Cerro Tapado Research Articles

Discussion of ‘Geomorphology, internal structure, and successive development of a glacier foreland in the semiarid Chilean Andes (Cerro Tapado, upper Elqui Valley, 30°08′ S, 69°55′ W.)’, by S. Monnier, C. Kinnard, A. Surazakov and W. Bossy, Geomorphology 207 (2014), 126–140

Recently, Monnier et al. (2014, Geomorphology 207, 126–140) suggested that diffraction events with air-wave velocities along profile CD were caused by an air-filled void at depth within one of the glaciers of the Cerro Tapado, Chile. This is not physically possible. The velocity at which radar travels to and from the scattering feature that causes a diffraction is at the velocity that surrounds and overlies the scatterer. Thus, their air-wave velocity features are on the surface of the glacier, not at depth. Their three-layer model for profile CD, therefore, is more appropriately a two-layer model, with a lower density layer, with more air and debris, overlying a denser layer, likely corresponding to the firn–ice transition. In addition, they carried out common mid-point (CMP) velocity profiles. While it is encouraging that they have made such an attempt, the results will be affected significantly by the surface and subsurface topography, truncated beds, unconformities, etc., because CMP profiles inherently assume flat-lying surface and subsurface boundaries. The CMP results, while useful, must therefore be treated with caution and assumed to be highly inaccurate and only be used as general guides to vertical velocity variations.

Geomorphology, internal structure, and successive development of a glacier foreland in the semiarid Chilean Andes (Cerro Tapado, upper Elqui Valley, 30°08′ S., 69°55′ W.) — Reply to Discussion by D.C. Nobes

We address the criticisms made by Nobes (2015, this issue) on the study by Monnier et al. (2014). We reiterate how an air-filled cavity was evidenced in a debris-covered glacier using ground-penetrating radar (GPR): by calculation of the interval velocity and not only relying on the velocity measured along diffraction events. Also, we show that common mid-point (CMP) measurements were correctly designed and carefully used and interpreted.

Geomorphology, internal structure, and successive development of a glacier foreland in the semiarid Chilean Andes (Cerro Tapado, upper Elqui Valley, 30°08′ S., 69°55′ W.)

We use geomorphological analysis, sedimentological survey, remote sensing, and ground penetrating radar (GPR) in order to understand the complex landform assemblage found in front of the Cerro Tapado glacier in the upper Elqui River catchment, semiarid Andes of Chile. The geomorphological analysis highlights prominent boundaries dividing the landform assemblage into (from the upper part to the lower part) an upper latero-frontal moraine complex, an upper debris-covered glacier, a lower debris-covered glacier, two rock glaciers, and a lower morainic complex. The sedimentological survey highlights the rather small size of the surface debris (in general <20cm) and the predominance of porphyritic rhyolite. Remote sensing data show that, between 1956 and 2010, considerable (>400m) receding of the glacier occurred, along with downslope displacements (dm–m·y−1) of most of the landform units and a significant evolution of the thermokarst features on the debris-covered glaciers. Considerable surface lowering occurred in the upper part of the assemblage, while localized bulging is seen along the morphological boundaries in the lower units. The GPR profiles highlight spectacular internal structure in the upper debris-covered glacier with up to 80m of buried ice. In the other landform units, the internal structure is less visible and more heterogeneous. The analysis of the radar wave velocity along the GPR profiles reveals the occurrence of air-filled and moist zones in the internal structure. The geomorphological assemblage is fundamentally characterized by its morphological, structural, and dynamical boundaries and defined as a young (probably <2000years) polygenetic construction with landform units having added to/overlapped one another. The rock glaciers do not derive from the present debris-covered glacier but preexist to it.

Glacier mass balance reconstruction by sublimation induced enrichment of chemical species on Cerro Tapado (Chilean Andes)

Abstract. A 36 m long ice core down to bedrock from the Cerro Tapado glacier (5536 m a.s.l, 30°08' S, 69°55' W) was analyzed to reconstruct past climatic conditions for Northern Chile. Because of the marked seasonality in the precipitation (short wet winter and extended dry summer periods) in this region, major snow ablation and related post-depositional processes occur on the glacier surface during summer periods. They include predominantly sublimation and dry deposition. Assuming that, like measured during the field campaign, the enrichment of chloride was always related to sublimation, the chemical record along the ice core may be applied to reconstruct the history of such secondary processes linked to the past climatic conditions over northern Chile. For the time period 1962–1999, a mean annual net accumulation of 316 mm water equivalent (weq) and 327 mm weq loss by sublimation was deduced by this method. This corresponds to an initial total annual accumulation of 539 mm weq. The annual variability of the accumulation and sublimation is related with the Southern Oscillation Index (SOI): higher net-accumulation during El-Niño years and more sublimation during La Niña years. The deepest part of the ice record shows a time discontinuity; with an ice body deposited under different climatic conditions: 290 mm higher precipitation but with reduced seasonal distribution (+470 mm in winter and –180 mm in summer) and –3°C lower mean annual temperature. Unfortunately, its age is unknown. The comparison with regional proxy data however let us conclude that the glacier buildup did most likely occur after the dry mid-Holocene.

Reconstructing Past Mass Balance and Climate Conditions from a Cerro Tapado Ice Core (Central Andes, Chile)

Reconstructing Past Mass Balance and Climate Conditions from a Cerro Tapado Ice Core (Central Andes, Chile)

Glaciers and Climate in the Andes between the Equator and 30° S: What is Recorded under Extreme Environmental Conditions?

Sublimation and melt disturb the environmental information obtained from ice core records in the Andes. In two case studies we demonstrate to what extent these post-depositional processes may remove major parts of the accumulated snow cover. Dark ash layers from the Tungurahua eruption changed the albedo of surface snow on Chimborazo glacier (6268 m, 1°30′ S,78°36′ W, Ecuador) between two ice core drilling campaigns and forced substantial melt. Re-distribution and washout of the chemical constituents shifted the concentration profiles obtained in December 1999 as compared to an equivalent core drilled in December 2000. The stable isotope records showed that approximately the water equivalent (weq) of an annual layer had melted, and that the percolating melt water penetrated within the firn layer to a depth of at least 16.5 m without refreezing. In the second example, from a site on the dry axis between the tropical and extra-tropical precipitation belts, significant loss of accumulated snow layers occurred by sublimation. A surface experiment at Cerro Tapado glacier (5536 m, 30°08′ S,69°55′ W, Chile) revealed that losses of ≈2 mm weq (≈5 mm snow) per day occurred during the dry period following the 1997/98 El Niño. This loss generally included the entire surface layer enriched in stable isotopes, and thus caused minimal disturbance of the isotopic signature (and hence climatic information) of the net accumulation, yet chemical constituents again experienced considerable changes in concentration. From annual layer counting and direct dating it is obvious that the major part of the accumulated ice on both glaciers is younger than 100 years; however, isotopic and chemical variations at least in the basal ice from Cerro Tapado clearly point to climate conditions different from the recent centuries. This evidence is supported by mass balance considerations derived from a glacier-climate model. The possibility of a third type of disturbance aside from sublimation and melting – in this case a significant hiatus in the environmental chronology – also deserves consideration for other icecore records from this region. Potential disruptions or discontinuities need to be carefully evaluated given the profound changes in climatic and glaciological conditions since the Last Glacial Maximum throughout Holocene times.

Potential for climate variability reconstruction from Andean glaciochemical records

AbstractIn order to select glacier sites suitable for the reconstruction of short-term climate variability, chemical constituents of shallow cores from Chimborazo, Ecuador, Illimani, Bolivia, and Cerro Tapado, Chile, were analyzed to determine if they represent the climate and atmospheric conditions of the particular area. Pronounced variations were observed in the Chimborazo core, which were attributed to the seasonal occurrence of wet and dry periods. Using chemical tracers, the Pacific and the Amazon basin were identified as the two principal sources of atmospheric moisture. In the Illimani record, seasonality is obvious but is less regular than in the Chimborazo core. Chemical tracers point to the Amazon basin as a moisture source. Post-depositional sublimation masks a possible atmospheric signal in the glaciochemical records from Cerro Tapado. the arid conditions and a prevailing high condensation level also cause little variability in the stable-isotope content. Irregular, more negative δ18O values occur during the humid phases of El Niño events. Despite alterations due to sublimation, chemical tracers indicate the Pacific Ocean as a main moisture source for precipitation preserved on Glaciar CerroTapado.

Effects of postdepositional processes on snow composition of a subtropical glacier (Cerro Tapado, Chilean Andes)

In order to study the effects of postdepositional processes on snow chemistry a surface snow experiment was performed on the Cerro Tapado summit glacier in northern Chile, a subtropical glacier site presumably influenced by strong sublimation. Chemical species irreversibly trapped in the snow were significantly enriched in the surface layer by sublimation of the water matrix (Cl−, SO42−, and K+) and by sublimation and dry deposition (Ca2+, Mg2+, Na+, CH3SO3−, and C2O42−). Species present in a volatile form, such as HCOOH, CH3COOH, and HNO3, were released from the snow. This indicates that not only particular meteorological conditions influence the modification of snow composition but so does the chemical form of the studied species (salts or volatile). From the enrichment in concentrations of irreversibly deposited chemical species the sublimation rate was quantified as 1.9 mm weq d−1, which agrees with the sublimation rates determined by lysimeters and by mass balance modeling.

Influence of sublimation on stable isotope records recovered from high‐altitude glaciers in the tropical Andes

Sublimation dominates the ablation process on cold, high‐altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30°08′S, 69°55′W) revealed a strong enrichment in the 2H and 18O content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of δ2H and δ18O help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2–4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.