Anthropogenic trigger for Late Holocene soil erosion in the Jebel Toubkal, High Atlas, Morocco
W.J. Fletcher1* & P.D. Hughes1
1Quaternary Environments and Geoarchaeology, Geography, School of Environment, Education and Development, The University of Manchester, Manchester, M13 9PL, UK
*Corresponding author, email@example.com
The Assif nImserdane valley, located in the Jebel Toubkal area of the High Atlas, Morocco, is a highly dynamic geomorphological setting. The valley was glaciated during the Late Pleistocene, and subsequently experienced a catastrophic rock avalanche leading to the formation of one of the largest mass movement landforms in North Africa. Recent research (Hughes et al., GSA Bulletin 126: 1093-1104) has dated the formation of the rock avalanche to the mid-Holocene at 4.5 0.5 ka. In this paper, we examine the sedimentological (organic matter content, magnetic susceptibility, particle size and XRF) and palaeoecological (pollen and spores, non-pollen palynomorphs (NPPs), microcharcoal and conifer tracheid fragments) record of a small infilled basin located adjacent to a Late Pleistocene moraine and close to the rock avalanche in the Arroumd sector. The deposits, primarily fine-grained and minerogenic with a low concentration of organic microfossils including pollen, coprophilous ascospores, wood and charcoal microfragments, are enriched in fine silts and ferrimagnetic minerals, consistent with erosional sources from surrounding slope soils. Three radiocarbon dates from the deposit indicate that the infill event occurred during the first millennium AD (after 430 - 640 AD). As such, the deposits point to a phase of slope instability and erosion that is not linked to either deglaciation processes or to the mid-Holocene rock avalanche. Instead, the nature and timing suggest that an anthropogenic trigger of degradation to the natural vegetation cover may be implicated, consistent with an increasing scale and intensity of pastoral activity in the southern High Atlas during the early Islamic period in Morocco. The record casts light on a previously undocumented phase of landscape instability in the dynamic setting of the Assif nImserdane valley, and highlights the potential for further exploration of small infilled basins in the High Atlas to illuminate the geoecological history of the region.
High Atlas, pollen, non-pollen palynomorphs, microcharcoal, anthropogenic impact, soil erosion
The Assif nImserdane in the Toubkal sector of the High Atlas, Morocco, represents a dynamic mountain landscape shaped by multiple forcing factors on Quaternary timescales. The understanding of the nature, timing and drivers of change in the High Atlas is a research priority, as for other Mediterranean and semi-arid mountain regions (Regato and Salmon, 2008; Garca-Ruiz, 2010). The Toubkal sector was the setting for some of the largest Pleistocene glaciers and ice fields in North Africa (Hughes et al., 2004; 2011; Hannah et al., 2016). The Assif nImserdane is also well-known as the location of one of North Africas largest mass movements and associated deposits, the Arroumd landform. Recent investigation employing cosmogenic isotope dating has demonstrated that the Arroumd rock avalanche events occurred during the mid-Holocene, ca. 4500 years ago (Hughes et al, 2014). Moraine features within the Assif nImserdane also yield similar mid-Holocene ages, despite corresponding altitudinally to Pleistocene moraine deposits in other locations of significantly greater antiquity, predating the Last Glacial Maximum (Hughes et al., 2004; 2011). The ages point to significant geomorphological modification of landsurfaces and landforms associated with the rock avalanche event and the inferred tectonic (earthquake) forcing.
To date, there are relatively few sedimentological or palaeoecological records from the High Atlas that can provide longer-term climatological or ecological context for the mid-Holocene Arroumd landslide. The most recent record is from Oukaimeden (2725 m, 31.2038N, 7.8566W; Figure 1) just 10 km to the northeast of Arroumd, where pollen, non-pollen palynomorph (NPP) and macrocharcoal data obtained from marsh and archaeological deposits span the last 4000 years (Ruiz et al., 2015). The study builds on pioneering work by Reille (1976). The composite pollen record derived from several clustered sites suggests increasing anthropic pressure through grazing and fire, against a backdrop of fluctuating hydrological conditions. Earlier studies have been limited by lack of dates, poor resolution and the limited length of the sediment record. For example, a pond at Tighaslant (2197 m a.s.l., 31.4368N, 7.4688W; Figure 1) just northwest of the Tizi nTichka (Bernard & Reille, 1987; Reille & Pons, 1992) provided one of the few pollen sequences for the High Atlas until the work of Ruiz et al. (2015). The Tighaslant record was from a short (