Late Quaternary palaeoceanographic record in giant piston cores off South Africa, possibly including evidence of neotectonism

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Quaternary International 148 (2006) 65–77

Late Quaternary palaeoceanographic record in giant piston cores off South Africa, possibly including evidence of neotectonism Amanda Raua,d, John Rogersb,, Min-Te Chenc a

Department of Oceanography, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa c Institute of Applied Geophysics, National Taiwan Ocean University, Keelung, 20224, Taiwan d Box 97235, Windhoek, Namibia

b

Available online 18 January 2006

Abstract Three giant piston cores were recovered in 1996 from the slope and rise off South Africa by the MARION DUFRESNE. The cores are MD962084, NW of Cape Town, MD962080, SSW of Cape Town and MD962077 off East London in the Natal Valley. MD962084 (35.28 m) contains a detailed record of the Quaternary variability of the Southern Benguela, whereas MD962080 (22.23 m) records latitudinal variations in the position of the Subtropical Convergence. Sediments of hypothermal periods are dominated by Neogloboquadrina pachyderma (right-coiling) and, in MD962080, higher proportions of very fine quartz sand, washed off the continental shelf during lowstands. In contrast, sediments of hyperthermal periods are dominated by Globorotalia inflata and increased amounts of the tropical Globorotalia menardii, which is also found in trace amounts in the hypothermal periods. This shows that Agulhas water transfer never completely ceased, even during equatorward excursions of the Subtropical Convergence during hypothermals. Three major turbidites of quartz sand characterise the Natal Valley core, MD962077 (35.54 m) and they have ages of 510, 340 and 250 ka. If they were triggered by major earthquakes, it is argued that an even bigger earthquake is overdue, 250 ka after the last one. r 2005 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction During 1996, the major, 6-week-long IMAGES-II research cruise of the French research vessel, the MARION DUFRESNE, led to the recovery of 23 giant piston cores, up to 40 m in length from the seafloor off southern Africa (Bertrand et al., 1997; Chen et al., 1998; Rogers, 1999). Three of these giant piston cores are the subject of this paper. Fig. 1 places the three cores in their oceanographic setting. Core MD962084 (henceforth referred to as ORS) lies off the west coast of South Africa on the relatively low gradients of the continental slope off the Olifants River in the Southern Benguela Region (SBR), within reach of coldwater filaments of the highly productive coastal-upwelling system of this major eastern boundary current of the SE Atlantic Ocean. The Benguela Current is considered the most important element of meridional equatorward heat Corresponding author. Fax: +27 21 650 3783.

E-mail address: [email protected] (J. Rogers).

transport in the South Atlantic, transporting warmer waters northward and westward by geostrophic flow as part of the global thermohaline circulation system (Gordon, 1986). The southern boundary of the Benguela system can be considered as the Agulhas Retroflection area (Shannon and Nelson, 1996) where the Agulhas Current turns back on itself (Fig. 1). The Retroflection loop can occlude to pinch off an Agulhas ring that will subsequently drift off into the South Atlantic Ocean, carrying with it its load of South Indian water masses (Fig. 1). The frequency and intensity of Indian Ocean water-mass transfer via Agulhas rings plays a crucial role in global thermohaline circulation (Gordon, 1986; Gordon et al., 1992), affecting Atlantic meridional overturning and, consequently, North Atlantic Deep Water formation (Broecker, 1987, 1991), modifying the intensity and, possibly timing, of glacial– interglacial climatic fluctuations. In contrast, Core MD962080 (henceforth referred to as ABS) lies on the steeper continental slope of the western margin of the Agulhas Bank, SW of the southern tip of the continent of Africa. The core site, in modern times, during the Holocene

1040-6182/$ - see front matter r 2005 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2005.11.007

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Fig. 1. Schematic of the Agulhas Current and Southern Benguela Region. Agulhas rings (1) and filaments (2) are shed at the Agulhas Retroflection (3) and are carried equatorward by the Benguela Current. The rest of the water of the Agulhas Current flows eastward along the Subtropical Convergence (STC) (4). Cool waters of the Benguela coastal upwelling system (BUS) are shown along the west coast of South Africa. AFR ¼ Agulhas Fracture Ridge. Core locations (2084 ¼ Core ORS, 2080 ¼ Core ABS and 2077 ¼ Core NV) are indicated by black triangles. (After Rau et al., 2002).

hyperthermal (interglacial), is within reach of warm-water eddies of the Agulhas Current, the major western boundary current of the SW Indian Ocean. Finally, Core MD962077 (henceforth referred to as NV), lies seaward of the Agulhas Current, in the Natal Valley, due south of Durban, under the influence of warm, subtropical Indian Ocean waters. All the cores, in the Holocene hyperthermal, lie on the equatorward side of the Subtropical Convergence (STC), but it remains to be seen whether this held true during Pleistocene hypothermals (glacials). It has been suggested (e.g. Berger and Wefer, 1996) that the influx of warm saline Indian Ocean water into the South Atlantic was shut off during glacial periods, by a substantial equatorward migration of the STC. 2. Methods The giant piston-cores were recovered with the CALYPSO coring system. Onboard, the casing of each core was cut into 1 m lengths, which were then capped, sealed and labelled. Each length was then analysed by non-destructive methods for sonic velocity, magnetic susceptibility and density variations, before being split in half, described, photographed in colour and tested for colour reflectance (Bertrand et al., 1997). The three cores, ORS, ABS and NV, were subsampled at 10-cm intervals for this study. Core ORS was recovered

from a depth of 1408 m (31144.50 S, 151310 E), Core ABS from a depth of 2488 m (361190 S, 191280 E) and Core NV from a depth of 3781 m (33110.10 S, 31114.80 S). The cores varied in length, from 35.28 m for ORS, to 22.23 m for ABS, to 35.54 m for NV. Each subsample was split into three, for isotope studies (O, C and N), micropalaeontological studies and sedimentological studies. Sixty specimens of the transitional planktonic foraminifer Globorotalia inflata were picked from the 250 to 350-mm fraction from the subsamples of each of the three cores to a depth of 30 m in ORS, to 15 m in Core ABS and to 24 m in Core NV. The oxygen isotope ratios for Core ORS were determined at the Department of Geological Sciences of the University of Bremen, whereas ABS and NV were analysed at the National Taiwan Ocean University in Keelung using standard procedures (Rau, 2002; Rau et al., 2002). Further details of the methodology and the derivation of the age-model have been fully described by Rau (2002) and Rau et al. (2002). Planktonic foraminifera in the 4125 mm fraction were picked from each subsample and, on average, 280 specimens were identified to species level (Rau, 2002; Rau et al., 2002). Separate subsamples were analysed sedimentologically. Each subsample was dried and weighed before wet-sieving through a 63 mm sieve. The coarse fraction, which can also be termed the sand fraction, because no particles exceeded

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2 mm in diameter, was then dried and weighed to obtain the mass of unleached sand and, by subtraction, the mass of unleached mud. From these data, the overall texture of each subsample was obtained. The unleached mud and the unleached sand were then treated separately with dilute ANALAR-grade hydrochloric acid to dissolve the calcium carbonate fraction. The acid-insoluble mud was then allowed to settle out and the supernatant water was decanted to remove dissolved salts. The process was repeated until the supernatant no longer tested acid with litmus paper. The acid-insoluble mud was then dried and weighed and its percentage calculated, both as a percentage of the unleached original subsample and as a percentage of the unleached mud-fraction. The acid-insoluble (leached) sand fraction was wet sieved, dried and weighed to obtain the mass of insoluble sand. The mass percentage of acid-insoluble sand was then calculated, both as a percentage of the original unleached subsample and as a percentage of the unleached sand fraction. The acid-insoluble sand fractions were then examined under a binocular microscope to identify the components. In the case of ABS (Rau et al., 2002), it was decided that there were sufficient numbers of biogenic siliceous components, namely benthic sponge spicules and planktonic radiolaria, to justify floating them off from the

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dominant quartz grains. This was achieved using a sodium polytungstate solution with a specific gravity of 2.5, just less than 2.65, the specific gravity of quartz. In addition, slightly magnetic grains of glauconite were removed using a Franz Isodynamic Magnetic Separator. Therefore, in the case of ABS, the plotted acid-insoluble sand fraction consists almost entirely of quartz grains (Rau et al., 2002, Fig. 4), whereas there are only traces of sponge spicules and radiolaria amongst the dominant quartz grains in the acidinsoluble sand fraction of Cores ORS and NV.

3. Oxygen isotope and micropalaeontological data from ORS and ABS The oxygen isotope data for ORS (Figs. 2 and 3) show that Marine Isotope Stage (MIS) 21.3 was reached at a depth of 30 m (about 838,000 years ago)(838 ka), whereas the data for ABS show that MIS 20.2 (almost 810 ka) was reached in a shorter depth of only 15 m, indicating a mean sedimentation rate of 3.6 cm/ka for ORS, compared to only 1.9 cm/ka for ABS. Off the east coast, in Core NV, MIS 16 (630 ka) was reached in a depth of 24 m (Chen, personal communication, 2003) for a mean sedimentation rate (3.8 cm/ka) slightly higher than that for ORS.

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Fig. 2. Oxygen isotopes versus depth for cores MD962084 (a), MD962080 (b) and MD962077 (c). ORS, Olifants River Slope; ABS, Agulhas Bank Slope; NV, Natal Valley.

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Seventeen species of planktonic foraminifera were picked and counted from the Olifants River Slope core and the Western Agulhas Bank Slope core. They are listed in Table 1 from Polar species at the bottom of the table to Tropical species at the top of the table. The micropalaeontological data of Cores ORS and ABS will now be presented, starting with those of Core ORS (Fig. 1). 3.1. Core ORS Factor analysis for core ORS (Fig. 4) indicates that three faunal components account for 95% of the total variance in the census data. The factor scores are shown in Table 2. Although the same dominant species appear in Core ABS (Fig. 5), to be discussed shortly, the assemblages are somewhat different in Core ORS from beneath the Benguela Current. The varimax factor components are plotted in Fig. 4. The Subantarctic Assemblage (Factor 1) (Fig. 4a) is exclusively defined by N. pachyderma (d) and accounts for 33% of the total variance. This cold end-member of the faunal population is dominant in waters in the vicinity of the STC (Be´ and Tolderlund, 1971) and also in mesotrophic, nutrient-rich upwelled filaments of the Benguela

Upwelling System (BUS) (Giraudeau and Rogers, 1994). The assemblage shows high-frequency fluctuations with no Glacial–InterGlacial (G–IG) cyclicity. There is an increasing trend from the bottom of the record to MIS 17. There is also a change in mean contribution of this factor to the total foraminiferal population, from relatively lower values in the period MIS 16–11 to slightly higher values in the middle part of the core studied (MIS 10–7) to lower values after 200 ka. The Mixed–Intermediate Assemblage (Factor 2) (Fig. 4b) is dominated by Gg. bulloides, with minor contributions from Gr. inflata, Gg. falconensis and Gr. scitula. This factor accounts for 31% of the total variance. Gg. bulloides is a common component of outer-shelf sediments on the seaward side of upwelling cells in the SBR, preferring the relatively lower nutrient levels away from upwelling filaments. Giraudeau and Rogers (1994) reported Gg. bulloides as the ‘‘intermediate’’ factor between upwelling and offshore regions. Gr. inflata defines the transitional zone between warm and cool temperate regions, with lower nutrient content and reduced primary productivity (Be´ and Tolderlund, 1971). Gr. inflata inhabits offshore regions in the southern Benguela (Giraudeau and Rogers, 1994). Gg. falconensis is the warmerwater variation of Gg. bulloides, occurring more frequently in sediments underlying subtropical waters. Gr. scitula is

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Table 1 Summary of main environmental characteristics of the planktonic foraminiferal species counted in core MD962084 (ORS) and MD962080 (ABS) Species

Characteristic

Water mass

Globorotalia menardii (Parker, Jones and Brady, 1865)

Peaks in tropical, equatorial surface waters, 22 1C, 35 potential salinity units (psu). Most abundant species in equatorial, subtropical, oligotrophic Indian Ocean waters, 14–30 1C, low salinity range,
35.5 psu. Most abundant species in tropical waters, 24–30 1C, tolerance for broad range of salinity Temperate to tropical waters, favours cooler waters than ecological variant Gs. sacculifer. Common in tropical productive environments, associated with western boundary currents, warm waters at depth and chlorophyll maximum. More common in mid-low latitudes, mixed-layer species with warm water at depth. Warmer variation of Gg. bulloides, underlies subtropical waters. Temperate, high salinity waters, below thermocline

Tropical

Globigerinoides ruber (alba) (d’Orbigny, 1839)

Globigerinoides sacculifer (Brady, 1877) Globigerinoides trilobus (Reuss, 1850) Neogloboquadrina dutertrei (d’Orbigny, 1839)

Globigerinella calida (Parker, 1962) Globigerina falconensis (Blow, 1959) Globorotalia crassaformis (Galloway and Wissler, 1927) Globorotalia hirsuta (d’Orbigny, 1839) Globorotalia scitula (Brady, 1882) Globorotalia truncatulinoides (d’Orbigny, 1839) Globorotalia inflata (d’Orbigny, 1839)

Globigerinita glutinata (Egger, 1893)

Globigerina bulloides (d’Orbigny, 1826) Turborotalia quinqueloba (Natland, 1938) Neogloboquadrina pachyderma (d) (Ehrenberg, 1861) Neogloboquadrina pachyderma (s) (Ehrenberg, 1861)

Southern transitional zone, minimal stratification, rare. Sparse, but widespread, cool deep waters (4100 m), high occurrence in plankton near Agulhas Current. Oligotrophic, high salinity waters, deep-dweller. Dominant in southern transitional zone north of STC, deep dweller (
200 m), low nutrient content, reduced primary productivity. Widespread, cosmopolitan, abundant under cool (10 1C) and warm (28 1C) conditions with deep-mixed surface layer adjacent to upwelling zones. Mixed subpolar–subtropical waters, productive environments offshore of upwelling cells. Intermediate-deep waters, limited water stratification. Cool, mesotrophic, high nutrient levels of upwelled filaments. Dominant south of STC

associated with cool, deeper waters, with a preference for areas with low seasonal salinity variation, low vertical temperature and density gradients (Be´ and Tolderlund, 1971). This factor is an indicator of intermediate conditions between warm oligotrophic waters and cool upwelled waters. Offshore waters with decreased productivity and nutrient content are indicated by this assemblage. The record shows a change in contribution from relatively higher values, prior to MIS 12, to relatively lower values for the period MIS 12–8, after which there is a general increase in contribution of this factor to the foraminiferal population (Fig. 4b). The Transitional Assemblage (Factor 3) (Fig. 4c), defined by Gr. inflata accounts for as much as 30% of the total variance. This monospecific assemblage reflects surface waters with temperatures in excess of 17 1C with a relatively low phytoplankton biomass. This assemblage is characteristic of outer-shelf and slope sediments of the SBR region (Giraudeau and Rogers, 1994), and shows lowfrequency fluctuations of the order of
200 ka (Fig. 4c). This record shows relatively lower contributions prior to 650 ka, followed by a period of consistently average contributions from MIS 16–12, then an increase in relative

Subtropical

Tropical Subtropical–tropical Warm-temperate, boundary currents Warm subtropical, mixedlayer Warm-temperate Warm-temperate, below thermocline Warm-temperate Cool-temperate, deep-waters Transitional, deep-waters Transitional, warm–cool temperate. Transitional/warm

Subpolar- cool subtropical Subpolar Subpolar Polar

contribution from MIS 11–8 and relatively lower values for the last 250 ka. There is an overall negative correlation between this factor and the previous factor with high abundances of the Intermediate Assemblage associated with low abundances of this Transitional Assemblage and vice versa. The summed abundances of the Subtropical and Tropical species, Gs. ruber (alba), Gs. trilobus, Gs. sacculifer, Gr. menardii and N. dutertrei, define the Tropical/Subtropical Assemblage (Fig. 4d). Neogloboquadrina dutertrei is associated with warm western boundary currents, with highest percentages occurring in the sediments of equatorial regions and along continental margins (Be´ and Tolderlund, 1971). Globigerinoides ruber (alba), Gs. trilobus and Gs. sacculifer are widespread species, dominating in subtropical and tropical waters. The dominance of Gs. ruber (alba) appears to increase in the oligotrophic Indian and Atlantic Central Waters (Be´ and Tolderlund, 1971). Although only a trace component of the foraminiferal population, Gr. menardii is an important indicator species of tropical Indian Ocean waters. This assemblage is not defined in the factor analysis, but it is considered to be important as a

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Subantarctic Factor 1 0.2 0.4 0.6 0.8

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Fig. 4. Planktonic foraminiferal assemblages as defined by factor analysis for the Agulhas Bank Slope core MD962080 (ABS). Shaded horizontal bars indicate glacial stages.

Table 2 Varimax factor score matrix for Core ORS Variable

Factor 1

Factor 2

Factor 3

N. pachyderma (s) N. pachyderma (d) Gg. bulloides Gr. scitula Ga. glutinata Gr. inflata Gg. falconensis Gr. hirsuta Gr. truncatulinoides Gs. ruber Gs. trilobus Gs. sacculifer N. dutertrei Gr. menardii Gr. crassaformis Ge. calida T. quinqueloba

0.028 0.859 0.149 0.071 0.128 0.082 0.080 0.024 0.151 0.020 0.283 0.057 0.304 0.005 0.006 0.071 0.022

0.028 0.313 0.671 0.311 0.157 0.341 0.347 0.007 0.121 0.049 0.237 0.049 0.106 0.006 0.010 0.044 0.053

0.033 0.081 0.207 0.124 0.053 0.910 0.061 0.045 0.165 0.172 0.180 0.008 0.079 0.009 0.044 0.016 0.020

possible indicator of the transfer of warm water from the Indian Ocean to the Atlantic Ocean. The oldest portion of the record (MIS 21–17) shows an increasing (warming) trend, which ceases at MIS 16 (Fig. 4d). There is a change in mean contribution of this factor from relatively lower values from MIS 16–12 to relatively higher values for the period MIS 11–7, after which there is a general decreasing trend for the last 200 ka of the record. The last 450 ka of the record (MIS 12–1) show a tendency to G–IG cyclicity, with slightly lower values for these warm-water species in glacial stages and slightly increased values in interglacial stages. 3.2. Core ABS Factor analysis for Core ABS (Fig. 5) indicates that three faunal components account for more than 90% of the total variance in the census data. The factor scores which give the species composition of each assemblage are shown

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Transitional Factor 1 0.3 0.6

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Fig. 5. Planktonic foraminiferal assemblages as defined by factor analysis for the Olifants River Slope core MD962084 (ORS). Shaded horizontal bars indicate glacial stages.

Table 3 Varimax factor score matrix for Core ABS Variable

Factor 1

Factor 2

Factor 3

N. pachyderma (s) N. pachyderma (d) Gg. bulloides Gr. scitula Ga. glutinata Gr. inflata Gg. falconensis Gr. hirsuta Gr. truncatulinoides Gs. ruber Gs. trilobus Gs. sacculifer N. dutertrei Gr. menardii Gr. crassaformis Ge. calida T. quinqueloba

0.057 0.137 0.139 0.038 0.236 0.928 0.037 0.017 0.027 0.020 0.160 0.048 0.087 0.004 0.001 0.017 0.047

0.156 0.097 0.208 0.086 0.605 0.262 0.043 0.086 0.144 0.125 0.480 0.326 0.341 0.013 0.050 0.077 0.126

0.123 0.959 0.207 0.023 0.024 0.166 0.057 0.017 0.034 0.029 0.074 0.003 0.033 0.002 0.013 0.050 0.036

in Table 3. Each factor can be characterised by a dominant taxon and one or two subordinate taxa. The varimax factor components, which give the composition of each sample in terms of the resultant assemblage, are plotted in Fig. 5. The Transitional Assemblage (Factor 1) (Fig. 5a), with 38% of the total variance, is defined by Gr. inflata, the most abundant species of the Transitional zone between Subantarctic and Subtropical waters (Be´ and Tolderlund, 1971). The present-day foraminiferal assemblage, dominated by Gr. inflata, occurs equatorward of the STC (Be´ and Tolderlund, 1971). The Transitional Assemblage (Factor 1) fluctuates independently of global G–IG cycles (Fig. 5a). Variations in abundances occur on shorter timescales, in response to local hydrographic changes. There is a change in mean contribution of this factor to the total foraminiferal population from relatively lower values prior to ca 450 ka to relatively higher values in the middle part of the core studied (MIS 11–6), to lower values in the younger portion of the core.

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Factor 2 (30% of the total variance) bears a cosmopolitan signature and is therefore termed the Cosmopolitan Assemblage (Fig. 5b). It is characterised by Ga. glutinata, with lesser contributions by Gs. trilobus, Gs. sacculifer and N. dutertrei. Globigerinita glutinata is one of the most widespread species, occurring over a wide range of temperatures and salinity. Gs. trilobus and Gs. sacculifer are abundant in subtropical and tropical waters. Neogloboquadrina dutertrei is associated with western boundary currents and exhibits a wide biogeographic range of Tropical and Subtropical environments (Be´ and Tolderlund, 1971). This assemblage shows the opposite trend to the Transitional Assemblage with higher relative contribution prior to 450 ka, lower relative contributions in the middle period (MIS 11–6) and higher relative contributions thereafter (Fig. 5b). A glacial–interglacial cyclicity develops from MIS 9 upcore, with higher values during warm periods. Factor 3 is a monospecific assemblage, exclusively defined by N. pachyderma (d) and accounting for 23% of the total variance (Fig. 5c). Neogloboquadrina pachyderma (d) is the cold-end member of the foraminiferal population and reflects cold Subantarctic waters. This Subantarctic Assemblage bears a unique signal. Firstly, there is no clear change in overall contribution for the period MIS 11–6 as seen in the previous factors. Secondly, a G–IG cyclicity is expressed throughout the top two thirds of the core, with higher values characterising cold periods (Fig. 5c). This relationship is not apparent prior to MIS 14. However, as in the case of the other factors, there is a change after 200 ka to an, on average, lower contribution of this factor. Although minor in the overall foraminiferal population in the core (thus not identified in the factor analysis), the Subtropical and Tropical species, Gs. ruber (alba), Gs. trilobus, Gs. sacculifer, Gr. menardii and N. dutertrei, need to be considered as potential indicators of warm surface waters over the core site. Their relative abundances were therefore summed to define a Tropical /Subtropical Assemblage (Fig. 5d). This Tropical/Subtropical assemblage exhibits a clear G–IG cyclicity after 350 ka. (Fig. 5d) with minimum abundances occurring during glacial periods MIS 10, 8, 6, 4 and 2 and peak abundances in warm periods. No such cyclicity is present in the older portion of the core. The bottom half of the record (MIS 21–12) displays, on average, higher abundances and lower frequency fluctuations than the middle interval (MIS 11–6). There is an average increase in relative abundances in the last 200 ka. 4. Discussion of the oxygen isotope and micropalaeontological data for cores ORS and ABS 4.1. Core ORS This core is today, in the Holocene Interglacial, situated in the SBR (not shown in Fig. 1) in an area influenced by nutrient-rich upwelled waters of the BUS, the filamentous

region of mesotrophic, old, upwelled waters, the oligotrophic waters of the South Atlantic gyre, as well as by the sporadic incursions of warm saline Indian Ocean waters carried by Agulhas rings (Fig. 1). The dominance of Transitional and Subantarctic planktonic foraminifera suggests that the core site has been under the influence of cool-temperate, mesotrophic mixed water masses. There is a general G–IG correlation of eutrophic species (N. pachyderma (d), N. dutertrei, and T. quinqueloba), with increased abundances in glacial periods. This implies a generally higher nutrient and productivity level in the glacial SBR. The negative correlation between the Intermediate and the Transitional Assemblages (Figs. 4b and c), may indicate changes in dominant water masses over the core site from offshore oligotrophic to intermediate conditions, associated with east–west movements in frontal zones of the BUS and also possibly movements in the Southern Ocean hydrological belts. Rapid increases in abundances of the Warm Cosmopolitan Assemblage at glacial terminations, together with increased abundances of the Tropical/Subtropical Assemblage in warm periods over the last 450 ka, suggest higher transfer of thermocline and intermediate waters from the Indian Ocean via the Agulhas Current during this period. The last 200 ka show a definite trend of either increasing or decreasing contributions in all of the assemblages (Figs. 4a–d). There is a rough G–IG cyclicity within the dominant fauna during this period. Giraudeau et al. (2001) report a change in planktonic foraminiferal distribution trend at 250 ka in the ODP Site 1087 core retrieved from the continental slope northwest of Core ORS. Variations in abundances of the dominant species change from an illdefined G–IG relationship prior to MIS 7 to a clear G–IG cyclicity thereafter. These shifts in distributional trends may, in part, be viewed as a local response to the global oscillation of the ‘‘Mid-Brunhes climatic event’’. 4.2. Core ABS This core, taken at 361160 S, lies in a zone influenced by waters from the Agulhas Current and the Agulhas Retroflection, as well as by waters from south of the STC and the southeast Atlantic Ocean (Fig. 1). The dominance of Transitional and Subantarctic planktonic foraminifera (Figs. 5a and 5c) suggests that the core site has been under the influence of cool-temperate mixed-water masses, with impacts from the warm waters of the Agulhas Current, as indicated by the strong presence of a warm-cosmopolitan assemblage. The planktonic foraminiferal record shows a number of changes in hydrographic conditions over the last 850 ka. In general, prior to 500 ka, the data record long-term, low-frequency changes in surface water conditions. The foraminiferal assemblage data indicate rapid changes to intensely cold conditions in glacial stages MIS 14 and 12. The faunal record suggests that MIS 14 (565–525 ka) was one of the coldest periods of the Pleistocene. Such cold

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periods are linked to increased advection of Subantarctic waters over the core site, possibly associated with equatorward displacements of the STC. The interval MIS 11–7 (425–185 ka) records a change in surface-water dynamics. Variations in abundances exhibit high-frequency fluctuations, as well as a shift in the mean contribution of each factor. The data from Core ABS (361160 S) should be compared with the data of previous researchers (e.g. Prell et al., 1979; Howard and Prell, 1992; Flores et al., 1999), who established that the STC (Fig. 1) lay poleward of 421S for this period, but also suggest an equatorward migration, together with an easterly shift in the Agulhas Retroflection during glacial stages. The increased abundances of cold-water planktonic foraminifera tend to support this theory. However, the decline in warm-water species is not large or rapid enough to indicate a collapse of the Agulhas Retroflection or a total lack of influence of Agulhas waters over the core site at this time. There is a change in surface-water conditions from 250 to 200 ka (MIS 8–7). Planktonic foraminiferal assemblages indicate that this shift represents a transition at the core location, from the presence of a mixed Subantarctic and Transitional surface-water mass, with limited variability on G–IG time-scales during the MIS 11–7 interval, to overall warmer conditions, as well as the development of a G–IG cyclicity for the period MIS 7–1 (Fig. 5). Data from numerous locations in the Southern Hemisphere show a transition to more ‘‘interglacial’’ conditions at about this time or earlier, in a change referred to as the ‘‘Mid-Brunhes climatic event’’ (Jansen et al., 1986). Flores et al. (1999) identify a warm episode (MIS 11–7) in the nannofossil assemblages of a core retrieved to the northwest of Core ABS and south of Cape Town, which they interpret to reflect the ‘‘Mid-Brunhes climatic event’’. Surface-water changes, following glacial–interglacial cyclicity, are mostly expressed at the site location by advection of cold subpolar waters during glacial intervals, as indicated by the N. pachyderma (d)-rich foraminiferal assemblage (Fig. 5c). This probably reflects equatorward shifts of the Antarctic Polar Front (APF) and associated subpolar waters during periods of Antarctic ice sheet growth. Despite being less abundant, the presence (always 45%) of Tropical and Subtropical planktonic foraminifera (Fig. 5d) throughout the length of the core studied, suggests that these advections of subpolar waters to the core location did not induce a drastic equatorward shift in the position of the STC or at least indicate that any substantial equatorward displacement of the STC was limited to south of the core location (361160 S). This is particularly significant, since it implies that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed by the STC during any of these periods. It is conceivable that a more equatorward location of this front could nonetheless force the Agulhas Current Retroflection eastward and limit the throughflow of warmer subtropical water, either as Agulhas rings or as Agulhas filaments.

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5. Acid-insoluble sand fraction in cores ORS, ABS and NV There is a great contrast between the distribution patterns of acid-insoluble sand upcore in Cores ORS, ABS and NV. The data have been plotted, both as a percentage of acid-insoluble sand as a percentage of the original unleached subsample (Fig. 6) and as a percentage of the unleached sand fraction (Fig. 7). Because the sand fractions were richer in calcium carbonate, chiefly in the form of planktonic foraminifera, when compared to the mud fraction (o63 mm), the latter approach emphasised the upcore variation in acid-insoluble sand. In Core ORS (Fig. 6a), despite its location (Fig. 1) closer to the Orange Delta (Bremner et al., 1990; Rogers and Bremner, 1991) and to the dust-bearing, winter-season, offshore adiabatic (berg) winds of the Namib Desert (Shannon and Anderson, 1982), the abundance of acidinsoluble sand is very low (o1%), throughout the core. The plot of acid-insoluble sand, as a percentage of the unleached sand fraction (Fig. 7a) shows no clear glacial–interglacial trend, values often being slightly higher (up to 15%) in interglacials, not in glacials as expected. In Core ABS, we plot the upcore distribution of acidinsoluble quartz sand, purified of biogenic silica (benthic sponge spicules and planktonic radiolaria) and of slightly magnetic, authigenic glauconite, the abundances of which were plotted in Fig. 4 of Rau et al. (2002), where biogenic silica was mainly less than 3% of the sand fraction with peaks of up to 7%. Glauconite was even less abundant (mainly o1% with peaks of up to 2%). In stark contrast to Core ORS, (mainly o1%), the acidinsoluble sand fraction of Core ABS reached peaks of up to 14% as a percentage of the unleached original subsample (Fig. 6b). A clear glacial–interglacial cyclicity is only apparent from MIS 14 (560 ka) upwards in Fig. 6b. Due to variations in mud-sand proportions (Rau et al., 2002, Fig. 3), with higher mud contents in the middle of the core, the quartz sand, plotted as a percentage of the unleached sand fraction is enhanced more (up to 70%) in the middle section than in the lower and upper sections of the core (Fig. 7b). A glacial–interglacial cyclicity, with peaks of quartz sand more abundant in the glacial periods, is only apparent from MIS 11–1 in Fig. 7b. A completely different pattern manifests itself for the acid-insoluble sand fraction from Core NV off the east coast of South Africa (Fig. 1). It is clearest in Fig. 6c, where three narrow peaks of acid-insoluble sand, plotted as a percentage of the unleached original subsample, are found in MIS 13, MIS 10 and MIS 8, the first in an interglacial and the other two in glacials. Two minor peaks are found in MIS 13 and MIS 12. All five layers, rich in quartz sand, within the dominant nannofossil ooze, were noted in the original logs published in the official Cruise Report and all were detected in the upcore plots of density, magnetic susceptibility and sonic velocity Bertrand et al. (1997). As shown in Fig. 6c, the highest quartzose-sand peak, in MIS 10, amounts to almost 40% of the original unleached

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subsample. The background values are o2%, similar to the values throughout Core ORS off the west coast. The pattern is enhanced when acid-insoluble sand, as a percentage of the unleached sand fraction, is plotted versus depth (Fig. 7c). The original narrow peaks (up to 85%) stand out clearly, but the background is much better defined and broadly resembles the enhanced plot for Core ABS. 6. Discussion of the acid-insoluble sand data Ignoring, for the moment, the narrow peaks of quartzrich, acid-insoluble sand in Core NV from the Natal Valley, it is clear from Fig. 6 that, overall, the abundance of acid-insoluble sand as a percentage of the original unleached subsample, in the most southerly core, Core ABS (Fig. 1), is far higher (up to 14%) than in the cores from lower latitudes, Core ORS(o1%) and Core NV(o2%). After discounting fluvial and aeolian sources, this high abundance of silici-clastic sand in Core ABS has been attributed, by Rau et al. (2002), mainly to storm-derived very fine quartz sand from the outer shelf, especially during

glacial lowstands, which would have shifted the coastline closer to the coresite, while lowering the wavebase closer to the seafloor by 130 m. The rarer quartz granules in Core ABS were attributed to ice-rafted detritus, on the grounds that, during the present Holocene Interglacial (MIS 1), icebergs were observed just equatorward of latitude 351S, due south of Cape Town and the Cape of Good Hope in a longitude of 181300 E (Fig. 1) in 1850 by officers of the Royal Navy (Needham, 1962). In other words, during the Holocene interglacial, icebergs were observed equatorward of the site of Core ABS. We therefore argue that icebergs were likely to have been much more abundant over the site during glacial periods and that warm waters of the Agulhas Retroflexion were still able to penetrate westward to melt such errant icebergs and to release their ice-rafted detritus. Ignoring the relatively insignificant abundances of acidinsoluble sand in Core ORS off the arid west coast, which seems to be little affected by input from either the Orange River or adiabatic berg winds, we now turn to Core NV off the east coast. As can be seen in Fig. 1, this core lies closer to the Holocene Highstand coastline and the coastline would have been even closer during each Pleistocene

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Lowstand, as the continental margin is both narrow and shallow. This is because it is a translational plate boundary (Dingle et al., 1983). In contrast to the semi-arid to arid west coast of Holocene South Africa, the east coast is traversed by many rivers, fed by abundant summer rainfall. This may explain why the background values of quartz sand in the Natal Valley are double, albeit only 2%, of those on the Olifants River Slope. The shallowest of the three major peaks of quartz sand in Core NV is found in a core depth of 10.5 m (MIS 8), deeper than similar quartz sands found in gravity cores, shorter than 10.5 m, recovered by the Professor Logachev in 1992 from the Natal Valley (Hartnady et al., 1992). Due to their content of shallow-water ostracods and benthic foraminifera and to their erosive bases, McKeown (1993) interpreted the sands as turbidites, although he had too few data to assign any age to them. We now suggest that the quartz sands in the nearby Core NV are also turbidites, especially the three major peaks seen in Figs. 6c and 7c, but also the minor peaks that stand out in Fig. 6c. The ages of these five peaks (Fig. 6c) range from 510 and 485 ka in MIS 13, to 455 ka in MIS 12, 335 ka in MIS 10 and, finally, 250 ka in MIS 8. We propose that the most likely triggers for the inferred turbidity currents were major

onshore, rift-related earthquakes in a core much closer to the Rift Valleys of East Africa than either Core ABS or Core ORS. Dingle and Robson (1985, p. 51) inferred such earthquakes as a possible explanation for the abundant, probably Pliocene–Pleistocene, major submarine slumps off the east coast of South Africa landward of the Natal Valley. On 31st December, 1932, there was an earthquake, with an intensity of 8 using the pre-Richter Mercalli Scale, the epicentre of which lay ‘‘yabout 25 miles off the coast near Cape St Lucia’’ (Krige and Venter, 1933, p. 108) in a latitude of 281300 S and a longitude of 321500 E, NE of Durban (Krige and Venter, 1933, Fig. 1). Hartnady (1990, 2002) has mapped the epicentres of recent earthquakes across the continent of Africa and, more specifically, in SE Africa, adjacent to the Natal Valley (Hartnady, 1990, Fig. 1). This paper seeks to draw a contrast, as stated above, between the turbidite-free Cores ORS and ABS off the west coast of South Africa and the turbidite-rich Core NV off the east coast of South Africa. The finer details of the turbidites of Core NV are the subject of a paper in preparation by Hartnady and Rogers. If major earthquakes were indeed the triggers for the five highlighted turbidites in Core NV, then the recurrence

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intervals, from the above data, are 25 ka (510–485 ka), 30 ka (485–455 ka), 120 ka (455–335 ka) and 85 ka (335–250 ka) with a mean recurrence interval of 65 ka. Simplifying it further, the recurrence intervals between the three major turbidites are 175 ka (510–335 ka) and 85 ka (335–250 ka) with a mean recurrence interval of 130 ka. The lack of a major turbidite in sediments younger than 250 ka can be interpreted in two ways, either major neotectonic activity has subsided from the Late Pleistocene into the Holocene or a much greater-magnitude earthquake can be expected as double the average recurrence interval has now elapsed.

Acknowledgements Carbon and oxygen isotopic measurements of specimens Globorotalia inflata from core MD962084 were performed at the Fachbereich Geowissenschaften, Universita¨t Bremen, Germany, under the supervision of Dr. Monica Segl and Professor Ralph Schneider. This research forms part of the IMAGES II-NAUSICAA project and was funded in part by the National Research Foundation (NRF, South Africa), the University of Cape Town, the FrancoSouth African Science and Technology Agreement and the Centre National de la Recherche Scientifique (CNRS, France).

7. Conclusions References This comparative study of Late Quaternary deep-sea sediments in three giant piston cores from the Olifants River Slope, the Agulhas Bank Slope and the Natal Valley off South Africa has shown that sediments of hypothermal (glacial) periods are characterised by assemblages of planktonic foraminifera dominated by the Subpolar right-coiling Neogloboquadrina pachyderma accompanied, in Core ABS, by higher amounts of glauconite and very fine quartz sand, probably swept seawards by storms, when the outer shelf was shallower during lowstands. Rare quartz granules were probably released from Antarctic icebergs melting in the Agulhas Retroflexion. In contrast, sediments deposited in hyperthermal (interglacial) periods are dominated by the Transitional Globorotalia inflata with minor amounts of the Tropical Globorotalia menardii. However, this Tropical planktonic foraminifer is found, albeit in trace amounts, even in sediments of hypothermal (glacial) periods, showing that, at the latitude of Core ABS (361160 S), warm Agulhas Current water was not completely choked off by any equatorward shift of the STC. Acid-insoluble sand, mainly very fine quartz sand, occurs in miniscule amounts on the Olifants River Slope off the west coast, in significant amounts, as discussed above, on the western Agulhas Bank Slope, but has a very different pattern in the Natal Valley. Three major and two minor peaks of acid-insoluble sand are attributed to triggering of turbidity currents by major onshore earthquakes with recurrence intervals, on average of 65 ka. It is suggested that either earthquake activity has subsided in the last 250 ka, when no major turbidites were found, or that a particularly high-magnitude earthquake is to be anticipated. The climatic variability is most apparent in the upper section of Core ABS. Both the end-members of the planktonic-foraminiferal assemblages (Subantarctic and Tropical) and the acid-insoluble (quartz) sand show glacial–interglacial cyclicity from MIS 11–1 (Figs. 5–7). The abundance of quartz sand and of the Subantarctic Assemblage increase during glacial periods, whereas the Tropical Assemblage increases during interglacials, therefore showing an antipathetic relationship.

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Prell, W.L., Hutson, W.H., Williams, D.F., 1979. The Subtropical Convergence and Late Quaternary circulation in the southern Indian Ocean. Marine Micropalaeontology 4, 225–234. Rau, A.J., 2002. A Late Quaternary history of Agulhas-Benguela interactions from two sediment cores on the western continental slope of South Africa. Unpublished Ph.D. Thesis, Department of Geological Sciences, University of Cape Town. 203pp. Rau, A.J., Rogers, J., Lutjeharms, J.R.E., Giraudeau, J., Lee-Thorp, J.A., Chen, M.-T., Waelbroeck, C., 2002. A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa. Marine Geology 180, 183–201. Rogers, J., 1999. Preliminary findings of the IMAGES-II Programme (International MArine Global changE Study) using giant piston cores from the continental slope and rise off southern Africa. South African Journal of Geology 102, 384–390. Rogers, J., Bremner, J.M., 1991. The Benguela Ecosystem. Part VII. Marine Geological Aspects. In: Barnes, M. (Ed.), Oceanography Marine Biology: Annual Review, vol. 29. Aberdeen University Press, Aberdeen, pp. 1–85. Shannon, L.V., Anderson, F.P., 1982. Applications of satellite ocean colour imagery in the study of the Benguela Current System. South African Journal of Photogrammetry, Remote Sensing and Cartography 13 (3), 153–169. Shannon, L.V., Nelson, G., 1996. The Benguela: large scale features and processes and system variability. In: Wefer, G., Berger, W.H., Siedler, G., Webb, D.J. (Eds.), The South Atlantic: Past and Present Circulation. Springer, Berlin, pp. 211–217.