Holocene fluvial valley fill sources of atmospheric mineral dust in the Skeleton Coast, Namibia

Abstract

Western Namibia is a significant global source of atmospheric mineral dust. We investigate the relationship between dust and source sediments, assessing the sustainability of dust flux. Remote sensing studies have highlighted specific ephemeral fluvial systems as important contributors to dust flux, including highlighting sections of valleys that are the origins of dust plumes in the period 2005-2008. Little is known however about the specific within-valley dust sediment sources, particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments, many of which have been reported in the region. As part of a region-wide analysis of aeolian dust flux, we investigate the sediment properties of atmospheric dust samples and valley sediments from the Huab valley, one of the principal regional dust sources. Trapped dust samples contain up to 88% very fine sand and silt when collected samples are disaggregated prior to analysis. Valley fill surface samples comprise 80% very fine sand and silt, and the surface of the modern ephemeral channel 30%. Valley fill sediments were sampled at depths up to 3.6 m below the present surface and reveal Holocene depositional ages from 0.6 ± 0.03 ka back to 9.79 ± 0.73 ka. These sediments contain 30% to 6% very fine sand and silt, with levels decreasing with depth and age. Aeolian bedforms in the valley system (nebkhas on the fill surface and climbing dunes on valley margins) indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces, with sandy bedforms being constructed from the coarser component of the fill sediments. The volume of valley fill sediment suggests dust sourced from Holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system. 10 cm depth subsurface sample, S5 and S6 are from the surface of an inner channel terrace and S7, S8, S9 and S10 are modern channel floor samples.

Influence of sampling approaches on physical and geochemical analysis of aeolian dust in source regions

“…The Huab is one of the most actively dust emitting ephemeral river systems in Namibia according to analyses of remote sensing, with consistent dust emission sites situated within the river at an upstream section associated with fluvial silt terraces, and at downstream stages where there are fine sediment deltaic deposits, as well as from the pans situated to the north and south (Vickery et al, 2013;. Dust emission tests with the PI-SWERL confirmed the high emission potential of the Holocene fluvial silt terraces , with no flood events recorded on these raised fill sediments for the last c.600 years from OSL chronology performed by Thomas et al (2017). In addition, Dansie et al (2017a) further confirmed the emission potential of the alluvial sediments in this system with ground-based measurements from seven stations situated throughout the river valley.…”

Assessing Landscape Dust Emission Potential Using Combined Ground‐Based Measurements and Remote Sensing Data

“…In the case of the stony systems class, a fundamental distinction could be made between stone pavement surfaces dominated by the presence of coarse lag gravel and portions of pavement where microdrainage channels (c. 0.1 m deep) were found (Figures d and e). In turn, the low‐relief alluvial class could also be divided between portions of ephemerally active river channel and valley fill terraces, the latter situated above the channel (Thomas et al, ; Figures f and g). The river valley fill terraces (within alluvial systems) were on average the most emissive landform (geometric mean E f : 0.0651 mg · m 2 · s), followed by the stony systems exhibiting drainage channels (geometric mean: 0.0318 mg · m 2 · s).…”

Is there evidence for global‐scale forcing of Southern Hemisphere Quaternary desert dune accumulation? A quantitative method for testing hypotheses of dune system development

“…When the general relationship of all Kalahari dunefield accumulation peaks is compared to the dust record, interesting observations emerge: AI peaks are broadly antiphase with high ocean dust accumulation. Stuut et al () regard the dust flux record as a proxy for windiness; however, studies of modern dust flux in southern Africa point towards strong associations between atmospheric dust and water‐lain sediments (Thomas et al , ), including antecedent flooding of playa basin (Bryant et al , ) and river valley (Vickery and Eckardt, ) source areas. Thus the dust record (Figure f) may not necessarily be a clear proxy for windiness but for source‐area precipitation, a proposition that is borne out by the strong paralleling of trends with the leaf wax humidity record of Collins et al (2014; Figure d).…”