A 1000-Year Carbon Isotope Rainfall Proxy Record from South African Baobab Trees (Adansonia digitata L.)

Abstract

A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab (Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.

Seasonal rainfall variability in southeast Africa during the nineteenth century reconstructed from documentary sources

“…The reconstruction incorporated all published annually-resolved climate records from southern Africa and surrounding oceans that spanned all or part of the 19th and 20th centuries. These included four documentary series (from Vogel, 1989;Endfield, 2002, 2008; and this study), three coral δ 18 O and/or Sr/Ca series (Zinke et al, 2004;Zinke et al, 2009), one treering width series (Therrell et al, 2006), a baobab tree-ring isotope series (Woodborne et al, 2015) and two rainfall series derived from ships' logs .…”

“…The reconstruction (Figure 4a) includes all the proxies used in Neukom et al (2014), the new KZN rainfall chronology presented here, and the ships' log-based reconstructions from . The baobab tree-ring data of Woodborne et al (2015) are excluded from the final reconstruction, as they do not introduce additional signal and lead to lower verification skill (see section S3.3).…”

“…In addition, we use our chronology, plus recent work by and Woodborne et al (2015), to extend the multi-proxy reconstruction developed by Neukom et al (2014), and consider rainfall variability within the southern African SRZ over the last 200 years.…”

Final Radiocarbon Investigation of Platland Tree, the Biggest African Baobab

“…The oldest dated A. digitata specimens were found to have ages greater than 2000 yr [14,15]. Dated growth rings of several African baobab specimens act as a proxy climate archive and have been used for past climate reconstruction in southern Africa [16,17].…”

Radiocarbon Dating of a Very Large African Baobab from Limpopo, South Africa: Investigation of the Sagole Big Tree

“…Dated growth rings of several investigated African baobab specimens, which may act as a proxy climate archive, were used for past climate reconstruction in southern Africa [13,14].…”