Kunihiko Nishiizumi

The soil production function and landscape equilibrium

Hilly and mountainous landscapes are partially to completely covered with soil under a wide range of erosion and uplift rates, bedrock type and climate. For soil to persist it must be replenished at a rate equal to or greater than that of erosion. Although it has been assumed for over 100 years that bedrock disintegration into erodable soil declines with increasing soil mantle thickness, no field data have shown this relationship. Here we apply two independent field methods for determining soil production rates to hillslopes in northern California. First, we show that hillslope curvature (a surrogate for soil production) varies inversely with soil depth. Second, we calculate an exponential decline of soil production rates with increasing soil depth from measurements of the in situ produced cosmogenic 10Be and 26Al concentrations in bedrock sampled under soils of different depths. Results from both methods agree well and yield the first empirical soil production function. We also illustrate how our methods can determine whether a landscape is in morphological equilibrium or not.

Cosmogenic nuclides, topography, and the spatial variation of soil depth

Abstract If the rate of bedrock conversion to a mobile layer of soil depends on the local thickness of soil, then hillslopes on uniform bedrock in a landscape approaching dynamic equilibrium should be mantled by a uniform thickness of soil. Conversely, if the depth of soil varies across an actively eroding landscape, then rates of soil production will also vary and, consequently the landscape will not be in morphologic equilibrium. The slow evolution of hillslopes relative to the tempo of climatic variations and tectonic adjustments would suggest that local morphologic disequilibrium may be expected in many landscapes. Here, we explore this issue of equilibrium landscapes through a previously developed model that predicts the spatial variation in thickness of soil as a consequence of the local balance between soil production and erosion. First, we confirm the assumption in the model that soil production varies inversely with the thickness of soil using two independent methods. One method uses the theoretical prediction that at local steady state (soil production equals removal), the depth of soil should vary inversely with hillslope curvature. The second method relies on direct measurements of in situ produced concentrations of cosmogenic 10 Be and 26 Al in bedrock at the base of the soil column. For our study site in Northern California, the two methods agree and yield the expression that the rate of soil production declines exponentially with the thickness of soil from 0.077 mm/year with no soil mantle to 0.0077 mm/year under 1 m of soil. We then use this function of soil production in a coupled production and diffusive model of sediment transport to explore the controls on the spatial variation of the depth of soil on four separate spur ridges (noses) where we measured the data for the function of soil production. Model predictions are sensitive to boundary conditions, grid scale, and run time. Nonetheless, we found good agreement between predicted and observed depths of soil as long as we used the observed function of soil production. The four noses each have spatially varying curvature and, consequently, have varying depths of soil, implying morphologic disequilibrium. We suggest that our study site has been subjected to a wave of incision and varying intensities of erosion because of tectonic and climatic oscillations that have a frequency shorter than the morphologic response time of the landscape.

Soil production on a retreating escarpment in southeastern Australia

The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.

Late Quaternary erosion in southeastern Australia: a field example using cosmogenic nuclides

Late Quaternary rates of apparent soil production, bedrock incision, and average erosion are determined for the southeastern highlands of Australia using in situ produced cosmogenic nuclide concentrations of 10 Be and 26 Al. Apparent soil production rates define a steep, inverse exponential function of soil depth with a maximum of 143 m Ma � 1 under zero soil depth. There were no observed soil depths between about 25 cm and zero, however, such that the maximum observed rate is about 50 m Ma � 1 . The Bredbo River catchment average erosion rate is 157 1mM a � 1 , and is similar to the average hillslope erosion rate of 167 1mM a � 1 . Bedrock incision rates average 9 mMa � 1 and suggest that the higher rates of hillslope erosion may be in response to a pulse of incision, perhaps generated by knickpoint propagation. Bedrock erosion rates inferred froma tor profile average 3.8 mMa � 1 , with higher rates on other, more weathered tor tops. An aboveground tor profile of nuclide concentrations is consistent with a simple model of rapid stripping of the surrounding saprolite, supporting the view that at least one episodic period of increased denudation has affected the landscape evolution of the highlands. We test this hypothesis by using a simple landscape evolution model to reasonably predict the spatial variation of soil depth as well as the emergence of tors. r 2001 Elsevier Science Ltd and INQUA. All rights reserved.

Radar-Enabled Recovery of the Sutter’s Mill Meteorite, a Carbonaceous Chondrite Regolith Breccia

The Meteor That Fell to Earth In April 2012, a meteor was witnessed over the Sierra Nevada Mountains in California. Jenniskens et al. (p. 1583) used a combination of photographic and video images of the fireball coupled with Doppler weather radar images to facilitate the rapid recovery of meteorite fragments. A comprehensive analysis of some of these fragments shows that the Sutters Mill meteorite represents a new type of carbonaceous chondrite, a rare and primitive class of meteorites that contain clues to the origin and evolution of primitive materials in the solar system. The unexpected and complex nature of the fragments suggests that the surfaces of C-class asteroids, the presumed parent bodies of carbonaceous chondrites, are more complex than previously assumed. Analysis of this rare meteorite implies that the surfaces of C-class asteroids can be more complex than previously assumed. Doppler weather radar imaging enabled the rapid recovery of the Sutter’s Mill meteorite after a rare 4-kiloton of TNT–equivalent asteroid impact over the foothills of the Sierra Nevada in northern California. The recovered meteorites survived a record high-speed entry of 28.6 kilometers per second from an orbit close to that of Jupiter-family comets (Tisserand’s parameter = 2.8 ± 0.3). Sutter’s Mill is a regolith breccia composed of CM (Mighei)–type carbonaceous chondrite and highly reduced xenolithic materials. It exhibits considerable diversity of mineralogy, petrography, and isotope and organic chemistry, resulting from a complex formation history of the parent body surface. That diversity is quickly masked by alteration once in the terrestrial environment but will need to be considered when samples returned by missions to C-class asteroids are interpreted.

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics.

Changes in Continental and Sea-salt Atmospheric Loadings in Central Greenland during the Most Recent Deglaciation: Model-based Estimates

By fitting a very simple atmospheric impurity model to high-resolution data on ice accumulation and contaminant fluxes in the GISP2 ice core, we have estimated changes in the atmospheric concentrations of soluble major ions, insoluble particulates and 10 Be during the transition from glacial to Holocene conditions. For many species, changes in concentration in the ice typically overestimate atmospheric changes, and changes in flux to the ice typically underestimate atmospheric changes, because times of increased atmospheric contaminant loading are also times of reduced snowfall. The model interpolates between the flux and concentration records by explicitly allowing for wet- and dry-deposition processes. Compared to the warm Preboreal that followed, we estimate that the atmosphere over Greenland sampled by snow accumulated during the Younger Dry as cold event contained on average four-seven times the insoluble particulates and nearly seven times the soluble calcium derived from continental sources, and about three times the sea salt but only slightly more cosmogenic 10 Be.

Holocene monsoonal dynamics and fluvial terrace formation in the northwest Himalaya, India

Aluminum-26 and beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley (northwest Himalaya) documents the close link between Indian Summer Monsoon (ISM) oscillations and intervals of enhanced fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and reached far into the internal parts of the orogen. The amplified sediment flux from these usually dry but glaciated areas caused alluviation of downstream valleys up to 120 m above present grade at ca. 9.9 k.y. B.P. Terrace formation (i.e., incision) in the coarse deposits occurred during century-long weak ISM phases that resulted in reduced moisture availability and most likely in lower sediment flux. Here, we suggest that the lower sediment flux during weak ISM phases allowed rivers to incise episodically into the alluvial fill.

The 36Cl–36Ar–40K–41K records and cosmic ray production rates in iron meteorites

This is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the last billion years, as recorded in these fossil detectors. We have performed new 36Cl, 26Al, 10Be, and noble gas measurements, including determination of the shielding parameter, S = 4He/21Ne, in samples with published K data. The K isotopic data, coupled to 36Ar and 36Cl concentrations permit selection of meteorites which have only experienced simple (constant geometry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. In order to carry out production rate calibrations based on 40K–41K data, we assume constancy of the cosmic flux during the interval 150 to 700 My ago. We note that meteorites with very old potassium ages cannot be included in this calibration, as these meteorites require distinct parameter sets. A calibration data set representing a total of 13 meteorites was used to compute long-term (0.5 Gy) average production rates. These average production rates of 36Cl from this particular calibration set are significantly (28%) lower than those determined for the recent (≤10 My) cosmic ray flux. We also document here the quality of the resulting potassium production rate parameter M0(S) with a calculated isochron for irons of group IVA.

Dating the Siple Dome (Antarctica) ice core by manual and computer interpretation of annual layering

The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core.The data were interpreted manuallyandwith acomputeralgorithm.The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10 Be stratigraphic ties to the dendrochronology 14 C recordandthedatedvolcanic stratigraphy.Thealgorithm interpretation ismorecon- sistent andbetter quantifiedthanthe tedious and subjective manual interpretation.