Calibration of δ2Hn-alkane and δ18Osugar for paleoclimate reconstructions in South Africa and its first application to peat sediments from Vankervelsvlei

Hydrogen isotope analyses of leaf wax n-alkanes (δ²H_wax) are widely applied to reconstruct paleoclimate changes. To date, it has proved difficult to disentangle past changes in the isotopic signal of precipitation (δ²H_p) and other fractionation factors, e.g. evapo-transpirative enrichment. Oxygen isotopes from hemicellulose sugars (δ¹⁸O_sugar) have been proposed to complement δ²H_wax and enable more robust paleohydrological reconstructions by coupling both isotopes. However, up to now, there is a lack of studies analysing both water isotopes in South Africa.

Therefore, we analysed δ²H_wax and δ¹⁸O_sugar from topsoils from South Africa to evaluate the coupled isotope approach on modern reference material as an initial step towards more robust paleohydrological reconstructions. The results indicate that δ²H_wax significantly correlates with δ²H_p values for growing season precipitation. However, no correlation exists between δ¹⁸O_sugar and growing season δ¹⁸O_p. While the apparent fractionation ε_{app 2H}, i.e. the difference between δ²H_wax and δ²H_p, is relatively constant and not affected by climate, ε_app 18O correlates significantly with both potential evapotranspiration and the aridity index, indicating a strong influence of evapo-transpirative enrichment on δ¹⁸O_sugar. Coupling δ¹⁸O_sugar and δ²H_wax facilitates the reconstruction of δ²H_p and δ¹⁸O_p in South Africa with a 1σ accuracy of ± ~27‰ and ± ~3.7‰, respectively, and relative humidity (RH) with a 1σ accuracy of ± ~17%.

In a second step, we applied the coupled isotope approach to a 14.6 m long sediment core to complement geochemical and sedimentological analyses. The core is from Vankervelsvlei, a fen near the southern Cape coast located 152 m above mean sea level within the year‑round rainfall zone of South Africa. Our results show relatively high values for δ²H_wax between 7,020 ⁺²⁰⁰/_‑270 and 4,770 ⁺²⁸⁰/_‑230 cal BP. Conventionally, this would be interpreted to indicate more arid conditions (referring to the ‘amount effect’ or enhanced evapo-transpirative enrichment). However, corresponding reconstructed RH values are high and point to more humid conditions. Thus, we interpret the higher δ²H_wax (as well as the isotopically positive reconstructed precipitation) to reflect changing moisture sources, i.e. more summer precipitation related to greater prominence of the Easterlies. Enhanced RH as well as increased wind speed inferred from high wind driven allochthonous input (Al, Sr, Ti), is associated with a maximum in obliquity during that time. Drier and less windy conditions are indicated between 4,770 ⁺²⁸⁰/_‑230 and 2,820 ⁺³⁵⁰/_‑330 cal BP, as suggested by lower δ²H_wax and reconstructed precipitation, low RH and reduced wind driven allochthonous input. Moister conditions persisted between 2,820 ⁺³⁵⁰/_‑330 and 1,620 ⁺⁴³⁰/_‑280 cal BP and are followed by a ~1 kyr dry period. Moisture levels have been increasing since 640 ⁺⁹⁰/_‑100 cal BP.