Marine Biodiversity in South Africa: An Evaluation of Current States of Knowledge
The two currents that meet at the southern tip of Africa are the cold Benguela The region stretching from southern Mozambique to Port St Johns is referred to as on the east coast, the fish on the west coast are usually silver or yellow- brown. The cold Benguela Current originates from upwelling of water from the cold depths of the Atlantic Ocean against the west coast of the continent. The two currents do not "meet" anywhere along the south coast of Africa. The Agulhas Current /əˈɡʌləs/ is the western boundary current of the southwest Indian Ocean. Agulhas Current are the East Madagascar Current (25 Sv), the . There are few places in the world where a country is positioned between The region stretching from southern Mozambique to Port St Johns is Off the west coast of South Africa, the cold Benguela Current flows The majority of fish found off the west coast tends to be silver to yellow-brown in colour.
Northward winds along the coast result in Ekman transport offshore and upwelling of nutrient rich deep water to the euphotic zone. The intensity of the upwelling event is determined by wind strength.
Pulses of upwelling induce biological production. In the Benguela system, phytoplankton growth requires a period of upwelling followed by a period of stratification and relatively calm waters. The phytoplankton bloom usually lags the upwelling event by 1 to 4 days and blooms for 4 to 10 days.
In order for zooplankton to have a continuous food supply, the phytoplankton blooms must not occur too far apart. Pulses of upwelling in the Benguela system regularly have a duration of 10 days, an optimal period for biological production. It is estimated that the annual new production in the Benguela system is 4. The Benguela oxygen minimum zone starts around a depth of m and is a few hundred meters thick.
Bacteria that use sulpher rather than oxygen reside in the oxygen minimum zone. Sardinops ocelata pilchard was intensely fished beginning in the s and peaking in with landings over 1. Since then, the Sardinops fishery has declined and the Engraulis capensis anchovy fishery has taken over. This local process together with the remote signal from the equatorial regions form the basis of the formation mechanism in which both processes sometimes reinforce each other.
A full synthesis of the paleoceanographic evidence is yet to emerge, but several aspects of the Agulhas paleo-time series reflect broad consensus. Based on published records, Agulhas leakage has been a persistent feature of the past 1. There is also general agreement about evidence for a more northerly position of the STF during glacial times Rau et al. Multiple-proxy data from planktonic foraminifers from sediment Core MD on the Agulhas Bank provide a detailed reconstruction of surface-ocean conditions in the area.
This pattern is consistent with the hypothesis that peak Agulhas leakage occurred during glacial terminations and plausibly aided the AMOC to shift to its full-strength interglacial mode Knorr and Lohmann,; Peeters et al.
A high-resolution faunal record of the accumulation rate of tropical species Globorotalia menardii at ODP Site in the southern Benguela region was used to infer that Agulhas leakage strengthened during every glacial termination of the past 1. Palaeoceanographic records from sediment Core MD However, the cyclicity also suggests that some of the apparent variability in Agulhas leakage could actually be due to changes in the Agulhas Current. Thus, to circumvent the potential problem that ALF Peeters et al.
Using this approach, Caley et al. The composition of terrigenous sediment in core tops of marine sediment cores around South Africa shows significant and systematic provenance variations Franzese et al. In a follow-up study, Franzese et al. The authors of these two studies infer a weaker Agulhas Current and less leakage, as well as an Agulhas trajectory very similar to modern.
These results are partly at odds with models that suggest that if the wind pattern is fixed but its strength reduced, a weaker Agulhas Current results in increased leakage De Ruijter, ; Dijkstra and De Ruijter, ; van Sebille et al.
However, if the winds shift northward and weaken during glacial periods, a weaker Agulhas Current could combine with reduced leakage. The equivocal comparison between data and models calls for further work including multiproxy analyses.
A key question concerning Agulhas leakage is How important are the locations of the STF and westerly wind belts? On the other hand, transient occurrences of subpolar species Neogloboquadrina pachyderma dextral suggest intermittent incursions of cold subantarctic waters in the Agulhas corridor during glacial periods Rau et al.
However, this picture is contrasted by substantial increases of ALF Peeters et al. Additionally, the records of Caley et al. Again the apparent contradictions of different data sets and between data sets and theory call for further work. Multiple-species stable isotope and alkenone data from ODP Siteslightly further north on the Atlantic margin, have also been taken to suggest that Agulhas leakage variability during MIS 11 was directly related to AMOC activity and wind forcing.
Site shows evidence for strengthened leakage at the end of MIS 11 that may have contributed to renewed ice-sheet build-up and the demise of warm conditions Dickson et al. Recent reconstructions detailing the multicentennial- to millennial-timescale hydrographic variability in the Agulhas corridor have been published for the last glacial cycle 80 ka to present; Core MD Dyez et al.
These studies confirm a connection of leakage maxima with glacial—interglacial climate transitions and extend the connections to millennial-scale salinity anomalies that appear to systematically coincide with accelerated warming at the end of the North Atlantic cold phases stadials.
These findings lend credence to earlier hypothetical considerations Lutjeharms, a and numerical models Knorr and Lohmann, ; Weijer et al. However, reanalyses of a freshwater LGM hosing experiment using a fully coupled atmosphere—ocean general circulation model Gong et al. This salt seesaw is analogous to the thermal bipolar seesaw, responding to reduced cross-hemispheric heat and salt exchange during times of weakened AMOC Simon et al.
Indeed, Simon et al. Accordingly, intervals with higher salinity in the Agulhas Current source region cannot necessarily be taken to indicate increased salt exchange through the Indian-Atlantic Ocean gateway into the South Atlantic Ocean, again calling for further study. The combined analysis of these multiple proxies on the same sediments from Expedition and the greater time interval available for study will help to elucidate the apparently contradictory scenarios from individual studies.
Benguela Current | Revolvy
Agulhas Current links with southern African climates and rainfall patterns Southern Africa spans a significant range of climatic and vegetation zones Figure F7 from the low-latitude tropical regime that is primarily under the influence of the Intertropical Convergence Zone ITCZ and Congo Air Boundary CAB to the high southern latitudes that are influenced by the northern extensions of the circum-Antarctic westerly wind belt Tyson, ; Chase and Meadows, Figure F3.
The Zambezi Catchment is the largest riverine catchment in southeastern Africa, with headwaters extending far into the continental interior. Because of the northern position of the African rainfall belt as well as the presence of mountain ranges of the East African Rift zone, annual rainfall amounts are higher across the Zambezi Catchment compared to the neighboring Limpopo Catchment to the south.
Although directly connected with the ITCZ, the convergence zone over tropical southern Africa is decoupled on occasion from the divide between dry trade winds in the east and wet monsoon in the west, as occurring, for instance, over West Africa in summer. Additionally, the interannual variability of precipitation in the historical record of South Africa is antiphased with that of tropical East Africa Goddard and Graham, It is apparent from modern observations that terrestrial precipitation records have very regional patterns, and thus understanding past variability requires records from a range of archives that span this variability.
Global land cover map of Africa. During the Late Pleistocene, shifts in African climate were paced by regional insolation changes driven by orbital precession deMenocal et al. These orbital changes resulted in antiphased summer insolation maxima between the Northern and Southern Hemispheres, driving an interhemispheric temperature contrast resulting in shifts of the ITCZ and leaving an imprint of the summer monsoonal rainfall Kutzbach et al.
Additionally, African climate was influenced by high-latitude forcing predominantly varying on longer obliquity and eccentricity timescales Tuenter et al. Whereas longer term climate evolution of the African continent is documented by a variety of terrestrial and marine records, the history of the northeastern and southeastern tropical Africa e.
Southern African records are sparse, fragmented, and often suffer from a lack of robust age frameworks e. Such information is important because variability is likely to have significantly influenced African biomes during the Pleistocene, and it appears that this has in turn affected the distributions of species as well as human demography Scott et al.
Lake Malawi, the southernmost of the East African Rift lakes in the southern tropics, provides the only available continuous continental climate record of the last 1. After the MPT, the record has strong ky periodicity, with warmer and wetter conditions in interglacial intervals and relatively cool and dry glacial intervals Johnson et al. This discrepancy in C3—C4 plants may be attributed to shifts in source areas within the Zambezi subcatchments, as inferred from magnetic mineral inventories Just et al.
Collectively, the evidence is consistent with the inference that the Zambezi Catchment experienced relatively wetter conditions during MIS 2 and the late Holocene, whereby Heinrich Stadial 1 and the Younger Dryas stand out as intervals of extreme wet conditions. The later interval is synchronous with the African Humid Period in northern Africa. Altogether, Zambezi Catchment records document precession-paced changes in the African monsoonal rainfall belt with enhanced rainfall during periods of increased Southern Hemisphere summer insolation, superimposed by wet conditions during high-latitude Northern Hemisphere abrupt cold events.
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On land the sediment record from Tswaing Crater, a meteorite impact north of Pretoria, provides the longest terrestrial record of climate variability in South Africa, spanning the last ky. Clastic particle size variations from Tswaing Crater have also been used to suggest that cyclic changes in precipitation were linked with the long-term effect of regional insolation changes driven by orbital precession Partridge, et al. X-ray fluorescence elemental data from Simon et al.
These excursions extend those recently identified in similar data from Core CDK, a little farther south, that were linked to periods of early modern human innovation during the middle Stone Age Ziegler et al. Some of the millennial-scale events recorded in the eastern South Africa runoff records are coincident with periods of enhanced dune activity and mobility in the Kalahari and Namib Deserts and are thus consistent with the suggestion of shifted rainfall patterns and wind regimes Stokes et al.
Long-term climate variability in eastern South Africa over the pasty. A particularly intriguing feature of records from both stalagmites in coastal caves in South Africa Bar-Matthews et al. Such variability demands highly resolved spatial and temporal records as well as numerical simulations.
Precession-only sensitivity experiments using a high-resolution, fully coupled ocean-atmosphere model altering the seasonal and latitudinal distribution of solar insolation show that during precession maxima Pmax higher Southern Hemisphere summer DJF insolation causes higher temperatures and lower surface pressure over the Southern Hemisphere, especially over land Figure F9.
Specifically, lower pressure during Pmax over South Africa and higher surface pressure over the southern Indian Ocean results in stronger easterly surface winds blowing toward eastern South Africa Figure F9.
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Over land, convection also increases, resulting in greater precipitation and greater net precipitation precipitation minus evaporation over nearly all of southern Africa during Pmax. The enhanced precipitation during Pmax and associated higher surface runoff is particularly pronounced in the KZN and Eastern Cape province region, compared to other regions further north or west, which may explain the considerable spatial variability observed in different core sites adjacent to land in the southwest Indian Ocean.
Results of idealized sensitivity modeling experiments. Density profile of the LGM ocean The circulation of the deep ocean at the LGM is a crucial piece of the puzzle to understand past variations in CO2 and glacial—interglacial cycles more generally. Thus, to understand the role of the deep ocean in ice age climate variability, we require a more robust assessment of ice age deep-ocean variability, using a suite of tracers characterized by wide-ranging sensitivity to surface forcing.
In the modern ocean, we use the pattern of temperature and salinity to map the distribution of deep-water masses. In addition, these values inform the balance of sea-atmosphere exchange i. It would be ideal to use the same strategy for the ice age ocean.
However, there are few available means to develop a full temperature-salinity plot for the LGM. Schrag and DePaolo and Schrag et al. This strategy has been expanded to include [Cl—], thus allowing an estimate of salinity Adkins et al.
The relative scarcity of the observations stems from the requirements of the approach: Nevertheless, despite the stringent requirements this approach is currently the most direct way to estimate temperature and salinity of the ice age deep ocean.
Temperature and salinity plot for modern and LGM values. Although sparse, the existing data imply that there were larger-than-modern density differences between deep-ocean water masses at the LGM, and most of the ocean was close to the freezing point and therefore differences in density were due primarily to salinity see also discussion in Miller et al. Most surprisingly, the salt gradient in the modern Atlantic Ocean, consisting of fresh southern-sourced waters and salty northern-sourced waters appears to have been reversed at the LGM.
Enhanced density differences between northern- and southern-sourced bottom waters imply greater vertical stratification. A stronger vertical stratification would help explain indications that there was a physically isolated reservoir of CO2 in the deep ocean at the LGM Broecker and Barker, In the years since the original work, analytical measurement of [Cl—] has been improved, several new sampling strategies have targeted increasing throughput, and the data analysis tools used to model the data have been improved.
The next most important step in constraining the temperature, salinity, and density structure of the LGM deep ocean is to obtain depth transects of these properties and just generally more data from different parts of the ocean. To this end, samples taken during Expedition sites ranging from to m water depth will be used to help constrain the processes that could have filled the LGM deep ocean and controlled its circulation.
Objectives Expedition Pliocene—Pleistocene objectives include the following: To establish the sensitivity of the Agulhas Current to changing climates of the Pliocene—Pleistocene in association with transient to long-term changes of high-latitude climates, ocean gateways, tropical heat budgets, and the monsoon system; To determine the dynamics of the Indian-Atlantic Ocean gateway circulation during such climate changes in association with changing wind fields and migrating ocean fronts; To examine the connection between Agulhas leakage and ensuing buoyancy transfer and shifts of the AMOC during major ocean and climate reorganizations during at least the last 5 My; and To address the impact of Agulhas variability on southern Africa terrestrial climates and notably rainfall patterns and river runoff.
Specific scientific objectives include the following: To determine the variability of the greater Agulhas Current system under contrasting climatic regimes: Agulhas Current variability under early Pliocene warm conditions, during the mid-Pliocene expansion of Northern Hemisphere ice sheets, and through the MPT; To establish the role of Agulhas leakage in transient and long-term shifts of the AMOC during these climatic transitions: Additionally, Expedition will complete an intensive interstitial fluids program aimed at constraining the temperature, salinity, and density structure of the LGM deep ocean, from the bottom of the ocean to the base of the main thermocline.
The Natal Valley is a sediment-filled basin between the coast of southeast Africa and Mozambique Ridge.
At its northern end, the shoaling valley passes into the extensive coastal plains of southern Mozambique Dingle et al. Terrigenous sediment supply into the basin is dominated by seasonal discharge from the numerous short but fast-flowing rivers that characterize the drainage system of the Natal coast e. The currents related to this northward flow have led to the formation of a network of elongate, north-south—orientated sediment drifts.
The northern section of the Agulhas Current system, overlying Site U, is extraordinarily stable due to the steep, nearly rectilinear continental shelf that topographically steers the current flow southward de Ruijter et al. Several high-resolution orbital- to millennial-timescale records from shallow-penetration marine sediment cores located near Site U Ziegler et al. In particular, Simon et al. This system can be plausibly linked to the influence of recirculation within the southwest Indian Ocean subgyre, as predicted from physical oceanography e.
Additionally, it has been shown that changes in terrestrial climate hydroclimate are related to regional precession-paced insolation changes and the effects associated with high-latitude abrupt climate forcing Simon et al.
When considered in combination with archaeological records, these records appear to suggest links between climate and human settlement in South Africa during the middle Stone Age Ziegler et al. Our primary objective was to recover a key Pliocene—Pleistocene sedimentary succession, including the early Pliocene warm period, mid-Pliocene expansion of Northern Hemisphere ice sheets, and the MPT, formed under the influence of the upper reaches of the Agulhas Current.
Site U consists of eight holes ranging in penetration depth from 3. Overall, cores were recovered at this site. The advanced piston coring APC system penetrated A total of The total time spent at Site U was 7. Expedition hole summary. Sedimentology The sediment at Site U consists of one lithologic unit.
Nannofossil-rich clay is more common downhole. The composition of these sandy layers is predominantly quartz with carbonate, and many have erosional bases. Sediment includes dark gray mottling that we interpret as bioturbation. The relative intensity of the bioturbation typically increases with depth throughout. Sediment at Site U also includes millimeter- to centimeter-scale green layers that contain pyrite and glauconite based on X-ray diffraction XRD and smear slide observations.
These layers represent diagenetic alterations. Significant drilling disturbance was observed in the first section of most of the cores and in the base of partial-stroke APC cores. Physical properties Despite the relatively homogeneous lithology, physical parameters vary along the entire sequence.
Porosity increases with depth from 0 to This decreasing trend toward the bottom is likely due to compaction. Variations in gamma ray attenuation GRA density reflect the same compaction pattern. This decreasing trend occurs in spite of compaction and thus likely reflects decreasing abundances of clay and magnetic minerals within the sediment. Fine-sand layers found throughout the cored section are clearly identified by peaks in magnetic susceptibility and GRA density values.
Spectral reflectance shows a marked cyclic pattern downhole and broadly corresponds with the trends in color spectrometry values.
Calcareous microfossils show good to moderate preservation in the upper m CSF-A of the site. Below this depth, preservation and fossil group abundances decrease. Evidence for reworking of calcareous nannofossils is found throughout the entire sequence, marked by the persistent presence of early Miocene species. The calcareous nannofossil assemblages are typical of tropical to subtropical settings, although the temperate species, Coccolithus pelagicus, is found throughout the cored interval.
Planktonic foraminifers also record a complete sequence of tropical biozones that span Miocene Biozone M13 to Late Pleistocene Subzone Pt1b.
The planktonic foraminifer fauna is dominated by Globorotalia inflata, which is indicative of transitional environments between subtropical and polar water masses, and in deeper parts of the sequence the assemblage contains elements of the globoconellid group that are typical of subtropical convergence zones.
Benthic foraminifer fauna is diverse and includes numerous suboxic components characteristic of a lower bathyal to abyssal environment. Siliceous microfossils are characteristic of a modern Indian Ocean assemblage and make up a minor component of the mudline sample. Below the mudline sample, sponge spicules are rare to trace until they disappear completely below Sample UAH-CC The age model for Site U is based on the biozonation of calcareous nannofossil and planktonic foraminifers.
Near the bottom there appears to be a higher accumulation rate, but more shore-based work will be required to refine the age model.