We report detailed chemical and isotopic data from a subglacial siliceous deposit on andesitic bedrock recently exposed by glacier retreat. Whereas a single, <1 μm, Si-rich layer covers the highly polished bedrock on the up-glacier (stoss) surfaces, distinct, lithified deposits commonly occur at the lee of small bedrock protuberances, on a scale <0.1 meter. The deposit is millimeters in thickness and consists of laminae tens to hundreds microns thick that differ from one another in color, rock-fragment abundance and chemical composition. Ca-rich laminae that are sufficiently enriched in uranium (~2–50 ppm) to permit U-series isotopic analysis suggest that the subglacial deposit formed 10–20 ka, much earlier than previously assumed. We conclude that (1) the siliceous deposit persisted for at least 10 000 years despite the intervening erosion and weathering, (2) distinct episodes of formation due to significant changes in hydrology and water chemistry are recorded in the deposit, and (3) a siliceous slurry may have existed at the ice-rock interface and influenced the local friction. This work reinforces earlier findings that subglacial chemical deposits can form and persist on geologic time scales and may have implications for the role of the cryosphere in the Earth's geochemical cycles and climate system.

Deglaciated bedrock surfaces in limestone areas often exhibit extensive patterning by solutional furrows and carbonate deposits that occur in close association with undulations in the bed topography. These features clearly result from subglacial dissolution and precipitation of calcite on the bed — induced, for instance, by melting and freezing in a regelation water film — but little is known about the observed morphology. In particular, it is intriguing that (i) the solutional furrows, whose formation requires explanation, are collectively organized into arcuate patterns, with characteristic spacing, and (ii) a fluted or “spiculed” surface texture is ubiquitous on the calcite deposits. Herein, we propose specific mechanisms for such patterning based on a theory where chemical processes in the water film are coupled to regelation physics. Solutional furrows reflect locally enhanced dissolution along stoss surfaces, where CO2-rich bubbles advected in the ice from up-glacier come into contact with the bed. The bubbles form as CO2 is exsolved from freezing film water at the lee of bed bumps. The flutings on the deposit are inherently the manifestation of a spatial instability at the interface where calcite precipitation occurs. Complex interactions underlie some of the striking glacier-bed features shaped by subglacial chemical processes.

To contribute to the understanding of the influence of climate on glacial erosion and on orogenic processes, we report contemporary glacial erosion rates from a calving glacier in the Southern Andes and elucidate the influence of ice dynamics on erosion. Using seismic profiles of sediments collected in a proglacial fjord and a documented history of retreat, we determine the time-varying sediment flux of Glaciar Marinelli as a measure of basin-wide erosion rates, and compare these rates with the annual ice budget reconstructed using NCEP–NCAR reanalysis climate data from 1950 to 2005. The rate of erosion of the largest tidewater glacier in Tierra del Fuego averaged 39 ± 16 mm a−1 during the latter half of the 20th century, with an annual maximum approaching 130 mm a−1 following a decade of rapid retreat. A strong correlation emerges between the variable rate of ice delivery to the terminus and the erosion rate, providing quantitative insight into the relationship between ice fluxes and glacial erosion rates. For Glaciar Marinelli, as for other calving glaciers for which suitable data exist, the marked retreat and thinning over the past 50 years have resulted in a period of accelerated basal sliding and unusually rapid erosion.

Recent studies of subglacially precipitated carbonate deposits and associated solutional furrows have provided interesting new insight on subglacial water films, as well as on chemical exchange at the glacier bed. Considerable information on the film thickness and its temporal and spatial variability has been gained by analyzing several properties of subglacial carbonate deposits including: (1) the morphology of surface features aligned parallel to ice flow, (2) the laminated structure, and (3) the size distribution of fine rock fragments presumably transported in the film prior to their incorporation in the deposits. Chemical analyses of water from pro-glacial streams, together with calculations of CaCO3 solubility and mass balance, show that the channelized water is chemically distinct from the film water in which CaCO3 precipitates, and that subglacial precipitation is not possible where there is a considerable water flux through the film in excess of that associated with regelation sliding. The principal implication of these studies is that a temperate cirque glacier is characteristically separated from its bed by a thin water film, probably micrometers in thickness; however, the film appears to occasionally thicken, at least locally by as much as a hundred fold in exceptional cases. Furthermore, the water flux and/or solute concentration in the basal film undergoes periodic, probably seasonal, variations possibly related to variations in the amount of water reaching and flowing through the basal film.

The spatial pattern of the formerly active processes of water flow, cavitation, abrasion, dissolution, and precipitation at the base of a small cirque glacier has been reconstructed by detailed mapping of surficial features on recently deglaciated limestone bedrock near the glacier terminus. Our interpretation of these features, which reflect basal conditions averaged over a period of several or several tens of years, leads us to the following conclusions:

1. 1. A nearly continuous, non-arborescent network of cavities and incised channels existed and probably acted as the primary drainage of melt waters. This network evolved through time as many channels were filled, perhaps intermittently, by basal ice.

2. 2. At least 20% of the glacier sole was separated from the bed by water-filled cavities. The rest of the glacier–rock interface characteristically comprises a very thin water film.

3. 3. Abrasion was locally intensified, relative to chemical alteration, in 5–10 m wide zones paralleling the ice-flow direction, perhaps as a result of locally enhanced sheet flow of subglacial water.

Preliminary results of a quantitative model of glacial abrasion are presented. The analysis, which is constructed within a framework of modern glaciological views of processes near to the bed, is aimed at modeling abrasion under a temperate glacier whose basal layers contain only occasional rock fragments. It does not simulate abrasion by debris-rich ice or by subglacial drift. Calculations of abrasion-rates reduce to evaluations of the forces pressing rock fragments against the glacier bed and of the rates at which they are moved along the bed. The estimated viscous drag induced by ice flow toward the bed due to basal melting is generally the dominant contribution to this contact force. Although the analysis shares several important elements with the pioneering study of Boulton ([c1974]), sufficient fundamental differences in the modeling lead to distinctly different conclusions. Several new results are noteworthy: (1) other parameters being equal, abrasion will tend to be fastest where basal melting is most rapid, (2) glacier thickness does not affect abrasion through its influence on basal pressures, and (3) lodgement of rock fragments is only possible if the sliding velocity is very low, equivalent to the rate of basal melting.

The glacier sliding theory of Nye is modified to include the effect of solutes in subglacial regelation waters on the sliding process. Motivation for this development stems from studies of subglacially formed chemical deposits that appear to be widespread on rock surfaces recently exposed by retreating temperate glaciers. These deposits indicate clearly that considerable local concentration of solutes commonly occurs subglacially as a result of the selective rejection of solutes into the melt during the freezing associated with regelation sliding. Because solutes accumulate where regelation waters refreeze, they tend to lower the temperature there and hence inhibit the heat transport away from these areas that is essential for regelation sliding. For a simple sinusoidal bed and solute distribution in the regelation water film, the modified theory shows that a maximum excess of solutes of, for example, several millimoles/1 of dissolved CaCO3 along lee surfaces relative to stoss surfaces impedes basal sliding significantly, especially if the bed roughness wavelength does not exceed about one meter.

A working hypothesis for the development and maintenance of sorted circles, which are finegrained soil domains delineated by borders of coarser material, involves clast motion relative to soil and intermittent circulatory motion of soil in the active layer. The hypothesis is compatible with the observed size, regularity, sorting, microtopography, surface soil motion, vegetation cover, and subsurface organic carbon content of soil patterns studied in Spitsbergen. Theoretical considerations of buoyancy-driven convection suggest that intermittent convection of thawed soils is plausible, and that the requisite bulk density decrease with depth arises naturally from moisture gradients that develop during the thaw consolidation of ice-rich soils. These considerations also help in understanding the dependence of sorted circles on soil characteristics and on moisture. Moreover, they point to the possible use of sorted patterns as paleoclimatic indicators based on the expected relation between pattern size and active layer depth.

The water and sediment output from Vitus Lake, in Front of Bering Glacier, was monitored starting in July 1994, Instrumentation was placed in the lake outlet to record stage, turbidity, conductivity and temperature and velocity of flow of river water. Two outburst floods punctuated the termination of the 1993–94 Bering Glacier surge in August 1994. The better-documented flood lasted about 10 d, during which the flood discharge averaged 1100 m3 s−1, in excess of normal discharge (1550 m3 s−1) during this part of the ablation season, and about 9.5 × 108 m3 of water drained from Bering Glacier. The excess water volume discharged during this flood corresponds to a 0.4 m thick layer of water extending over the 2500 km2 of Bering Glacier that was surging in early summer. The suspended-sediment flux from Vitus Lake during the summer of 1994 was two orders of magnitude less than rates of sediment production by other fast-moving glaciers in southern Alaska. This implies that most of the sediment produced is being stored in Vitus Lake, under the glacier, or in both locations.

Bathymetric and sub-bottom acoustic data were collected in Laguna San Rafael, Chile, to determine sediment yields during the Little Ice Age advance and subsequent retreat of San Rafael Glacier. The sediment volumes and subaqueous landforms imaged are used to interpret the proglacial dynamics and estimate erosion rates from a temperate tidewater glacier over a complete advance–retreat cycle. Sediment yields from San Rafael Glacier averaged 2.7 × 10 m/a since the end of the Little Ice Age, circa AD 1898, corresponding to average basin-wide erosion rates of 23 ± 9 mm/a; the highest erosion rates, 68 ± 23 mm/a, occurred at the start of the retreat phase, and have since been steadily decreasing. Erosion rates were much lower during glacial advance, averaging at most 7 mm/a, than during retreat. Such large glacial sediment yields over two centuries of advance and retreat suggest that the contribution of sediments stored subglacially cannot account for much of the sediment being delivered to the terminus today. The detailed sub-bottom information of a proglacial lagoon yields important clues as to the timing of erosion, deposition and transfer of glacigenic sediments from orogens to the continental shelves, and the influence of glacier dynamics on this process.

Magnetic susceptibility profiles χ(z) in loess sequences reflect a combination of two climatically modulated processes: dust deposition and pedogenic development. Prominent soils with high χ values, for example, likely reflect periods of slow dust deposition, and warm wet conditions favorable for rapid chemical weathering. To refine our understanding of the climate records contained in loess, we develop a numerical model as a tool for exploring quantitatively the integrated record of the temporal variation in rates of loess aggradation and soil development contained in χ profiles. In our model, the aggrading loess is pedogenically altered in a reactive zone near the ground surface. The strength of the χ signal is dictated by both the depth-dependent intensity of pedogenic processes and the rate of dust accumulation, which dictates the total time a loess parcel spends in the near-surface reactive zone. The model can accommodate both pedogenic production of magnetically susceptible minerals and arrival of magnetically susceptible grains as eolian dust. To explore the model performance and develop a sense of time and length scales implicit in χ profiles, we first examine simple synthetic cases with idealized steady and cyclic climatic forcing. Reported χ profiles in three Chinese loess sequences at varying distances from the western China dust source are then modeled in two illustrative ways: (i) by imposing a specific dust deposition rate history that is proportional to the dust accumulation rate history reported from western Pacific deep-sea cores, allowing the time variation of pedogenic rates to be calculated directly from χ profiles; and (ii) by imposing both dust accumulation and pedogenic rate histories that are independently scaled by the deep-sea δ18O history, which reflects global climate cycles to which regional climate forcing is linked. We find that the zone of pedogenic activity is roughly 0.5–1.0 m thick, both deposition rate and pedogenic intensity have varied dramatically over the last 140,000 yr, the age structure of the Luochuan loess sequence is best fit by driving the model with the δ18O record, and environmental conditions must have been anomalously favorable for pedogenesis during isotope stage 3 at all three sites. Finally, we advocate the assembly of a variety of data types at a suite of sites within any loess field that taken together will better constrain the temporal and spatial patterns of climatically modulated deposition and pedogenic processes.

Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km2, with a maximum depth of 680 m and contained an estimated 832 km3 of water; the smaller lake contained an estimated 80 km3 of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 14C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 14C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 X 106 m3 s–1. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.

Magnetic susceptibility (MS) of surface sediment varies systematically across the Loess Plateau in central China, decreasing exponentially from >200×10−8 m3/kg at the northern margin of the Qinling Shan to ≤30×10−8 m3/kg near the southern margin of the Mu Us Desert. MS correlates highly with loess median grain size (r 2=0.79), which decreases south-southeastward across the plateau. It also correlates with mean annual temperature (MAT) and mean annual precipitation (MAP) (r 2=0.58 and 0.60, respectively), and with their product MAT×MAP (r 2=0.83), which is considered a measure of potential pedogenic activity. Because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel, it is difficult to determine their relative influence on MS. A simple MS model, based on the observed spatial variation in loess thickness, permits quantitative assessment of the effect of the dust accumulation rate on the MS signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals. The model suggests that 84% of the loess MS variance is dictated by the diluting effect of dust and 10–11% is associated with meteorological factors, primarily precipitation. The observed and modeled relationships support hypotheses that attribute variations in MS in the loess-paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils, both of which are controlled by monsoon climate.

A model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice-rock interface on glacier sliding. An increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed. Abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content. A similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness. Sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed.

Polygonal patterned ground in polar regions of both Earth and Mars has received considerable attention. In comparison with the size, shape, and arrangement of the polygons, the diverse micro-relief and topography (termed here simply “relief”) of polygonal patterned ground have been understudied. And yet, the relief reflects important conditions and processes occurring directly below the ground surface, and it can be observed readily in the field and through remote sensing. Herein, we describe the relief characteristic of the simplest and relatively young form of patterned ground in the Dry Valleys of Antarctic. We also develop a numerical model to examine the generation of relief due to subsurface material being shouldered aside contraction cracks by incremental sand wedges growth, and to down-slope creep of loose granular material on the surface. We model the longterm subsurface deformation of ice-cemented permafrost as a non-linear viscous material. Our modeling is guided and validated using decades of field measurements of surface displacements of the permafrost and relief. This work has implications for assessing the activity of surfaces on Earth and Mars, and much larger scale potential manifestations of incremental wedging in icy material, namely the distinct paired ridges on Europa.

To explore links between glacier dynamics, sediment yields and the accumulation of glacial sediments in a temperate setting, we use extensive glaciological observations for Columbia Glacier, Alaska, and new oceanographic data from the fjord exposed during its retreat. High-resolution seismic data indicate that 3.2 × 108 m3 of sediment has accumulated in Columbia Fjord over the past three decades, which corresponds to ~5 mm a−1 of erosion averaged over the glaciated area. We develop a general model to infer the sediment-flux history from the glacier that is compatible with the observed retreat history, and the thickness and architecture of the fjord sediment deposits. Results reveal a fivefold increase in sediment flux from 1997 to 2000, which is not correlated with concurrent changes in ice flux or retreat rate. We suggest the flux increase resulted from an increase in the sediment transport capacity of the subglacial hydraulic system due to the retreat-related steepening of the glacier surface over a known subglacial deep basin. Because variations in subglacial sediment storage can impact glacial sediment flux, in addition to changes in climate, erosion rate and glacier dynamics, the interpretation of climatic changes based on the sediment record is more complex than generally assumed.