
succession of new ground, and sulfur can limit some components of
wetland ecosystems. But over the long term, the limiting soil nutrient
is phosphorus extracted by weathering from minerals such as apatite
(Vitousek et al., 1997a; Chadwick et al., 1999). Life has an especially
voracious appetite for common alkali (Na+ and K+) and alkaline earth
(Ca2+ and Mg2+) cations, supplied by hydrolytic weathering, which is in
turn amplified by biological acidification (Schwartzmann and Volk, 1991;
see Chapter 5.06). These mineral nutrients fuel photosynthetic fixation
and chemical reduction of atmospheric CO2 into plants and plantlike
microbes, which are at the base of the food chain. Plants and
photosynthetic microbes are consumed and oxidized by animals, fungi, and
other respiring microbes, which release CO2, methane, and water vapor to
the air. These greenhouse gases absorb solar radiation more effectively
than atmospheric oxygen and nitrogen, and are important regulators of
planetary temperature and albedo (Kasting, 1992). Variations in solar
insolation ( Kasting, 1992), mountainous topography ( Raymo and
Ruddiman, 1992), and ocean currents ( Ramstein et al., 1997) also play a
role in climate, but this review focuses on the carbon cycle. The carbon
cycle is discussed in detail in Volume 8 of this Treatise.The greenhouse
model for global paleoclimate has proven remarkably robust (Retallack,
2002), despite new challenges ( Veizer et al., 2000). The balance of
producers and consumers is one of a number of controls on atmospheric
greenhouse gas balance, because CO2 is added to the air from fumaroles,
volcanic eruptions, and other forms of mantle degassing (Holland, 1984).
Carbon dioxide is also consumed by burial as carbonate and organic
matter within limestones and other sedimentary rocks; organic matter
burial is an important long-term control on CO2 levels in the atmosphere
(Berner and Kothavala, 2001). The magnitudes of carbon pools and fluxes
involved provide a perspective on the importance of soils compared with
other carbon reservoirs ( Figure 1). (6K)Figure 1. Pools and fluxes of
reduced carbon (bold) and oxidized carbon (regular) in Gt in the
pre-industrial carbon cycle (sources Schidlowski and Aharon, 1992;
Siegenthaler and Sarmiento, 1993; Stallard, 1998). Before
industrialization, there was only 600 Gt (1 Gt=1015g) of carbon in CO2
and methane in the atmosphere, which is about the same amount as in all
terrestrial biomass, but less than half of the reservoir of soil organic
carbon. The ocean contained only ˜3 Gt of biomass carbon. The deep
ocean and sediments comprised the largest reservoir of bicarbonate and
organic matter, but that carbon has been kept out of circulation from
the atmosphere for geologically significant periods of time (Schidlowski
and Aharon, 1992). Humans have tapped underground reservoirs of fossil
fuels, and our other perturbations of the carbon cycle have also been
significant ( Vitousek et al., 1997b; see Chapter 8.10).Atmospheric
increase of carbon in CO2 to 750 Gt C by deforestation and fossil fuel
burning has driven ongoing global warming, but is not quite balanced by
changes in the other carbon reservoirs leading to search for a "missing
sink" of some 1.8±1.3 GtC, probably in terrestrial organisms,
soils, and sediments of the northern hemisphere (Keeling et al., 1982;
Siegenthaler and Sarmiento, 1993; Stallard, 1998). Soil organic matter
is a big, rapidly cycling reservoir, likely to include much of this
missing sink.During the geological past, the sizes of, and fluxes
between, these reservoirs have varied enormously as the world has
alternated between greenhouse times of high carbon content of the
atmosphere, and icehouse times of low carbon content of the atmosphere.
Oscillations in the atmospheric content of greenhouse gases can be
measured, estimated, or modeled on all timescales from annual to eonal
(Figure 2). The actively cycling surficial carbon reservoirs are
biomass, surface oceans, air, and soils, so it is no surprise that the
fossil record of life on Earth shows strong linkage to global climate
change (Berner, 1997; Algeo and Scheckler, 1998; Retallack, 2000a).
There is an additional line of evidence for past climatic and
atmospheric history in the form of fossil soils, or paleosols, now known
to be abundant throughout the geological record ( Retallack, 1997a,
2001a). This chapter addresses evidence from fossil soils for global
climate change in the past, and attempts to assess the role of soils in
carbon cycle fluctuations through the long history of our planet.
(30K)Figure 2. Variation in atmospheric CO2 composition on a variety of
timescales ranging from annual to eonal (a) Keeling et al., 1982;
reproduced from Carban Dioxide Review 1982, 377-385 (b) Petit et al.,
1999; reproduced by permission of Nature Publishing Group from Nature
1999, 399, 429-436 (c) Retallack, 2001d; reproduced by J. Geol. 2001,
109, 407-426 (d) Berner and Kothavala, 2001; reproduced by permission of
American Journal of Science from Am. J. Sci. 2001, 301, 182-204.
We measured selected properties of a naturally saline and sodic soil in pedons that had been reclaimed a minimum of 0, 5, 8, 15, or 25 yr. More than 7.2 x 104 kg of soluble salt were leached from the upper 100 cm of the soil after 5 yr, with little or no additional removal with time. A steady-state soil solution composition was reached after approximately 15 yr based on electrical conductivity values of the saturation extracts. The sodium adsorption ratios (SAR) of the steady-state solutions were either greater or less than the SAR of the irrigation water and appeared to be related to mineral weathering. Smectite minerals were unstable in the reclaimed soils and were transformed to kaolinite. This may have been the cause for the reduction in water-dispersible clay in soils that had been reclaimed for 15 yr or more.
(C) Williams & Wilkins 1985. All Rights Reserved.
A biomantle is a differentiated zone in the upper part of soils produced largely by bioturbation, but often aided by subsidiary processes. One such process is loss of fine particles and redistribution of coarse ones from evolving biomantles. This is accomplished by rainwash and wind sorting of unprotected surface mounds and by vertical and/or lateral eluviation (lessivage) through-flow processes. Also subsidiary are the various processes by which iron, manganese, and other concretions come to reside in some biomantles. Concretions may form in biomantles directly from metal-bearing solutions, they may enter biomantles from upslope via creep and slope processes, or they may enter biomantles from below as the landscape down-wastes.
Faunalmantles are biomantles produced largely by burrowing animals (faunalturbation), and floralmantles are produced largely by tree uprooting (floralturbation). Faunalmantles and floralmantles may be one-, two-, or multilayered, as differentiated by one or more observable or measurable soil properties. Chief among these properties are particle size and biologically produced soil fabric (biofabric). Complex biomantles result where faunal-mantles and floralmantles are conjoined or coevolve on the same tract or landscape.
Stone zones, stone lines, or coarse-textured layers comprise the lower members of two-layered and multilayered faunal-mantles, and stone pavements comprise the upper members of two-layered and multi-layered floralmantles. Prehistoric and historic artifacts may be components of some floralmantle pavements and of some faunalmantle some lines and stone zones.
(C) Williams & Wilkins 1990. All Rights Reserved.
This article looks at the paleoclimatic record of winds recorded in the deposits of dust grains in the deep sea. Such grains are carried to the sea by the wind. The strength of the wind is indicated by the size of dust grains observed, larger grains requiring stronger winds to effect transport from arid regions. The number of the dust grains provides an indication of the aridity of land masses at the times in question. The record indicates that wind varies on the Milankovitch cycles of orbital variability, but also on time scales shorter than the 100kyr cycles associated with glaciation and aridity.
A considerable amount of progress in the understanding of the factors that control the genesis of calcic soils has been made by integration of field and laboratory studies supplemented by numerical modeling. Modeling of pedogenic carbonate accumulation using a compartment model strategy has produced simulated depth functions similar to those observed in field investigations and emphasizes the significance of varying eolian dust flux and soil pCO2 on the magnitude and depth of carbonate accumulation. Recent studies have provided significant new information concerning factors that influence carbonate accumulation in soils, which should significantly enhance prospects for numerical simulation of calcic soils in complex circumstances (e.g., soils subjected to glacial-to-interglacial climatic change). Such studies include carbonate dissolution rates, seasonal variation and depth variation of soil pCO2, soil-available water-holding capacity and evapotranspiration in arid climates, and isotopic composition of pedogenic carbonate. Isotopic studies also show that carbonate dissolution and precipitation occur in a chemically open system, demonstrating that calculations of carbonate solubility using models assuming a chemically open soil system are reasonable. Additional data from new studies regarding influences of presence of gypsum and other soluble salts on carbonate dissolution and accumulation enable evaluation of the coprecipitation of such materials on calcic soil genesis.
Soils are dynamic components of terrestrial ecosystems that historically have been viewed as economic resources by government and private interests. The large-scale conversion of many sections of the United States to agriculture and urban land uses, combined with the growing awareness of the role of soils in global biogeochemistry and ecology, ultimately requires an assessment of the remaining distribution of undisturbed soils in the country. Here we conduct the first quantitative analysis of disturbed and undisturbed soil distribution in the USA using a GIS-based approach. We find that a sizable fraction (4.5%) of the nation's soils are in danger of substantial loss, or complete extinction, due to agriculture and urbanization. In the agricultural belt of the country, up to 80% of the soils that were naturally of low abundance are now severely impacted (greater than 50% conversion to agricultural/urban uses). Undisturbed soils provide ecosystem services that warrant their preservation, including a somewhat complex relationship with rare or endangered plants. The known and unknown attributes of undisturbed soils suggests the need for an integrated biogeodiversity perspective in landscape preservation efforts.
The distribution and residence time of cosmogenic 10Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10Be varies with soil horizon clay content, that the residence time of 10Be in these horizons exceeds 105 years, and that to a rough approximation the inventory of 10Be in a thoroughly sampled soil profile fits the equation: N = (q − Em)(1 − e−λι)/λ where q is delivery rate, E is erosion rate, m is the concentration of 10Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.
Soils of continental Antarctica are forming in one of the most severe terrestrial environments. Continuously low temperatures and the scarcity of water in the liquid state result in the development of desert-type soils. In an earlier experiment to determine the degree to which radioactive Na(Cl-36) would migrate from a shallow point source in permafrost, movement was observed. To confirm this result, a similar experiment involving (Na-22)Cl was conducted. Significantly less movement of the Na-22 ion was observed. Ionic movement in the unfrozen interfacial films at mineral surfaces in frozen ground is held to be important in chemical weathering in Antarctic soils.
The alpha particle x-ray spectrometer on the Spirit rover determined major and minor elements of soils and rocks in Gusev crater in order to unravel the crustal evolution of planet Mars. The composition of soils is similar to those at previous landing sites, as a result of global mixing and distribution by dust storms. Rocks (fresh surfaces exposed by the rock abrasion tool) resemble volcanic rocks of primitive basaltic composition with low intrinsic potassium contents. High abundance of bromine (up to 170 parts per million) in rocks may indicate the alteration of surfaces formed during a past period of aqueous activity in Gusev crater.