ALBERT

Description: No abstract is available for this article.

Description: Aeolian-derived soils are found throughout the world. Soil evolution processes in aeolian-dominated landscapes differ from processes in bedrock-weathering landscapes by a number of key aspects including the lack of: (1) soil-production depth-dependency; (2) surface armoring; and (3) grain size self-organization in the soil profile. We use here a soil evolution model (mARM5D) to study the differences between aeolian and bedrock-weathering dominated landscapes by analyzing soil evolution on a hillslope under various aeolian and bedrock soil supply settings subject to fluvial and diffusive sediment transport. The model simulates spatial and temporal variation in soil particle size distribution (PSD) and profile depth for each grid-cell on the landscape, as a function of physical weathering, aeolian deposition, and diffusive and fluvial sediment transport. Our results indicate that surface armoring plays a major role in soil evolution. Under bedrock-weathering dominated conditions, armoring reduces soil erosion and in conjunction with depth-dependent soil production, leads to steady-state soil grading and depth and a relatively uniform soil distribution. In contrast, aeolian-dominated landscapes tend to have considerable spatial variability in soil depth and PSD. Our results also indicate that in contrast with diffusive transport, which is assumed to be PSD-independent, fluvial sediment transport is strongly influenced by the soil-production mechanism (aeolian or bedrock-weathering). Based on the results presented here we propose that aeolian-dominated landscapes are more responsive to environmental changes (e.g. climatic and anthropogenic) compared with bedrock-weathering landscapes. We further propose that this sensitivity may help explain the patchy soil distribution that is often observed in aeolian-dominated regions.

Description: Supply and transport of sediment in catchments involve processes with fundamental consequences for river management, land use and the prediction of climate-driven sediment fluxes. In the present study we addressed spatial variability in the water routes through the surface and subsurface of a catchment and the suspended sediment discharge ( Q s ) over a mountain–piedmont system. We analyzed daily suspended sediment concentration ( C s ) and water discharge ( Q ) measurements at stations located in different topographic settings (mountain and piedmont) in the Biobío River basin (southern-central Andes, 37-39°S). In steep catchments, the Q vs. Q s relationship has a marked seasonal hysteresis. In the piedmont, Q s is proportional to Q , with no seasonal hysteresis. The contrast in the hysteresis pattern between catchments with different topographies is explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table is near the surface. In the mountains, groundwater storage results in large seasonal variations in the proportion of Q that flows at the surface and transports sediment from the hillslopes, producing hysteresis. By separating the total Q into two components (direct discharge, Q d and baseflow, Q b ), we observed the response of Q s to the fraction of water that quickly leaves the catchment after a rainfall event ( Q d ). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Q s into a linear relationship with Q d over the entire catchment and lead usto propose that sediment mobilization to the river along the Biobío catchment is primarily controlled by overland flow. Our findings highlight the importance of an adequate hydrological model for understanding the erosion and transport processes of a catchment, and which can be applied to other natural and modeled mountain–piedmont systems.

Description: [1]  Measurements of the loss or gain of ice mass from large ice sheets is presently achieved through satellite-based techniques such as GRACE (Gravity Recovery and Climate Experiment). The accuracy of these satellite-based measurements to changes in modern ice sheet mass depends on our knowledge of present day glacio-isostatic crustal uplift rates caused by past ice sheet changes. To improve models of glacio-isostatic rebound in East Antarctica we investigated ice histories along Rayner Glacier, Enderby Land, and a little explored sector of the ice sheet where GRACE data had suggested significant mass gain during the last decade. [2]  Observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the Rayner Glacier, Enderby Land, ice heights lowered by at least 300 m, and the calving margin retreated by at least 10 km in the early Holocene (~6 to 9 ka BP). The magnitude and timing of deglaciation is consistent with ice histories used to model the post-glacial rebound corrections for present-day GRACE mass trends. These observations strengthen the body of evidence that suggests ice mass gain in Enderby Land is presently partly offsetting mass loss in other parts of Antarctica.

Description: [1]  The tidally modulated, stick–slip events of Whillans Ice Stream in West Antarctica produce seismic energy from three locations near the grounding line. Using ice velocity records obtained by combining time series from co-located broadband seismometers and GPS receivers installed on the ice stream during the 2010–2011 and 2011–2012 austral summers, along with far-field seismic recordings of elastic waves, we locate regions of high rupture velocity and stress drop. These regions, which are analogous to “asperities” in traditional seismic fault studies, are areas of elevated friction at the base of the ice stream. Slip events consistently initiate at one of two locations: near the center of the ice stream, where events associated with the Ross Sea high tide originate, or a grounding-line spot, where events associated with the Ross Sea low tide initiate, as well as occasional high-tide events following a skipped low-tide event. The grounding-line site, but not the central site, produces Rayleigh waves observable up to 1000 km away, through fast expansion of the slip area. Grounding-line initiation events also show strong directivity in the downstream direction, indicating initial rupture propagation at 1.5 km/s, compared to an average of 0.150 km/s for the entire slip event. Following slip initiation, additional seismic energy is produced from two sources located near the grounding line: firstly, at the downstream end of Subglacial Lake Engelhardt and secondly, toward the farthest downstream extent of the ice stream. This evidence suggests that the stronger, higher friction material along the grounding line controls motion throughout the stick–slip region.

Description: [1]  We examine the relationship of channel steepness to incision rate from channels eroding into a previously tilted, planar, and progressively exhumed unconformity surface cut across erosionally resistant limestone bedrock. Key to this analysis is the calibrated substitution of down-slope and downstream distance for time of exposure of resistant rocks. Channel and unconformity slopes are measured from a suite of channels developed on resistant Paleozoic limestone exhumed by the removal of Cenozoic sediments from the Baybeiche Range bordering the Naryn basin in the western Tian Shan. The compiled data set, sampling five orders-of-magnitude of upstream drainage area (0.03 to 227 km 2 ), is used to derive the exponent, n , relating channel steepness to channel incision rate, and the ratio, K / V of the rate constant for channel incision of the resistant substrate, K , to the erosion rate, V , of the cover strata. We show that for a typical value of intrinsic concavity (slope-area exponent, θ  = 0.5), erosion rates that are proportional to specific stream power ( n  = 1) satisfy the data set. However, valley-width data suggest that the intrinsic concavity is higher ( θ  = 0.8) and that the channel-incision data can also be fit if erosion is proportional to basal shear stress ( n  = 2/3). Our results do not support values of n significantly greater than one. Using 36 Cl exposure age-dating of the unconformity surface, we independently demonstrate that theCenozoic cover strata have been progressively stripped downward from the unconformity surface at a vertical rate of 1 to 2 m/kyr. Using V  = 1 m/kyr, we constrain the rate constant, K , to between 6 ± 1 and 9 ± 2 × 10 − 4 kyr − 1 for incision of resistant limestone bedrock in this field setting.

Description: [1]  Low-lying barrier islands are ubiquitous features of the world's coastlines and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. To varying degrees, the barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. And these interactions are modulated by changes in storminess associated with changes in our climate. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2-m high sand berm constructed between June, 2010 and April, 2011 along the northern Chandeleur Islands, LA. The berm was monitored using satellite and aerial remote sensing and topographic and bathymetric surveys. Over the study period, from November 2010 through September 2011, the central portion of the berm was altered and ultimately destroyed by the passage of winter and tropical storms. We show that berm-length evolution is well predicted using a statistical model that was fit to the observations by estimating two parameters describing the rate of berm-length change. The model considers wave-driven runup, tides, and storm surge to evaluate the probability and duration of berm overwash. Specifically, computed overwash probabilities predict episodic berm erosion associated with major storm events when overwash is likely to occur. The episodic erosion is superimposed on a constant berm-length change rate that persists even when there is no overwash. Using the calibrated model, the analysis is extended to a 16-year time series of storm climatologies that includes both intra- and inter-annual variability of overwash events and potential variations in berm-length erosion. For a 2-m high feature on the Chandeleur Islands, overwash is expected to occur, on average, 4 days (96 hours) every year. Variability in interannual storminess produces as many as 10 or as few as 1 days of overwash conditions per year. The dependence of overwash frequency on feature elevation (e.g., the height of a berm, dune, or other superimposed feature) indicated that an increase in feature elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to 0.5 event-days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using either (or both) past and future climates based on readily available observational or modeled data.

Description: [1]  To permit the tracking of turbulent flow structures in an Eulerian frame from single-point measurements, we make use of a generalization of conventional two-dimensional quadrant analysis to three-dimensional octants. We characterize flow structures using the sequences of these octants and show how significance may be attached to particular sequences using statistical mull models. We analyse an example experiment and show how a particular dominant flow structure can be identified from the conditional probability of octant sequences. The frequency of this structure corresponds to the dominant peak in the velocity spectra and exerts a high proportion of the total shear stress. We link this structure explicitly to the propensity for sediment entrainment, and show that greater insight into sediment entrainment can be obtained by disaggregating those octants that occur within the identified macro-turbulence structure from those that do not. Hence, this work goes beyond critiques of Reynolds stress approaches to bed-load entrainment that highlight the importance of outward interactions, to identifying and prioritizing the quadrants/octants that define particular flow structures.

Description: We present an inverse modelling approach to reconstruct annual accumulation patterns from ground-penetrating radar (GPR) data. A coupled surface energy balance - snow model simulates surface melt and the evolution of subsurface density, temperature and water content. The inverse problem consists of iteratively calibrating accumulation, serving as input for the model, by finding a match between modelled and observed radar travel times. The inverse method is applied to a 16-km GPR transect on Nordenskiöldbreen, Svalbard, yielding annual accumulation patterns for 2007–2012. Accumulation patterns with a mean of 0.75 m w.e. a −1 contain substantial spatial variability, with a mean annual standard deviation of 0.17 m w.e. a −1 , and show only partial consistency from year to year. In contrast to traditional methods, accounting for melt water percolation, refreezing and runoff facilitates accurate accumulation reconstruction in areas with substantial melt. Additionally, accounting for horizontal density variability along the transect is shown to reduce spatial variability in reconstructed accumulation, whereas incorporating irreducible water storage lowers accumulation estimates. Correlating accumulation to terrain characteristics in the dominant wind direction indicates a strong preference of snow deposition on leeward slopes, whereas weaker correlations are found with terrain curvature. Sensitivity experiments reveal a non-linear response of the mass balance to accumulation changes. The related negative impact of small-scale accumulation variability on the mean net mass balance is quantified, yielding a negligible impact in the accumulation zone and a negative impact of −0.09 m w.e. a −1 in the ablation area.

Description: Flow of glacial ice in the West Antarctic Sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice, but our understanding of the physical processes that generate this morphology is incomplete. Here, we study the thermal and mechanical properties of ice-stream margins, where flow transitions from rapid to stagnant over a few kilometers. Our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation-induced melting. We propose a 2D model that represents a cross-section through the ice-stream margin perpendicular to the downstream flow direction. We limit temperature to the melting point to estimate melt rates based on latent heat. Using rheology parameters as constrained by laboratory data and observations, we conclude that a zone of temperate ice is likely to form in active shear margins.