Mid- and low latitude effects on eastern South African rainfall over the Holocene

Quaternary Science Reviews 229 (2020) 106088

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Mid- and low latitude effects on eastern South African rainfall over the Holocene Charlotte Miller*, Annette Hahn, Diederik Liebrand, Matthias Zabel, Enno Schefuß MARUM e Center for Marine Environmental Sciences, University of Bremen, Leobener Str. 8, 28359, Bremen, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 April 2019 Received in revised form 30 September 2019 Accepted 17 November 2019 Available online xxx

Climate variability and its forcings in eastern South Africa during the late Quaternary remain poorly understood with data suggesting temporal variability and spatial heterogeneity. To constrain the variability and the drivers of past climate, we explore vegetation (C3/C4) and hydrological change using stable carbon and hydrogen isotopes of plant-waxes and X-ray fluorescence (XRF) analysis in marine core GeoB20610-2 offshore the Limpopo River. We find evidence for two climatic phases: an initial arid phase (c. 7e3 cal kyr BP), followed by a wetter phase (c. 3e0 cal kyr BP). To put our findings into a regional context, we investigate a latitudinal transect of sites along eastern South Africa and divide the region into three distinct hydro-climatic zones, with significantly different climate drivers. During the last c. 3 cal. ka BP, wet climatic conditions dominated the northern summer rainfall zone (SRZ) and the southern South African zone (SSAZ), whereas arid conditions prevailed in the central and eastern SRZ. In the northern SRZ the climate was driven by local insolation, where heightened insolation resulted in more precipitation. The aridity during the last c. 3 cal. ka BP in the central and eastern SRZ and the increased humidity in the SSAZ are attributed to an equatorward displacement of the southern hemisphere westerlies, the South African high-pressure cell and the South Indian Ocean Convergence Zone (SIOCZ). For the Holocene we find little evidence for an Indian Ocean SST control on climate. The results for the Limpopo River catchment region highlight the spatial heterogeneity of Holocene climate conditions in eastern South Africa and indicate significant latitudinal differences in climate drivers. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Eastern South Africa n-alkanes Hydrogen isotopes Carbon isotopes XRF analysis Southern hemisphere westerlies Tropical easterlies Insolation South indian ocean convergence zone

1. Introduction Numerous different climate-forcing mechanisms have been proposed to explain the observed climate change in eastern South Africa, during the late Quaternary. Low Indian Ocean SSTs were held responsible for the arid conditions during the Last Glacial Maximum (LGM; 26.5e19 kyr BP; Clark et al., 2009) at Mfabeni peat bog (Baker et al., 2017; Miller et al., 2019) and during older glacial periods observed within a marine core from Maputo Bay (Dupont et al., 2011). Conversely, in the same marine core, SST forcing has been shown to have had little effect on eastern South African hydroclimate (Caley et al., 2018). On orbital timescales, eastern South African climate variability has also been associated with insolation changes, driven by orbital precession (Simon et al., 2015). During the late Holocene (the last c. 4.2 ka BP; Walker et al., 2012),

* Corresponding author. Present address: Leeds Trinity University, Brownberrie Ln, Horsforth, Leeds, LS18 5HD, United Kingdom. E-mail address: [email protected] (C. Miller). https://doi.org/10.1016/j.quascirev.2019.106088 0277-3791/© 2019 Elsevier Ltd. All rights reserved.

wet conditions in eastern South Africa have been linked to strong summer insolation which resulted in stronger atmospheric convection, increased moisture transport by the tropical easterlies and higher precipitation in the region (e.g. Chevalier and Chase, 2015; Schefub et al., 2011). On shorter timescales, North Atlantic abrupt cooling events appear to correspond with wet conditions in eastern South Africa. During these cooling events the tropical rainbelt and the associated rain-bearing systems may shift southwards, which has been suggested to strengthen monsoonal precipitation in the southern hemisphere resulting in humid conditions in eastern South Africa (Schefub et al., 2011; Simon et al., 2015; Ziegler et al., 2013). Dry conditions in the eastern South African SRZ during the LGM have been related to a northward shift of the southern hemisphere westerlies, resulting from an equatorward advance of Antarctic sea-ice (e.g. Chase et al., 2017; Chevalier and Chase, 2015; Miller et al., 2019). Records spanning the LGM, within the winter rainfall zone (WRZ), such as at Driehoek Vlei (Meadows and Sugden, 1993), Pakhuis Pass (Scott, 1994) and Elands Bay Cave (Baxter, 1996), indicate increased winter rainfall, providing support

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for an equatorward displacement of the southern hemisphere westerlies at the LGM (Chase and Meadows, 2007). Moreover, temporal shifts in climate driving mechanisms from mid-latitude drivers (westerlies) to low-latitude drivers (insolation) across the glacial-interglacial transition have been invoked (Chevalier and Chase, 2015). Most palaeoenvironmental studies focus particularly on the Last GlacialeHolocene transition (e.g. Chase et al., 2017; Chevalier and Chase, 2015; Chevalier and Chase, 2016; Miller et al., 2019), however significant climate changes have also been documented during the Holocene. For the late Holocene, palaeorecords within the SRZ and those within the year-round zone (YRZ) and WRZ, appear to show contrasting climatic signals (e.g. Baxter, 1989; Carr et al., 2015; Finch and Hill, 2008; Humphries et al., 2016; Miller et al., 2019; Neumann et al., 2010). Records from the WRZ and YRZ, such as Eilandvlei (Wündsch et al., 2018), Seweweekspoort (Chase and Meadows, 2007), Verlorenvlei (Carr et al., 2015), Klaarfontein (Meadows and Baxter, 2001) and Cecilia Cave (Baxter, 1989) document higher moisture availability over the last c. 3 ka BP. Conversely, records from the SRZ suggest arid conditions during the late Holocene (Humphries et al., 2016, 2017; Miller et al., 2019; Neumann et al., 2010). The spatially heterogeneous nature of climate variability in eastern South Africa and the proposed climate-driving mechanisms, render the region (and this time period) an important focus for palaeoclimate research. Here we present new data from marine core GeoB20610-2, which offers the unique opportunity to explore climate and environmental change in the Limpopo River catchment over the last c. 7 cal. ka BP (Fig. 1). We use the stable carbon (d13Cwax) and hydrogen isotopes (dDwax) of plant-wax n-alkanes in order to infer past vegetation and hydrological variability. Subsequently, for a regional perspective, we compare the d13Cwax, dDwax and XRF record from core GeoB20610-2 with a series of other palaeoenvironmental records, forming a latitudinal transect of sites along the South African east coast. 2. Study region The terrigenous material in core GeoB20610-2 consists of suspended particle load that was delivered to the ocean by the Limpopo, Incomati and Lusutfu rivers (Fig. 2; Schüürman et al., 2019). Consequently, we interpret changes we see in GeoB20610-2 as Limpopo, Incomati and Lusutfu catchment-derived signals. The catchment of all three rivers lies within the SRZ, where 66% of the mean annual precipitation falls between October and March, in austral summer (Fig. 1; Chase and Meadows, 2007). During austral summer, intense heating of the South African interior results in the formation of a low-pressure cell, which draws moisture from the Indian Ocean, resulting in monsoonal precipitation delivered by the tropical easterlies (Fig. 1; Chase and Meadows, 2007). Generally, the stronger the low pressure system, the higher the amount of monsoonal precipitation in eastern South Africa (Munday and Washington, 2017). During austral winter, a high-pressure cell develops over the continental interior, leading to dry conditions in the SRZ in eastern South Africa. With the strength of the low-pressure system (and thus also the monsoon) closely linked to insolation fluctuations (Munday and Washington, 2017), core GeoB20610-2 is ideally suited to record past changes in summer insolation. The majority of the vegetation comprising the Limpopo, Incomati and Lusutfu River catchments is of C4 type (Fig. 2; Dupont et al., 2011; Olson et al., 2001; White, 1983). It is only near the mouths of the rivers where the rivers drain areas of predominantly C3-type vegetation (Fig. 2; Dupont et al., 2011; Kersberg, 1985). Sedges (Cyperaceae), of which 20e60% are of C4-type in this region (Stock et al., 2004), are common river floodplain vegetation.

3. Methods The 3.21-m-long marine sediment core GeoB20610-2 (25
2.6960 S, 34
43.543’ E) was retrieved at 59 m water depth, c. 28 km from the coastline and 123 km from the Limpopo River mouth (Figs. 1 and 2). The sediments of core GeoB20610-2 consist of both terrestrial and marine components. From 321 to 220 cm the core is silty-sand and from 220 to core top it consists of clayey-silt (Supplementary Fig. 1). The chronology of the core is established by twelve 14C AMS (accelerator mass spectrometry) dates on bivalves, gastropods, urchins and bulk sediment (Fig. 3; Supplementary Fig. 2, Supplementary Table 1). The ages are calibrated using OXCAL (Bronk Ramsey, 2016) and Marine 13 e Modelled ocean average calibration curve (Reimer et al., 2013). A reservoir age of 121 ± 16 14C years (Maboya et al., 2017) was applied. The age model is based on the median age estimation (Supplementary Fig. 2), and indicates that the core spans the last 6.9 cal ka BP. Potentially, a short hiatus may exist at c. 220 cm, at the lithological boundary between the silty-sand and clayey-silt. However, no evidence was found for a non-depositional horizon or erosional surface. If a hiatus is present, this would still fall within the 95% uncertainty for the interval between 189 cm and 241 cm (Supplementary Fig. 2). Hence, on our (presumed continuous) age model, sedimentation rates vary between 0.4 and 2.3 cm/kyr, with the highest sedimentation rates toward the top of the study section (i.e., the upper 52 cm). The average sampling resolution for d13Cwax and dDwax between 317 and 217 cm is 480 years and between 217 and 0 cm is 95 years, as it was necessary to combine samples between 317 and 217 cm core depth (due to low n-alkane concentrations). For the isotope analysis, freeze-dried bulk sediment samples were homogenised and ground with a pestle and mortar. Prior to extraction, squalane was added as an internal standard. Lipids were extracted from c. 9 g of sediment with a DIONEX Accelerated Solvent Extractor (ASE 200) at 100
C and at 1000 psi for 5 min (repeated 3 times) using a dichloromethane (DCM):methanol (MeOH) (9:1, v/v) mixture. The remaining water was removed by elution over a Na2SO4 column with hexane. Elemental sulfur was removed from the total lipid extracts using copper turnings. After saponification, by the addition of 6% KOH in MeOH, and extraction of the neutral fractions by hexane, silica column chromatography was used to split the neutral fractions into hydrocarbon, ketone and polar fractions, eluting with hexane, DCM and DCM:MeOH (1:1), respectively. To obtain the saturated hydrocarbon fraction from the hydrocarbon fraction, the hydrocarbon fractions were eluted with hexane over AgNO3-impregnated silica columns. To quantify the concentration of long-chain n-alkanes, the saturated hydrocarbon fractions were measured using a Thermo Fisher Scientific Focus gas-chromatograph (GC) with flame-ionization-detection (FID) equipped with a Restek Rxi 5 ms column (30 m  0.25 mm x 0.25 mm). The GC oven temperature was programmed at 60
C, held for 2 min, increased at 20
C/min to 150
C and then at 4
C/min to 320
C and held for 11 min. The split/splitless inlet temperature was set at 260
C. Comparison with an external standard containing C19eC34 n-alkanes at a concentration of 10 ng/ml, that was run every 5 samples, enabled the determination of n-alkane concentrations. A quantification uncertainty of <5% was determined through replicate analyses of the external standard. The stable carbon isotopic composition (d13C in ‰ VPDB) of long chain n-alkanes (d13Cwax) was measured using a Thermo Trace GC equipped with an Agilent DB-5MS column (30m  0.25 mm x 0.25 mm) coupled to a Finnigan MAT 252 isotope ratio monitoring mass spectrometer via a combustion interface operated at 1000
C. The GC temperature was programmed from 120
C (hold time: 3 min), followed by heating at 5
C/min to 320
C (hold time:

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Fig. 1. a) Rainfall seasonality map of South Africa with the position of the tropical rainbelt, the major ocean and atmospheric currents and the Congo Air Boundary (CAB). Red/ orange ¼ summer rainfall zone (SRZ). Green ¼ year-round rainfall zone (YRZ). Blue is winter rainfall zone (WRZ). The blue arrows are oceanic circulation and the white arrows are atmospheric circulation. Map is courtesy of B. Chase (Chase et al., 2017). Red star is the location of core GeoB20610-2 (this study). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

15 min). The d13C values were calibrated against external CO2 reference gas. Samples were run in duplicate when n-alkane concentrations were adequate for multiple runs. As n-alkane concentrations were low it was not possible to get isotope data from individual samples between 317 and 217 cm. Consequently, samples from these depths were combined to obtain sufficiently high nalkanes concentrations for reliable d13C and dD data (data plotted as the average sample depth; Figs. 4 and 5). An external standard was analysed every 6 runs. The internal standard (squalane, d13C ¼ 19.9‰), yielded an accuracy of 0.3‰ and a precision of 0.1‰ (n ¼ 21). The long-term precision and accuracy of the external n-alkane standard was 0.2 and 0.1‰, respectively. For d13C the average precision of the n-C31 alkane in replicates was 0.1‰ (n ¼ 21). The stable hydrogen isotopic composition (dD in ‰ VSMOW) of

long chain n-alkanes (dDwax) was measured using a Thermo Trace GC equipped with an Agilent DB-5MS column (30m  0.25 mm x 1 mm) coupled via a pyrolysis reactor (operated at 1420
C) to a Thermo Fisher MAT 253 isotope ratio mass spectrometer (GC/IRMS). The GC temperature program was similar to that used for the d13C analysis. The dD values were calibrated against external H2 reference gas. The H3þ factor varied around 5.2 ppm nA1 during analyses and was monitored daily. After every sixth sample measurement an n-alkane standard containing 16 externally calibrated alkanes was measured. The long-term precision and accuracy of this external n-alkane standard mixture was 2.8 and < 1‰, respectively. When n-alkane concentrations permitted multiple runs, samples were run in duplicate. Squalane, the internal standard (dD ¼ 180‰; ±2), yielded an accuracy of 4.1‰ and a precision of 0.6‰ (n ¼ 20). For dD the average precision of the n-C31

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Fig. 2. The modern-day Limpopo, the smaller river catchments to the south and the vegetation biomes over which they drain. River catchment map is adapted from Asante et al. (2007). Vegetation biomes are from Olson et al. (2001). The location of core GeoB20610-2 (this study) is shown by the red star. Li ¼ Limpopo. In ¼ Incomati. Ma ¼ Matola and Lu ¼ Lusutfu. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3. The relationship between sediment depth (in cm) and age in core GeoB20610-2. The age-model was calculated using Bacon 2.2 software (Blaauw and Christen, 2011).

alkanes in replicates was 1.2‰ (n ¼ 20). To determine the source and the level of degradation of the organic matter we calculated the carbon preference index (CPI;

Equation (1); Bray and Evans, 1961). The CPI reflects the molecular distribution of odd-to-even n-alkanes. The generally high CPI values of all the samples (Fig. 4), especially those between 3 and

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Fig. 4. Records of climate and environmental change from core GeoB20610-2. a) K/Al XRF data with lower values representing an increased input of chemically weathered materials, as a result of increased humidity. b) Fe/Ca XRF data with higher values representing increased fluvial vs. marine input. c) CPI, with higher values indicating terrestrially derived n-alkanes from higher plants and low organic matter degradation. d) Concentrations of long-chain, odd-numbered n-alkanes (n-C25 to n-C35) in the sediments, with higher concentrations ¼ more organic input/less degradation. e) Hydrogen isotope composition of C31 n-alkanes, reflecting changes in precipitation amount/ET. f) Stable carbon isotopic composition of C31 n-alkanes, reflecting changes in C3/C4 vegetation type.

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0 cal kyr BP, indicate a high contribution of odd-numbered n-alkanes from terrestrial plants with low levels of degradation (Eglinton and Hamilton, 1967), meaning that the n-alkanes within core GeoB20610-2 are suitable for compound specific isotope analysis. Equation (1): The CPI was calculated following Bray and Evans (1961):


X X CPI ¼ 0:5* Ceven2632 Codd2733 =  X X þ Codd2733 = Ceven2834 With Cx the amount of each homologue. For the elemental composition, samples were measured using a PANalytical Epsilon3-XL XRF spectrometer equipped with a rhodium tube, filters and a SSD5 detector. To quantify elemental counts a calibration was used based on certified standard materials (GBW07309, GBW07316, MAG-1) (cf. Govin et al., 2014).

4. Proxy interpretation To reconstruct contributions by plants using different photosynthetic pathways, we analysed the stable carbon isotopic composition of long chain n-alkanes (d13Cwax) derived from higher plants (Fig. 4f). The high abundances of the n-C31 alkane (n-C31) enabled reliable isotope measurements and therefore, we use n-C31 for both d13Cwax and dDwax. The n-C31 is derived from the leaf wax coating of terrestrial higher plants (e.g., Diefendorf and Freimuth, 2017). The d13Cwax on late Quaternary timescales is mainly controlled by fractionation due to the plant carbon fixation pathway (C3/C4/Crassulacean acid metabolism (CAM) e.g., Diefendorf and Freimuth, 2017), which differs by vegetation type. Grasses exploit either the C3 or the C4 photosynthetic pathway. In the SRZ, C4 grasses are known to be the most prevalent (Vogel et al., 1978). Shrubs and trees use the C3 photosynthetic pathway. CAM plants have overall intermediate fractionation, due to their large (during the day) and small (during the night) CO2 fixation modes (Farquhar et al., 1989). CAM plants are highly adapted to waterdeficient environments such as deserts, with their distribution related to stress-tolerance (Diefendorf and Freimuth, 2017). The biomass productivity of CAM plants is low when compared to that of C3 and C4 plants (Black, 1973), and thus C3 and C4 terrestrial plants are likely the major contributors to the long chain n-alkanes found within core GeoB20610-2. The differences in fractionation amount, depending on the carbon fixation pathway, result in different d13Cwax isotopic signatures: d13Cwax of C4 plants is c. 20‰, C3 plants is c. 36‰ (e.g., Diefendorf and Freimuth, 2017). Consequently, high (low) d13Cwax values indicate a higher contribution of C4 (C3) type vegetation. To reconstruct continental hydrological changes we analysed the stable hydrogen isotope composition of long chain n-alkanes (dDwax; Fig. 4e). dDwax is a useful tool for reconstructing past hydrological conditions (e.g.,Collins et al., 2013; Schefub et al., 2005). dDwax predominantly reflects the isotopic composition of leaf water

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used during lipid biosynthesis (Sachse et al., 2012), which is mainly controlled by the isotopic composition of precipitation (dDprecip). The dDprecip in tropical areas is strongly controlled by the amount effect, with a negative correlation evident between monthly precipitation amount and dDprecip (Dansgaard, 1964). In the Limpopo, Incomati and Lusutfu River catchments, evapotranspiration (ET) is likely a secondary factor controlling the dD of soil as well as leaf waters. Isotopic enrichment of soil and leaf waters occurs during times of low humidity and serves to amplify the signal of the amount effect. A modern-day calibration study of surface samples across eastern South Africa suggests that precipitation amount and ET are the primary controls on sedimentary dDwax values in the region (Hahn et al., 2018). Here, dDwax signatures indicate increasing rainfall amounts towards the north, paralleling mean annual precipitation gradients. Soil samples taken along climatic transects in central and eastern South Africa also indicate that dDwax correlates significantly with annual dDprecip and mean annual precipitation amount (Herrmann et al., 2017). Consequently, precipitation amount and ET are likely the main controls of the isotopic composition of soil and leaf waters, and subsequently of the leaf waxes within core GeoB20610-2. Thus, we interpret shifts to higher dDwax values in the sediments of core GeoB20610-2 as an indication of aridity, resulting from reduced precipitation amount and increased ET. A similarity is evident between the GeoB20610-2 dDwax record and the northern precipitation stack (Fig. 5c and e; Chevalier and Chase, 2015). The increasingly negative GeoB20610-2 dDwax values correspond to higher precipitation in this stack. This supports our interpretation that precipitation amount and ET are the primary controls on sedimentary dDwax values for core GeoB20610-2. Our results display the same dDwax and d13Cwax anti-correlation (here, R2 ¼ 0.4) that is documented in the nearby marine core MD96-2048 (Caley et al., 2018). Consequently, we interpret higher d13Cwax values (more C4-type vegetation) as either, i) an increase in riverine transport of plant wax n-alkanes from the interior catchment (where C4 type vegetation is prevalent; Fig. 2), or ii) an inundation of the floodplains with an expansion of flooded grasslands, resulting in more C4-type Cyperaceae input. Both scenarios are consistent with an increase in precipitation and discharge, and hence higher d13Cwax values. A similar argumentation was previously proposed for the Zambezi catchment, where a comparison of Zambezi dDwax values with their d13Cwax values indicates that nalkanes with depleted dD values (wetter conditions) show a stronger C4 plant signal (Schefub et al., 2011). As elemental ratios are thought to be insensitive to dilution effects, we use the XRF-derived elemental ratios Fe/Ca and K/Al to support the palaeoclimatic interpretations based on the organic geochemistry (Fig. 4a and b). Fe is linked to the siliciclastic terrigenous component of the sediment, whereas Ca represents the marine-derived carbonate component. Fe/Ca ratios are therefore used widely to trace changes in terrigenous input, with higher values of Fe indicating more fluvial input (Govin et al., 2012 and references therein). During intervals of aridity and low chemical weathering rates, soils are comprised of easily weathered elements such as K, with K-rich illites common as clay minerals. When

Fig. 5. Records of climate and environmental change from core GeoB20610-2 compared with Holocene regional records and orbital insolation. a) December-January-February (DJF) insolation for 28
S (Laskar et al., 2004). Sites within the northern SRZ: b) Hydrogen isotope compositions of the C31 n-alkanes in GeoB9307-3, reflecting rainfall changes in the Zambezi catchment (Schefub et al., 2011). c) The northern regional precipitation stack based on pollen records from Wonderkrater, Tswaing Crater, Rietvlei Dam and Tate Vondo (Chevalier and Chase, 2015). d) Concentrations of long-chain, odd-numbered n-alkanes (n-C25 to n-C35) for GeoB20610-2 (this study), with higher concentrations ¼ more discharge. e) Hydrogen isotope composition of C31 n-alkanes within GeoB20610-2 (this study). Sites within the central and eastern SRZ: f) The central and eastern regional precipitation stack based on pollen records from Equus Cave, Braamhoek, Blydefontein, Florisbad and Lakes Eteza and Mfabeni (Chevalier and Chase, 2015). g) Stable carbon isotopic composition (weighted mean) of C29eC31 n-alkanes from Mfabeni (Miller et al., 2019). Sites within the SSAZ: h) Fe/K XRF record from CD154 10-06P representing run-off variability (Simon et al., 2015). i) Bulk carbon isotope data from Verlorenvlei indicating moisture availability (Carr et al., 2015). j) Extent of Antarctic sea-ice, based on the abundance of Fragilariopsis curta and Fragilariopsis cylindrus at site PS2090/ODP1094 (SW of Cape Town; Bianchi and Gersonde, 2004). k) UK0 37 derived SSTs from GIK16160-3 in the Mozambique Channel (Wang et al., 2013). Holocene sub-divisions are based on Walker et al. (2012).

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Fig. 6. Schematic of the proposed climate driving mechanisms explaining the Holocene temporal and latitudinal environmental variability in eastern South Africa. Blue squares (orange squares) ¼ site wet (dry). Top zone is the Northern SRZ: 1) Zambezi catchment (core GeoB9307-3; Schefub et al., 2011), 2) Tate Vondo (Scott, 1987), 3) core GeoB20610-2 € m et al., 2009), 7) Mfabeni (Miller et al., 2019), (this study), 4) Wonderkrater (Scott, 1982), 5) Rietvlei (Scott, 1983). Middle zone is the central and eastern SRZ: 6) Braamhoek (Norstro 8) Lake Eteza (Neumann et al., 2010). The southern-most zone is the SSAZ: 9) CD154 10-06P, (Simon et al., 2015), 10) Blydefontein (Scott et al., 2005), 11) Eilandvlei (Quick et al., 2018), 12) Verlorenvlei (Carr et al., 2015) and 13) Cecilia Cave (Baxter, 1989). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

conditions are more humid, soils become depleted in water-soluble elements, such as K, which easily leach from the top soils (Cohen, 2003; Croudace and Rothwell, 2015). In turn, the sediment becomes enriched in insoluble elements and the clay mineral smectite, which is K depleted (Govin et al., 2012). Consequently a decrease in K/Al represents increased humidity. When using the K/ Al ratio as a chemical weathering proxy, Al is used for normalisation as it is abundant and stable in clay minerals (e.g. Shala et al., 2014). Sedimentary leaf wax concentrations are associated with changes in vegetation cover, transport strength, dilution by other sedimentary components and preservation. The K/Al (Fig. 4a), Fe/Ca (Fig. 4b) and leaf wax concentration data (Fig. 4d) show similar trends as the dDwax (Fig. 4e) record and the northern precipitation stack (Fig. 5c). Consequently, increases in Fe/Ca and leaf wax concentration and decreases in K/Al are interpreted to be commonly controlled by increases in precipitation/river discharge. 5. GeoB20610-2 results and interpretation The long-chain n-alkanes extracted from core GeoB20610-2 have CPI values ranging from 2 to 8, averaging around 5 (Fig. 4c). The carbon chain lengths within the samples range from C17eC35, with C31 having the highest abundance. The concentrations of the long-chained odd-numbered n-alkanes (n-C25 to n-C35) within core GeoB20610-2 range from 200 to 5200 ng/sample (Fig. 4d). The elevated concentrations of n-C31 (especially during the last c. 3 cal. ka BP) and relatively high CPI values (averaging around 5; Fig. 4c and d), indicate that the long chain n-alkanes, over the last c. 3 cal. ka BP at least, were derived from terrestrial higher plants, with little degradation. The lower CPI values between 7 and 3 cal kyr BP indicate the presence of degraded plant waxes due to low terrestrial plant wax input. Trends in d13Cwax and dDwax are rather gradual over the last c. 7 cal. ka BP, however, a decrease in K/Al (Fig. 4a) and an increase in Fe/Ca (Fig. 4b) ratios, increased CPI values (Fig. 4c), and increased n-

alkane concentrations (Fig. 4d) imply higher contributions of terrestrial organic matter and more river influence due to higher discharge after c. 3 cal. ka BP. Wetter climatic conditions are indicated by a shift to lower dDwax values (6‰) at c. 3 cal. ka BP (Fig, 4e). Furthermore, an increased contribution of waxes from C4-type vegetation after c. 3 ka BP (1‰ positive shift in d13Cwax values; Fig. 4f) suggests increased riverine transport from the interior catchments and/or an expansion of the flooded grasslands on the river flood plains. Overall, the data from core GeoB20610-2 depicts two distinct climatic phases over the last 7 cal ka BP: an initial arid phase (c. 7e3 cal kyr BP), followed by a wetter phase (c. 3e0 cal kyr BP). 6. Eastern South Africa: hydro-climatic zones and climate drivers By comparing the results obtained from core GeoB20610-2 to published data, we identify three distinct latitudinally separated hydro-climatic zones along the eastern South African coastline, namely the YRZ and the SRZ, which can be subdivided into the northern SRZ and the central/eastern SRZ (Chevalier and Chase, 2015) (Figs. 1 and 5). Each of these zones appears to have different climatic drivers (Figs. 5 and 6). The northern SRZ, which includes the Limpopo River catchment source areas of material deposited at site GeoB20610-2 (this study; Fig. 5d and e), displays increasingly wetter climatic conditions during the late Holocene (Fig. 5bee, Fig. 6a). The wetter conditions we observe during the Late Holocene from core GeoB20610-2 are also evident in records comprising the northern precipitation stack (Fig. 5c; Wonderkrater, Tswaing Crater, Rietvlei Dam and Tate Vondo; Chevalier and Chase, 2015). In addition, the hydrological record from the Zambezi catchment displays wetter late Holocene conditions, where gradually lower dDwax values during the late Holocene appear to correlate with increasing local summer insolation (Fig. 5a and b; Fig. 6; Schefub et al., 2011). Climate trends

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documented in core GeoB20610-2 and the broader northern SRZ correlate well with local summer insolation. Within this mechanism, high local insolation drives stronger atmospheric convection, which leads to increased monsoon-driven summer precipitation and more river runoff (Fig. 6a; Schefub et al., 2011; Chevalier and Chase, 2015). The hydrological response over the Limpopo River catchment area does not appear to be linearly related to changes in local insolation, but is instead marked by a threshold response. In this scenario, the amount of precipitation may have reached a critical threshold at c. 3 cal. ka BP to influence the vegetation and hydrological proxies and for discharged terrestrial material to reach the core site. In the central and eastern SRZ, between c. 25e30
S, the late Holocene was characterised by a relative decrease in humidity compared to the early/mid Holocene (Fig. 5f and g & Fig. 6). Palaeoenvironmental records from Lake St Lucia (Humphries et al., €m et al., 2009), Lake Eteza (Fig. 6; 2016), Braamhoek (Fig. 6; Norstro Neumann et al., 2010), Mfabeni (Fig. 5g; Fig. 6; Miller et al., 2019), and the records comprising the central and eastern precipitation stack (Fig. 5f; Chevalier and Chase, 2015) all show evidence for dry conditions during the late Holocene. This aridification in the central and eastern SRZ was not driven by local summer insolation changes, but probably by an equatorward displacement of the southern hemisphere westerlies, the high-pressure cell and the SIOCZ (Fig. 6a; Miller et al., 2019). In South America palaeo-evidence for an equatorward displacement of the westerlies during the late Holocene is abundant (e.g. Gilli et al., 2005; Lamy et al., 2001; Mayr et al., 2007; Schimpf et al., 2011). Furthermore, these data are supported by climate modelling output, which also indicates a northward shift of the westerlies during the late Holocene (Hudson and Hewitson, 2001). An increase in sea-ice from c. 8 cal. ka BP, with maximum sea ice occurring during the late Holocene (Fig. 5j), as indicated by diatom data (Fragilariopsis curta and F. cylindrus) from PS2090/ODP1094 in the South Atlantic, may have been responsible for displacing the southern westerlies northward (Fischer et al., 2007). This displacement is probably associated with a northeastward migration of the SIOCZ (Fig. 6a; Cohen and Tyson, 1995; Cook, 2000). Within this hypothesis, tropical temperate troughs (TTTs), which constitute the main rain producing mechanism in eastern South Africa, preferentially form at the SIOCZ (Hart et al., 2018; Tyson and Preston-Whyte, 2000). Consequently, a northeastward displacement of the SIOCZ during the late Holocene may have shifted the precipitation related to TTTs away from the central and eastern SRZ towards the northern SRZ (Fig. 6a; Cook, 2000). This interpretation is strengthened by the evidence from marine core GeoB20610-2 (within the northern SRZ), which shows an increase in rainfall over the Limpopo River catchment during the Late Holocene. Furthermore, rainfall variability in the central and eastern SRZ is highly correlated to the latitudinal position of the South African high-pressure cell, which controls the amount of moisture advection onto the continent (Dyson and Van Heerden, 2002; Vigaud et al., 2009). A more northerly location of the highpressure cell, associated with a northward shift of the westerlies, during the late Holocene would have reduced monsoonal penetration onto the continent, lengthening the dry season and leading to increased aridity in the central and eastern SRZ (Fig. 6a). The stronger, more northerly-displaced westerlies that characterize the Late Holocene atmospheric circulation patterns were also likely responsible for the increased moisture availability documented at sites south of 30
S, such as at CD154 10-06P (Fig. 5h; Fig. 6; Simon et al., 2015), Cecilia Cave (Fig. 6; Baxter, 1989), Verlorenvlei (Fig. 5i; Fig. 6; Carr et al., 2015), Seweweekspoort (Chase et al., 2017), Klaarfontein (Meadows and Baxter, 2001), Eilandvlei (Fig. 6; Quick et al., 2018) and Blydefontein (Fig. 6; Scott et al., 2005). Although today these sites have different rainfall regimes,

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situated within the modern-day WRZ, YRZ and southern SRZ (Fig. 5h and i; Fig. 6), for the Late Holocene (<3 ka BP) we group these sites all into the SSAZ, because they show similar climatic trends. Today the frontal systems, related to the southern hemisphere westerlies are a major source of precipitation in the WRZ (Fig. 6; Mason and Jury, 1997; Singleton and Reason, 2007; Tyson, 1986). Nevertheless, the westerlies do bring rainfall further north, especially within the YRZ. With more northerly-displaced westerlies, we would expect an increase in both the frequency and the northward propagation of these rain-bearing westerlies (Fig. 6a; Reason and Rouault, 2005), increasing the precipitation amount at sites not just within today’s WRZ but also further to the north into the YRZ and southern SRZ. After the frontal systems pass the most southern tip of Africa they bring rain from the Indian Ocean and Agulhas Current region (Gimeno et al., 2010). Despite the dominance of winter precipitation for the SSAZ, variations in summer rainfall amount and the tropical easterlies are also of critical importance in determining moisture availability at these sites (Chase et al., 2015). Furthermore, it is also difficult to disentangle the influence of the Agulhas Current within the WRZ from the displacement of the westerlies and changes in the extent of Antarctic sea ice, with poleward shifts of the westerlies, thought to be linked to increased Agulhas flow offshore the southern Cape (Quick et al., 2018). Importantly, on these Holocene timescales, we find no evidence that abrupt northern hemisphere cold events drive climatic variability in this region as was previously suggested by Simon et al. (2015) and Ziegler et al. (2013). Greenland temperatures remain high over the last c. 7 ka BP (Andersen et al., 2004) and show no consistent relationship with aridity in southeastern Africa. Consequently, on these timescales, we can explain the climatic variability evidenced within the SSAZ by more northerly-displaced westerlies which resulted in more frontal rainfall. Contrary to the LGM, where low Indian Ocean SSTs appear to have been important in controlling aridity in eastern South Africa (Chevalier and Chase, 2015; Miller et al., 2019), we see no clear evidence that SST (e.g. Fig. 5k; Sonzogni et al., 1998; Wang et al., 2013), drove mid/late Holocene climatic change in any of the climatic zones identified. 7. Conclusions The data from this study (core GeoB20610-2), are thought to reflect the integrated environmental and hydroclimatic conditions of the Limpopo, Incomati and Lusutfu catchment areas. The results provide new evidence for two separate climatic phases over the last 7 cal ka BP: an initial phase of aridity (c. 7e3 cal kyr BP), and subsequently a wetter phase (c. 3e0 cal kyr BP). A comparison with the data from core GeoB20610-2 with nearby study sites indicates that these exhibit similar patterns of climate change on these timescales and thus we group these sites into the northern SRZ. Importantly, our data provides evidence that the Limpopo River catchment likely evolved climatologically as part of the northern SRZ. Comparing the data from core GeoB20610-2 to other available palaeo-records along the eastern South African coast indicates that eastern South Africa (during the late Holocene) can be divided into three geographically distinct hydro-climatic zones, each having different main drivers (Figs. 5 and 6). Firstly, the northern SRZ, where rainfall was dominated by monsoonal variability and controlled by changes in summer insolation. Secondly, the central and eastern SRZ, which is strongly affected by the location of the southern hemisphere westerlies, the South African high-pressure cell and the SIOCZ, and experienced dry (wet) conditions due to equatorward (poleward) displacement of these systems. And lastly, the SSAZ, where northerly-displaced westerlies

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during the late Holocene resulted in increased humidity. Importantly, supporting previous findings of Chevalier and Chase (2015), our data point to a strong antiphase relationship between hydro-climates in the northern SRZ and those in the central and eastern SRZ. These proposed climate-driving mechanisms account for a large part of the temporal and latitudinal environmental variability evident in mid/late Holocene palaeoenvironmental records from eastern South Africa. On these Holocene timescales, we see no strong evidence that changes in SSTs were responsible for climatic change in any of the climatic zones identified. Numerous climate models suggest that the southern westerlies will move poleward in response to anthropogenic climate warming (Russell et al., 2006; Toggweiler and Russell, 2008; Toggweiler et al., 2006). If these models are correct, then in the coming centuries, we propose a likely increase in rainfall in the central and eastern SRZ and a decrease in rainfall in the northern SRZ and the SSAZ. Indeed, the record-breaking Cape Town drought from 2015 to 2017, was recently attributed to the poleward shift of the southern westerlies (Sousa et al., 2018). Author contributions CM conducted d13Cwax and dDwax analyses. AH conducted the XRF analyses. Interpretation was carried out by CM, ES, AH, DL and MZ. Data availability All data within the manuscript are available on Mendeley Data and Pangaea. Acknowledgements This research was funded by the Bundesministerium für Bildung und Forschung (BMBF; RAiN project 03G0840 A/B). CM is also grateful for financial support via a Fellowship at WWU Münster, DE. We thank R. Kreutz for assistance with d13Cwax and dDwax data acquisition and the captain and crew of cruise M123. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.quascirev.2019.106088. References Andersen, K.K., Azuma, N., Barnola, J.M., Bigler, M., Biscaye, P., Caillon, N., Chappellaz, J., Clausen, H.B., Dahl-Jensen, D., Fischer, H., Flückiger, J., Fritzsche, D., Fujii, Y., Goto-Azuma, K., Grønvold, K., Gundestrup, N.S., Hansson, M., Huber, C., Hvidberg, C.S., Johnsen, S.J., Jonsell, U., Jouzel, J., Kipfstuhl, S., Landais, A., Leuenberger, M., Lorrain, R., Masson-Delmotte, V., Miller, H., Motoyama, H., Narita, H., Popp, T., Rasmussen, S.O., Raynaud, D., Rothlisberger, R., Ruth, U., Samyn, D., Schwander, J., Shoji, H., Siggard€rnsdo ttir, A.E., Svensson, A., Andersen, M.L., Steffensen, J.P., Stocker, T., Sveinbjo Takata, M., Tison, J.L., Thorsteinsson, T., Watanabe, O., Wilhelms, F., White, J.W.C., North Greenland Ice Core Project m, 2004. High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature 431, 147e151. Asante, K.O., Macuacua, R.D., Artan, G.A., Lietzow, R.W., Verdin, J.P., 2007. Developing a flood monitoring system from remotely sensed data for the Limpopo basin. IEEE Trans. Geosci. Remote Sens. 45, 1709e1714. Baker, A., Pedentchouk, N., Routh, J., Roychoudhury, A.N., 2017. Climatic variability in Mfabeni peatlands (South Africa) since the late Pleistocene. Quat. Sci. Rev. 160, 57e66. Baxter, A., 1996. Late Quaternary Palaeoenvironments of the Sandveld, Western Cape Province, South Africa. University of Cape Town, Cape Town, South Africa. Baxter, A.J., 1989. Pollen Analysis of a Table Mountain Cave Deposit. University of Cape Town, Cape Town. Bianchi, C., Gersonde, R., 2004. Climate evolution at the last deglaciation: the role of the Southern Ocean. Earth Planet. Sci. Lett. 228, 407e424.

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