High-resolution record of river discharge in SE Africa

Marine sediment core CD154-17-17K (33° 16.13′ S, 029° 07.29′ E, 3,333 m water depth) was retrieved from ∼95 km off the Eastern Cape coast near the mouth of the Great Kei river (Fig. 1). The initial chronology of the core is established through eight radiocarbon dates in the upper part of the core and graphically correlating the planktonic foraminiferal (Globigerinoides ruber) oxygen isotope (δ18O) record to the Antarctic deuterium (temperature) record of the EPICA Dome C ice core35. The planktonic δ18O record reflects the combined influence of ambient sea-surface temperature variability in the Agulhas Current and global ice volume changes and shows a good fit with the long-term temperature variability in Antarctica (Fig. 3). The sediment core spans a time period of approximately the last 100,000 years with an average sedimentation rate of 4 cm kyr−1. Elemental concentrations across the whole core were obtained with an X-ray fluorescence (XRF) core scanner (ITRAX). The relative element intensity counts obtained from the XRF scanning were calibrated to concentrations using a suite of individual samples analysed for absolute bulk elemental composition. The major oxide elemental ratios in sediments from CD154-17-17K are very similar to the ratios in the suspended load of rivers in South Africa36 that drain similar rock types as the Great Kei river, suggesting that the terrestrial material is of local origin. The most proximal source for terrestrial material to CD154-17-17K is the Great Kei river, which is ∼520 km long and has a catchment area of 20,566 km2, forming the southern border of the Transkei coast of the Eastern Cape (Fig. 1). Several other rivers also enter the Indian Ocean to the north of our core site. These include the Mbashe, Umzimvubu and Umtata rivers, as well as the Tugela, the largest in the KwaZulu-Natal Province. These rivers are all typical brown-water rivers, characterized by high sediment loads. Their sediments, in particular those derived from the latosol-type soils, derived from mudstones and sandstones of the Karoo Supergroup and associated intruded dolerites (‘Ironstone’) within the catchment areas, are notably rich in iron (Fe) oxides. Consequently, the Fe/Ca ratio recorded in CD154-17-17K can be used as a first-order indication of relative changes in the amount of fine (Fe-rich) terrigenous components supplied to the core site from regional river discharge. However, we use the Iron/Potassium (Fe/K) ratio as a more reliable proxy, as it is independent of possible variations in biogenic carbonate input. Fe/K ratios serve as indicator of changes between humid and dry conditions34,37. Govin et al.37 demonstrate, that the spatial distributions of Fe/K in marine core-top samples reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas. In tropical humid regions, high precipitation promotes intense chemical weathering of bedrocks38, resulting in highly weathered soils whose geochemical signature, rich in Fe, is transferred to marine sediments by fluvial input. In contrast, K derives from potassium feldspar or illite, which are both characteristic of drier regions with low chemical weathering rates39. Govin et al.37 find that low Fe/K values indicate dominant deposition of only slightly weathered particles originating from relatively dry areas on the subtropical African margin. Conversely, suspended material from the major river systems exhibits high Fe/K ratios. In addition, the spatial Fe/K distribution observed along the African continental margin and in African dust- and river-suspended samples reflects the spatial distribution of African soil types. High Fe/K values recorded in the tropics reflect the presence of intensively weathered soils enriched in Fe over the adjacent continent. Govin et al.37 conclude therefore, that Fe/K ratios of surface sediments can be used to reconstruct African and South American climatic zones. Fe/K ratios have also been applied as a proxy for fluvial versus aeolian input, with high values indicating an increased supply of river-suspended material relative to dust deposition34. Together, these proxies provide a high-resolution record of variable hydrological conditions in the Eastern Cape.

Figure 3: Age model construction. (a) Graphical correlation of G. ruber δ18O record of CD154-17-17K (red) to the EPICA Dome C deuterium record (purple) (EPICA) on speleothem-timescale of ref. 24. Age control points as dots, red dots are based on radiocarbon dates and blue dots are based on the tuning of the foraminiferal δ18O record. (b) δ18O splice from Chinese speleothems (green) (Hulu and Sanbao Cave)20,48. (c) Fe/K ratio of CD154-17-17K on the initial age model. (d) Fe/Ca ratio of CD154-17-17K on the initial age model. Stippled lines show the fine-tuning of the initial age model through graphical correlation of Fe/K ratio of CD154-17-17K to δ18O splice from Chinese speleothems (Hulu and Sanbao Cave). Full size image

Millennial-scale climate variability in the Eastern Cape

The Fe/Ca and Fe/K records are prominently punctuated by a series of abrupt events, indicating pulses in river input and more humid conditions (Fig. 3). Within the uncertainty of our age model, it is possible to link each individual event in the Fe/K record to a corresponding millennial-scale cold event in the Northern Hemisphere temperature as documented in Greenland Ice core records25. The amplitude of the long-term variability appears to be larger in the Fe/Ca record than in the Fe/K ratio, which may indicate that variable carbonate production and preservation may have an additional role on these longer timescales. Sea-level changes may have had an impact as well, with increased valley river incision during sea-level low stands and, in consequence, a larger discharge of terrigenous sediments. However, these processes have no effect on the ratio of the terrigenous elements Fe and K.

Modelling studies confirm that remote atmospheric forcing during Northern Hemisphere cold events is a key driver of hydrological variability in South Africa, resulting in wetter conditions during these events40,41,42. A mean southeastward shift in the positions of the South Indian and South Atlantic Ocean anticyclones would lead to increased rainfall over the Eastern Cape. In contrast, palaeoclimate reconstructions from Lake Malawi15, Lake Tanganyika43 and the Sahel zone34 indicate much dryer conditions associated with Northern Hemisphere cold events, reflecting the southward shift of the ITCZ during these periods. Similarly, West Africa experienced weaker monsoonal precipitation during these events29. Evidence from an ocean–atmosphere general circulation model44 suggests that this linkage also occurs because a weakened meridional overturning circulation in the Atlantic Ocean leads to a bipolar seesaw warming response in the equatorial South Atlantic Ocean28, and consequently a reduction in West African summer monsoonal winds and rainfall over West Africa29. While large parts of sub-Saharan Africa faced severe dry conditions during North Atlantic cold events45, South Africa apparently experienced more humid conditions. Precipitation in the Eastern Cape is strongly dependent on the Agulhas Current sea-surface temperatures, with a warm current providing supply of low-level moisture and buoyancy to facilitate the occurrence of deep convection and rainfall, as the onshore flow reaches the coast46. Associated with the bipolar seesaw response, the Southern Ocean experiences a southward shift of the subtropical front during North Atlantic cold events26, and a southward shift of the southern Hemisphere Westerlies is associated with an increase in the wind stress curl in the South Indian Ocean and warming in the Agulhas Current47.

The East Asian summer monsoon also responded sensitively to these glacial cooling events in the Northern Hemisphere. Oxygen isotope records in speleothems from Chinese Caves provide a high-resolution and precisely dated (U/Th) record of East Asian–Indian Monsoon intensity20,48,49 and indicate weakened monsoon intensity during Northern Hemisphere cold events. When compared directly, our record of Eastern Cape riverine input (humid versus dry conditions) and the Chinese speleothem record show a remarkable similarity in the structure of millennial-scale events (Fig. 3) that also fit dynamically with the interhemispheric signal propagation in that they indicate opposite wet–dry successions between the records. We take advantage of the precise absolute dating of the speleothem record to further fine-tune the age-scale of CD154-17-17K by synchronizing transitions into and out of the millennial excursions in both records. The average age difference between the initial and this fine-tuned age model is only 0.07 kyr. Absolute age differences for individual tie points are generally within 1 kyr and always less than ±1.8 kyr and are therefore well within the age uncertainty of the initial low-resolution planktonic foraminiferal δ18O-based age model. We adopt the speleothem-based ages in the following detailed comparison of millennial-scale climate events with the archaeological record and the dates of well-documented phases in behavioural and technological innovation such as the SB and HP industries of the MSA7.