Louis A. Scuderi

Fluvial form in modern continental sedimentary basins: Distributive fluvial systems

Analysis of more than 700 modern continental sedimentary basins that are both endorheic (internally drained) and exorheic (externally drained) and cover a wide range of climatic and tectonic settings shows that sedimentation is dominated by distributive fluvial systems (DFSs). Facies distributions on DFSs are different from those of rivers in degradational settings, yet rivers in non-aggradational settings are commonly used to develop fluvial facies models. DFS rivers typically decrease in size downstream, are not confined to valleys, and form a radial pattern from an apex. Confined rivers are present in specific locations in sedimentary basins, including basin axial positions, areas between adjacent DFSs, and valleys incised into the DFS. DFSs and adjacent axial fluvial systems develop in a predictable manner that allows interpretation and prediction of fluvial architecture at the basin scale.

A 2000-year tree ring record of annual temperatures in the Sierra Nevada mountains

Tree ring data have been used to reconstruct the mean late-season (June through January) temperature at a timberline site in the Sierra Nevada, California, for each of the past 2000 years. Long-term trends in the temperature reconstruction are indicative of a 125-year periodicity that may be linked to solar activity as reflected in radiocarbon and auroral records. The results indicate that both the warm intervals during the Medieval Warm Epoch (∼A.D. 800 to 1200) and the cold intervals during the Little Ice Age (∼A.D. 1200 to 1900) are closely associated with the 125-year period. Significant changes in the phase of the 125-year temperature variation occur at the onset and termination of the most recent radiocarbon triplet and may indicate chaotic solar behavior.

Tree-ring evidence for climatically effective volcanic eruptions

Abstract Ringwidth variations from temperature-sensitive upper timberline sites in the Sierra Nevada show a marked correspondence to the decadal pattern of volcanic sulfate aerosols recorded in a Greenland ice-core acidity profile and a significant negative growth response to individual explosive volcanic events. The appearance of single events in the mid-latitude tree-ring record, in connection with ice-core evidence from the arctic and historical records from the Mediterranean, indicates that the majority of these events represent climatically effective volcanic eruptions, producing temperature decreases on the order of 1°C for up to 2 yr after the initial eruption. Clusters of climatically effective volcanic events may serve as a trigger to glaciation and are consistently associated with lowered ringwidths and late-Holocene glacier advance in the Sierra Nevada. The tree-ring record strongly suggests forcing of solar radiation receipt and temperatures by increased volcanic aerosols, especially during the Recess Peak advances and Matthes (Little Ice Age) advances from 1400 to 1850 A.D. Intervals with an absence of significant volcanic aerosol production or historically documented eruptive activity correspond to intervals of significantly increased indexed ringwidth values, minimal numbers of severe annual negative ringwidth anomalies, and an absence of glacial deposits in the southern Sierra Nevada.

Groundwater sapping as the cause of irreversible desertification of Hunshandake Sandy Lands, Inner Mongolia, northern China

Significance In contrast to earlier assertions that deserts in northern China are 106 years old, our multidisciplinary investigation in the Hunshandake Sandy Lands, located in the eastern portion of China’s desert belt, shows that this desert is ca. 4,000 years old. This study documents dramatic environmental and landscape changes in this desert during the last 10,000 years. For the first time to our knowledge we present a case of desertification mainly triggered by changes in the hydrological and geomorphological system, associated with climate change at ca. 4.2 ka. Our research on the human–environment interactions in the Hunshandake suggests Chinese civilization may be rooted in the marginal areas in the north, rather than in the middle reaches of the Yellow River. In the middle-to-late Holocene, Earth’s monsoonal regions experienced catastrophic precipitation decreases that produced green to desert state shifts. Resulting hydrologic regime change negatively impacted water availability and Neolithic cultures. Whereas mid-Holocene drying is commonly attributed to slow insolation reduction and subsequent nonlinear vegetation–atmosphere feedbacks that produce threshold conditions, evidence of trigger events initiating state switching has remained elusive. Here we document a threshold event ca. 4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia, northern China, associated with groundwater capture by the Xilamulun River. This process initiated a sudden and irreversible region-wide hydrologic event that exacerbated the desertification of the Hunshandake, resulting in post-Humid Period mass migration of northern China’s Neolithic cultures. The Hunshandake remains arid and is unlikely, even with massive rehabilitation efforts, to revert back to green conditions.

Chronotopographic analysis directly from point-cloud data: A method for detecting small, seasonal hillslope change, Black Mesa Escarpment, NE Arizona

The advent of high-resolution, precise, back-pack portable terrestrial lidar scanners (TLS) provides a revolutionary new tool for obtaining quantitative, high-resolution (2-mm to 30-mm point spacing) measurements of landscape surface features. Moreover, data collected using these instruments allow observation of geomorphic processes in systems that can experience change on a daily basis. We have introduced TLS techniques in ongoing investigations of semiarid landscapes associated with weakly cemented sandstones along part of the Black Mesa escarpment of NE Arizona. Clay-cemented, Jurassic sandstones exposed along this escarpment are sensitive to moisture, and thus climate, via hydration-expansion weathering of interstitial clay. Sediment shed from weathered slopes has caused locally rapid valley floor aggradation and upper basin slope vertical denudation rates of 2–3 mm/yr over 10- to 100-yr timescales, as indicated by dendrochronology coupled with soil geomorphic analysis. These rates suggest rapid hillslope denudation rates. Employing the University of New Mexico Lidar Laboratory Optech Ilris 3D TLS, we are constructing a high-resolution model of two major basins along the escarpment. Focusing on a single, small (30 × 60 m) area of a mostly non-vegetated, steep slope (>35°), we demonstrate in this paper a method of comparative analysis of point-cloud data sets that can detect subcentimeter change resulting from a single season of monsoon precipitation along the escarpment. Using repeat scans can provide an empirical evaluation of single season erosion rates in the study site, and because our observations are geospatial in nature, we can also document the parts of the slopes that make the greatest contribution to local valley floor aggradation. In demonstrating the utility of this method, we expect that continued investigation of this site will provide insight to the key processes associated with soil-mantled versus bedrock-dominated slopes during modern escarpment retreat and hillslope modification, which, in turn, may further elucidate the impacts of Holocene climate change on this rapidly evolving landscape.

Recognition and importance of amalgamated sandy meander belts in the continental rock record

Meandering fluvial channels and their meander belts are common in modern continental sedimentary basins, yet compose a minor constituent of the reported fluvial rock record. Here we document exhumed amalgamated meander belt deposits from the upper Jurassic Morrison Formation, Utah (United States). The size of the amalgamated meander belt (9000 km2) is significantly larger than any documented previously and comparable in size to those from modern sedimentary basins. We describe a representative outcrop of sandy point bar deposits that shows features considered characteristic of both braided and meandering fluvial systems. Lateral accretion sets compose <5% of the outcrop area, yet point bar morphology is clearly visible in plan view. We suggest that difficulties in the identification of sandy, amalgamated meander belt deposits indicate that they have gone largely unrecognized in the rock record. Their recognition has important implications for basin-scale reconstructions of fluvial systems and interpretation of tectonic setting.

The composite nature of physical geography Moving from linkages to integration

This editorial is the product of the Progress in Physical Geography lecture at the April 2013 meeting of the Association of American Geographers. The paper was presented by George Malanson, the North American Editor, and the co-authors presented critiques based on a draft. Subsequently, the manuscript was developed and revised based on discussion at the meeting and additional exchange among the co-authors.

SOLAR INFLUENCES ON HOLOCENE TREELINE ALTITUDE VARIABILITY IN THE SIERRA NEVADA

Studies utilizing treeline altitude and growth variability over time contribute to our understanding of the timing and magnitude of Holocene climate change and the response of alpine treelines to this change. Analysis of treeline annual growth at a site in the southern Sierra Nevada, California, indicates that much of the past climate variability on timescales of centuries to millennia during the Holocene may be attributed to periodic changes in solar irradiance. Analysis of treeline-altitude variation in the Sierra Nevada and White Mountains of California suggests that this periodic change is superimposed on a generally decreasing temperature trend over the past 5000 years. This has led to a general stepped decrease in treeline altitude in both areas with treeline falling significantly following downturns in reconstructed solar activity levels and stabilizing rather than rising during upturns in solar activity. Treeline-altitude decrease generally lags changes in solar radiation by 400 to 500 years, sugg...

Lightning, precipitation and vegetation at landscape scale

We investigated the question “Is there a relationship between seasonality in precipitation and vegetative cover in Pole Canyon, NM?” GIS and statistical methods were used to determine the degree of association between either summer or winter precipitation and percent canopy cover for trees, graminoids and total vegetation. Monsoon (summer) precipitation was predicted for the years 1986–1994 from lightning strike and relative humidity data by multiple regression. Winter precipitation, the percent of annual precipitation that occurs during winter, and vegetative cover were derived from the Forest Service Terrestrial Ecosystem Survey. Vegetation and precipitation data were ranked and classified (e.g., high, medium, low) and cross-tabulations were generated to compare the spatial distribution of vegetation classes within each precipitation class.Results indicate that seasonality in precipitation affects the distribution and spatial pattern of vegetation at landscape scales. Winter precipitation is a key factor that influences the distribution and spatial pattern of tree cover. Monsoon precipitation may affect the spatial pattern of graminoid cover where Bouteloua gracilis dominates. Winter precipitation may affect the distribution and spatial pattern of graminoid cover where Festuca arizonica dominates. Some of the unexplained relationships may be due to competition between trees and graminoids for moisture and other limiting factors. The importance of temperature was implicit in the division between summer (monsoon) and winter seasons. Annual precipitation, elevation, topography and edaphic factors probably contributed to the observed relationships.

Controls on the apex location of large deltas

The majority of sediment transport to the worlds oceans is routed via large deltas. We examine controls on delta apex location using a database of 84 of the worlds largest deltas. Of the dataset, 94% of apices are controlled by either bedrock valleys (80%) or Pleistocene alluvial valleys (14%), suggesting that the principal control on modern apex development is valley exit and/or bedslope-mediated avulsion and not hydrodynamic backwater length. Valley exit control on large delta apex location may have been as important in the rock record as it is today, and should be considered as a key control on delta development. Supplementary material: Tabulated data on backwater length and apex type for studied deltas available at https://doi.org/10.6084/m9.figshare.c.3469770