Dryland dunes and other dryland environmental archives as proxies for Late Quaternary stratigraphy and environmental and climate change in southern Africa

Abstract

The Namib Desert and the Kalahari constitute the drylands of southern Africa, with the current relatively humid portions of the latter having experienced periodically drier conditions during the Late Quaternary. This study explores the range of dryland archives and proxies available for the past ~190 ka. These include classic dryland geomorphological proxies, such as sand dunes, as well as water-lain sediments within former lakes and ephemeral fluvial systems, lake shorelines, sand ramps, water-lain calcrete and tufa sediments at the interface of surface hydrological and hydrogeological, speleothems and groundwater hydrogeological records, and hyrax middens. Palaeoenvironmental evidence can also be contained within geoarchaeological archives in caves, overhangs and rockshelters. This integration of records is undertaken with the aim of identifying a (or a number of) terrestrial regional chronostratigraphic framework(s) for this time period within southern Africa, because this is missing from the Quaternary stratigraphy lexicon. Owing to a lack of long, near-continuous terrestrial sequences in these drylands, the correspondence between nearby terrestrial records are explored as a basis for parasequences to build this chronostratigraphy. Recognising the modern climatological diversity across the subcontinent, four broad spatial subdivisions are used to explore potential sub-regional parasequences, which capture current climatic gradients, including the hyper-arid west coast and the decrease in aridity from the southwest Kalahari toward the north and east. These are the Namib Desert, the northern Kalahari, the southern Kalahari and the eastern fringes of the southern Kalahari. Terrestrial chronostratigraphies must start from premise that climate-driven environmental shifts may have occurred independently to those in other terrestrial locations and may be diachronous compared to the marine oxygen isotope stratigraphy (MIS), which serves as a global-scale master climatostratigraphy relating to global ice volume. The fragmented nature of preserved evidence means that we are still some way from producing unambiguous parasequences. There is however, a rich record to consider, compile and compare, within which seven broad wetter intervals are identified, with breaks between these inferred to be relatively drier, and some also have proxy evidence for drying. The onset and cessation of these wetter intervals does not align with MIS: they occur with greater frequency, but not with regular periodicity. Precession-paced insolation forcing is often invoked as a key control on southern African climate, but this does not explain the pacing of all of the identified events. Overall, the pattern is complex with some corresponding wetter intervals across space and others with opposing west-east trends. The evidence for drying over the past 10 ka is pronounced in the west (Namib Desert), with ephemerally wet conditions in the south (southern Kalahari). The patterns identified here provide a framework to be scrutinised and to inspire refinements to proposed terrestrial chronostratigraphies for southern Africa. Considering changes across this large geographic area also highlights the complexity in environmental responses across space as we continue to test a range of hypotheses about the nature of climatic forcing in this region.

Provenance of Kalahari Sand: Paleoweathering and recycling in a linked fluvial-aeolian system

“…312different proxies in a range of archives, including aeolian sand dunes, former lake shorelines, pan 313 deposits and fringing lunette dunes, fluvial sediments, tufa carbonates, speleothems, groundwater 314 and rock shelter deposits (animal and human middens and rock art) (table 1 inStone, 2021b). The315 Quaternary dynamics of aeolian-fluvial interactions can be put into context by combining these 316 different archives.…”

Detrital zircon in an active sedimentary recycling system: Challenging the ‘source‐to‐sink’ approach to zircon‐based provenance analysis

“…Linear dune formation may have started as early as ca 4 Ma, and was certainly well‐advanced when dunes were cut by the Auob River at ca 1.0 Ma (Miller, 2014). Subsequent periods of dune development in late Pleistocene and Holocene time are recorded by cosmogenic nuclide and luminescence dating (Matmon et al ., 2015; Thomas & Burrough, 2016; Stone, 2021). At present, dunes are mainly dormant or partially active, with limited remobilization of sand along dune crests (Telfer & Thomas, 2007).…”

“…Lancaster, 1981; Thomas & Shaw, 1991). Dunes may have started forming already in the Pliocene, and have remained intermittently active in the Pleistocene and Holocene (Miller, 2014; Thomas & Burrough, 2016; Stone, 2021). In the area sampled in this study, the dunes have a general north‐west/south‐east orientation, whereas in the south‐westernmost part of the dune field, dunes approach a WNW–ESE direction (Lancaster, 1981; Heine, 1982).…”

“…Lancaster, 1981;Thomas & Shaw, 1991). Dunes may have started forming already in the Pliocene, and have remained intermittently active in the Pleistocene and Holocene (Miller, 2014;Thomas & Burrough, 2016;Stone, 2021) (Hall et al, 2018); WGr -Areas with outcrop of Neoproterozoic Witvlei Group, otherwise not visible on the scale of this map (Prave et al, 2011).…”

Reconstructing rainfall using dryland dunes: Assessing the suitability of the southern Kalahari for unsaturated zone hydrostratigraphies

“…Dryland sand dunes, and particularly linear dunes, have been a focus of palaeoenvironmental reconstruction in drylands for more than 50 years based on their geomorphology and sediment accumulation history (see review by Telfer and Hesse, 2013 and the INQUA Dunes Atlas chronological database (Lancaster et al, 2016)), including in southern Africa (Stone, 2021). When considered as part of the dryland hydrogeological system, they are part of the unsaturated zone (USZ) where sediment pores are only partly saturated, and act as a conduit for moisture to the underlying groundwater table.…”