Craig Rasmussen

Quantifying the climatic and tectonic controls on hillslope steepness and erosion rate

Hillslopes in humid regions are typically convex to concave in profile and have a relatively thick, continuous regolith cover. Conversely, hillslopes in arid regions are typically cliff-dominated and have a relatively thin, discontinuous regolith cover. The difference between these two end-member slope forms is classically attributed to climate, but climate, tectonics, and lithology all play a role. In this paper, we describe a mathematical model for hillslope gradient and regolith thickness using basic climatic and tectonic input data for a given rock type. The model first solves for the regolith thickness on a planar slope segment in topographic steady state using the soil production function and a prescribed uplift/incision rate. The climatic and lithologic controls on soil production rates are quantified using an empirical energy-based model for the physical weathering of bedrock. The slope gradient is then computed by balancing uplift/incision rates with sediment fluxes calculated using a nonlinear depth- and slope-dependent sediment transport model. The model quantifies the ways in which, as aridity and uplift/incision rates increase, regolith thicknesses decrease and hillslope gradients increase nonlinearly until a threshold condition is reached, beyond which bare, cliff-dominated slopes form. The model can also be used to estimate long-term erosion rates and soil residence times using basic input data for climate and regolith thickness. Model predictions for erosion rates closely match cosmogenically derived erosion rates in granitic landscapes. This approach provides a better quantitative understanding of the climatic and tectonic controls on slope form, and it provides a simple, widely applicable method for estimating long-term erosion rates and the thickness of regolith cover on hillslopes.

Effects of a biochar-amended alkaline soil on the growth of romaine lettuce and bermudagrass

Abstract Biochar from pine forest waste (PFW) was used in greenhouse pot experiments to evaluate plant growth using two levels (2% and 4% wt/wt) of biochar amendments applied to an alkaline, loamy sand soil. Biochar soil additions induced a large decrease in the soil bulk density (from 1.59 to 1.26 g cm−3) and large to moderate increases in gravimetric and volumetric soil-water contents, respectively, under pot and field moisture capacity conditions. The growth of romaine lettuce was initially adversely affected in the 4% biochar-amended soil. However, bermudagrass benefited from the biochar addition with increased biomass production and enhanced drought resistance. Both plant species showed statistically significant increases (compared with controls) in biomass yields at the 2% but not at the 4% biochar application rate. An incubation study indicated that soil microbial activity, as measured by evolved CO2, was significantly suppressed (−31% compared with the control) in the presence of biochar over a period of 4 months. The data indicated that addition of PFW biochar induced a species-dependent plant response and produced an overall decrease in microbial mineralization of organic materials. Vegetables such as lettuce may benefit from a period of excess irrigation, to leach any potentially toxic biochar-introduced salts or organic compounds, before seeding. Conversely, warm season grasses may adapt quickly to a soil amended with PFW biochar with increased biomass production and drought resistance.

Calibration and testing of upland hillslope evolution models in a dated landscape: Banco Bonito, New Mexico

[1] In this study we tested upland hillslope evolution models and constrained the rates of regolith production, colluvial transport, and eolian deposition over geologic time scales in a dated volcanic landscape in northern New Mexico using field measurements of regolith thickness; geochemical analyses of regolith, bedrock, and regional dust; numerical modeling of regolith production and transport; and quantitative analyses of airborne light detection and ranging (lidar) digital elevation models (DEMs). Within this volcanic landscape, many topographically closed basins exist as a result of compressional folding and explosion pitting during eruption. The landscape has evolved from an initial state of no regolith cover at 40 ± 5 ka to its modern state, which has highly weathered regolith ranging from 0 to 3+ m, with local thickness values controlled primarily by topographic position. Our models constrain the maximum rate of regolith production in the study area to be in the range of 0.02 to 0.12 m kyr ! 1 and the rate of colluvial transport per unit slope gradient to be in the range of 0.2 to 2.7 m 2 kyr ! 1 , with higher values in areas with more aboveground biomass. We conclude that a depth! dependent colluvial transport model better predicts the observed spatial distribution of regolith thickness compared to a model that has no depth dependence. This study adds to the database of estimates for rates of regolith production and transport in the western United States and shows how dated landscapes can be used to improve our understanding of the coevolution of landscapes and regolith cover.

Climatic and landscape controls on water transit times and silicate mineral weathering in the critical zone

The critical zone (CZ) can be conceptualized as an open system reactor that is continually transforming energy and water fluxes into an internal structural organization and dissipative products. In this study, we test a controlling factor on water transit times (WTT) and mineral weathering called Effective Energy and Mass Transfer (EEMT). We hypothesize that EEMT, quantified based on local climatic variables, can effectively predict WTT within—and mineral weathering products from—the CZ. This study tests whether EEMT or static landscape characteristics are good predictors of WTT, aqueous phase solutes, and silicate weathering products. Our study site is located around Redondo Peak, a rhyolitic volcanic resurgent dome, in northern New Mexico. At Redondo Peak, springs drain slopes along an energy gradient created by differences in terrain aspect. This investigation uses major solute concentrations, the calculated mineral mass undergoing dissolution, and the age tracer tritium and relates them quantitatively to EEMT and landscape characteristics. We found significant correlations between EEMT, WTT, and mineral weathering products. Significant correlations were observed between dissolved weathering products (Na+ and DIC), 3H concentrations, and maximum EEMT. In contrast, landscape characteristics such as contributing area of spring, slope gradient, elevation, and flow path length were not as effective predictive variables of WTT, solute concentrations, and mineral weathering products. These results highlight the interrelationship between landscape, hydrological, and biogeochemical processes and suggest that basic climatic data embodied in EEMT can be used to scale hydrological and hydrochemical responses in other sites.

Beyond clay: towards an improved set of variables for predicting soil organic matter content

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.

Semi-Automated Disaggregation of a Conventional Soil Map Using Knowledge Driven Data Mining and Random Forests in the Sonoran Desert, USA

Conventional soil maps (CSM) have provided baseline soil information for land use planning for over 100 years. Although CSMhave been widely used, they are not suitable to meet growing demands for high resolution soil information at field scales. The authors present a repeatable method to disaggregate CSM data into ~30-meter resolution rasterized soil class maps that include continuous representation of probabilistic map uncertainty. Methods include training set creation for original CSM component soil classes from soil-landscape descriptions within the original survey database. Training sets are used to build a random forest predictive model utilizing environmental covariate maps derived from ASTER satellite imagery and the United States Geological Survey (USGS) National Elevation Dataset. Results showed agreement at 70 percent of independent field validation sites and equivalent accuracy between original CSM map units and the disaggregated map. Uncertainty of predictions was mapped by relating prediction frequencies of the random forest model and success rates at validation sites.

Factors Influencing Observed Variations in the Structure of Bacterial Communities On Calcite Formations in Kartchner Caverns, AZ, USA

Kartchner Caverns is an oligotrophic subterranean environment that hosts a wide diversity of actively growing calcite speleothems (secondary mineral deposits). In a previous study, we demonstrated that bacterial communities extracted from these surfaces are quite complex and vary between formations. In the current study, we evaluated the influence of several environmental variables on the superficial bacterial community structure of 10 active formations located in close proximity to one another in a small room of Kartchner Caverns State Park, Arizona, USA. Physical (color, dimensions) and chemical (elemental profile and organic carbon concentration) properties, as well as the DGGE-based bacterial community structure of the formations were analyzed. While elemental concentration was found to vary among the formations, the differences in the community structure could not be correlated with concentrations of either organic carbon or any of the elements evaluated. In contrast, the locations of formations within a distinct region of the cave as well as the relative location of specific formations within a single room were found to have a significant influence on the bacterial community structure of the formations evaluated. Interestingly, Canonical Correspondence Analysis suggests an association between the observed drip pathways (drip lines) feeding the formations (as determined by the patterns of soda straws and small stalactites that reveal water flow patterns) and the bacterial community structure of the respective formations. The results presented here indicate that a broad range of formations fed by a diversity of drip sources must be sampled to fully characterize the community composition of bacteria present on the surfaces of calcite formations in carbonate caves.

Vegetation effects on soil organic carbon quality in an arid hyperthermic ecosystem.

Abstract Arid lands occupy substantial global land area and thus may play an important role in the terrestrial carbon cycle. This study examined a facet of arid land carbon cycling by examining variation in plant and soil organic carbon quality and the physical partitioning of carbon into aggregate and mineral-associated fractions for soils dominated by Prosopis velutina (mesquite), Larrea tridentata (creosote), and a combination of Bouteloua barbata, Bouteloua aristidoides, Aristida adscensionis, and Cynodon dactylon (mixed grass) vegetation types in an arid hyperthermic ecosystem. We used a combination of density fractionation to quantify physical distribution of organic carbon, in addition to isotopic, thermal, and spectroscopic techniques to quantify carbon quality in each fraction. Data indicated that most of the soil carbon across all vegetation types was concentrated in free light fractions, with little role for aggregate occlusion or mineral adsorption. Differential thermal analysis and derivative thermogravimetry indicated that all vegetation types were dominated by thermally labile material (exothermic peak and mass loss at ∼350°C), with the greatest differences in carbon quality noted among the respective plant materials. Diffuse reflectance Fourier transform infrared spectroscopy also indicated that the most substantial variation in organic carbon chemical quality was among the various plant materials. In particular, the creosote plant material exhibited a distinct high-temperature exothermic peak near 515°C, the greatest specific enthalpy (20 kJ g−1 biomass), and a relatively greater proportion of aromatic components as determined by diffuse reflectance Fourier transform infrared spectroscopy. Furthermore, the data indicated substantial alteration of chemical and thermal properties as organic material progressed from plant material to the soil fractions with a convergence on organic material dominated by polysaccharides and substantial reduction in specific enthalpy in the soil fractions.

A new geological slip rate estimate for the Calico Fault, eastern California: implications for geodetic versus geologic rate estimates in the Eastern California Shear Zone

ABSTRACT Accurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.

Signatures of obliquity and eccentricity in soil chronosequences

Periodic shifts in Earth’s orbit alter incoming solar radiation and drive Quaternary climate cycles. However, unambiguous detection of these orbitally driven climatic changes in records of terrestrial sedimentation and pedogenesis remains poorly defined, limiting our understanding of climate change-landscape feedbacks, impairing our interpretation of terrestrial paleoclimate proxies, and limiting linkages among pedogenesis, sedimentation, and paleoclimatic change. Using a meta-analysis, we show that Quaternary soil ages preserved in the modern record have periodicities of 41 and 98 kyr, consistent with orbital cycles. Further, soil ages predominantly date to periods of low rates of climatic change following rapid climate shifts associated with glacial-to-interglacial transitions. Soil age appears linked to orbital cycles via climate-modulated sediment deposition, which may largely constrain soil formation to distinct climate periods. These data demonstrate a record of widespread orbital cyclicity in sediment deposition and subsequent pedogenesis, providing a key insight into soil-landscape evolution and terrestrial paleo-environment changes. Plain Language Summary Over the past 2.6 million years, the Earth’s climate has cycled at regular intervals in concert with orbital variations. Climate variations have driven changes in the rates of erosion and deposition of new sediment, but detection of these orbitally driven climate cycles has remained elusive in soil systems. We demonstrated that soils were preserved to the present at the same intervals as known orbital climate cycles using a meta-analysis of soil chronosequences. We further tied dominant periods of soil formation to periods of relatively low rates of past climate change or periods of relatively stable, unchanging climate that enable soil formation. Our results provide a better understanding of how climate change impacts landscapes, which could greatly enhance our understanding of the impact of future climate change on soil resources and new insights into past environmental changes.