Chronostratigraphic palaeo-climate phasing based on southern African wetlands: From the escarpment to the eastern seaboard

This paper aims to identify chronostratigraphic palaeo-climatic boundaries based on proxy indications from mountain- and coastal wetlands in eastern South Africa and Lesotho. Phase boundaries were identified from timing of climate change inferred by proxies, as well as regime shifts in climate variability. Sometimes magnitude and/or frequency of change was also considered. Summarizing the common palaeo-climatic indications suggest the following chronostratigraphic climate phases: 25 to 18 ka, 18 to 15 ka, 15 to 11.5, 11.5 to 8 ka, 8 to 5.5 ka, 5.5 to 2 ka and 2 to 0 ka. The most robust boundaries were identified at 18 ka, 15 ka and 2 ka, i.e. these boundaries were supported by several proxies/sites. The other boundaries were less clearly detected from available proxies/sites and should be regarded tentative. The timing of a climate shift often coincides at coast and mountain sites. However, the climate conditions within each chronostratigraphic phase sometimes vary between coast and inland sites. The 25 to 18 ka phase was cool and dry with strong and frequent storms, followed by the ca. 18 to 15 ka period when conditions were less severe but still generally cool and dry. At ca. 15 to 11.5 ka several proxies infer warmer climate, with less winter rains. During 11.5 to 8 ka a general increase in wetness is inferred, followed by warming over the 8 to 5.5 ka phase. Between 5.5 and 2 ka a successive change towards wetter is indicated, although timing differ between sites. After 2 ka generally a more variable climate is seen, often with high magnitude shifts between dry and wet. The data resolution, i.e. the number of available wetland records, increases with time from very low during glacial times, to highest resolution during late Holocene. Geographically, sites in the mountain region are overrepresented compared to coastal sites. A comparison with coastal lake records suggests a more variable climate at coastal sites compared to mountain sites during mid- and late Holocene, although different proxy resolution and methodology cannot be ruled out as an explanation. A case study compares multiproxy records from Drakensberg (Sekhokong, Ntsikeni) and the coast (Mfabeni), discussing advantages and problems associated with proxy-comparisons within and between sites.

Place, space and time: resolving Quaternary records

Principles of place, space and time can frame an understanding of the context and interpretation of Quaternary palaeo-records, and this is particularly the case for the varied proxies used for late Quaternary climate and environmental reconstruction in southern Africa. Place refers to the specific topographic setting or context of any one record, which has implications for the operation of physical processes in the landscape that control the accumulation of different records. Space refers to the spatial scale or footprint of any one record or proxy, and this varies from one proxy to another. Time refers to not only the time period covered by individual records, but also the temporal resolution of the record, which depends on accumulation rates and availability and quality of any radiometric dating. These three principles are discussed specifically in the context of the Quaternary of southern Africa and through the papers that form this special issue, but are also relevant globally. Future research directions in Quaternary research in southern Africa are identified, including opportunities for refining regional chronostratigraphies.

Macroseismic survey of the 6 February 2016 KwaZulu-Natal, South Africa earthquake

On the 6th of February 2016 at 11:00 hours local time (0900 UTC), KwaZulu-Natal was struck by an earthquake of local magnitude ML=3.8. The epicentre of the earthquake was located offshore in the Durban Basin. The earthquake shaking was widely felt within the province as well as in East London in the Eastern Cape province and was reported by various national media outlets. Minor structural damage was reported. A macroseismic survey using questionnaires was conducted by the Council for Geoscience (CGS) in collaboration with the University of KwaZulu-Natal (UKZN) which yielded 41 intensity data points. Additional intensity data points were obtained from the United States Geological Survey (USGS) Did You Feel It? programme. An attempt was made to define a local intensity attenuation model. Generally, the earthquake was more strongly felt in low-cost housing neighbourhoods than in more affluent suburbs.

Late Quaternary deep marine sediment records off southern Africa

High-resolution mapping, sampling and analysis of upper Quaternary southern African continental margin sediments recovered from beyond the Last Glacial Maximum shoreline (>130 m water depth) have expanded our understanding of how marine and terrestrial records are linked over glacial-interglacial climatic cycles. This paper synthesises data currently available from the deep seafloor around southern Africa and, specifically, core sites that demonstrate terrestrial sedimentological connectivity. Several proxies and case studies reveal the evolution of depositional systems, palaeoceanography and palaeoclimate over the last 191 kyr. Hydroacoustic mapping and investigations of submarine canyons have been carried out primarily on the eastern and southwestern margins, while palaeoceanographic productivity and microfossil assemblages have been applied most extensively on the western marine and southern Agulhas Bank. Studies on the western margin indicate that enhanced productivity, less oxygenated bottom waters and reduced marine faunal diversity in the transition to glacial periods, while glacial terminations are associated with reduced productivity and more oxygenated bottom waters. These changes, linked to palaeoceanography and late Quaternary sea-level fluctuations, influence the sedimentary record and sedimentation rates. On the eastern margin, sediment fluxes applied as proxies for rainfall offshore of the Great Kei, Umzimvubu, Limpopo and Zambezi rivers indicate that the southern African climate responds to changes in orbitally-modulated insolation and in particular, to the ~23 kyr precessional cycle, where the proxy records keep pace with this and then diverge at ~80 to 70 kyr. Since the penultimate glacial (Marine Isotope Stage/MIS 6), more humid conditions observed in southern Africa, as the Northern Hemisphere entered phases of rapid cooling, were potentially driven by a combination of warming in the Agulhas Current and shifts of the subtropical anticyclones. Broadly, the sedimentary records reviewed suggest fluctuations in climate and oceanographic circulation that are strongly correlated with the global benthic δ18O record, suggesting sensitivity to high-latitude forcing, and a strong influence of late Quaternary glacial-interglacial cycles despite these marine sites being far-removed from terrestrial environments.

Pollen, charcoal and phytolith records from the Late Quaternary of southern Africa: vegetation and climate interpretations

Although the lack of Late Quaternary pollen, phytolith and charcoal records for southern Africa has been bemoaned by many, there are a surprising number of publications by a relatively small group of researchers. Previous comprehensive reviews covered the research up to 2016 and 2018 so this paper only considers a selection of more recent studies, with a focus on the three types of botanical remains (pollen, phytoliths, micro- and macro-charcoal). The newer works use the traditional approaches of identification of vegetation and the use of modern analogues to reconstruct past climate and relate the results to other works and proxies. Sibudu Cave, with its long record and multi-proxy record is presented as a case study because the vegetation, climate and human behaviour are well integrated together. A more recent and general trend to provide emphasis on identifying climate driving forces and re-interpretation of data have a tendency to obscure the original sound research.

Paradise lost: large mammal remains as a proxy for environmental change from MIS 6 to the Holocene in southern Africa

Analyses of faunal remains are a key means of inferring palaeoenvironmental change. In this paper, the use of faunal remains as a proxy for environmental conditions from Marine Isotope Stage 6 to the Holocene in southern Africa is reviewed. The focus of this review is on large herbivore abundance and how these fluctuate temporally and regionally in accordance with palaeo-climatic shifts. Here, southern Africa is divided into four eco-regions loosely based on climatic, biotic and zoogeographic traits: the Cape Floristic Region, the arid and semi-arid region, the savanna and grassland region, and the wetter eastern region. The relative abundance of large herbivores within these regions are noted, and temporal trends are inferred. On the whole, most eco-regions maintain similar herbivore compositions over time showing the regional ecological resilience of these taxa to local-scale environmental change. Yet some changes in faunal frequencies are apparent. The Cape Floristic Region shows evidence of significant faunal turnover from the Late Pleistocene to the Holocene. Here, grazers are significantly more abundant during glacial periods, probably linked to the terrestrial expansion of the palaeo-Agulhas coastal plain. Shifts in ungulate abundance in the currently xeric central interior, also indicate wetter periods in the Pleistocene. Holocene faunas are generally similar to historic distributions but shifts between xeric and mesic periods are also evident.

Characterising the late Quaternary facies stratigraphy of floodplains in South Africa

South African river floodplains and their alluvial deposits reflect a diversity of geological and geographical drivers. We use a genetic geomorphic classification system originally developed for dryland wetlands to characterise geomorphic processes and potential successions of sedimentary fill for South African floodplains. Using case studies from the literature, we consider differences between alluvial rivers and mixed bedrock-alluvial rivers in the context of macro-scale geomorphic setting, and evaluate the impact of the setting on floodplain persistence and potential as a palaeo-environmental archive. Sedimentary facies associations represented in South African floodplains, including lateral and oblique accretion, channel, channel infill, levee vertical accretion, floodplain vertical accretion and debris flow deposits, are also evaluated. Floodplains of South Africa’s interior are typically mixed bedrock-alluvial as channel beds are set upon or close to bedrock and sediment thickness is limited. By contrast some floodplains in tectonic basin settings have sediment deposits exceeding 30 m in thickness. The resulting rivers are alluvial, and thus able to adjust their width, depth and slope to accommodate changes in discharge and sediment supply. Similarly, coastal floodplain rivers are alluvial due to downcutting during the last glacial maximum and subsequent sedimentary infilling as sea levels rose. When considering the potential of floodplains as palaeoarchives of environmental change, two considerations emerge. First, floodplain stratigraphy is not a response to a single variable due to complex process-form feedbacks. Rather, floodplain stratigraphy is an outcome of both autogenic and allogenic processes. Second, most South African floodplains are zones of sediment recycling, and as such, preservation potential is typically low. Thus, although floodplain settings of the interior may be a few million years old, the sediment within them may be only thousands to tens of thousands of years old. Our review indicates that research has historically focused on meandering river and mixed bedrock-alluvial anabranching river floodplains, while understanding of other floodplain sub-types remains limited.

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

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.

Cenozoic stratigraphy of South Africa: current challenges and future possibilities

The Cenozoic stratigraphy of South Africa has developed over the past 166 years since geological mapping of the region was initiated. The current status of lithostratigraphy and the global chronostratigraphic framework is discussed in the context of the diverse Cenozoic regolith across the region. Geological mapping here utilizes lithostratigraphy to organise the Cenozoic deposits, although some extensive units are characterised informally using lithological descriptors. Although there are no formal biostratigraphic units, the allied use of “Land Mammal Ages” compiled from fossil type sites is described. An analogous archaeological cultural-historical “technocomplex” stratigraphy is outlined to subdivide stone age cultural material commonly associated with Quaternary deposits and has often been used as a relative dating framework. A summary table of Cenozoic regolith is presented, differentiating deposits into their terrain morphologically defined Geomorphic Province context as a means of correlating similar deposits across the subcontinent. For mapping units based on lithological characteristics, the use of lithodemic nomenclature to characterise units in each geomorphic province is proposed as a temporary measure to enhance inter-regional correlation and encourage further research that could lead to formal lithostratigraphic descriptions.