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

Stephan Woodborne,Adrian Patrut,Mathieu Rouault,Iain Robertson,Grant Hall,Michele Hofmeyr,Neil J Loader,Shang-Ping Xie

doi:10.1371/journal.pone.0124202

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.

Highlights

Much of the Southern African rainfall is of convective origin forced by large-scale dynamics during the Austral summer [1]
The age models used for the different trees and core samples are presented in S1 Fig. The use of an interpolated radiocarbon chronology is not uncommon for isotopic analysis of trees [38], but it introduces a degree of temporal error
The sampling method imposes a further element of complexity in the chronology, and a distinction is made between the trees that yielded individual ring samples, and the cores that were subdivided into aliquots for the isotope analysis

Summary

Introduction

Much of the Southern African rainfall is of convective origin forced by large-scale dynamics during the Austral summer [1]. The summer rainfall region experiences dramatic inter-annual changes leading to severe droughts or floods that affect agricultural productivity, . Research Grant for Unrated Researchers number CSUR13092647960.

Methods

Results

Conclusion