A Regional Stable Carbon Isotope Dendro-Climatology from the South African Summer Rainfall Area.

Lucas C.R Silva,Stephan Woodborne,Jemma Finch,Grant Hall,Adrian Patrut,Patience Gandiwa

doi:10.1371/journal.pone.0159361

Abstract

Carbon isotope analysis of four baobab (Adansonia digitata L.) trees from the Pafuri region of South Africa yielded a 1000-year proxy rainfall record. The Pafuri record age model was based on 17 radiocarbon dates, cross correlation of the climate record, and ring structures that were presumed to be annual for two of the trees. Here we present the analysis of five additional baobabs from the Mapungubwe region, approximately 200km west of Pafuri. The Mapungubwe chronology demonstrates that ring structures are not necessarily annually formed, and accordingly the Pafuri chronology is revised. Changes in intrinsic water-use efficiency indicate an active response by the trees to elevated atmospheric CO2, but this has little effect on the environmental signal. The revised Pafuri record, and the new Mapungubwe record correlate significantly with local rainfall. Both records confirm that the Medieval Warm Period was substantially wetter than present, and the Little Ice Age was the driest period in the last 1000 years. Although Mapungubwe is generally drier than Pafuri, both regions experience elevated rainfall peaking between AD 1570 and AD 1620 after which dry conditions persist in the Mapungubwe area until about AD 1840. Differences between the two records correlate with Agulhas Current sea-surface temperature variations suggesting east/west displacement of the temperate tropical trough system as an underlying mechanism. The Pafuri and Mapungubwe records are combined to provide a regional climate proxy record for the northern summer rainfall area of southern Africa.

Highlights

Radiocarbon dating of baobab (Adansonia digitata L.) trees in southern Africa has demonstrated the existence of a fused, multi-stem architecture that may be one of the reasons why some of these trees achieve ages in excess of 1000 years [1]
This approach worked for the age models from the Leokwe B baobab (LKB), Mapungubwe C baobab (MPC), Luna, and Schroda baobabs and all of the assigned ages fall within the one-sigma calibration range of the AMS radiocarbon dates
Previous age modeling of baobab rings [2, 24] showed that radiocarbon dates and ring counts for baobabs in the Kruger National Park were consistent with annual ring formation

Summary

Introduction

Radiocarbon dating of baobab (Adansonia digitata L.) trees in southern Africa has demonstrated the existence of a fused, multi-stem architecture that may be one of the reasons why some of these trees achieve ages in excess of 1000 years [1]. The Pafuri record was used to contextualize the archaeological trajectory of the Iron Age in southern Africa [5] and elucidate climate response to forcing over longer time scales than can be obtained from the instrumental record It revealed that the Medieval Warm Period was relatively wet, the Little Ice Age was relatively dry, and that inter-annual variability in rainfall is linked to sea-surface temperature regimes in the Agulhas Current Core region and the Indian Ocean Dipole Moment Index [2]. Changing iWUE in trees is the physiological mechanism underlying the use of stable carbon isotope ratios as a proxy for climate, but the distortion of the record by elevated atmospheric CO2 in recent centuries needs to be considered. By comparing a climate record from Mapungubwe with that from Pafuri it is feasible to test if the E/W displacement of the TTT rainfall system is driven by changes in SST in the Agulhas Current

Materials and Methods

Results and Discussion

Conclusions