H. Curtis Monger

Classification of paleosols

Despite increased interest in paleosols during the past decade, no satisfactory classification is in current use. Presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record. Emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope. The classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes: organic matter content, horizonation, redox conditions, in situ mineral alteration, illuviation of insoluble minerals/compounds, and accumulation of soluble minerals. The most prominent of these six features/processes provides the key to classifying a paleosol into one of nine orders. Four of the order names are borrowed from Soil Taxonomy (Histosol, Spodosol, Oxisol, Vertisol), whereas the other five order names are presented here for the first time (Calcisol, Gypsisol, Gleysol, Argillisol, Protosol). The orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol. The classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

Disentangling Complex Landscapes: New Insights into Arid and Semiarid System Dynamics

Abstract Although desertification is a global phenomenon and numerous studies have provided information on dynamics at specific sites, spatial and temporal variations in response to desertification have led to alternative, and often controversial, hypotheses about the key factors that determine these dynamics. We present a new research framework that includes five interacting elements to explain these variable dynamics: (1) historical legacies, (2) environmental driving variables, (3) a soil-geomorphic template of patterns in local properties and their spatial context, (4) multiple horizontal and vertical transport vectors (water, wind, animals), and (5) redistribution of resources within and among spatial units by the transport vectors, in interaction with other drivers. Interactions and feedbacks among these elements within and across spatial scales generate threshold changes in pattern and dynamics that can result in alternative future states, from grasslands to shrublands, and a reorganization of the landscape. We offer a six-step operational approach that is applicable to many complex landscapes, and illustrate its utility for understanding present-day landscape organization, forecasting future dynamics, and making more effective management decisions.

Microbial precipitation of pedogenic calcite

Pedogenic calcite in desert soils has become increasingly important as an indicator of paleoclimate, landscape stability, and landscape age. This study indicates that calcic and petrocalcic horizons in desert soils are not simply the result of inorganic precipitation of calcite. Soil microorganisms were found to be involved in calcite precipitation in a typical desert soil near Las Cruces, New Mexico. Fossilized remains of calcified fungal hyphae and Microcodium structures are abundant in the petrocalcic horizon. Soil bacteria and fungi precipitated calcite when cultured on a Ca-rich medium. In an experiment where soil columns were irrigated with Ca-rich solutions, calcite formed in soils containing soil microorganisms, but no calcite formed in sterile soils. Thus, biomineralization of calcite by soil microorganisms appears to be an important mechanism of unknown magnitude.

Calcium Carbonate Features

Publisher Summary The precipitation of calcium carbonate is common in soils and regoliths, especially in soils of arid environments. The precipitation and the accumulation of calcium carbonate in soils and regoliths are very complex phenomena. As they are linked to the interaction between the lithosphere, the biosphere, and the atmosphere, pedogenic carbonates may be important proxies of paleoenvironmental changes. The study of the morphological expression and hierarchical organisation of calcitic pedofeatures in thin sections allows us to partially decipher the climatic, geochemical, and biological influences on the precipitation of carbonates in soils. With the contribution of submicroscopic techniques (e.g., SEM), progress has been made in the understanding of the relation between the biological activity and the precipitation of carbonate, and many calcitic features seem to be linked to biological processes, in a direct or indirect manner. The carbonates may represent a highly active phase, undergoing intense transformation such as recrystallisation, dissolution, and secondary carbonate precipitation.

Plio-pleistocene pumice floods in the ancestral Rio Grande, southern Rio Grande rift, USA

Abstract At least four times during the late Pliocene and early Pleistocene pyroclastic eruptions in the Jemez volcanic field, northern Rio Grande rift, flooded the ancestral Rio Grande with gravel-sized pumice. Following as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Rio Grande rift. A combination of reversal magnetostratigraphy and single-crystal sanidine 40 Ar/ 39 Ar dating constrains the ages of pumice-clast conglomerates at 3.1, ∼2.0, 1.6, and 1.3 Ma. The coarsest pumice beds (cobbles, boulders) were deposited as antidune-like bedforms in a fluvial channel and as a crevasse-splay sheet. Granule and pebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clasts in the gravel fraction (60–100%) suggests very rapid transport downriver, probably in a few days or weeks. The two older pumice-clast conglomerates correlate with the Puye Formation in the Jemez volcanic field, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.

Scale and the isotopic record of C4 plants in pedogenic carbonate: from the biome to the rhizosphere

The 13C/12C ratio in pedogenic carbonate (i.e., CaCO3 formed in soil) is a significant tool for investigating C4 biomes of the past. However, the paleoecological meaning of delta13C values in pedogenic carbonate can change with the scale at which one considers the data. We describe studies of modern soils, fossil soils, and vegetation change in the Chihuahuan Desert of North America and elsewhere that reveal four scales important for paleoecologic interpretations. (1) At the broadest scale, the biome scale (hundreds to millions of km2), an isotopic record interpreted as C3 vegetation replacing C4 grasslands may indicate invading C3 woody shrubs instead of expanding C3 forests (a common interpretation). (2) At the landscape scale (several tens of m2 to hundreds of km2), the accuracy of scaling up paleoclimatic interpretations to a regional level is affected by the landform containing the isotopic record. (3) At the soil-profile scale (cm2 to m2), soil profiles with multiple generations of carbonate mixed together have a lower-resolution paleoecologic record than soil profiles repeatedly buried. (4) At the rhizosphere scale (microm2 to cm2), carbonate formed on roots lack the 14-17 per thousand enrichment observed at broader scales, revealing different fractionation processes at different scales. A multi-scale approach in dealing with delta13C in pedogenic carbonate will increase the accuracy of paleoecologic interpretations and understanding of soil-geomorphic-climatic interactions that affect boundaries between C4 and C3 vegetation.

Sequestration of inorganic carbon in soil and groundwater

Movement of CO 2 from the atmosphere into land via photosynthesis and root respiration, the subsequent formation of bicarbonate in soil, and its storage in groundwater or precipitation as CaCO 3 in dryland soils are major processes in the global carbon cycle. Together, inorganic carbon as soil carbonate (∼940 PgC) and as bicarbonate in groundwater (∼1404 PgC) surpass soil organic carbon (∼1530 PgC) as the largest terrestrial pool of carbon. Yet, despite general agreement about its huge size as a carbon pool, controversy about the potential of inorganic carbon to sequester atmospheric CO 2 remains unresolved. We suggest that the controversy stems from the absence of a lexicon and propose a classification scheme that uses (1) calcium source illustrated by two widely recognized chemical reactions, and (2) the concept of carbonate “generations.” When calcium is derived from preexisting carbonate, an equilibrium reaction occurs that does not sequester carbon in soil carbonate but does sequester carbon in groundwater until bicarbonate precipitates as CaCO 3 . When calcium is derived from silicate minerals, a unidirectional reaction occurs that sequesters carbon in both soil carbonate and groundwater. The generations concept shows that carbon sequestration occurs only in the first generation when calcium is released directly from silicates. This classification not only enhances communication and provides a framework for quantifying amounts of fossil fuel carbon that can be sequestered within a geoengineering context, it provides more precise language for discussing the terrestrial carbon cycle through geologic time.

Classification of paleosols: Discussion and reply

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Banded vegetation-dune development during the Medieval Warm Period and 20th century, Chihuahuan Desert, New Mexico, USA

With the advent of systematic high-resolution satellite photography, striking geometric shapes of banded vegetation several km2 in size, but not apparent from the ground, have been documented for many areas of the arid and semiarid world. Banded vegetation, in which dense perennial vegetation alternates with bands of bare soil may originate from geomorphic processes, ecological self-organization, or human land use. In the Chihuahuan Desert of New Mexico prominent arc-shaped bands of vegetation and dunes occur along the contact of a piedmont slope (bajada) and basin floor. The origin and chronology of this banded vegetation-dune complex was investigated using early aerial photography (1936–1942), landscape photography (1918), vegetation and soil surveys (1858, 1918), soil stratigraphy, 13C/12C ratios, and 14C dating. These methods reveal two periods of eolian deposition. The first began in the Medieval Warm Period (ca. AD 900–1300) and was followed by a period of landscape stability during the Little Ice Age (ca. AD 1500 to 1850). The second began in the late-1800s when widespread desertification occurred throughout the American Southwest. Banded vegetation was initiated after formation of erosional scarplets that functioned as obstacles upon which eolian sand accumulated, thus becoming a dam to overland flow and causing strips of vegetation to form. Banded vegetation in this study is an emergent pattern produced by a coupled ecologic-geomorphic-climatic system. The stratigraphic record produced by this system enables us to compare current ecological responses to climate change with baseline prehistorical responses to climate change.

Landform influences on the resistance of grasslands to shrub encroachment, Northern Chihuahuan Desert, USA

In arid and semiarid regions, vegetative boundaries are often strikingly similar to landform boundaries. However, it is not well documented whether landforms exert an influence on the resistance of desert grassland to shrub encroachment. Dominant grassland communities have been displaced by woody shrubs over the last 150 years in the Jornada Basin, southern New Mexico. Digital vegetation maps from 1858, 1915–1916, 1928–1929, 1938, and 1998, in conjunction with a detailed landform map, were analyzed in a Geographical Information System. The generated time series maps and spatial data compiled from these datasets were used to quantify the extent and rate that grasslands were replaced by shrubs on eight contiguous landforms. From this assessment, we generated a resistance index that revealed desert grasslands were least resistant (most susceptible) to shrub expansion on sandy landforms and bajadas and most resistant to shrub invasion on ephemerally flooded playas. This study demonstrates that landforms both provide the broad-scale background for detailed mechanistic studies and affect the sensitivity of grasslands to shrub encroachment.