Browsing by Author "Compton, John S"
Now showing 1 - 8 of 8
Results Per Page
Sort Options
- ItemOpen AccessBathymetry of the South African Continental Shelf(2013) de Wet, Willem Myburgh; Compton, John SSouth Africa has an extensive coastline offshore of which lies the prominent South African continental shelf, a relatively flat extension of the onshore coastal plain. The continental shelf is host to major mineral and petroleum deposits, home to South Africa’s major sea fisheries and full of navigation hazards. Therefore, knowledge of the seafloor features, or bathymetry, of the continental shelf is essential to understanding both its long-term geological evolution and present-day use for resources and navigation. Unfortunately there has been little advancement in our knowledge of the South African continental shelf since the marine studies of the 1970’s and 1980’s which culminated in the “Bathymetry around Southern Africa” map of Dingle et al. (1987). Although bathymetric mapping equipment and techniques have greatly improved during the last few decades, very little high resolution bathymetric data of the South African continental margin are currently available for scientific use, with the majority of the high resolution multi-beam echo-sounding bathymetric surveys being undertaken by privately owned mineral exploration and mining companies (such as De Beers, Alexkor, Petro SA, Petroleum Agency of South Africa, etc.), the Council for Geoscience and the South African Navy and Hydrographic Office. More recent advances in satellite altimetry have had a major impact on ocean floor bathymetric mapping especially in deep ocean areas where the sea surface generally reflects the underlying bathymetry. The Department of Agriculture, Forestry and Fisheries (DAFF) annually collect single-beam echo-sounding data in order to monitor the abundance of fish species along the South African continental shelf and along with that collect seafloor bathymetry as an additional benefit. The aim of this project is to create a detailed bathymetric map of the continental shelf of South Africa by using digital single-beam echo-sounding data collected by the Fisheries Division of the DAFF over the last two decades. The bathymetric dataset of ±7 million single-beam echo-sounding data points was manually processed, gridded and exported to produce a detailed bathymetric map of the entire South African continental shelf between the Orange River mouth and Kosi Bay complemented by Satellite Altimetry data from the ETOPO 1 – 1 Arc-Minute Global Relief Model (Amante and Eakins 2009) for the deep ocean area adjacent to the continental shelf. The single- beam bathymetric data were collected by the F.R.S. Africana II and F.R.S. Algoa vessels using SIMRAD EKS-38, EK 400, EK 500 and more recently the EK 60 single-beam echo-sounders along with a the SIMRAD ES38B split beam transducer. The West Coast and South Coast margins have the greatest bathymetric detail due to DAFF’s Cape Town base of operations, whilst the East Coast margin is less detailed due to fewer research campaigns in this area. The Bathymetric Map of the South African Continental Margin produced in this thesis reveals several new and more detailed bathymetric features. New bathymetric features include the northern extension of the Olifants Valley submarine canyon, details of the rocky inner shelf related to glacial period sea level lowstands, as well as the coast parallel wave cut terraces and palaeo dune ridges on the middle shelf between Cape Seal and Cape Recife. Other prominent bathymetric features such as Childs Bank, Cape Canyon, Cape Point Valley, the offshore submerged river valleys of the Breede and Gouritz Rivers and the east-west trending, basement anticlinal ridges situated at the southernmost extent of the Agulhas Arch were revealed in greater detail by this study. The underlying geology, physical Oceanography, drainage patterns as well as eustatic sea-level fluctuations can all be linked to the bathymetry of the continental shelf, which is why this thesis examines the influences of each of these factors on the seafloor morphology around the South African coastline.
- ItemOpen AccessGlacial-interglacial variations of the water masses in the southeast Atlantic Ocean derived from foraminiferal neodymium isotope ratios(2017) Von Koslowski, Rebecca; Compton, John SVariations in the global climate over time have long been associated with changes in the meridional overturning circulation of the oceans. It is now commonly believed that, during the Last Glacial Maximum (LGM), the transport of North Atlantic Deep Water (NADW) to the southeast Atlantic Ocean was reduced. A popular method to trace these ambient changes is the stable isotope systematics of neodymium (given as εNd). In this study εNd data were measured on mixed planktic and bulk foraminifera from two gravity cores, GeoB8336-6 and GeoB8342-6, retrieved from the Cape Basin at water depths of 3524 and 3521 meters from the western continental slope of South Africa. The samples were prepared following the protocol presented by the Cambridge group (Tachikawa, Piotrowski, & Bayon, 2014). Planktic and bulk foraminifera samples taken from the same core depth interval had the same εNd ratios within error, which suggests that bulk foraminifera may provide a quick way to reconstruct ambient bottom water values. However, more research is needed to further support these findings. While the Holocene samples' εNd ratios (εNd(N36/6a) -10.7±0.3 and εNd(N42/6a mean) -10.2±0.4) lie within the range of modern Eastern NADW (εNd(modern ENADW) -10.9±1.2), glacial samples yield significantly more radiogenic εNd ratios (εNd(N42/6b) -7.7/ εNd(N36/6b) -8.1). This indicates the greater influence of southern-sourced water masses and thus provides further evidence for a reduction of NADW during the last glacial. MIS3 samples show ratios that lie in between those observed in the LGM and Holocene, and it is hypothesized that ocean circulation during MIS3 was comparable to that during the transition from the LGM to the Holocene (Termination I).
- ItemOpen AccessHolocene evolution of the Anichab Pan on the south-west coast of Namibia.(Wiley, 2007) Compton, John SCoastal sediment-filled depressions (pans) are one of the few areas that contain Quaternary records of sea-level and palaeoenvironmental change along the western margin of southern Africa. Anichab is a 128 km2 salt-encrusted pan on the hyper-arid southern coast of Namibia with an emergent, well-preserved and in-place mid-Holocene mollusc assemblage. The molluscs are typical of subtidal sands on the sheltered side of offshore islands but include several warm-water species no longer found living along this coast. The Holocene evolution of the pan was largely influenced by changes in sea level and supply of sand along the coast. Calibrated radiocarbon ages of mollusc shells indicate a maximum Holocene sea level of ca 2 m above mean sea level (msl) from 7Æ0 to 6Æ3 ka and a return to near present-day sea level by 5Æ3 ka. The pan surface is 2 m below msl and has been emergent since 4Æ9 ka from the build up of sandy beaches and coastal dunes. A thin (1–4 cm) halite crust occurs over much of the pan surface but a layer of halite-cemented sand up to 40 cm thick is restricted to the central pan. Gypsum occurs near the subsurface brine interface and is limited by calcium to the edges of the pan. Nodules of calcite-cemented sand are forming in brackish, relatively high alkalinity subsurface waters in the southeast corner of the pan and nodules of aragonite-cemented sand are forming in brines 1 m below the central pan surface. Although modern dolomite has been reported from coastal lagoons of Brazil and Australia, carbonate cements are a minor feature of Anichab Pan and dolomite was restricted to a single reworked nodule most likely of Late Pleistocene age. Therefore, Anichab Pan does not appear to be a modern analogue to extensive, mixed-water dolomite cements found in Upper Pleistocene sediment-filled depressions on the Namibian shelf
- ItemRestrictedThe mid-Holocene sea-level highstand at Bogenfels Pan on the southwest coast of Namibia(Elsevier, 2006) Compton, John SThe radiocarbon ages of mollusc shells from the Bogenfels Pan on the hyper arid southern coast of Namibia provide constraints on the Holocene evolution of sea level and, in particular, the mid-Holocene highstand. The Bogenfels Pan was flooded to depths of 3 m above mean sea level (amsl) to form a large subtidal lagoon from 7300 to 6500 calibrated radiocarbon years before present (cal yr BP). The mollusc assemblage of the wave sheltered lagoon includes Nassarius plicatellus, Lutraria lutraria, and the bivalves Solen capensis and Gastrana matadoa, both of which no longer live along the wave-dominated southern Namibian coast. The radiocarbon ages of mollusc shell from a gravely beach deposit exposed in a diamond exploration trench indicate that sea level fell to near or 1 m below its present-day position between 6500 and 4900 cal yr BP. The rapid emergence of the pan between 6500 and 4900 cal yr BP exceeds that predicted by glacio-isostatic models and may indicate a 3-m eustatic lowering of sea level. The beach deposits at Bogenfels indicate that sea level rose to 1 m amsl between 4800 and 4600 cal yr BP and then fell briefly between 4600 and 4200 cal yr BP before returning to 1 m amsl. Since 4200 cal yr BP sea level has remained within one meter of the present-day level and the beach at Bogenfels has prograded seaward from the delayed arrival of sand by longshore drift from the Orange River. A 6200 cal yr BP coastal midden and a 600 cal yr BP midden 1.7 km from the coast indicate sporadic human utilization of the area. The results of this study are consistent with previous studies and help to refine the Holocene sea-level record for southern Africa.
- ItemOpen AccessNeogene to quaternary foraminifera from the western margin of southern Africa(2019) Bergh, Eugene; Compton, John SThe western margin of southern Africa underwent major palaeoceanographic changes since the initiation of the Benguela Upwelling System during the Neogene. Microfossils in marine sediments provide key proxies in our understanding of how the margin evolved. Fossil shells (tests) of foraminifera (singlecelled protists) from twenty cores from the Namibian shelf (199 to 309 m water depth) and three cores from the western slope (874 to 3631 m water depth) of South Africa were studied to determine the middle Miocene to Quaternary stratigraphy, palaeoenvironment and palaeoceanography of the western margin of southern Africa. Cores from the Namibian shelf recovered middle Miocene calcareous mud in erosional contact with overlying Pliocene to Pleistocene phosphatic sediments. Strontium isotope stratigraphy and planktic foraminifera biostratigraphy provide age control of the Namibian shelf sediments. The planktic indicator species Globoquadrina dehsicens and Globigerinoides bisphericus support strontium isotope stratigraphy results for the olive-green mud unit of the northern Namibian shelf indicating an age of 16 to 14 Ma, and the overlying Plio-Pleistocene age of the phosphorite-rich unit supported by planktic indicator species Globorotalia truncatulinoides and Globorotalia (Globoconella) inflata. Middle Miocene foraminifera reflect a warmer, oligotrophic, subtropical, deeper environmental setting in contrast to the shallower depositional environment, cooler conditions and a eutrophic bottom water setting indicated by Pleistocene foraminifera in the phosphatic units. The palaeoenvironment on the Namibian shelf was progressively shoaling during the Pleistocene as sea level amplitudes increased. An Uvigerina spp.- dominated association occurs in deeper shelf deposits dated to the early Pleistocene and the Ammonia beccarii association occurs in shallower shelf deposits of the late Pleistocene to Holocene. The planktic and benthic foraminiferal stable oxygen isotope records, colour reflectance (L*) and non-carbonate mineral counts provide age control on cores from the western slope of South Africa, whose records extend to just beyond Glacial Termination (GT) II. Sediment and benthic foraminiferal accumulation rates were higher during interglacial periods and lower during glacial periods. The major planktic species in the slope cores include Globorotalia (Globoconella) inflata, Globigerina bulloides and Neogloboquadrina incompta. Principal component analysis (PCA) reveals that the major factors influencing planktic foraminiferal abundances are upwelling intensity, the penetration of colder waters during glacial periods and the inflow of subtropical waters from the South Indian Ocean during interglacial periods. The major benthic species in the slope cores include Uvigerina peregrina, Uvigerina hispidocostata and Cibicidoides wuellerstorfi indicating the delivery of organic matter and oxygen availability to have the largest influence on the benthic foraminiferal faunal composition. Uvigerina spp. on the slope show increased relative abundances during periods of lower oxygen conditions. Bottom water masses identified by Nd (neodymium isotopic compositions) values recorded by foraminifera, along with the stable carbon isotope composition and abundance of the benthic foraminifer C. wuellerstorfi indicate shifts from Southern Component Water to North Atlantic Deep Water during GT II and I. Variation in Nd values in an upper slope core (874 m water depth) indicate Antarctic Intermediate Water (AAIW) influence was stronger during glacial periods compared to interglacial periods.
- ItemRestrictedA reworked mid-Holocene lithic assemblage at Dunefield Midden 1, Elands Bay, South Africa(South African Archaeological Society, 2006) Orton, Jayson; Compton, John SThe stratigraphy and context of open air archaeological sites on the West Coast can be complicated by reworking through strandline migration from relative changes in sea level (Compton and Franceschini 2005) and deflation by strong seasonal winds (Kandel et al. 2003). The Elands Bay area, located 180 km north of Cape Town on the West Coast, is rich in both cave and open air archaeological sites which together offer valuable insights into human utilization of the area. The majority of well-preserved West Coast open air sites are late Holocene in age (<1600 cal BP, calibrated years before present). Although there is increasing evidence of mid-Holocene occupation along several parts of the West Coast (Kaplan 1994; Jerardino and Swanepoel 1999; Jerardino and Yates 1996; Compton and Franceschini 2005; Halkett 2003; Orton and Halkett 2005), such evidence has, until now, not been discovered in the immediate Elands Bay vicinity (Parkington et al. 1988). This is despite more than 30 years of intensive survey and research in the area that has produced significant contributions to both the body of knowledge on Western Cape archaeology and that on coastal hunter-gatherers more generally (e.g. Jerardino 1996; Manhire et al. 1984; Parkington 1977, 1980b, 1988, 1990; Parkington et al. 1988; Sealy and van der Merwe 1986, 1988). The Dunefield Midden 1 (DFM 1) open air site is situated 3.2 km northeast of Elands Bay Cave and 0.5 km inland of the present-day beach in a deflated part of the active Holocene dunefield recently stabilized by alien acacias (32 degrees18'04.9'S: 18 degrees 20'54.0'E; Fig. 1). In situ limpet-dominated shell middens at DFM 1 include several hearths dated to between 770 and 450 cal BP (Parkington et al. 1992; Orton 2004). In this paper, the stratigraphy, several radiocarbon dates and an abraded stone artefact assemblage are interpreted to indicate an earlier, reworked mid-Holocene occupation at the DFM 1 site (Orton 1998, 2002, 2004).
- ItemOpen AccessSand transport along the Western Capecoast: Gone with the wind?(2003) Franceschini, Giuliana; Compton, John S; Wigley, Rochelle ABeach sand sample were collected between Cape Town and St Helena Bay in order to study sediment composition and transport along the Western Cape coastline. Between Cape Town and Saldhana Bay, the beaches are a mixture of terrigenous and carbonate material. Those between Saldanha Bay and Shelley Point are carbonate-rich. North of Shelley Point, terrigenous-rich beaches were found. The decrease in terrigenous material from Cape Town to Saldanha Bay is a reflection of reduced delivery of terrigenous sand by rivers. The low content of terrigenous material in beach sands between Saldanha Bay and Shelley Point is related to the high biogenic CaCO3 in the rocky coastal area. In St Helena Bay the high percentage of terrigenous material is contributed by the Berg River. Coastal dune composition also reflects the mineralogy of the beach sand. This work indicates that the terrigenous material delivered by rivers to False Bay and Table Bay move northwards by longshore drift while being mixed with carbonate shell fragments.
- ItemRestrictedA tenfold increase in the Orange River mean Holocene mud flux: implications for soil erosion in South Africa(SAGE Publications, 2010) Compton, John S; Herbert, Caren T; Hoffman, M Timm; Schneider, Ralph R; Stuut, Jan-BerendSoil erosion poses a major threat to sustainable agriculture in southern Africa but is difficult to quantify. One measure of soil erosion is the sediment flux of rivers. The Orange River is the principal source of sediment to the western margin of South Africa with an estimated mean mud flux over the last 11 500 years (the Holocene epoch) of 5.1 (3.2–7.4) million metric tons/year (Mt/yr). A total of 43 gigatons (Gt; 1015 g) representing 72% of the Holocene mud flux has accumulated on the shelf in the Orange River prodelta and mudbelt, a clayey finesilt deposit focused on the inner to middle shelf. Only 8% (5 Gt) of the mud flux occurs in Holocene calcareous ooze on the slope. Comparison of the clay to mud ratio of offshore deposits with Orange River suspended sediment and catchment soils indicates that 20% (11 Gt) of the Holocene mud flux has been lost as clay beyond the margin. The Orange River mud flux prior to the building of large dams (1930–1969) is ten times greater than the mean Holocene mud flux and is reconciled with estimates of soil erosion within the catchment. A tenfold increase in the Orange River mud flux implies up to a hundredfold increase in total soil erosion depending on the extent of mud storage over periods of decades to centuries within the catchment. Erosion has shifted from areas of high relief and rainfall of the Drakensberg escarpment during the Holocene to intensely cultivated lands of low relief having moderate to high rainfall in the eastern catchment and to a lesser extent, grazing areas of the southern Orange River catchment.