South African Continental Shelf Research Articles

A middle Miocene to Quaternary sedimentary and palaeoenvironmental record from the western continental shelf of South Africa

The sedimentary record of the western South African continental shelf is condensed compared to the continental slope and contains erosional unconformities, owing to periods of non-deposition, eustatic sea-level fluctuations, episodic uplift and intensified continental aridity. Despite this, the sedimentary record of the continental shelf provides important information on the depositional history and palaeoenvironmental evolution of the region. A core retrieved from the western shelf of South Africa was analysed for its sedimentary composition, lithological variation, foraminiferal content and its relation to the palaeoenvironment of the region. Four depositional facies were identified along the core, namely quartzitic sand, sandy mud, and glauco-phosphatic sand and a glauco-phosphatic gravel. The basal facies consisting of quartzitic sand is interpreted to have been deposited between 15.90 and 14.60 Ma, corresponding to the timing of the Mid-Miocene Climatic Optimum (MMCO). The highly quartzitic nature of the sediments indicate a high terrestrial influence from fluvial sources. The overlying sandy mud facies was deposited between 14.60 and 13.90 Ma based on planktic foraminiferal biostratigraphy. Foraminiferal analyses of these two facies that were deposited in the Langhian stage of the middle Miocene point to subtropical sea surface conditions and mesotrophic benthic environments. Sea level was noticeably higher during the MMCO and part of the cooling period following the MMCO. An erosional surface that spans 10.77 Myr, equal to the late Miocene (13.90 Ma) to early Pliocene (3.13 Ma), marks the boundary between the two Langhian facies and the overlying two Pleistocene facies, consisting of coarser grained glauco-phosphatic gravelly sand units. The Pleistocene environment on the shelf is interpreted to contrast with the Langhian environment, where cooler, shallower conditions and a more eutrophic benthic environment was prevalent, during a time that Benguela upwelling intensified with higher frequency and higher amplitude sea level fluctuations. Palaeobathymetric interpretations indicate that middle Miocene sea-level in the region were up to 77 m higher than present day and 101 m lower in the Pleistocene, in-line with previous global studies. Glauco-phosphatic content that increase up-core also marks the shallowing of the environment under high productivity conditions.

Bathymetry of the South African continental shelf

Bathymetry of the South African continental shelf

Shallow seamounts represent speciation islands for circumglobal yellowtail Seriola lalandi

Phenotypic plasticity in life-history traits in response to heterogeneous environments has been observed in a number of fishes. Conversely, genetic structure has recently been detected in even the most wide ranging pelagic teleost fish and shark species with massive dispersal potential, putting into question previous expectations of panmixia. Shallow oceanic seamounts are known aggregation sites for pelagic species, but their role in genetic structuring of widely distributed species remains poorly understood. The yellowtail kingfish (Seriola lalandi), a commercially valuable, circumglobal, epipelagic fish species occurs in two genetically distinct Southern Hemisphere populations (South Pacific and southern Africa) with low levels of gene-flow between the regions. Two shallow oceanic seamounts exist in the ocean basins around southern Africa; Vema and Walters Shoal in the Atlantic and Indian oceans, respectively. We analysed rare samples from these remote locations and from the South African continental shelf to assess genetic structure and population connectivity in S. lalandi and investigated life-history traits by comparing diet, age, growth and maturation among the three sites. The results suggest that yellowtail from South Africa and the two seamounts are genetically and phenotypically distinct. Rather than mere feeding oases, we postulate that these seamounts represent islands of breeding populations with site-specific adaptations.

A trawl survey of seafloor macrolitter on the South African continental shelf

Demersal trawls provide an index of seafloor macrolitter abundance, but there are no published data from sub-Saharan Africa. We collected litter items from 235 trawls conducted to assess fish abundance off South Africa. Only 17% of trawls contained litter (3.4 items·km−2, 2.1 kg·km−2 but only 0.2 kg·km−2 excluding four megalitter items). Plastic items predominated (88%), of which 77% floated once cleaned of epibionts. One LDPE bag manufactured three months before being caught carried pelagic goose barnacles Lepas anserifera, confirming that biofouling leads to rapid sinking of floating plastics. Fishery/shipping wastes comprised 22% of litter items (98% by mass; 73% excluding megalitter items); the remainder was general waste – mostly packaging or other single-use items – that could come from land- or ship-based sources. Litter was more abundant in deep water close to Cape Town. The annual demersal trawl survey is a useful way to monitor seafloor litter off South Africa.

CO2 fluxes in the South African coastal region

Abstract The air-sea exchange of CO 2 , its distribution and trends in the South African continental shelf over 8 years (2005–2012) has been studied using data from 28 journeys of a volunteer observing ship (VOS). Sea water properties, strongly controlled by the oceanographic dynamic, showed a complex distribution pattern between Cape Town and Durban. Three different upwelling cells were identified, which dominated the regional variability, together with the presence of eddies, filaments and the core of the Agulhas Current. From west to east the variation in CO 2 fugacity ( f CO 2 ) followed the temperature increase, resulting in oversaturated (> 600 μatm) or undersaturated (~ 215 μatm) waters. After removing seasonality, seawater f CO 2 decreased while sea surface temperature (SST) and salinity did not show a clear trend. From 18°25′E to 20°00′E (the west region), surface water was undersaturated and presented an averaged flux of − 4.27 ± 0.07 mol m − 2 year − 1 . The south region (20°00′E-25°30′E) reached a value of − 4.39 ± 0.08 mol m − 2 year − 1 , while between 25°30′E and 28°30′E (the south-east region) the averaged flux was − 5.03 ± 0.11 mol m − 2 year − 1 . Between 28°30′E and 31°15′E (the east region) the flux was the lowest (− 1.65 ± 0.04 mol m − 2 year − 1 ). The full studied area acted as a sink with a mean value of − 3.83 ± 0.04 mol m − 2 year − 1 , decreasing at an annual rate of 0.13 ± 0.16 mol m − 2 year − 1 from 2005 to 2012 related to the highest rate of increase in f CO 2,sw over that in f CO 2,atm . The estimated carbon uptake by the coastal region with a surface area of 99 × 10 9 m 2 was − 4.6 ± 0.1 TgC year − 1 .

Sea-air carbon dioxide fluxes along 35°S in the South Atlantic Ocean

The oceans play an important role in absorbing a significant fraction of the atmospheric CO2 surplus, but there are still uncertainties concerning several open ocean regions, such as the under-sampled South Atlantic Ocean. This study assessed the net sea-air CO2 fluxes and distribution of sea-surface CO2 fugacity (fCO2sw) along the 35°S latitude in the South Atlantic, during 2011 spring and early summer periods. Underway CO2 molar fraction, temperature, salinity and dissolved oxygen measurements were taken continuously from South American to South African continental shelves. Values of both satellite and discrete in situ chlorophyll-a concentration along the ship's track were used as ancillary data. Both fCO2sw and difference in sea-air fugacity (ΔfCO2) showed high variability along the cruise track, with higher values found on the continental shelf and slope regions. All ΔfCO2 values were negative, implying that a sinking process was occurring during the cruise period, with an average net CO2 flux of −3.1±2.2mmol CO2 m−2 day−1 (using Wanninkhof, 1992). Physical variables were the main drivers of fCO2sw variability in South American continental shelf and open ocean regions, while the biological factor dominated the South African continental shelf. Algorithms for estimating fCO2 and temperature-normalized fCO2 were developed and applied separately to the three defined sub-regions: the South American shelf, the open ocean and the South African continental shelf, with the regional temperature-normalized fCO2 models showing better results.

Submerged shorelines and landscape features offshore of Mossel Bay, South Africa

Abstract Coastal geomorphic systems have been studied widely to understand the responses of shorelines to fluctuating sea levels. Submerged shorelines, remnant of Pleistocene sea-level lowstands, are well preserved on the South African continental shelf. This paper describes work undertaken to better understand offshore coastal environments now submerged by high sea levels off the South African south coast near Mossel Bay, offshore of the Pinnacle Point archaeological locality. Multibeam bathymetry and side-scan sonar reveal evidence of past sea-level fluctuations and submerged coastal landscape features on the seabed. These results form the basis of an ongoing palaeoenvironmental reconstruction for this part of the shelf. We describe seven significant geomorphic features that show a submerged environment that differs significantly to the immediate adjacent coastal plain. However, these features are comparable to other stretches of the present South African shoreline that serve as modern analogues. We propose that features on the continental shelf primarily reflect geological substrate, gradients and Pleistocene sea-level fluctuations. Early modern humans were likely to have had a different set of resources to use in this Pleistocene landscape compared to those available along the presently exposed coast.

Testing the alkenone D/H ratio as a paleo indicator of sea surface salinity in a coastal ocean margin (Mozambique Channel)

Reconstructing past ocean salinity is important for assessing paleoceanographic change and therefore past climatic dynamics. Commonly, sea water salinity reconstruction is based on planktonic foraminifera oxygen isotope values combined with sea surface temperature reconstruction. However, the approach relies on multiple proxies, resulting in rather large uncertainty and, consequently, relatively low accuracy of salinity estimates. An alternative tool for past ocean salinity reconstruction is the hydrogen isotope composition of long chain (C37) alkenones (δDalkenone). Here, we have applied δDalkenone to a 39ka sedimentary record from the Eastern South African continental shelf in the Mozambique Channel, close to the Zambezi River mouth. Despite changes in global seawater δD related to glacial – interglacial ice volume effects, no clear changes were observed in the δDalkenone record throughout the entire 39ka. The BIT index record from the same core, which provides information on relative contributions of soil organic matter (OM) vs. marine input, indicates high soil OM input during the glacial and low input during the Holocene. This suggests a more pronounced freshwater influence at the core location during the glacial, resulting in alkenones depleted in D during that time, thereby explaining the lack of a clear glacial-interglacial alkenone δD shift. The correlation between the BIT index and δDalkenone during the glacial period suggests that increased continental runoff potentially changed the growth conditions of the alkenone-producing haptophytes, promoting coastal haptophyte species with generally more enriched δDalkenone values. We therefore suggest that the application of δDalkenone for reconstructing past salinity in coastal settings may be complicated by changes in the alkenone-producing haptophyte community.

Investigation of CO2 storage potential of the Durban basin in South Africa

Abstract The mitigation of anthropogenic carbon dioxide (CO2) emissions through technologies such as Carbon Capture and Storage (CCS) has been internationally identified as one of the major technical approaches that can be used to combat global climate change in fossil fuel dominated countries. The South African Centre of Carbon Capture and Storage (SACCCS) was established in 2009 to investigate the potential for CCS in the country. Results from a country-scale assessment of CO2 storage potential suggest that South Africa has a theoretical storage capacity of ∼150 Gt, of which 98% occurs offshore in Mesozoic sedimentary basins preserved on the submerged continental shelf. This paper will discuss the progress associated with the ongoing investigation of CO2 storage potential in the 10 000 km2, offshore Durban basin on the east coast of South Africa. With the use of existing data and information the geological development and CO2 storage suitability of the offshore Durban basin is assessed. The basin is structurally complex, hosting a number of horst and graben structures but is dominated by the Tugela Cone, a Tertiary-age, deep-water fan complex that is located seaward of the continental shelf. Although not formally given formation status, sequence stratigraphic correlation can be applied to the Jurassic to Cretaceous succession, separating it into four formations based upon onshore sequences in the Zululand basin to the north. Formations are namely; Syn-Rift, Makatini, Mzinene, St Lucia. Cenozoic sediments range in thickness from ∼1300 m to ∼2000 m comprising a variety of lithologies that cap the sequence. Re-analysis of existing legacy 2D seismic and exploration borehole data has been undertaken in the context of CCS to assess the CO2 prospectivity, geological evolution, and depositional architecture of this portion of the South African continental shelf.

First record of a vagrant Commerson’s dolphin, Cephalorhynchus commersonii, at the southern African continental shelf

Commerson’s Dolphins Cephalorhynchus commersonii are distributed in two disjunct populations, one around southern South America and the other around the sub-Antarctic Kerguelen Islands. These populations have been shown to be morphologically and genetically distinct and movement between the two populations is unlikely. We report on the sighting of a single specimen over the South African continental shelf on 13 November 2004. To our knowledge this is the first record of this species in South African waters and such a vast distance (over 4000 km) from the known distribution limits.

First record of a vagrant Commerson's dolphin, Cephalorhynchus commersonii, at the southern African continental shelf

Commerson's Dolphins Cephalorhynchus commersonii are distributed in two disjunct populations, one around southern South America and the other around the sub-Antarctic Kerguelen Islands. These populations have been shown to be morphologically and genetically distinct and movement between the two populations is unlikely. We report on the sighting of a single specimen over the South African continental shelf on 13 November 2004. To our knowledge this is the first record of this species in South African waters and such a vast distance (over 4000 km) from the known distribution limits.

Stock separation and life history of Argyrozona argyrozona (Pisces: Sparidae) on the South African east coast

Fishery independent biomass surveys and commercial linefish catch returns were used to elucidate the spatial patterns of carpenter Argyrozona argyrozona distributed along the South African continental shelf. Two distinct areas of abundan ce ere determined, one on the central and the other on the eastern Agulhas Bank. Tagging studies revealed little exchange between them. Nurseries were identified in Algoa Bay on he eastern Agulhas Bank and on the central Agulhas Bank (CAB). Early juveniles (<100mm total length) on the CAB were found offshore in the vicinity of the Alphard Bank. They were found to move inshore with growth and then back offshore as they approached maturity. Juveniles in Algoa Bay dispersed both eastwards and westwards with growth. Otoith readability and growth rates varied between regions, with fish from the Eastern Cape (Port Elizabeth and Port Alfred combined) having the lowest average percentage error (4.82 vs 5.33 and 7.03) and the slowest growth rates. Size-at-50 % mturity (L50) varied regionally, female fish in the Eastern Cape maturing at a smaller size (L50 = 206mm fork length) than in the Tsitsikamma National Park (L50 = 292mm) or the CAB (L50 = 267mm). Mass-at-length varied between regions, with carpenter in the Park having the highest mass-at-length and those in the Eastern Cape having the lowest. Based on the distribution of carpenter, variability in otolith readability, mass-at-length, variation in growth and size-at-maturity, it is concluded that carpenter exist as two separate stocks and should be managed accordingly.

Species identification of pelagic fish schools on the South African continental shelf using acoustic descriptors and ancillary information

Stock assessment of exploited pelagic species often relies on hydroacoustic surveys, but a major limitation of this approach stems from an inability to distinguish between the echoes of co-occurring species. We make acoustic measurements of morphometric, energetic, and bathymetric features of anchovy (Engraulis capensis), sardine (Sardinops sagax), and round herring (Etrumeus whiteheadi) schools, to test the hypothesis that schools of each species can be distinguished on the basis of such acoustic descriptors. School measurements were extracted using commercially available software from acoustic data collected by a vertical echosounder during trawling operations of pelagic stock surveys of the South African continental shelf, November 1997, 1998 and 1999. Principal components analysis of 18 school descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples suggested that schools of the three species differed in energetic features and position in the water column, but not in morphometrics. Discriminant function analysis indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). Sardine were distinguished from anchovy and round herring primarily on the basis of school depth and energy, and anchovy from the two other species based on an index of school altitude. Correct classification was not improved by including the ratios of measurements of school features made at a 65 dB processing threshold relative to measurements made at 60 dB. Inclusion of ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of inter-specific differences in habitat use, as well as in schooling behaviour. 2001 International Council for the Exploration of the Sea

Species identification of pelagic fish schools using acoustic descriptors and ancillary information

It is shown that schools of similarly sized and behaving pelagic fish species can be accurately identified to species over a large spatial area, on the order of the broad spatial-scales typical of standard assessment surveys, and over a time-span of three years. Acoustic measurements of morphometric, energetic, and bathymetric features of anchovy, sardine, and round herring schools were extracted using commercially available software from acoustic data collected by a conventional single-frequency narrow-band echosounder during trawling operations of pelagic stock surveys of the South African continental shelf (November 1997, 1998 and 1999). Discriminant function analysis of 18 descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). The three species were differentiated primarily on the basis of school bathymetric position and backscattered energy. Including ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) in analysis improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of interspecific differences in habitat use, as well as in schooling behavior. a)Currently at MIT/Woods Hole Joint Program.

Trends in the abundance and distribution of anchovy and sardine on the South African continental shelf in the 1990s, deduced from acoustic surveys

The South African pelagic resources have been monitored acoustically since 1983. The results of these surveys are currently used to determine catch limits for anchovy Engraulis capensis and sardine Sardinops sagax. Two main surveys are conducted annually, in winter to determine the strength of the year’s recruitment and in summer to estimate the size of the adult stock. The results of the surveys conducted between 1990 and 1997 are presented and discussed in the context of the time-series of stock abundance. Distribution patterns, size structure and biomass levels are related to general trends in the life-history strategies of anchovy and sardine, as well as the major environmental features in the southern Benguela in the 1990s. The biomass of adult anchovy appears to be driven largely by the strength of the early recruitment, corresponding to fish spawned in the first half of the summer spawning season. The biomass of adult sardine, however, is better explained on the basis of the biomass of 2-year-old and older fish in the previous year’s survey. Anchovy tend to move east with age, whereas sardine appear to move both north, along the south African west coast, and east with increasing age.

Determination of Composition and Vertical Structure of Fish Communities Using in situ Measurements of Acoustic Target Strength

A split-beam echo-sounder was used to estimate in situ acoustic target strengths (TS) of fish from a number of different fish communities on the South African continental shelf. The TS and size distributions (obtained by aimed trawling) were used to describe the size structure and vertical distribution of these communities. TS distributions obtained from several monospecific populations of juvenile Cape hake (Merluccius capensis), the dominant species present, were self-consistent, and there was good correspondence between modes in the TS and length distributions of juvenile hake, adult round herring (Etrumeus whiteheadi), anchovy (Engraulis capensis) recruits, pelagic goby (Sufflogobius bibarbatus), horse mackerel (Trachurus trachurus capensis), and ribbon fish (Lepidopus caudatus). Average TS values for all these species, both per individual and normalized by weight, are presented and compared with published values. The use of TS information in studies of the small-scale community structure and dynamics of fish populations is discussed. It is concluded that the method can be effective on low-density, multispecific assemblages such as those in our study, avoiding many of the pitfalls of conventional net sampling.

The role of acoustic surveys in the assessment of pelagic fish resources on the South African continental shelf

Acoustic data on the abundance and distribution of anchovy Engraulis capensis, pilchard Sardinops ocellatus and round herring Etrumeus whiteheadi on the South African continental shelf have been collected from 21 echo-integrator surveys between 1984 and 1991. Most effort has been concentrated on estimating adult biomass of anchovy and pilchard in November (spring) and anchovy recruitment in autumn. Distribution maps from all surveys are presented and the biomass estimates considered most reliable documented. A series of distribution maps tracing movements of three anchovy year-classes over a four-year period is presented to illustrate the usefulness of the surveys in migration studies. The major findings of the survey programme have been that anchovy are generally considerably more abundant and widespread than was thought to be the case prior to the surveys, that the pilchard resource has recovered substantially in recent years, and that the round herring resource, about which little was known prior to th...

Geophysics in oil exploration

The reflection seismic technique is the most widely used geophysical method world-wide, being used mainly in oil exploration. It is an echo-sounding technique fo r mapping the tectonic structure o f layered sedimentary rocks. Seismic waves are induced in the ground, typically fro m an explosive source, and they are reflected at each geological boundary where there is a change in acoustic impedance (acoustic impedance is the product o f density and seismic velocity). The reflected waves are detected at the surface using geophones on land, or hydrophones at sea. The outputs fro m the detectors are recorded on digital magnetic tape and are then computer-processed. A field technique called common-depth-point recording is used so that noise and multiple reflections can be cancelled by “stacking”, which is carried out during the computer processing. In the computer various types o f filters are applied to the seismic results to remove noise, to improve resolution, and to correct geometrical distortion which occurs in areas o f complex tectonic structure. The final output is displayed as a seismic record section, which is made up o f many individual seismic traces, and resembles a cross-section through the earth, showing the structure o f the sedimentary rocks. Very high-quality reflection seismic results have been obtained on the South African continental shelf, and have been extremely valuable in siting oil exploration wells.

The Cape Fold Belt of South Africa; tectonic control of sedimentation

South Africa's Cape Fold Belt is a little known orogen involving sedimentary sequences of Palaeozoic and early Mesozoic age, deformed in at least two orogenic pulses with a climax in the middle of the Triassic. The relationship between pre-, syn-, and post-orogenic sedimentation and the fold belt is outlined and placed into a broad perspective. Among the principal conclusions, the following are of particular significance: i. only the northern foreland area of the fold belt is available for study on land in South Africa—the greater part of the fold belt is submerged beneath the waters of the South African continental shelf and those of the Falklands Plateau, ii. given these restrictions, the known features of the fold belt (excluding the anomalous Cedarberg foldings) are compatible with the view that it results from conventional plate tectonic processes, iii. the north-south striking Cedarberg branch of the western Cape appears to be independent of the main fold belt, iv. a conformable transition from flysch-like to molasse-like facies is clearly preserved, v. the outpouring of the extensive tholeiitic basalts of the Drakensberg volcanics may possibly be related to a late stage in the history of the fold belt rather than to the effects of the breakup of Gondwanaland, vi. the later Mesozoic sediments within the fold belt are closely comparable with the sediments of the Newark-type basin model of Potter & Pettijohn.

Seismic refraction observations in the Transkei Basin and adjacent areas

Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.