Near-surface Ground Ice Research Articles

Formation processes of shallow ground ice in permafrost in the Northeastern Qinghai-Tibet Plateau: A stable isotope perspective

The Source Area of the Yellow River (SAYR) on the Northeastern Qinghai-Tibet Plateau (QTP) stores substantial amounts of ground ice, which plays a significant role in understanding the hydrological processes and past permafrost evolution on the QTP. However, little is known about the initial sources and controlling factors of the ground ice in the SAYR. In this study, for the first time, ground ice stable isotope data (δ18O, δD, and d-excess) are presented, along with cryostratigraphic information for nine sites is integrated into three cryostratigraphic units (palsa, thermo-gully, and lake-affected sites) in the central SAYR. The ground ice in the nine sites exhibited diverse structures, ice contents, and stable isotopes due to differences in the initial water sources, ice formation mechanisms, soil types, and climate conditions. All of the freezing lines of ground ice are below those of the precipitation, streams, and lakes in most cases, suggesting the freezing of liquid water. The near-surface ground ice (NSGI) originated from precipitation, active layer water, and precipitation-fed springs. The NSGI was formed by quick freezing at the thermo-gully site (TG-1). In contrast, the formation of the NSGI at the palsa site (Palsa-1) experienced a slow segregation process during the permafrost aggradation. The NSGI was formed by quick freezing at the lake-affected sites under colder climate conditions. Conversely, the deep-layer ground ice (DLGI) at the lake-affected sites was fed by isotopically negative water and lake water occurred during a colder climate period. The DLGI at the TG-1 and Palsa-1 formed via similarly slow segregation of supra-permafrost water (mixed with precipitation), but had opposite water migration directions. The stable isotope compositions of the DLGI at the lake-affected sites became gradually more positive with decreasing distance from WL Lake, emphasizing the large influence of the lake changes on the growth of ice. The two end-member mixing model estimated that the contributions of paleo-lake water to the DLGI ranged from 9.8 % to 63.4 % towards the lake at the lake-affected sites, while the meltwater from past permafrost/ground ice contributed 36.6–90.2 % of the total input. A conceptual diagram of the δ18O trajectories of ground ice was constructed, the water migration patterns and ground ice formation processes between the palsa, thermo-gully, and lake-affected sites were clarified. The results of this study emphasize the influence of lake changes and past permafrost evolution on ground ice growth and improve our understanding of permafrost changes on the QTP.

High‐resolution stable isotopic signals of ground ice indicate freeze–thaw history in permafrost on the northeastern Qinghai–Tibet Plateau

AbstractUnderstanding the mechanism of formation of ground ice and the freeze–thaw history of permafrost is essential when assessing the future of permafrost in a changing climate. High‐resolution ground ice records, integrating stable isotopes (δ18O, d‐excess, and δ13C), hydrochemistry (EC and pH) data, and cryostratigraphy at a depth of 4.8 m from two contrasting permafrost profiles (P‐1, P‐2) in the Source Area of the Yellow River (SAYR) on the northeastern Qinghai–Tibet Plateau (QTP), were investigated. The results suggested significant depth variations in the stable isotopes and hydrochemistry of the ground ice. The near‐surface ground ice (NSGI) and deep‐layer ground ice (DLGI) were characterized in terms of variations in stable isotopes and known modern active layer data. By synthesizing the measured δ18O and the modeled isotopic fractionation processes during freezing, we suggest that both the NSGI and DLGI in P‐1 were mainly formed by the segregation mechanism during permafrost aggradation. The NSGI in P‐2, however, exhibited quick freezing origins compared with the predominant ice segregation processes for the DLGI. By combining the evolution of various stable isotopes and hydrochemistry with 14C age data, four historical freeze–thaw stages were identified. Specifically, one thawing–refreezing stage (2.8–2.2 m), one freezing aggradation stage (2.2–1.6 m), and two permafrost aggradation–degradation cycle stages (4.8–2.8 m; 1.6–0.7 m) were differentiated, which emphasize the importance of climate‐induced freeze–thaw transitions and differing permafrost aggradation processes on ground ice formation and resultant isotope hydrochemical behaviors. This study is the first to use high‐resolution data in ground ice to interpret the freeze–thaw history of permafrost in the SAYR. These findings are important for further understanding of past permafrost evolution and projected future permafrost degradation trends on the QTP, and provide an alternative method to explore permafrost history.

Permafrost geomorphology

Abstract Permafrost geomorphology came of age in 1965–2000, evolving from a field-based subdiscipline largely based on qualitative observation to one of detailed process measurement, field experimentation, and analytical modelling. In this, the development of permafrost geomorphology exemplified the change in geomorphology as a whole. Fundamental advances were made in understanding the development of ice wedges and ice-wedge polygons and the genesis and growth of closed-system pingos, especially by J. Ross Mackay working in the continuous permafrost terrain of the western Canadian Arctic coastlands. Mackay also investigated moisture movement in relation to the ground thermal regime, which led to seminal insights regarding ground freezing, frost heave, near-surface ground ice development, and hummocky micro-relief. Others used these ideas to examine frost shattering of rocks and the development of sorted circles, as most well known from Svalbard. In the second half of the period, measurements of permafrost creep and deformation of hillslopes and rock glaciers were presented, complementing antecedent measurements of solifluction in the active layer. Significant investigations of thermokarst development foreshadowed the preoccupation with permafrost thaw that now gathers so much attention.

Oriented-lake development in the context of late Quaternary landscape evolution, McKinley Bay Coastal Plain, western Arctic Canada

Oriented lakes—characterized by elongate forms, central basins and shallow littoral shelves—are common features of circum-arctic coastal lowlands. The environmental conditions, geological processes and chronology associated with the development of oriented lakes, however, are little known but essential for understanding how such Arctic lowlands evolve. Using combined techniques of field and drill-log stratigraphy and sedimentology, luminescence and radiocarbon dating methods and geomorphic mapping, we reconstruct the landscape evolution leading toward oriented-lake formation on the McKinley Bay Coastal Plain of western Arctic, Canada—a region with over 900 oriented lakes. Most lakes with deep central basins are inherited from a preglacial braidplain (ca. 73–27 ka) and alluvial braided-channel network that extended beyond the glacial limit (ca. 18.6–14.3 ka). Eolian erosion, active during the lateglacial and postglacial period (ca. 12.8–1.9 ka), reworked fluvial deposits. Eolian processes modified existing basins and created other shallow deflationary basins, as small barchanoid dunes migrated under cold, dry paraglacial conditions between about 12.8 and 10.7 ka. Vegetation cover developed at the onset of the early Holocene climatic optimum ca. 10.7 ka, and parabolic dunes were active between 9.6 and 4.6 ka. Thus, oriented lakes developed in basins conditioned by fluvial and eolian processes. In the absence of much near-surface ground ice, lateral expansion of deep-basin lakes and shallow stabilized deflationary basins predominated during the late Holocene through wind-induced wave and current processes. Overall, this sequence of oriented-lake formation does not support a thaw-lake cycle but, rather, small-basin evolution of a periglacial landscape.

Factors controlling near surface ground-ice characteristics in a region of warm permafrost, Beiluhe Basin, Qinghai-Tibet Plateau

Ground ice is a key component of permafrost, and its melt induced by climate change and anthropogenic disturbance has been causing increased ground surface subsidence, thermal erosion, and engineering problems. However, the distribution and quantity of ground ice in permafrost have yet to be investigated in detail on the Qinghai-Tibet Plateau (QTP), and consequently, an assessment of the nature of impacts associated with permafrost degradation is challenging. In this study, variation in near-surface ground ice content of the upper 2–3 m of the permafrost layer was examined by drilling 72 boreholes at eight sites in Beiluhe Basin, QTP, an area with relatively warm (near 0 °C) permafrost. High ground ice contents occur at most sites, but visible ice was absent at one site, where the vegetation cover has transitioned from a meadow to a sparsely-covered grassland. The moisture content within the active layer (surface to 2 m depth) increases with depth at most sites, and the higher moisture contents were associated with greater near-surface ground ice contents. The gravimetric moisture content (Mg) in permafrost typically ranged from 8% to 500%, and ~76% of samples were classified as ice rich (Mg ≥ 20%). The mean excess-ice content in near-surface permafrost was ~19% for all boreholes. At six flat sites, the minimum mean excess-ice content was about zero, and the mean maximum was ~22% at an alpine grassland site. The mean excess-ice content at a sunny sloping site was much higher (~27%) than at a north-facing shady site (10%) and the ice was distributed differently with depth. The mean subsidence ratio at the eight sites was from 0.05 to 0.44. The volumetric ice content varied from 1% to 70% in samples from the different sites, with an average value of ~16%. Topographically controlled moisture availability, slope direction, and fine-particle content are important controls on ground ice content in Beiluhe Basin. This study provides fundamental information about the spatial distribution of ground ice on QTP, which is important for future assessments of thermal erosion potential and infrastructure instability in the region.

Exploring near-surface ground ice distribution in patterned-ground tundra: correlations with topography, soil and vegetation

For informed predictions on the sensitivity of Arctic tundra landscape to permafrost thaw, we aimed to investigate the distribution pattern of near-surface ground ice and its influencing factors in Northeast Siberia. Near-surface permafrost cores (60 cm) were sampled along small-scale topographic gradients in two drained lakebeds. We investigated which factors (vegetation, hydrological and soil) correlated strongest with ice content and explored its spatial heterogeneity at different scales (1 to 100 m). The ice content was highest in the depressions of the wet lakebed and lowest at the slopes of the dry lakebed. In the wet lakebed the ice content increased with depth, while in the dry lakebed the vertical distribution depended on topographical position. Spatial variability in ice content was similar at different scales, stressing strong influence of local drivers. 0–60 cm ice content correlated strongest with soil moisture of the overlying unfrozen soil, while 0–20 cm ice content correlated strongest with vegetation characteristics. Our study implies that vegetation effect on microclimate is strong enough to affect near-surface ice distribution, and that ice-rich tundra may be highly sensitive to thaw once climate warming offsets the protective impact of vegetation.

Delineating the hydrological processes and hydraulic connectivities under permafrost degradation on Northeastern Qinghai-Tibet Plateau, China

Climatic warming has resulted in permafrost degradation and it is expected to alter the hydrological processes and hydraulic connectivity on the Qinghai-Tibet Plateau (QTP). Some important and pending issues for understanding the hydrological processes in permafrost regions are how much melting water from thawing permafrost can feed the stream water and how the hydraulic connectivities will be altered under continual permafrost degradation. In this study, the Source Area of the Yellow River locating on the Northeast QTP was studied by using stable isotopic method and field hydrological observation. Results exhibited significant seasonal hydrological variations of stream water, thermokarst lakes, and ground ice. Hydrograph separation suggested that precipitation is the main contributor to stream water in ice-free months, accounting for 53.5% and 52.2% of the streams on average for the two branches, respectively. The second source is springs, which contributed about 29.8% and 17.9%. However, the recharge from melting ice is also important; it exported an average of 16.7% and 13.2% to the surface stream. Conceptual model and stable isotopes emphasized the remarkable hydraulic connectivities between the precipitation, stream water, thermokarst lakes, spring, and the near-surface ground ice. Current findings provide a basic understanding of the log-term hydrological processes under permafrost degradation, and can offer an efficient way to assess future hydrological changes and water resource protection.

δ18O,δD and d-excess signatures of ground ice in permafrost in the Beiluhe Basin on the Qinghai-Tibet Plateau,China

In this paper, stable isotope (δ 18 O, δD) investigations were completed in ground ice from a deep borehole in the Beiluhe Basin on northern Qinghai-Tibet Plateau to unravel the isotopic variations of ground ice and their possible source water. The δ 18 O and δD of ground ice show distinctive characteristics compared with precipitation and surface water. The near-surface ground ice is highly enriched in heavier isotopes (δ 18 O and δD), which were gradually depleted from top to bottom along the profile. It is suggestive of different origin and ice formation process. According to isotopic variations, the ice profile was divided into three sections:the near-surface ground ice at 2.5 m is frozen by the active-layer water which suffered evaporation. It is possible that ground ice between 3 and 4.2 m is recharged by the infiltration of snowmelt. From 5 to 6 m, the ground ice show complex origin and formation processes. Isotopic variations from 6 to 11.1 m and 20.55 m indicate different replenishment water. The calculated slope of freezing line ( S =6.4) is larger than the experimental value (5.76), and is suggestive of complex origin and formation process of ground ice.

Review: Impacts of permafrost degradation on inorganic chemistry of surface fresh water

Abstract Recent studies have shown that climate change is impacting the inorganic chemical characteristics of surface fresh water in permafrost areas and affecting aquatic ecosystems. Concentrations of major ions (e.g., Ca 2 + , Mg 2 + , SO 4 2 - , NO 3- ) can increase following permafrost degradation with associated deepening of flow pathways and increased contributions of deep groundwater. In addition, thickening of the active layer and melting of near-surface ground ice can influence inorganic chemical fluxes from permafrost into surface water. Permafrost degradation has also the capability to modify trace element (e.g., Ni, Mn, Al, Hg, Pb) contents in surface water. Although several local and regional modifications of inorganic chemistry of surface fresh water have been attributed to permafrost degradation, a comprehensive review of the observed changes is lacking. The goal of this paper is to distil insight gained across differing permafrost settings through the identification of common patterns in previous studies, at global scale. In this review we focus on three typical permafrost configurations (pervasive permafrost degradation, thermokarst, and thawing rock glaciers) as examples and distinguish impacts on (i) major ions and (ii) trace elements. Consequences of warming climate have caused spatially-distributed progressive increases of major ion and trace element delivery to surface fresh water in both polar and mountain areas following pervasive permafrost degradation. Moreover, localised releases of major ions and trace elements to surface water due to the liberation of soluble materials sequestered in permafrost and ground ice have been found in ice-rich terrains both at high latitude (thermokarst features) and high elevation (rock glaciers). Further release of solutes and related transport to surface fresh water can be expected under warming climatic conditions. However, complex interactions among several factors able to influence the timing and magnitude of the impacts of permafrost degradation on inorganic chemistry of surface fresh water (e.g., permafrost sensitivity to thawing, modes of permafrost degradation, characteristics of watersheds) require further conceptual and mechanistic understanding together with quantitative diagnosis of the involved mechanisms in order to predict future changes with confidence.

Unraveling of permafrost hydrological variabilities on Central Qinghai-Tibet Plateau using stable isotopic technique

Permafrost degradation on the Qinghai-Tibet Plateau (QTP) will substantially alter the surface runoff discharge and generation, which changes the recharge processes and influences the hydrological cycle on the QTP. Hydrological connections between different water bodies and the influence of thawing permafrost (ground ice) are not well understood on the QTP. This study applied water stable isotopic method to investigate the permafrost hydrological variabilities in Beiluhe Basin (BLB) on Central QTP. Isotopic variations of precipitation, river flow, thermokarst lake, and near-surface ground ice were identified to figure out the moisture source of them, and to elaborate the hydrological connections in permafrost region. Results suggested that isotopic seasonalities in precipitation is evident, it is showing more positive values in summer seasons, and negative values in winter seasons. Stable isotopes of river flow are mainly distributed in the range of precipitation which is indicative of important replenishment from precipitation. δ18O, δD of thermokarst lakes are more positive than precipitation, indicating of basin-scale evaporation of lake water. Comparison of δI values in different water bodies shows that hydrology of thermokarst lakes was related to thawing of permafrost (ground ice) and precipitation. Near-surface ground ice in BLB exhibits different isotopic characteristics, and generates a special δD-δ18O relationship (freezing line): δD=5.81δ18O–23.02, which reflects typical freezing of liquid water. From isotopic analysis, it is inferred that near-surface ground ice was mainly recharged by precipitation and active layer water. Stable isotopic and conceptual model is suggestive of striking hydrological connections between precipitation, river flow, thermokarst lake, and ground ice under degrading permafrost. This research provides fundamental comprehensions into the hydrological processes in permafrost regions on QTP, which should be considered in investigating the influence of thawing permafrost on the hydrological cycle on QTP.

Stable Isotopic Stratification and Growth Patterns of Ground Ice in Permafrost on the Qinghai-Tibet Plateau, China

Stable isotope analysis of ground ice from a deep borehole in permafrost in the Kunlun Mountain Pass on the Qinghai-Tibet Plateau, China, was carried out to assess the water source and origin of the ground ice. Five sections were isotopically distinguished in the borehole. The ground-ice samples had δ18O values ranging from −20.2 to −8.5‰ (mean: −11.0‰), δD values from −151.9 to −73.1‰ (mean: −84.9‰) and D-excess values from −16.7 to 9.8‰ (mean: 2.7‰). The isotopic contents and the freezing line (Sδ 18 O-δD = 6.86) of ground-ice samples in the five sections showed significant deviations from modern isotopic values of regional precipitation and surface water, indicating discrepancies in the replenishment of source water and different climatic conditions during periods of ice formation. Near-surface ground ice (0–3 m depth) (D-excess: 9.8‰) formed mainly from modern precipitation, whereas deep ground ice was related to the source water and freezing conditions during ice formation. Ground ice from 3.8 to 9.6 m and from 9.6 to 16 m is segregated in origin and a stable freezing front at 9.6 m was identified. Ground ice from 16 to 50 m was replenished by precipitation in the period during ground-ice formation. Between 50 and 91.3 m, ground ice formed in an unstable climate and a palaeo-permafrost table at 91.3m was distinguished. Copyright © 2016 John Wiley & Sons, Ltd.

Physical and temporal factors controlling the development of near-surface ground ice at Illisarvik, western Arctic coast, Canada

Near-surface permafrost was sampled in summer 2010 at 26 sites in the Illisarvik drained-lake basin and nine sites in the surrounding tundra on Richards Island, NWT, to investigate the growth of segregated near-surface ground ice. Permafrost and ground ice have developed in the lake basin since drainage in 1978. The lake bed soils are predominantly silts of varying moisture and organic-matter contents, with sandier soils near the lake margins. Excess-ice contents in the basin were also variable, and ice enrichment was observed to a maximum depth of 60 cm below the 2010 permafrost table. Shrub-covered, wet areas had the highest mean excess-ice content in the top 50 cm of permafrost (10%), while grassy, dryer areas (4%) and poorly vegetated marginal areas (<1%) were less enriched with ice. Site wetness was the most important variable associated with near-surface excess-ice content in the lake basin. Silt content was a secondary variable. Mean excess-ice content in the top 50 cm of permafrost at tundra sites (25%) was much greater than in the basin, with ice enrichment to greater depths, likely a result of the time available for permafrost aggradation since the early Holocene climatic optimum.

Morphological and mechanical characterization of gullies in a periglacial environment: The case of the Russell crater dune (Mars)

Gullies on terrestrial sand dunes are rare, and their presence on Mars, as well as their mechanical properties, and the quantity of fluid required for their formations currently remain poorly understood. This study focuses on gully morphologies on the Russell megadune (54.5°S; 12.7°E) using High Resolution Imaging Science Experiment (HiRISE) images and Digital Terrain Models (DTM). Based on the scenario of ground ice melting in a periglacial environment, we propose to test the hypothesis that Martian gullies on dunes are debris flows. This implies a flow with a significant proportion of liquid water (>10% in volume). We used an original method to study Martian gullies based on empirical equations from terrestrial debris flows in order to calculate the physical properties of Martians flows. We observe a decrease in viscosity induced by the relative increase of fluid concentration (from 28% to 39%) during the flow advance. The total estimated volume of eroded and deposited material range from ∼14850m3 to ∼18890m3. The volume of liquid water required to generate one gully ranges from 4450m3 to 6900m3. The calculated results for Martian gullies are consistent with terrestrial studies on debris flows. Based on a morphological description and on the estimated physical parameters, we propose a model for gully formation on Martian dunes. The melt water from near-surface ground ice is incorporated in the debris flow and water concentration increases during its propagation. The increase of water concentration in the debris flow can be explained by a progressive increase of water/ice content in the permafrost downstream. Consequently, the lack of a final deposit at the front of the gullies tends to demonstrate that the flow became relatively highly concentrated in liquid downstream and all the water could have been lost in the final stage of the flow. This process could explain the lack of terminal lobes at the front of the gullies. We conclude that a process of formation similar to terrestrial debris flows is plausible. The large amount of liquid water involved requires formation of Martian gullies on dunes during a warmer climatic episode.

Landforms indicative of ground-ice thaw in the northern high latitudes of Mars

Abstract The confirmation of near-surface ground ice and perchlorates at the Phoenix landing site suggest that high-latitude ground-ice thaw may be more easily achieved than previously envisaged, providing the potential to drive significant, distinctive morphogenesis. We describe the results of a survey of 23 High Resolution Imaging Science Experiment (HiRISE) images covering 337° of longitude between latitudes 59°N and 79°N in which such morphogenesis is apparent, confirming that thaw has been a regionally important morphological agent. Some of the strongest geomorphological indicators of cyclical ground-ice thaw described are assemblages of sorted landforms, including clastic patterned ground resulting from cryoturbation of ice-rich regolith and lobate forms reflecting solifluction. Also described are braided gully-fan systems sourced at thermokarst pits and channels that have evolved from enlarged thermal contraction cracks. Not only are these landforms indicative of thaw and flowing liquid but the incision of solifluction lobes by thermokarst gullies demonstrates that thaw has been responsible for polycyclic morphogenesis. The presence of these landforms across the high northern latitudes of Mars indicates that the regional importance of thaw has been underestimated. This in turn has important implications for the development of better climate models and the search for life on Mars.

Southern high latitude dune fields on Mars: Morphology, aeolian inactivity, and climate change

In a study area spanning the martian surface poleward of 50° S., 1190 dune fields have been identified, mapped, and categorized based on dune field morphology. Dune fields in the study area span ∼ 116 400 km 2, leading to a global dune field coverage estimate of ∼904 000 km 2, far less than that found on Earth. Based on distinct morphological features, the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding. The six dune field classes occur in specific latitude zones, with a sequence of reduced activity and degradation progressing poleward. In particular, the first signs of stabilization appear at ∼ 60° S., which broadly corresponds to the edge of high concentrations of water-equivalent hydrogen content (observed by the Neutron Spectrometer) that have been interpreted as ground ice. This near-surface ground ice likely acts to reduce sand availability in the present climate state on Mars, stabilizing high latitude dunes and allowing erosional processes to change their morphology. As a result, climatic changes in the content of near-surface ground ice are likely to influence the level of dune activity. Spatial variation of dune field classes with longitude is significant, suggesting that local conditions play a major role in determining dune field activity level. Dune fields on the south polar layered terrain, for example, appear either potentially active or inactive, indicating that at least two generations of dune building have occurred on this surface. Many dune fields show signs of degradation mixed with crisp-brinked dunes, also suggesting that more than one generation of dune building has occurred since they originally formed. Dune fields superposed on early and late Amazonian surfaces provide potential upper age limits of ∼ 100 My on the south polar layered deposits and ∼ 3 Ga elsewhere at high latitudes. No craters are present on any identifiable dune fields, which can provide a lower age limit through crater counting: assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age, their estimated crater retention age is <∼10 000 years. An average-sized uncratered dune field (94 km 2) has a crater retention age <∼8 My. This apparent youth suggests that present-day climate conditions are responsible for the observed degradation and reduced level of aeolian activity. A lack of observed transport pathways and the absence of large dune fields in the largest basins (Hellas and Argyre Planitiae) are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on Mars.

Dating martian climate change

Geological evidence indicates that low-latitude polygonally-patterned grounds on Mars, generally thought to be the product of flood volcanism, are periglacial in nature and record a complex signal of changing climate. By studying the martian surface stratigraphically (in terms of the geometrical relations between surface landforms and the substrate) rather than genetically (by form analogy with Earth), we have identified dynamic surfaces across one-fifth of martian longitude. New stratigraphical observations in the Elysium–Amazonis plains have revealed a progressive surface polygonisation that is destructive of impact craters across the region. This activity is comparable to the climatically-driven degradation of periglacial landscapes on Earth, but because it affects impact craters—the martian chronometer—it can be dated. Here, we show that it is possible to directly date this activity based on the fraction of impact craters affected by polygon formation. Nearly 100% of craters (of all diameters) are superposed by polygonal sculpture: considering the few-100 Ma age of the substrate, this suggests that the process of polygon formation was active within the last few million years. Surface polygonisation in this region, often considered to be one of the signs of young, ‘plains-forming’ volcanism on Mars, is instead shown to postdate the majority of impact craters seen. We therefore conclude that it is post-depositional in origin and an artefact of thermal cycling of near-surface ground ice. Stratigraphically-controlled crater counts present the first way of dating climate change on a planet other than Earth: a record that may tell us something about climate change on our own planet. Parallel climate change on these two worlds—an ice age Mars coincident with Earth’s glacial Quaternary period—might suggest a coupled system linking both. We have previously been unable to generalise about the causes of long-term climate change based on a single terrestrial example—with the beginnings of a chronology for climate change on our nearest planetary neighbour, we can.

Integrating GPR and CCRI techniques: Implications for the identification and mapping of ground ice on Mars

Background: Since the early flyby missions of the 1970s, the study of Mars has largely focused on understanding the role of water (liquid or solid) in the geological evolution of the planet as well as its potential to support biotic activity. Data generated by the Mars Odyssey neutron and gamma ray spectrometers indicate large reservoirs of hydrogen (possibly H2O ice) in the near surface of Mars at latitudes greater than 50°. Additionally, Viking Orbiters and the Mars Orbiter Camera have revealed numerous landforms, possibly related to ground ice and permafrost processes (e.g., polygonal terrain, pingo-like mounds, thermokarst depressions, debris-aprons, and rock glacier-like features). However, despite observational evidence, an accurate identification and mapping of near-surface ground ice remains an open research question. Recently, the use of geophysical methods for investigating the Martian subsurface has witnessed growing interest among planetary scientists. Research involving the design and testing of geophysical instruments has focused primarily on Ground Penetrating Radar (GPR) and, to a lesser extent, seismic sounding, Time Domain Electromagnetic and Surface Nuclear Magnetic Resonance sounding. Approach: The purpose of this study is to investigate the occurrence of ground ice in an area analogous to what may be found on Mars, the ice cored terrain surrounding Athabasca Glacier, Alberta, Canada; and to examine the combined applicability of two geophysical methods such as GPR and Capacitively Coupled Resistivity Imaging (CCRI) in the investigation of the ice-rich Martian environment. Results and Implications: Our results show that GPR and CCRI techniques effectively complement each other by resolving different characteristics of the subsurface. While GPR clearly showed the subsurface structures and interfaces, CCRI provided diagnostic information about the subsurface lithologies. The two main implications of this combined application for the exploration Mars are: (a) it improves the ability to identify and map Martian ground ice; and, (b) it produces a more accurate description of the subsurface properties.

Recent low-latitude freeze–thaw on Mars

Outside polar latitudes, features corresponding to surface thaw have yet to be identified on Mars. The youthful gully landforms observed at mid-high latitude [Malin, M., Edgett, K., 2000. Science 288, 2330–2335] are the nearest candidate, but the source (and nature) of the gully carving agent remains controversial [e.g., Musselwhite, D.S., Swindle, T.D., Lunine, J.I., 2001. Geophys. Res. Lett. 28, 1283–1285; Mellon, M.T., Phillips, R.J., 2001. J. Geophys. Res. 106, 1–15; Knauth, L.P., Burt, D.M., 2002. Icarus 158, 267–271; Costard, F., Forget, F., Mangold, N., Peulvast, J.P., 2002. Science 295, 110–113; Christensen, P.R., 2003. Nature 422, 45–48; Treiman, A.H., 2003. J. Geophys. Res. 108]. At higher obliquity than the present epoch, near-surface ground ice should be present globally [Mellon, M.T., Jakosky, B.M., 1995. J. Geophys. Res. 100 (E6), 11781–11799], populated by condensation of atmospheric water vapour in the top few metres of the regolith, or emplaced as dusty ice sheets reaching down towards the equator. The latitudinal restriction of these gullies to regions poleward of ±30° appears to argue against a thaw component to their formation—since ground ice is present and stable at all latitudes at high obliquity, the current (low) obliquity regime should result in ground ice thaw at low latitudes, where insolation and daytime temperatures are currently greatest, and this is not observed. A previously undescribed meltwater sequence in the Cerberus plains, at 20° N/187° E, shows that comparable, but much more continuous, and mappable melting and surface runoff have occurred in the geologically recent past at near-equatorial latitudes on Mars. Polygonal ground in the Cerberus plains is seen by the Mars Global Surveyor Mars Orbiter Camera (MOC) to suffer sequential, regional-scale volatile-loss consistent with thaw of near-surface ground ice under periglacial conditions. This degradation is continuously sampled by a single MOC strip, showing an icy landscape undergoing thaw modification and collapse, and may form the first evidence of equatorial wet-based glaciation during late Amazonian time, with indications of melting within the last million years. The dissolution and re-formation of polygonal ground links this landform to freeze–thaw processes, providing the conclusion to a question that has been the subject of debate for three decades—whether Mars' polygonal grounds require ice to form—and a consistent explanation for the fate of the water that carved the great outflow channels, much of which may still reside as ground ice in the regolith. This thaw occurs in the Cerberus Formation; deposits that are considered to be magmatic in origin, and the type formation for late-stage, “plains-style” volcanism on Mars [Keszthelyi, L., McEwen, A.S., Thordarson T., 2000. J. Geophys. Res. 105, 15027–15049]. By superposing large numbers of small impact craters, polygonal ground in the Cerberus plains sustains previous suggestions of a non-magmatic origin for this and other landforms in the region [Page, D.P., Murray, J.B., 2006. Icarus 183, 46–54]. Together, these periglacial landforms document evidence of climate change much younger than is currently recognised by crater counts, with important implications for age constraints on young surfaces and absolute age determinations by this method. It is tentatively suggested that this melting may be occurring today, with a striking correspondence between permafrost thaw in the Cerberus plains, the high atmospheric methane flux currently observed over this region [Mumma, M.J., Novak, R.E., DiSanti, M.A., Bonev, B.P., Dello Russo, N., 2004. Bull. Am. Astron. Soc. 36, 1127; Krasnopolsky, V.A., Maillard, J.P., Owen, T.C., 2004. Icarus 172, 537–547; Formisano, V., Atreya, S., Encrenaz, T., Ignatiev, N., Giuranna, M., 2004. Science 306, 1758–1761], and the only latitude zone on Mars—equatorward of 30° N—where melting of ground ice is thought possible in the current climate [Haberle, R.M., McKay, C.P., Schaeffer, J., Cabrol, N.A., Grin, E.A., Zent, A.P., Quinn, R., 2001. J. Geophys. Res. 106 (E10), 23317–23326; Lobitz, B., Wood, B.L., Averner, M.M., McKay, C.P., 2001. Proc. Natl. Acad. Sci. 98, 2132–2137]. Low-latitude polygonal ground as transient, and hydrologically active over wide areas transforms our understanding of the recent climatic evolution of Mars, supporting models of atmospheric water-ice migration [Mischna, M., Richardson, M.I., Wilson, R.J., McCleese, D.J., 2003. J. Geophys. Res. 108 (E6). 5062], complex, volatile stratigraphies [Clifford, S.M., Parker, T.J., 2001. Icarus 154, 40–79], and hypothesised, geologically recent ‘ice ages’ [Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003. Nature 426, 797–802]. The temporal coincidence of glacial epochs on the Earth and Mars during the Quaternary and latest Amazonian would suggest a coupled system linking both [Sagan, C., Young, A.T., 1973. Nature 243, 459].

The Structure and Dynamics of Earth Hummocks in the Subarctic Forest Near Inuvik, Northwest Territories, Canada

ABSTRACTSurface microrelief, permafrost table configuration, ground-ice, and soil organic-matter contents are described for three sites near Inuvik, N.W.T., that are characterized by collapsed, poorly developed, and well-developed earth hummocks, respectively. The diameters of collapsed earth hummocks were significantly greater than those of well-developed vegetated hummocks. Hummock relief and interhummock distance increased along the continuum of forms, with the widest spacing and greatest relief measured at the site with well-developed vegetated hummocks. A bowl-shaped permafrost table mirrored the surface relief of most hummocks, but the collapsed hummocks were underlain by a planar or domed permafrost table. Segregated ice lenses parallel to the permafrost table, and small bodies of intrusive ice, were observed beneath the developing and well-developed hummocks. The configuration of the permafrost table and hummock relief, long-term observations of active-layer and hummock change, and hummock response to surface manipulation indicate that formation of a bowl-shaped permafrost table in association with organic accumulation and development of near-surface ground ice thrusts soils inward and upward causing hummock growth, whereas thaw subsidence may cause outward spreading and hummock collapse. Reaction-wood rings in black spruce trees growing on hummocky terrain indicate that tree tilting was associated with active-layer thinning and hummock growth. Cessation of reaction wood coincided with a period of active-layer deepening, degradation of ground ice, and outward spreading of the hummocks. In subarctic forests, hummock dynamics may be driven by ecological change associated with the fire cycle or climate change.

Young (late Amazonian), near-surface, ground ice features near the equator, Athabasca Valles, Mars

A suite of four feature types in a ∼20 km 2 area near 10° N, 204° W in Athabasca Valles is interpreted to have resulted from near-surface ground ice. These features include mounds, conical forms with rimmed summit depressions, flatter irregularly-shaped forms with raised rims, and polygonal terrain. Based on morphology, size, and analogy to terrestrial ground ice forms, these Athabascan features are interpreted as pingos, collapsing pingos, pingo scars, and thermal contraction polygons, respectively. Thermal Infrared Mapping Spectrometer (THEMIS) data and geological features in the area are consistent with a sedimentary substrate underlying these features. These observations lead us to favor a ground ice interpretation, although we do not rule out volcanic and especially glaciofluvial hypotheses. The hypothesized ground ice that formed the mounds and rimmed features may have been emplaced via the deposition of saturated sediment during flooding; an alternative scenario invokes magmatically cycled groundwater. The ground ice implicit in the hypothesized thermal contraction polygons may have derived either from this flooding/ground water, or from atmospheric water vapor. The lack of obvious flood modification of the mounds and rimmed features indicates that they formed after the most recent flood inundated the area. Analogy with terrestrial pingos suggests that ground ice may be still extant within the positive relief mounds. As the water that flooded down Athabasca Valles emerged via a volcanotectonic fissure from a deep aquifer, any extant pingo ice may contain evidence of a deep subsurface biosphere.