The sediments of MD96-2048 were retrieved in the middle of the Delagoa Bight (Fig. 1) from the southern Limpopo cone forming a depot centre that has been build up at least since the Late Miocene (Martin, 1981). The site collects terrestrial material including pollen and spores mostly from the rivers that discharge into the Delagoa Bight of which the Limpopo River is the biggest draining large areas of northern South Africa and southern Mozambique. Apart from the offshore winds descending from the interior plateau, so-called Berg winds, the predominant wind direction is landward (Tyson and Preston-Whyte, 2000) and aeolian input of terrestrial material is probably minor. Thus, pollen source areas would cover the region north of the Delagoa Bight in southern Mozambique and the region west of the Lebombo hills and the Drakensberg escarpment (Dupont et al., 2011a).

A wide variety of measurements have been performed on MD96-2048 sediments. Caley et al. (2011, 2018a) recorded stable oxygen isotopes of benthic foraminifers (Planulina wuellerstorfi) providing a stable oxygen stratigraphy and age model aligned to the global stack LR04 (Lisiecki and Raymo, 2005) for the past 2200 kyr. Trace element (Mg/Ca ratios) of the planktic foraminifer Globigerinoides ruber sensu stricto and foraminifer assemblages were combined to produce a robust sea surface temperature (SST) record (Caley et al., 2018a). High-resolution (0.5 cm) X-ray fluorescence (XRF) scanning has been performed over the total core length, of which the iron–calcium ratios, ln(Fe/Ca), were used to estimate fluvial terrestrial input variability (Caley et al., 2018a). At millennial resolution, higher plant leaf wax (n-alkane) concentrations and ratios and compound-specific stable carbon isotopes (δ13Cwax) provided a record of vegetation changes in terms of open versus closed canopy and C4 versus C3 plants of the past 800 kyr (Castañeda et al., 2016a). A very low-resolution leaf wax deuterium isotopic record was generated (Caley et al., 2018a) and, in conjunction with other high-resolution proxies including ln(Fe/Ca), was used to reconstruct rainfall and Limpopo River runoff during the past 2.0 Myr.

Modern climate is seasonal with the rainy season in summer (November to March). Yearly precipitation ranges from 600 mm in the lowlands to 1400 mm in the mountains, whereby rains are more frequent along the coast under the influence of SSTs (Jury et al., 1993; Reason and Mulenga, 1999). Annual average temperatures range from 24 to 16 ∘C, but in the highlands clear winter nights may be frosty.

The modern vegetation of this area belongs to the forest, Highveld grassland and savannah biomes and also includes azonal vegetation (Fig. 1) (Dupont et al., 2011a, and references therein). The natural potential vegetation of the coastal belt is forest, although at present it is almost gone; north of the Limpopo River mouth, rain forests belong to the Inhambane phytogeographical mosaic and south of the Limpopo River the forest belongs to the Tongaland–Pondoland regional mosaic (White, 1983). The vegetation of the northern part of the Tongaland–Pondoland region is the Northern Coastal Forest (Mucina and Rutherford, 2006). Semi-deciduous forest is found in the Lebombo hills (Kersberg, 1996). Afromontane forest and Highveld grasslands grow along the escarpment and on the mountains. The savannahs of the Zambezian phytogeographical region, including, e.g. the Miombo dry forest, occur further inland (White, 1983). Azonal vegetation consists of freshwater swamps, alluvial and seashore vegetation, and mangroves (Mucina and Rutherford, 2006).

Pollen percentages of Cyperaceae (sedges) and Poaceae (grasses) are plotted in Fig. 2 together with the δ13Cwax of the C31 n-alkane and XRF-scanning data, ln(Fe/Ca), the natural logarithm of elemental ratios of iron over calcium. Comparing the records of Cyperaceae and δ13Cwax reveals that high relative amounts of C4 plant material co-varied with increased representation of sedges. They also co-varied with higher terrestrial input indicated by ln(Fe/Ca) and increased precipitation as suggested by deuterium of the C31 n-alkane (Caley et al., 2018a). We substantiated the correlations for the Brunhes Chron between pollen percentages, leaf waxes and elemental ratios in Table 1. Leaf wax data are after Castañeda et al. (2016b) including average chain length (ACL) of the C27–C33 n-alkanes, the ratio of C31 / (C31+ C29) and δ13Cwax. XRF ln(Fe/Ca) ratios are from Caley et al. (2018c). Significant correlation is found between the leaf wax parameters and Cyperaceae data but not between δ13Cwax (indicative of C4 inputs) and Poaceae pollen percentages – although a correlation exists between Cyperaceae and Poaceae percentages. While the sedge pollen percentages fluctuate between 10 % and 50 % (mostly > 20 %), the percentages of grass pollen are always lower than 20 %. Such low grass pollen values have not been found adjacent C4 grass dominated biomes (mainly savannahs) on the western side of the continent (Dupont, 2011). It is, therefore, likely that in sediments of MD96-2048 the C4 component of the plant wax originated from C4 sedges rather than from C4 grasses.

South Africa has 68 species of Cyperaceae (sedges) of which 28 use the C4 pathway (among the 10 Cyperus species, 8 are C4) predominantly growing in the northern part of the country (Stock et al., 2004). They are an important constituent of tropical swamps and riversides (Chapman et al., 2001). An inventory of six modern wetlands between 500 and 1900 m in KwaZulu-Natal shows that C4 grasses dominate the dry surroundings of the wetlands at all altitudes (Kotze and O'Connor, 2000). In the wet parts of the wetlands, however, C4 sedges may make up to 60 % of the vegetation cover at 550 m. At higher altitudes the coverage of C4 sedges declines (Kotze and O'Connor, 2000).

Cyperaceae pollen concentration (Fig. 3) and percentages correlate with ln(Fe/Ca) and with δ13Cwax (Table 1, Fig. 2). The ratios of terrestrial iron over marine calcium can be interpreted as a measure for terrestrial input, which in this part of the ocean is mainly fluvial. Correlation between increased fluvial discharge and increased C4 vegetation as well as increased Cyperaceae pollen has been reported from sediments off the Zambezi (Schefuß et al., 2011; Dupont and Kuhlmann, 2017). Moreover, a fingerprint of C4 sedges was found in Lake Tanganyika (Ivory and Russel, 2016). As a consequence, material (leaf waxes and pollen) from the riverine vegetation is probably better represented than that from dry and upland vegetation. These results corroborate the reinterpretation of the δ13Cwax record, in which the increased representation of C4 plants (n-alkanes enriched in 13C) is instead attributed to stronger transport of material from the upper Limpopo catchment and the extension of swamps containing C4 sedges under more humid conditions (Caley et al., 2018a). Previously, Castañeda et al. (2016a) had interpreted increased C4 inputs as reflecting increased aridity.

Relatively low values of Cyperaceae pollen and Fe/Ca ratios are found for most interglacials of the Brunhes Chron (Figs. 2 and 3), which could be interpreted as an effect of sea-level highstands. However, Caley et al. (2018a) demonstrated that the fluvial discharge is not related to sea-level changes. From the bathymetry of Delagoa Bight, a strong influence of sea level is also not expected because the shelf is not broad and the locality of core MD96-2048 is relatively remote on the Limpopo cone in the centre of the clockwise flowing Delagoa Bight lee eddy. The eddy transports terrestrial material northeastwards before it is taken southwards (Fig. 1) (Martin, 1981) and likely has not changed direction during glacial times. Thus, fluvial discharge was probably low during interglacials (among other periods), which might be the combined result of more evapotranspiration and less precipitation. Despite drier conditions, the representation of woodland and dry forest is relatively high during the interglacial periods (Fig. 3; see also next section).

Palynological results have been published for the past 350 kyr (Dupont et al., 2011a) providing a detailed vegetation record for the past three climate cycles. Pollen and spore assemblages could be characterized initially by three endmembers via endmember modelling (EM1, EM2, EM3). The assemblage of EM1 was dominated by Podocarpus (yellow wood) pollen percentages being more abundant during the non-interglacial parts of MIS 5, 7 and 9. EM2 was characterized by pollen percentages of Cyperaceae (sedges), Ericaceae (heather) and other plants of open vegetation and abundant during full glacial stages. EM3 constituted of pollen from woodland, forest and coastal vegetation and was interpreted to represent a mix of several vegetation complexes.

We repeated the endmember modelling for the extended record covering the entire Brunhes Chron and the two early Pleistocene windows. The analysis of the extended dataset gave compatible results with the previous analysis (Dupont et al., 2011a). The main difference is that the longer sequence allowed to distinguish two assemblages of interglacial vegetation. In terms of analysis, the cumulative increase of explanatory power lessened after four (instead of three) endmembers, and a model with four endmembers was chosen. We used the scores of the different pollen taxa on the endmember assemblages for our interpretation of the endmembers (list of taxa and scores in Tables S1 and S2 in the Supplement). This interpretation is summarized in Table 2. To distinguish between the previous and current analyses (which show strong similarities), we have given new names to the endmember assemblages reflecting our interpretation: E-heathland, E-Mountain-Forest, E-Shrubland and E-Woodland. A selection of pollen percentage curves are plotted together with each endmember's fractional abundance in Figs. S2–S5 in the Supplement.  

E-Heathland. Of the four endmember assemblages (Fig. 3), one endmember had a counterpart in EM2 (Dupont et al., 2011a) of the previous analysis. Not only composition but also the fractional abundances, which were high during glacial stages, are very much alike. We name this endmember “E-Heathland”, which is dominated by Cyperaceae (sedges) pollen percentages, followed by Ericaceae (heather) pollen and hornwort (Anthocerotaceae) spores (Table 2). Also Lycopodium (clubmoss) spore, Restionaceae and Stoebe-type pollen percentages score highest on this endmember. The E-Heathland assemblage represents a fynbos-like open vegetation growing during full glacials. Other pollen records from SE Africa also indicate an open ericaceous vegetation with sedges and Restionaceae during glacial times (Scott, 1999; Dupont and Kuhlmann, 2017). The record of MD96-2048 testifies that this type of open glacial vegetation has regularly occurred for at least 2 million years.

In summary, the endmember analysis indicates a very stable open ericaceous vegetation with partially wet elements such as sedges and Restionaceae characterizing the landscape of full glacials (when global temperatures and pCO2 were lowest). During the less extreme parts of the glacials, mountain Podocarpus forest was extensive as in most mountains of Africa (Dupont, 2011; Ivory et al., 2018). On the other hand, interglacials were characterized by coastal shrubs. In the course of the Brunhes, the woody component, which was relatively weak before the MBE, became more and more important, reflecting the same long-term trend found in the leaf wax records (Castañeda et al., 2016a). It is likely that the Miombo dry forest and woodland migrated into the region in the successive interglacials of the Brunhes Chron. Particularly during Interglacials 9e and 1, the area might have been more forested than during the older interglacials of the Brunhes Chron.

The region of the Limpopo River becoming more and more wooded in the course of successive interglacials (Castañeda et al., 2016a) somewhat paralleled the conditions around Lake Malawi (Johnson et al., 2016). However, around Lake Malawi, forested phases of either mountain forest, seasonal forest or Miombo woodland alternating with savannahs occurred during both glacial and interglacial stages (Ivory et al., 2018). Also in contrast to the Lake Malawi record, the MD96-2048 Poaceae pollen percentages fluctuated little and remained relatively low (less than 20 %), indicating that savannahs were of less importance in the Limpopo catchment area and the coastal region of southern Mozambique.

The trend to increased woodland in SE Africa after the MBE, noted at both Lake Malawi and in the Limpopo River catchment (Johnson et al., 2016; Caley et al., 2018a, this study), contrasts with the trend around Lake Magadi at the Equator. At Lake Magadi, a trend to less forest marks the Mid-Brunhes Transition (Owen et al., 2018). Antiphase behaviour of SE African climate with that of west and east Africa emphasizes the importance of the average position of the tropical rain belt shifting southwards during globally cold periods as has been inferred from Holocene to last glacial records of Lake Malawi (Johnson et al., 2002; Scholz et al., 2011). Our results confirm this relationship existed over the entire Brunhes Chron.

The Lake Malawi pollen records and Lake Magadi pollen records in Kenya (Johnson et al., 2016; Owen et al., 2018) do not show much of a glacial–interglacial rhythm and are dominated by the precession variability in tropical rainfall (e.g. Clement et al., 2004). Obviously, in the tropical climate of the Southern Hemisphere north of ∼15∘ S, the hydrological regime had more effect on the vegetation than changes in temperature, while further south the impact of glacial–interglacial variability on the vegetation increased.

While the hydroclimate of the region shows precession variability (Caley et al., 2018a), the vegetation shows a glacial–interglacial rhythm (Fig. S1 in the Supplement), indicating that besides hydrology, temperature and/or atmospheric CO2 levels were important drivers of the vegetation development. Combining the results of the pollen assemblages with stable carbon isotopes and elemental information indicates that during interglacials the region of SE Africa (northern South Africa, Zimbabwe and southern Mozambique) was less humid. This is in accordance with other palaeoclimate estimates for the region (see reviews by Simon et al., 2015; Singarayer and Burrough, 2015).

The interglacial woodlands (represented by E-Woodland, Fig. 4) would probably have grown under warmer and drier conditions than the glacial mountain forest (represented by E-Mountain-Forest). The increase in maximum pCO2 levels during the post-MBE interglacials might have favoured tree growth, as higher pCO2 levels would have allowed decreased stomatal conductivity and thus relieved drought stress (Jolly and Haxeltine, 1997). Woodlands would have expanded at the cost of mountain forest during MIS 11c, 9e, 5e and 1, and to a lesser extent during 7e and 7c, when temperatures and pCO2 were high (Fig. 4). It might be only after interglacial pCO2 levels rose over ∼270 ppmv that Miombo woodland could fully establish in the area during the warm and relatively dry post-MBE interglacials.

The glacial stages showed the expansion of either mountain forest or heathland. The record indicates extension of mountain forests in SE Africa during those parts of the glacial stages with low temperatures and atmospheric pCO2 exceeding ∼220 ppmv (Fig. 5). If low temperatures were the only driver of the extension of mountain forests, further spread into the lowlands during the coldest glacial phases should be expected. Instead, when pCO2 dropped below ∼220 ppmv during those colder glacial periods, mountain forest declined, in particular during MIS 18, 16, 14, 8, 6 and 2. A picture emerges of cool glacial stages in SE Africa in which tree cover broke down when atmospheric pCO2 became too low. Additionally, mountain forests were important during Interglacials 19c, 17c, 15e, 15a, 13a and 7e, in which pCO2 and Antarctic temperatures were subdued.

With an inverse modelling technique, Wu et al. (2007) estimated the climate inputs for the vegetation model BIOME4 using as information the biome scores of pollen records from equatorial east African mountains. Wu et al. (2007) found that lowering of the tree line under glacial conditions (1–3 ∘C lower temperatures, less precipitation, 200 ppmv pCO2) depended hardly on temperature but primarily on increased aridity and somewhat on lower pCO2, whereby lower pCO2 amplified the effects of water limitation. However, Izumi and Lézine (2016) found contrasting results using pollen records of mountain sites on both sides of the Congo basin. At any rate, the lack of trees in the southeast African mountains during glacial extremes is unlikely the result of drought, because our record indicates that climate conditions in SE Africa were less dry during glacials than during interglacials (the post-MBE interglacials in particular). Instead, C4 sedges being an important constituent of the ericaceous fynbos-like vegetation increased during glacials when atmospheric pCO2 and temperatures were low (Fig. 5). However, low temperatures are not particularly favourable for C4 sedges as indicated by the altitudinal distribution of C4 sedges in modern wetlands of KwaZulu-Natal (Kotze and O'Connor, 2000). We presume, therefore, that the extension of C4 sedges during the colder phases of the glacials is the result of low atmospheric CO2 concentrations rather than of low temperatures.

Pollen records of ericaceous vegetation suggest an extensive open vegetation existing in the east African mountains (e.g. Coetzee, 1967; Bonnefille and Riollet, 1988; Marchant et al., 1997; Debusk, 1998; Bonnefille and Chalié, 2000) and in SE Africa and Madagascar (e.g. Botha et al., 1992; Scott, 1999; Gasse and Van Campo, 2001; Scott and Tackeray, 1987) during the last glacial. In our study, ericaceous fynbos-like vegetation (E-Heathland) was found for those parts of the glacials having lower (less than ∼220 ppmv) atmospheric pCO2 (Fig. 5). Exceptions were found for MIS 12 and 14 when the difference of pCO2 with that of the preceding stage was small (Bereiter et al., 2015). Dupont et al. (2011a) argued that increase of C4 vegetation as the result of low pCO2 was unlikely because no extension of grasses was recorded. However, this argument is flawed if sedges dominantly constituted the C4 vegetation in the area. We also note that in many parts of South Africa, no substantial increase of C4 grasses occurred but that many sites suggest an expansion of C3 grasses during the Last Glacial Maximum (Scott, 2002).

As climate was wetter during most of the glacials in this part of the world, the question arises about the climatic implication of the ericaceous fynbos-like vegetation (represented by E-Heathland, Fig. 5) extending during full glacials over the mountains of South Africa – and correlating with the SST record (see also the correlation between SST and EM2 in Dupont et al., 2011a). The correlation with SST, however, is problematic. Singarayer and Burrough (2015) argued that the control of the Indian Ocean SSTs on the precipitation of South Africa shifted from a positive correlation during the interglacial to a negative correlation during the Last Glacial Maximum. They invoked the effects of the exposure of the Sunda Shelf (Indonesia) and Sahul Shelf (Australia) on the Walker circulation causing a wetter region over the western Indian Ocean but also weaker easterly winds to transport moisture inland. To question the link between SST and precipitation in SE Africa even further, Caley et al. (2018a) found that the precession signature in the river discharge proxy (ln(Fe/Ca); see also Fig. S1 in the Supplement) was absent in the SST record from the same core. SE Africa would have been more humid during glacials when the temperature difference between land and sea increased.

The increase in C4 vegetation during relative cool and humid climate would be in conflict with the idea that C4 plants are more competitive in hot and dry climates (Ehleringer et al., 1997; Sage, 2004). However, this idea is mainly based on the ecology of grasses and the development of savannahs, while the C4 vegetation expansion in SE Africa during cool and humid phases seems to be driven by sedges. A survey of the distribution of C4 sedges in South Africa revealed that those Cyperaceae do not have the same temperature constraints as C4 grass species (Stock et al., 2004). More important, South African C4 sedges appear to have evolved under wetland conditions rather than under aridity. C4 Cyperus species even occur in the wettest parts of lower-altitude wetlands in KwaZulu-Natal (Kotze and O'Conner, 2000).