Holocene geomorphic activity in coastal Greenland at glacier equilibrium line altitudes

Holocene Geomorphic Activity in Coastal Greenland at Glacier Equilibrium Line Altitudes

This project aims at investigating terrestrial geomorphic activity within a critical altitudinal range of great geomorphic importance and climatic sensitivity, within a key geographical region for understanding Holocene land-atmosphere-ocean interaction in the North Atlantic region.

Research on global climate change indicates that, in general, land surfaces will heat or cool more rapidly than oceans and that temperature variations especially within high-latitudinal regions will be pronounced (Houghton et al., 1996), mainly due to the operation of various, interrelated climatic feedback mechanisms (Kellogg, 1973).

Analysis of instrumentally recorded land and marine temperatures indicate that the planet surface has heated about 0.5^oC since the late 19th century (Jones and Briffa, 1992). Northern Hemisphere surface temperature observations, however, indicate the observed warming not to be equal everywhere, nor equal throughout the seasons. The greatest warming has occurred over central Siberia and NW Canada, especially during the winter, while other areas such as the North Atlantic region including Greenland and Iceland have cooled in the latter half of the 20th century, also mainly during the winter (Jones and Briffa, 1992; Houghton et al., 1996; Humlum and Christiansen, 1998a, 1998b). In general, the North Atlantic region is emerging as a geographical focus of several arctic-global interactions, primarily through the exchange of near-surface and deeper waters involved in thermohaline processes.

A pronounced climatic contrast across the North Atlantic conveys an additional significance to this region. Usually, warm winters in northern Europe correspond to cold winters in Greenland and the Canadian Arctic, and vice versa. This is the well-known Greenland/North European Seesaw reviewed 1978 by van Loon and Rogers. Warm winter weather in Europe goes along with a deeper Icelandic Low and a stronger Azores High, corresponding to a positive North Atlantic Oscillation (NAO), and enhancing westerly wind flow onto Europe, and cold meridional winds in Greenland (Trenberth, 1993, Dickson 1998). However, in certain years both Europe and Greenland experience extraordinary warm or cold winters. Warm winters on both sides of the North Atlantic are typically associated with higher than normal pressures over the Atlantic south of 55^oN and a broad region of lower than normal pressures throughout the Arctic, resulting in strong westerly flow onto Europe and an absence of strong meridional flow anywhere from the polar regions (van Loon and Rogers, 1978). In contrast, cold winters on both sides of the North Atlantic are remarkable for very small cyclonic activity north of 55^oN (van Loon and Madden, 1983).

In recent times shipboard observations of sea surface temperatures since 1945 indicate a predictable pattern of warming and cooling in the North Atlantic of around 1^oC over a roughly 11-12 year period, superimposed upon a gradually falling trend since about 1930 (Humlum and Christiansen, 1998a, 1998b).

On longer time scales, close correspondence has emerged between the d¹⁸O record in the Greenland Summit ice cores and North Atlantic sea surface temperatures. This suggest that, for the last 80 ka at least, the atmosphere and the North Atlantic ocean surface were a coupled system that was repeatedly undergoing massive reorganizations on timescales of centuries or even less (Bond and Lotti, 1995). Relating these major hemispherical (and perhaps global) climatic changes to the terrestrial non-glacial proxy record around the North Atlantic has proved to be more difficult, however (Lowe and Walker, 1997).

Palaeoclimatic information from coastal Greenland is therefore important in order to investigate operation of the North Atlantic ocean-atmosphere coupling back in time, but also to obtain information on contrasts between the coastal land climate in Greenland and the ice sheet climate such as registered by ice core analysis (Dansgaard et al., 1993; Meese et al., 1994; Karlén and Kuylenstierna, 1996; Humlum, 1999). While palaeotemperatures modeled from oxygen isotopes reveal a rather subdued temperature effect at the GISP2 site for the Little Ice Age period (AD 1100-1920), the limited evidence available suggests the coastal climate to have been much more variable during this period (Humlum, 1999, 2000). As an example, local glaciers within coastal W Greenland have experienced significant Holocene fluctuations, partly due to variations in air temperature, but also due to variations in precipitation and wind regime, both during the early Holocene (Ingólfsson et al., 1990) as well as in the late Holocene (Humlum, 1987, 1998d).

Traditionally, most terrain elements within a landscape are viewed upon as representing semi-permanent features, with only few elements, such as glaciers, representing exceptions from this rule. However, modern geomorphic research have shown that the landscape organization is much more complex, and that there are intimate links between climate, vegetation and geomorphic processes. Any landscape should therefore be considered as consisting of a number of elements currently adjusting according to present climate, and a number of relict features, produced under past climatic conditions.

Whether a climatic change has impact on the physical landscape depends on the character, magnitude and duration of the change, and on the properties of the landforms considered. The rate of adjustment can be studied by measuring the sediment flux through and within river basins or smaller catchments, as well as by mapping of recent landform changes and dating of older forms or by studying sedimentary records. By this, analytic studies of landscapes may yield important information on the character and duration of past and present environments. However, in high-relief, cold-climate landscapes, the geomorphic activity and sensivity to climatic change appears to be especially pronounced at a certain, critical altitude.

Field work carried out in various parts of Greenland since 1980 have documented the overwhelming geomorphic importance of altitudes close to the glacier equilibrium line altitude (ELA) ±250 m a.s.l. (Humlum, 1982, 1986, 1987, 1988a, 1988b, 1992, 1996, 1997a, 1997b, 1998b, 1998c, 1999 and 2000). Within this critical range the weathering rate of bedrock appears to be at maximum, leading to extraordinary high rates of talus production and new sediment in general. Derived characteristic landforms are talus sheets, protalus ramparts, avalanche deposits, rock glaciers and moraines.

Intense bedrock weathering at Qivitut, Diskofjord, Disko Island, early September 1999

To obtain more detailed knowledge on the geomorphic and sedimentologic significance of this critical altitudinal range for large-scale, long-term landscape evolution, a preliminary inventory of about 400 individual sites close by the local ELA was carried out in central West Greenland by means of field studies and analysis of aerial maps and topographic maps (Humlum, 1998e). The estimated Holocene debris volume was compared with the area of the source headwall above, using available knowledge on the internal depositional landform structure. The results of this inventory suggest that especially the presence of rock glaciers is diagnostic for very high rock weathering rates (2-5 mm/yr). Headwalls associated with glacier-derived rock glaciers are related to extraordinary weathering rates (5-15 mm/yr), and thereby delimits critical ranges regarding aspect and altitude, for the maximum Holocene headwall weathering. The relations of this intensive rock weathering zone to other regional concepts such as glaciation level and glacier equilibrium line altitude were investigated by various statistical means. The upper limit of the zone is close to the modern regional glaciation level, and the lower limit is close to the late Holocene range of equilibrium line altitudes, as represented by modern and Little Ice Age ELA’s (Humlum, 1985, 1987, 1988a). Both lower and higher headwall altitudes result in lower weathering rates.

The high-weathering zone is highly sensitive towards any climatic change, weather represented by variations in air temperature, precipitation or in wind regime. Large interannual variations in topoclimate derived from the local net radiation balance are closely related to variations in the mean summer snow cover, which shows high interannual variations at the ELA.

Weathering can be divided into those processes involving only physical changes (physical weathering) and those that involve chemical reactions and the formation of new minerals (chemical weathering). Although the differences between these two types of weathering are distinct in theory, in practice they rarely operate separately; rather, the effects of one aid the operation of the other. Bedrock shattered through physical weathering will be more liable to chemical weathering because of the increased surface area, while chemical weathering along micro fractures will assist physical weathering processes.

Within the critical geomorphic altitudinal range both types of weathering apparently operate at very high rates. The above landforms demonstrates this to be the case for physical weathering, while the importance of chemical weathering in this environment is proved by oxidation, solution, deposition of calcium carbonate, desert varnish, cavernous weathering, granular disintegration, etc.

Temperature is an important factor in rates of chemical weathering through the direct effect it has on the rate of chemical reactions. Despite the importance of chemical weathering in polar regions was stressed as early as 1897 by Tarr, there still remains a tendency to regard this process as completely negligible in cold-climate regions. This notion is, however, being gradually dispelled as observations on the contrary slowly increase (see, e.g., Rapp, 1960; Washburn, 1969; Darmody and Thorn, 1997a). Hydration is probably the single most widespread chemical bedrock weathering process. An increased absorption by water of oxygen and, especially, of CO₂ at the low average temperature of the thaw season counteracts the lowered chemical activity normally accompanying low temperatures, as was recognized as early as 1949 by Williams. In short, the widespread availability of liquid water from melting snow accumulations, hydration and absorption of CO₂ may be the environmental background for the importance of chemical weathering within the critical geomorphic zone.

Within a broader global climatic change context especially the chemical aspect of weathering may be of some interest through the process of carbonation, by which substantial amounts of CO₂ is fixed from the atmosphere. Carbonation plays a particularly important role in the weathering of calcareous rocks such as limestones, but is also widespread for other bedrock types containing albite (feldspar).

There is still a significant lack of understanding as to the natural CO₂-cycle, and the CO₂ content is only growing less than half the rate expected by the Kyoto Protocol (Kerr, 1997; Hansen, 1998). It is therefore believed that major, but still unknown, sinks of CO₂ must exist in order to balance the added carbon budget. Most likely, the uptake by CO₂ sinks such as the North Atlantic ocean (Takahshi, 1998), vegetation and weathering phenomena have increased. Removal of atmospheric CO₂ by way of geomorphic processes such as chemical weathering in high-relief, cold-climate regions may prove of considerable importance in this respect.

The empirical altitudinal relation between the zone of maximum bedrock weathering and glaciological concepts such as the regional glaciation level and ELA, suggests the net accumulation of snow to exercise some direct or indirect control on the rate of bedrock weathering, presumably by representing a moisture source. Water plays a vital role in nearly all mechanisms of physical and chemical weathering.

A previous study (Humlum, 1992) has demonstrated the ability of common Greenlandic bedrock types (gneiss and basalt) to exchange significant amounts of moisture with the surrounding air masses, even on a time scale of just few hours. Field measurements show this phenomenon not to be confined to above-freezing air temperatures with liquid precipitation, but it operates very efficiently also for below-freezing temperatures with solid precipitation, snow drifting, fog or just periods with significant variations in the relative air moisture content. Meteorological events such as these are likely to be frequently met for headwalls rising to altitudes close to the ELA. Such rock free faces are exposed to a high mean degree of water saturation, and significant variations of this, conditions known to promote rapid rock breakdown (Wiman, 1963; Potts, 1970; White, 1976; McGreevy, 1981; Whalley et al., 1982; Fukuda, 1983; McGreevy and Whalley, 1985; Hall, 1986, 1988; Humlum, 1992, 1998b, 2000).

The geomorphic high-weathering zone is close to the ELA, and therefore moves vertically in the landscape (as does the ELA), according to prevailing climate.

Incipient glaciation requires the existence of an equilibrium line at the terrain surface. From a geomorphic point of view, glacier equilibrium lines are important because they represent the lowest boundary of climatic glaciation (Humlum et al., 1996). The climate that prevails at glacier equilibrium lines is therefore considered just sufficient to maintain the existence of glaciers (e.g. Ohmura et al. 1992). Average accumulation on a glacier approximates closely to accumulation at the equilibrium line, while ablation is most usefully predicted by mean summer temperature. There is therefore a close, but non-linear relationship between accumulation and mean summer temperature at the equilibrium line (Sutherland, 1984). This correlation between glacier mass-balance input and air temperature is known to apply both spatially and temporally within many regions with modern glaciation (Robin 1977; Oerlemans 1982; Muszynski and Birchfield 1985; Bromwich 1988; Fortuin and Oerlemans 1990), and can therefore also be applied to delimit essentials of the typical climate within the above geomorphic critical altitude (Humlum, 1998c).

Within the larger North Atlantic region comparatively little palaeoclimatic information is available from coastal land regions in Greenland, and the project is therefore carried out at two main study sites in W and E Greenland, respectively. These sites have been chosen because of availability of 1) good geomorphic/geologic maps and 2) comparatively long series of meteorological data, and 3) a strategic location with respect to the North Atlantic atmosphere-ocean coupling issue.

In W Greenland the main study site will be Disko Island (70oN, main study site 1999), which presently marks the northernmost penetration of warm water of Atlantic origin along West Greenland. The position at the junction of Davis Strait and Baffin Bay with different water masses, makes the palaeoclimatic history of this central West Greenland region especially interesting in terms of geomorphic responses to regional and global climatic variations. This region is well investigated, and various types of data are at hand. The Arctic Station (Faculty of Science, Univ. Copenhagen) and the modern research vessel “Porsild” will provide the logistic base for the planned investigations. Automatic meteorological stations (two) have been run in Disko Island by the Arctic Station since 1990 and 1993 (Humlum, 2000). Meteorological data series from Godhavn and Jakobshavn goes back to 1923 and 1873, respectively. Bedrock types are gneiss, sandstone, shale and basalt

In SE Greenland the Ammassalik Island (65oN; main study site 2001) will be the main study site. This locality is situated within the western North Atlantic region (the Denmark Strait), not far from Iceland, where manifold data on the Holocene climatic evolution are available. The Denmark Strait is an area of great contrast in water masses. In the east warm and saline Atlantic Water is flowing towards the north, while along the western side cold and fresh Polar Water flow southwards. The Denmark Strait Overflow provides part of the deep water ventilating the Atlantic Ocean. The Sermilik Station (Geogr.Inst., Univ.Copenhagen) will provide the logistic base for investigations in this study region. Two automatic meteorological stations have been run by the Sermilik Station in the area since 1992 and 1994, respectively, and there is now established a dated framework for the Holocene geomorphic activity within this area (Christiansen et al., 1998). From the village Ammassalik meteorological observations are at hand from 1894. Bedrock types are gneiss, granite and diorite.

Main research activities

At each study site an inventory of the geomorphology will is being carried out by means of aerial photographs, to select smaller research sites for detailed field study. At each site a geomorphic mapping will be carried out in order to identify geomorphic units diagnostic for high bedrock weathering (talus sheets, protalus ramparts, rock glaciers, moraines, etc.). Test areas for estimating modern bedrock weathering and transport rates will be established. Digital Terrain Models (DTM’s) will be constructed, to model topoclimate under different meteorological situations (Humlum, 1997b), and from this, to assess geomorphic effects (Humlum, 1998c). Overall, the approach will be that of a mapping, monitoring and modeling research scheme, with main emphasis on periglacial phenomena.

Physical weathering will be investigated by establishing sediment traps and by means of recurrent photography. Chemical weathering will be studied by means of comparing conductivity and chemistry of precipitation with water from streams draining individual test areas, and by investigations on the weathering rind on exposed bedrock and talus fragments. Also the chemical weathering of soils of different age will be compared.

The Holocene debris volume is being estimated by surveying. Characteristic grain size distribution will be investigated, as especially grain sizes smaller than 1 mm contribute significantly to the total surface area exposed to chemical weathering. Also historical evidence and instrumental data series on late Holocene meteorological variations will be incorporated in the study.

Dating of various terrain units will be attempted by investigations on depth of chemical weathering rinds, Smith Hammer rebound values, lichenometry, tephrochronology, luminescence AMS ¹⁴C, etc. Miniature dataloggers will be installed to gain insight into topoclimate. Special attention will be paid to measurements of air, surface and ground temperatures, and to the duration of the snow cover.

Implications for marine sedimentology

Although especially rock glaciers are suggested to represent diagnostic forms for extraordinary high geomorphic activity, their sedimentological role are somewhat particular (Humlum, 2000).

While normal glaciers act as temporary storage units for new sediment, which soon is released and transferred further downvalley by melt water streams, this is not the case for rock glaciers. Water draining from rock glaciers tend to be without much sediment (ignoring solutes), and most of the debris produced from the headwall above stay within the rock glacier body for an extended period, as much as several thousands of years, even though the rock glacier on a local scale is highly efficient in transporting new talus away from the foot of the headwall. The debris is stored within rock glaciers during interglacials, and is transferred significantly further downvalley only in connection with the evolution of big valley glaciers during glacial periods. This has implications for the interpretation of interglacial and interstadial marine sedimentology along coasts with numerous rock glaciers in the hinterland.

Implications for global climatic change and palaeoclimatology

Within the current debate on global change context the chemical aspect of bedrock weathering within the critical geomorphological zone presumably is of some importance, as CO₂ is fixed from the atmosphere by this phenomena. This may contribute towards recognizing the nature of still unknown sinks of CO₂. Apparently, the role of high geomorphic activity in cold-climate, high-relief regions has been more or less overlooked in this context.

Mapping of the Holocene palaeowind regime has implications for the timing of deep water formation in the Greenland Sea. This also has implications for understanding the role of the North Atlantic Ocean as an important sink for CO₂ (Takahshi, 1998).

As demonstrated by Briffa and Jones (1993), summer air temperatures are the most atypical of all the various seasonal averages, especially in the Northern Hemisphere, where most of the global surface warming observed since the middle of the 19th century has occurred in winter, spring and autumn, but only little in summer. Inferring climate change on the basis of observations biased toward summer conditions is therefore questionable regardless of spatial scale. Winter responsive data, in general, represent a much more powerful source of information (Humlum and Christiansen, 1998a, 1998b). Therefore, in an overall global change context, the occurrence of geomorphic cold-climate phenomena associated the physical weathering (frost weathering) of bedrock and accumulation of snow should be considered with special interest, because they are controlled mainly by winter season climate.

Autumn (late August 2000) on Disko Island

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