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Late Quaternary climate and environmental change in the summer rainfall region of South Africa: A study using trees and wetland peat cores as natural archives
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis contributes with information on past climate and environmental changes in South Africa’s summer rainfall region. The study is based on multi-proxy analyses on wetland peat cores and analyses of stable isotope composition (δ13C, δ18O) and wood anatomy in cross sections from subtropical trees (Breonadia salicina). The peat archive covers the last 16 ka (ka; 1000 cal yrs BP) and was analysed in terms of fossil pollen, charcoal, diatoms, phytoliths and stable isotope composition. The peat record infers relatively wet climate conditions at c. 13.7-12.8 ka, 10.5-9.5 ka and 2.5-0.5 ka, and drier conditions at c. 16-13.7 ka, 12.8-10.5 ka and during mid-Holocene. Tentatively, cooler late Pleistocene temperatures shifted towards warmer after c. 9.5 ka. The study of B. salicina demonstrates the paleo-climatic value of subtropical trees despite absence of annual tree rings. An age model was constructed from radiocarbon dating and calibration by wiggle matching. δ13C in B. salicina shows a co-variation with annual rainfall amounts, suggesting that it may be considered a regional climate-proxy. δ18O is mainly influenced by local factors, but acts as a useful complement when interpreting δ13C. Together with other regional, high-resolution records, the 600 year long δ13C-record suggests dry climate conditions in northern South Africa during the 1700s and mid-1500s AD. Inferred climate and environmental changes are suggested being a response to expansion, contraction and latitudinal shifts of the tropical, subtropical and mid-latitude atmospheric circulation cells. An observed inverse humidity pattern between southern and equatorial Africa suggests that ENSO-like teleconnections may be a possible forcing mechanism in a decadal to centennial time perspective.

Place, publisher, year, edition, pages
Stockholm: Institutionen för naturgeografi och kvartärgeologi , 2008. , 108 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 11
Keyword [en]
paleoclimate, Late Quaternary, pollen, stable isotopes, diatoms, phytoliths, dendroclimatology, South Africa
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-7375ISBN: 978-91-7155-581-6 (print)OAI: oai:DiVA.org:su-7375DiVA: diva2:198149
Public defence
2008-03-17, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 13:00
Opponent
Supervisors
Available from: 2008-02-25 Created: 2008-02-15Bibliographically approved
List of papers
1. Rainfall driven variations in δ13C and wood anatomy of Breonadia salicina trees from South Africa between AD1375 and 1995
Open this publication in new window or tab >>Rainfall driven variations in δ13C and wood anatomy of Breonadia salicina trees from South Africa between AD1375 and 1995
2005 (English)In: South African Journal of Science, ISSN 0038-2353, E-ISSN 1996-7489, Vol. 101, no 3/4, 162-168 p.Article in journal (Refereed) Published
Abstract [en]

This study demonstrates the potential of deriving palaeoenvironmental information from carbon isotope composition (δ<sup>13</sup>C) and wood anatomy properties along the growth radii of two Breonadia salicina trees from Limpopo province, South Africa. An age model, based on AMS dating and 'wiggle-match' dating of the wood, shows that the data series from the two trees span AD 1375-1995 and 1447-1994, respectively. Shifts in the trees' δ<sup>13</sup>C composition and wood anatomy resemble the indications of climate change observed in regional palaeoclimatic studies, and the parts of the B. salicina record from the last century show similarities with the observed variations in annual rainfall in the region. We propose that changes in carbon isotope composition and wood anatomy indicate variations in regional rainfall during the period of tree growth. Both the δ<sup>13</sup>C and the wood anatomy records of B. salicina signify dry conditions in the early 1400s, mid-1500s, 1700s and early 1900s. The wettest conditions were during the late 1400s and in the 1600s.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-24703 (URN)
Note
Part of urn:nbn:se:su:diva-7375Available from: 2008-02-25 Created: 2008-02-15 Last updated: 2017-12-13Bibliographically approved
2. A 600 year long δ18O-record from cellulose of Breonadia salicina trees, South Africa
Open this publication in new window or tab >>A 600 year long δ18O-record from cellulose of Breonadia salicina trees, South Africa
2008 (English)In: Dendrochronologia, no 26, 21-33 p.Article in journal (Refereed) Published
Abstract [en]

The oxygen isotope composition in cellulose was analysed along the growth axis of two 600-year-old Breonadia salicina trees (Matumi) from subtropical South Africa, with the aim of testing the isotope variations as a regional climate proxy. The visible tree rings in B. salicina are not annual and therefore radiocarbon analysis was applied to produce an age model for the isotope record, covering the time period between 1375 and 1995 AD. Before _1600 AD, a co-variation is evident between the variations in d18Ocellulose and previously published d13Ccellulose, indicating that a common factor is responsible for both the carbon and the oxygen isotope signals in the tree cellulose. Between _1600 and _1900 AD, the correlation between the isotope series weakens, concurrently with drastic changes in growth rates and average ring production rates. Possibly, this phase was a result of extreme changes in the growth environment, affecting the cellulose production rate, the source water d18O signal and the proportion of different types of oxygen isotope fractionation processes affecting the final isotope composition in the tree cellulose. We suggest that during the lifetime of the two B. salicina trees, different factors have governed the oxygen isotope signal in the cellulose. Both regional climate conditions and site-specific factors associated with the riparian growth environment have had an impact on the oxygen isotope variations in the two trees, clearly complicating the possibilities to utilise a d18Ocellulose record from this tree species for paleoclimatic reconstruction.

Identifiers
urn:nbn:se:su:diva-24704 (URN)000259432000004 ()
Note
Part of urn:nbn:se:su:diva-7375Available from: 2011-01-17 Created: 2008-02-15 Last updated: 2011-01-17Bibliographically approved
3. Reconstruction of environmental and climate changes at Braamhoek wetland, eastern escarpment South Africa, during the last 16 000 years with emphasis on the Pleistocene-Holocene transition
Open this publication in new window or tab >>Reconstruction of environmental and climate changes at Braamhoek wetland, eastern escarpment South Africa, during the last 16 000 years with emphasis on the Pleistocene-Holocene transition
Show others...
2009 (English)In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, Vol. 271, 270-258 p.Article in journal (Refereed) Published
Abstract [en]

A paleo-environmental record covering the last 16 ka (16 000 cal yrs BP) from the eastern areas of the summer rainfall region in South Africa is presented. This area is until now sparsely investigated due to the lack of well preserved natural archives. For this study, we used a peat section from a wetland situated close to the Drakensberg escarpment, where the high annual rainfall amounts supported a continuous peat accumulation since c. 16 ka. One peat core was analysed in terms of fossil pollen composition, carbon and nitrogen content, isotope composition (δ13C, δ15N) and microscopic charcoal concentration. The greatest degree of temporal resolution was achieved from the late Pleistocene and early Holocene section, where proxy-records indicate relatively dry conditions between ca. 16-13.7 ka, 12.8-10.5 ka, 9.5-8.2 ka, and wet conditions between c. 13.7-12.8 ka and 10.5-9.5 ka. A weak moisture signal is also evident at c. 8.2-7.5 ka. The late Pleistocene to early Holocene period was relatively cool, while conditions became generally warmer after 11-8 ka. The interpretation of the mid- and late-Holocene sequence is limited due to a slow accumulation and low sample resolution, but the available data suggest relatively dry conditions until c. 1.5 ka, followed by more humid conditions until c. 0.5 ka. We suggest that the millennial scale variability within the record can be attributed to shifts in the circulation systems dominating the region, i.e. the latitudinal movements of the inter-tropical convergence zone (ITCZ) and the dynamics of the mid-latitude low pressure belts.

Place, publisher, year, edition, pages
Elsevier, 2009
Keyword
paleoclimate, South Africa, palynology, Holocene
Identifiers
urn:nbn:se:su:diva-24705 (URN)doi:10.1016/j.palaeo.2008.10.018 (DOI)000263013200005 ()
Available from: 2011-01-17 Created: 2008-02-15 Last updated: 2011-01-17Bibliographically approved
4. Siliceous microfossils as Late Quaternary paleo-environmental indicators at Braamhoek wetland, South Africa
Open this publication in new window or tab >>Siliceous microfossils as Late Quaternary paleo-environmental indicators at Braamhoek wetland, South Africa
2010 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 20, no 5, 747-760 p.Article in journal (Refereed) Published
Abstract [en]

A peat-sequence covering the last 16 ka (16 000 cal. yr BP) from Braamhoek wetland, eastern South Africa, was analysed in terms of phytolith and diatom composition. The fossil peat was rich in phytoliths, while diatoms were less prominent, probably as a result of degradation during wetland plant growth associated with silica uptake. With this study we present the first continuous phytolith and diatom record from South Africa covering the Late Pleistocene and Holocene period. The phytolith assemblages indicate a clear dominance of C

3-grasses within the wetland throughout the sequence. The fossil diatom record infer changes in past moisture conditions. Unlike the modern wetland, which is dominated by benthic and aerophilic diatoms, the Late Pleistocene– early Holocene wetland favoured growth of planktonic species. Abundance of planktonic diatoms suggests three main phases when water depth was deeper than today; at c.13.6 ka, 11.3 ka and 10.4–10.0 ka. These indications of past fluctuations in humidity mostly provide confirmation of previously published indications of pollen, charcoal fragments and isotopes in the same core, but the siliceous microfossil data also help to refine the paleo-environmental interpretation of the sequence.

Keyword
diatoms, Holocene, paleoclimate, phytoliths, South Africa
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-24706 (URN)10.1177/0959683610362810 (DOI)
Note
Part of urn:nbn:se:su:diva-7375Available from: 2008-02-25 Created: 2008-02-15 Last updated: 2017-12-13Bibliographically approved

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