POLYNOMIAL AND POWER FUNCTIONS FOR GLACIAL VALLEY CROSS-SECTION MORPHOLOGY

An empirical evaluation of glacial trough cross-section shape is performed on seven vertical cross-sections in three Sierra Nevada valleys glaciated during the late Quaternary. Power and second-order polynomial functions are fitted by statistical regression. Power functions are very sensitive to subtle valley-bottom topographic features and require precise specification of the valley-bottom-centre location. This dependency is problematic given under-representation of valley bottoms by conventional contour-sampling methods, and the common alteration of valley-bottom morphology by non-glacial processes. Power function exponents vary greatly in response to these and other non-genetic factors and are not found to be reliable indicators of overall valley morphology. Second-order polynomials express overall valley shape in a single robust function. They are applied to both bedrock- and sediment-floored glacial valleys with negligible statistical bias except where side-slopes are stepped or convex-upward or where valley form is asymmetrical. They can describe alluviated or severely eroded valleys, and can objectively identify indi-vidual components of polymorphic valleys, because valley bottom and centre locations need not be specified. Mathematical expressions of parameters useful for geomorphic measurements and glaciological modelling are analytically derived from the polynomials as functions of the three polynomial coefficients. These parameter equations provide estimates of valley side-slopes, mean and maximum depth, midpoint location, width, area, boundary length, form ratio and symmetry.     

Assessing paraglacial processes at Nexpayantla Gorge (Popocatépetl volcano,Central Mexico) using OSL and 14C

We evaluate the paraglacial activity in Nexpayantla, a subtropical mountainous gorge in Popocatépetl volcano (Central Mexico), fully deglaciated in the 20th century. Glacial advances are evidenced by the presence of moraines. Fluvio-glacial terraces and an alluvial megafan resulted from the gorge deglaciation. Current reworking of the glacigenic material is done by landslides and debris flows produced on the moraines and terraces. To study the different phases of mobilization of glacigenic sediment, we used an approach based on the study of the optically stimulated luminescence (OSL) signals obtained from a portable OSL (POSL) reader in samples extracted from both glacigenic and paraglacial deposits. The luminescence (POSL) results obtained at moraines increase as altitude decreases, which is expected for deglaciated valleys where the oldest moraines are located at lower elevations. We evaluate the grade of luminescence signal reset of the glacigenic sediments during the proglacial stage, and the subsequent deglaciation phases. Our results indicate that there is a marked transition between glacial and fluvially dominated processes at Nexpayantla Gorge. We find that the grade of luminescence signal resetting in the paraglacial deposits is a good indicator to trace paraglacial stages and the beginning of exhaustion of the paraglacial activity in mountain areas. OSL ages confirm that the oldest fluvio-glacial terraces found at the middle sector of Nexpayantla Gorge are ~2 ka, which is also supported by an AMS ¹⁴C age. OSL dating was found challenging, since quartz grains have low sensitivity because of their volcanic origin; POSL signals, however, are in good agreement with the location and distribution of geomorphic markers. We propose that luminescence data obtained from the POSL unit can be useful to provide information about sediment mobilization in paraglacial environments during different climatic pulses – even for the case where mineral grains have low sensitivity, such as in volcanic sediments. © 2020 John Wiley & Sons, Ltd.     

Paraglacial modification of slope form

The morphological consequences of paraglacial modification of valley-side drift slopes are investigated at six sites in Norway. Here, paraglacial slope adjustment operates primarily through the development of gully systems, whereby glacigenic sediment is stripped from the upper drift slope and redeposited in debris cones downslope. This results in an overall lowering of average gradient by up to 4·5° along gully axes. In general, slope profile adjustment appears to be characterized by a convergence of slope profiles towards an ‘equilibrium form’ with an upper rectilinear slope gradient at 29°± 4° and a range of concavities of approximately 0·0 to 0·4. After initial rapid incision, further gully deepening is limited, but gullies become progressively wider as sidewall gradients decline to c. 25°, after which parallel retreat appears to predominate. The final form of mature paraglacial gully systems consists of an upper bedrock-floored source area, a mid-slope area of broad gullies whose sidewalls rest at stable, moderate gradients, and a lower slope zone where gullies discharge onto the surfaces of debris cones and fans. Some gullies appear to have attained this final form and have stabilized following exhaustion of readily entrainable sediment within decades of gully initiation. At most sites, paraglacial activity has transformed steep drift-mantled valley sides into gullied slopes where an average of c. 2–3 m of surface lowering has taken place. At the most active sites, these average amounts imply minimum erosion rates averaging c. 90 mm a⁻¹ since gully initiation, which highlights the extreme rapidity of paraglacial erosion of deglaciated drift-mantled slopes. Copyright © 1999 John Wiley & Sons, Ltd.