Marie-Françoise André

New insights into rock weathering from high-frequency rock temperature data: an Antarctic study of weathering by thermal stress

A major limitation of many weathering studies has been the acquisition of rock temperature data at insufficiently frequent intervals for the meaningful determination of the rate of change of temperature (ΔT/t). Equipment and/or logistical constraints frequently facilitate temperature measurement at only hourly intervals or, at best, every 10 min. Such data are not adequate for the determination of ΔT/t required for the evaluation of the freeze–thaw mechanism or thermal stress fatigue. Recent undertakings at different sites in Antarctica (and at other cold-region locations) provide rock temperature measurements at 1-min intervals, which indicate that the perception of the weathering regime would be very different from that generally assumed from longer-interval geomorphological data. These data clearly show that thermal stress fatigue and thermal shock may be more active components of the Antarctic weathering regime than have generally been recognised; the aridity of the study area limits the role of freeze–thaw weathering. Values of ΔT/t of ≥2 °C min−1 that suggest thermal stress fatigue/shock is operative were recorded; observations of rock flaking are thought to reflect the impact of thermal stress. Further, the data show that contrary to general perceptions, the southern aspect can, in summer, experience higher rock surface temperatures than the north-facing exposure. An examination of rock fracture patterns found in the field shows great similarity to fracture patterns developed in the laboratory as a direct result of thermal shock. The argument is made that greater cognizance should be given to thermal effects.

Holocene Rockwall Retreat in Svalbard: A Triple-Rate Evolution

Volumetric calculations of slope deposits, direct measurements of rockwall retreat and chronological control based on lichenometry provide a wide range of rockwall retreat rates in Svalbard (0–1580 mm ka−1) that appears consistent with previous evaluations from other Arctic areas. In northwest and central Spitsbergen (79°N), a triple-rate rockwall retreat is suggested for the last two millennia: very slow biogenic flaking (2 mm ka−1), moderate retreat due to frost shattering (100 ka−1) and rapid retreat associated with post-glacial stress relaxation (c. 1000 mm ka−1). Examination of the distribution of various processes indicates that the Holocene retreat of most rockwalls has not exceeded one or two metres. Bedrock conditions appear to be the main control on retreat rates. The massiveness of igneous and metamorphic outcrops, widespread in Arctic shield areas, largely accounts for the slowness of rockwall retreat, which on these lithologies is primarily due to chemical and biological processes. More rapid rates are usually associated with stress relaxation following glacial surges or with local frost susceptibility of bedrock, often where faulting has induced high joint density. At such sites, rockwall retreat rates are of the same order of magnitude as those reported from Alpine areas (1000–3000 mm ka−1) where both bedrock weakening due to tectonic stresses and the greater height of steep rockwalls account for the more rapid rockwall retreat rate.

The geomorphic impact of glaciers as indicated by tors in North Sweden (Aurivaara, 68° N)

Abstract Geomorphological investigations carried out on 15 tor-like features located on the Aurivaara plateau (North Sweden, 68° N) provide new insights in the greatly debated age of these landforms. Erratics and till trapped deep in the tor joints support a pre-Weichselian age for tor formation. Moreover, the occurrence of various weathering stages in allochtonous material, the joint width up to 1.5 m (requiring long-term weathering), and the frequent association of tors with pediment-like forms, suggest pre-Quaternary tor formation. The juxtaposition of fresh erratics and in situ old weathering features (mushroom rocks, concentrically weathered well-rounded corestones, and grus) indicates a predominantly cold-based regime for the Scandinavian ice sheet, with erratics carried by the overlying moving ice being repeatedly deposited on tor summits during deglaciation phases. The relationships between tors and ice action indicated for the Aurivaara plateau result in the proposal of a morphodynamical succession of five tor subtypes ranging from the preservation of well-rounded corestones still embedded in grus (suggesting negligible glacial erosion) to the almost complete removal of tor features by ice scouring. A comparison with tors in similar geological and topographical contexts from the unglaciated Dartmoor area allows a tentative evaluation of an average overall glacial erosion of 0–10 m on the northern Sweden plateaus, in sharp contrast with the 190 m overdeepening of the nearby Tornetrask basin. Thus, this case study of Swedish tors provides additional support to the recent interpretations of relict landscapes in previously glaciated areas and is in accordance with the classical «model» of glacial selective erosion established in the Nordic and Arctic mountains.

Temperature observations in Antarctic tafoni: implications for weathering, biological colonization, and tafoni formation

Tafoni have long been recognized, measured and discussed within the Antarctic context. However, with respect to the formative processes tafoni still remain somewhat of an enigma. In terms of the weathering attributes of tafoni, one problem is the monitoring of environmental conditions without the transducers themselves altering that environment. The application of ultra-small thermocouples provides an avenue for monitoring of rock surface temperatures without influence on the tafoni environment. At an Antarctic site temperatures were measured both inside and outside of a tafone, with data at 20 second intervals. These data show a spatial variability that may help explain tafoni development, at least in terms of weathering. Humidity data indicate that moisture conditions are very low such that water-based weathering processes are temporally and spatially constrained. The presence of several episodes of extreme temperature variations indicates that thermal stress may be an important contributor to weathering here. It is argued that the absence of any endolithic communities (at this site) within the sandstone, in which the tafoni develop, is a reflection of weathering rates that exceed the ability of organisms to invade and colonize the rock. At the present time, weathering appears to be primarily in the form of granular disintegration and flaking.

From climatic to global change geomorphology: contemporary shifts in periglacial geomorphology

Abstract Periglacial geomorphology developed in the 1940s–1960s as a branch of climatic geomorphology, focusing first on Quaternary studies and palaeoenvironmental reconstructions, then on current geomorphic activity in cold regions. The ‘periglacial fever’ of the 1960s–1970s was dominated by the ‘freeze–thaw dogma’: periglacial areas were regarded as necessarily submitted to efficient frost-driven processes ruling over the geomorphic activity. Such a view was severely criticized in the 1980s–1990s based both on monitoring studies and on time–space multiscale approaches that pointed to the need to cross the ‘smokescreen of the periglacial scenery’ to search for the real past and present processes responsible for the landform geometry. The role of non-cold-related processes in the making of ‘periglacial’ landcapes was re-evaluated, and the necessity to better take into account the rock properties and the pre-Quaternary history of slope systems was emphasized. Whereas the part of the cold-related processes was being minimized, the interest of genuine periglacial landforms as geoindicators of climate change was growing, providing a new legitimacy to periglacial geomorphology. Polar and Alpine regions are nowadays considered as key observatories of ongoing climate change, and periglacial geomorphologists are involved in the detection, monitoring and prediction of environmental changes. Finally, the evolution of ‘periglacial geomorphology’ over the past six decades is in accordance with the development of the whole geomorphology. Based on the quantitative and technological revolution, it tends to find a balance between the functional and historical approaches.

Optical rock properties and weathering processes in polar environments (with special reference to Antarctica)

As a result of the “freeze-thaw dogma,” the polar scientific community has, for a long time, emphasized the importance of physical properties of rocks (porosity, jointing, etc) as a primary control on rock weathering. More recently, due to growing interest in chemically driven processes operating in cold areas, attention has been drawn to the chemical rock properties. Surprisingly, the optical properties of rocks have either been ignored or only alluded to in most rock weathering studies. Based on the available Antarctic biological and geomorphological literature, it is now appropriate to consider these optical properties as exerting a potentially significant influence and to promote a Manichean view in which the light-colored and translucent rocks (e.g., the emblematic Beacon sandstones) are considered from the perspective of biogenic weathering, whereas the dark rocks (e.g., the dolerites of the Dry Valleys) are viewed as being influenced by thermal weathering. Field observations and monitoring carried out from Labrador to Antarctica, lead, however, to a much more subtle appreciation, for it appears necessary to: (1) integrate the optical properties within a corpus of rock properties (within which some operate synergistically and others antagonistically with those optical properties); (2) to take into account the impact of scale (e.g., macro vs. micro); and (3) to consider the nature and role of lithophytic communities involved in bioweathering.

Some further observations regarding « cryoplanation terraces » on Alexander Island

Abstract Landforms with the appearance of cryoplanation terraces were studied on Alexander Island in an attempt to better understand their formation and growth. Developed on sub-horizontal sedimentary rocks, with 360° exposure around a nunatak, the terraces show a distinct equatorward orientational preference and an increase in terrace size with elevation. Available data fail to indicate any evidence of freeze-thaw weathering and information relating to present-day debris transport is singularly absent. Thermal data from the rock exposures showed variability that could cause thermal fatigue but no rates of change of temperature commensurate with thermal shock were recorded. Terrace development appears to be connected with lithological differences in the local sandstones, with growth along sedimentary junctions. Although presently in a permafrost environment, the available information on these landforms does not appear to be compatible with that generally accepted for cryoplanation terraces.

Impact of cement repointing on rates of sandstone decay in medieval churches of the French Massif Central

Abstract Three Romanesque churches of the Bourbonnais region in the French Massif Central have been investigated. These date back to the twelfth century, and are mainly built of red and white sandstones. Their ashlar blocks were extensively repointed in the mid-twentieth century with cement-rich mortar. In order to quantitatively assess the impact of repointing on the decay rates of the sandstones, the following methods have been used: dating of reference surfaces (‘zero datum levels’) based on stone-dressing marks, stone-by-stone and terrestrial LiDAR (Light Detection And Ranging) surveys, and petrographical analyses including on-site non-destructive tests. This study reveals that, between the twelfth century and the mid-twentieth century, intrinsic geological factors have been a key control on weathering rates. Stone surface recession has proceeded, on average, 500 times faster in the poorly cemented soft white sandstone compared to the iron-cemented red sandstone (4 mm/century v. 0.008 mm/century). Since the mid-twentieth century, the replacement of the original lime mortar with a cement-rich mortar has resulted in a marked increase in weathering rates and resulting surface recession of the poorly cemented white sandstone: this increase is fivefold where cement-lime mortar (‘bastard mortar’) has been used and 10-fold where harder cement-rich mortar has been applied. By contrast, the application of cement-lime mortar between the ashlars composed of iron-cemented red sandstone had no deleterious effects. This study confirms that the use of cement-based mortar should be avoided on soft and porous masonries. It also stresses the fact that accelerated stone surface recession rates are systematically associated with heritage sites that are subject to significant anthropogenic effects caused by inappropriate interventions or excessive pressure on the monuments or their environment.

Karstic valleys, Vineyards and Palafittic Settlements of the External Jura

The Jura massif, which gave his name to the Jurassic period, comprises a folded mountainous eastern part (internal Jura) and a hilly and tabular western part, which is the focus of this contribution. The external Jura is a part of the charming French countryside displaying both original geomorphological landscapes as well as distinctive local products. It is especially the case of the fold-and-thrust belt overriding the Bresse graben, which bears the Jura vineyards mainly growing on Triassic and Liassic marls and providing the unique Château-Chalon ‘yellow wine’. By contrast, the Jura plateaus include karstic plateaus with picturesque valleys called ‘reculees’ and glacially formed lakes. During the Holocene, these lake shores were inhabited by Neolithic cultural groups representative of the earliest European agrarian societies, who built pile-dwelling settlements inscribed in 2011 by UNESCO as World Heritage Sites.