Late Holocene diatom community responses to climate variabiligy along the southern Cape coastal ...

336 Pages·2014·17.28 MB·English

by  Kelly Kirsten

Late Holocene diatom community responses to
climate variability along the southern Cape coastal
plain, South Africa

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Kelly Kirsten
Thesis Presented for the Degree of
DOCTOR OF PHILOSOPHY
in the Department of Environmental and Geographical Science
UNIVERSITY OF CAPE TOWN
February 2014
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published without full acknowledgeement of the source.
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ABSTRACT

In recent decades an emphasis on understanding the long-term variability of ecosystems and their
responses to environmental changes has been undertaken. This has mainly been achieved through the
identification and analysis of fossil material in lake sediments. In the South Africa, the relative paucity of
available sites and the semi-arid nature of the region have led to incomplete or discontinuous
palaeoclimatic records. However, lakes along the southern Cape coastal plain have shown promise.
With this in mind palaeo-records from neighbouring rainfall regimes were used to evaluate the
spatial and temporal patterns of climatic fluctuations during the late Holocene. Diatom analysis was
conducted on sediment cores retrieved from coastal lakes at two localities, namely the Wilderness Lake
Complex, near Knysna, as a representative of the year-round rainfall zone, and Princessvlei on the Cape
Flats, as a representative of the winter rainfall zone of the southern African coastal zone. Additionally,
oxygen isotopic analysis was performed on the silicate frustules from Princessvlei. On the basis of this
evidence, it can be stated with some confidence that salinity is the driving mechanism governing the
state of the Wilderness lake systems, which in turn is a response to sea level changes and climatic
fluctuations, in particular moisture availability. Alternatively, the Princessvlei system is governed by
changes in nutrients and water turbidity, which have been shown to be proxies for moisture availability
and wind. This makes moisture availability a primary controlling factor over the environment along the
southern coastal plain, with shifts between wet and dry periods occurring rapidly. Notable shifts in
precipitation coincide with global climatic phenomena, including the Medieval Climate Anomaly (~1200
– 700 cal yrs BP) and the Little Ice Age (~700 – 125 cal yrs BP). Marine transgressions during the late
Holocene are also recorded in the diatom assemblage particularly in the Wilderness region, as is the
system’s recovery during the subsequent regressions. However, anthropogenic forcings during the last
few centuries have resulted in considerable modifications in the natural functioning of all systems,
altering flow dynamics and nutrient influx. The outcome of this study shows that diatom analysis is a
powerful tool in the reconstruction of past environments under conditions wherein other typical
palaeoenvironmental proxies may be less adequately represented.
Keywords
Diatoms, late Holocene, southern Cape coastal plain, palaeoclimate, sea level changes

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ACKNOWLEDGEMENTS

This project would not have been possible without the assistance, belief and motivation of my
supervisor Professor Mike Meadows. Thanks are also extended to the copious number of people who
were bombarded with emails relating to diatom analysis and statistics (special mention to fellow
diatomist, Liandra Bertolli) as well as to the conservationists at the SANParks Sedgefield office and Cape
Nature (Princessvlei). This degree has afforded many opportunities, particularly through various
laboratory visits to Friedrich Schiller University of Jena and Forschungszentrum Jülich which were
wonderful and interesting experiences.
The Institute of Geography at FSU-Jena provided various additional data to compliment this research,
including grain size analysis and seismic profiles for the Wilderness Lakes. In particular, I’d like to thank
Dr. Torsten Haberzettl, Bastian Reinwarth, Sarah Franz, Thomas Kasper and Siegfried Clausnitzer as well
as acknowledging their insightful discussions during the interpretation stages. I’m grateful to Dr.
Andreas Lücke for hosting me at the Forschungszentrum Jülich where the isotopic analysis was
conducted. The moral support and encouragement recieved from friends, family and departmental
colleagues is something I will always be grateful for, especially to my eternally present office mate,
Lynne Quick and my father who has supported me for all these years.
The generous funding received from the Deutscher Akademischer Austauschdienst (DAAD), National
Research Foundation (NRF), the University of Cape Town’s Postgraduate Funding Office and the
Bundesministerium für Bildung und Forschung (BMBF) (the Wilderness sites form part of a bilateral -
South African/German - project between the University of Cape Town and Friedrich Schiller University of
Jena) greatly assisted in the successful conclusion of this project. The financial assistance of the National
Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed and
conclusions arrived at, are those of the author and are not necessarily to be attributed to the NRF.

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TABLE OF CONTENTS
ABSTRACT ............................................................................................................................................................... I
KEYWORDS............................................................................................................................................................. I
ACKNOWLEDGEMENTS .......................................................................................................................................... II
LIST OF FIGURES...................................................................................................................................................V II
LIST OF TABLES ...................................................................................................................................................XVI
CHAPTER 1. INTRODUCTION ............................................................................................................................ 1
1.1. THE SIGNIFICANCE OF PALAEOENVIRONMENTAL RESEARCH IN SOUTH AFRICA ....................................... 2
1.2. RESEARCH QUESTIONS: .......................................................................................................................... 5
1.3. AIMS AND OBJECTIVES: .......................................................................................................................... 6
1.4. THESIS STRUCTURE: ............................................................................................................................... 7
CHAPTER 2. THE ENVIRONMENT OF THE SOUTHERN CAPE COASTAL PLAIN ..................................................... 8
2.1. CONTEMPORARY ENVIRONMENTAL SETTING ......................................................................................... 8
2.2. OCEANIC CIRCULATION ........................................................................................................................ 10
2.2.1. Agulhas System ........................................................................................................................... 10
2.2.2. Benguela System ......................................................................................................................... 13
2.3. CONTEMPORARY CLIMATE DYNAMICS ................................................................................................. 13
2.3.1. Ocean-to-Atmosphere Feedbacks ................................................................................................ 14
2.3.2. Long-term Climate Variability ...................................................................................................... 16
2.4. LATE HOLOCENE PALAEOENVIRONMENT.............................................................................................. 20
2.4.1. Palaeoclimate ............................................................................................................................. 20
Winter Rainfall Zone (WRZ) .......................................................................................................................................20
Year-round Rainfall Zone (YRZ) ..................................................................................................................................24
Summer Rainfall Zone (SRZ) .......................................................................................................................................28
2.4.2. Palaeo-Oceanic Influences ........................................................................................................... 32
2.5. CONCLUSIONS ..................................................................................................................................... 35
CHAPTER 3. PROXIES AND DEPOSITIONAL ENVIRONMENTS........................................................................... 36
3.1. DEPOSITIONAL ENVIRONMENTS........................................................................................................... 37
3.2. LAKES AND DIATOMS ........................................................................................................................... 38
3.3. LAKE CHARACTERISTICS........................................................................................................................ 41
3.4. LIMITATIONS: ...................................................................................................................................... 45
3.5. CONCLUSIONS ..................................................................................................................................... 46

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CHAPTER 4. METHODS ................................................................................................................................... 47
4.1. SEDIMENT SAMPLING: ......................................................................................................................... 47
4.1.1. The Wilderness Cores................................................................................................................... 50
4.1.2. The Princessvlei Core ................................................................................................................... 54
4.2. CHRONOLOGY ..................................................................................................................................... 56
4.3. SEDIMENT ANALYSES ........................................................................................................................... 57
4.4. DIATOM ANALYSIS: .............................................................................................................................. 58
4.5. ISOTOPIC ANALYSIS .............................................................................................................................. 59
4.6. STATISTICAL ANALYSES ........................................................................................................................ 61
4.7. CONCLUSIONS ..................................................................................................................................... 64
CHAPTER 5. WILDERNESS REGION AND ITS PALAEO-DIATOM ASSEMBLAGES ................................................ 65
5.1. REGIONAL SETTING .............................................................................................................................. 65
5.1.1. Regional Climate ......................................................................................................................... 67
5.1.2. Hydrology.................................................................................................................................... 69
Wilderness Lake Complex ..........................................................................................................................................71
Swartvlei Complex.....................................................................................................................................................74
5.1.3. Vegetation .................................................................................................................................. 76
5.1.4. Human Impact............................................................................................................................. 77
5.1.5. Summary ..................................................................................................................................... 78
5.2. RESULTS: EILANDVLEI ........................................................................................................................... 79
5.2.1. Chronology: ................................................................................................................................. 81
5.2.2. Assemblage Zonation .................................................................................................................. 85
5.2.3. Statistical Anaylsis ....................................................................................................................... 92
5.2.4. Indice de Polluosensibilité ............................................................................................................ 97
5.3. RESULTS: SWARTVLEI: .......................................................................................................................... 98
5.3.1. Chronology: ................................................................................................................................. 99
5.3.2. Assemblage Zonation ................................................................................................................ 100
5.3.1. Statistical Analysis ..................................................................................................................... 103
5.3.2. Indice de Polluosensibilité .......................................................................................................... 109
5.4. CONCLUSIONS: ...................................................................................................................................... 109
CHAPTER 6. HYDROLOGICAL DEVELOPMENT OF THE WILDERNESS EMBAYMENT ........................................ 111
6.1. MARINE EMBAYMENT PHASE (~4000 – 3450 CAL YRS BP): ..................................................................... 111
6.2. LAGOON PHASE (~3450 – 2000 CAL YRS BP):.......................................................................................... 113
6.2.1. 2200 – 2000 cal year BP ............................................................................................................. 116

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6.3. COASTAL LAKE PHASE (2000 CAL YRS BP - PRESENT: ................................................................................. 118
6.3.1. Swart and Wilderness Catchment Dynamics ............................................................................... 122
Medieval Climate Anomaly (MCA) ........................................................................................................................... 122
Little Ice Age (LIA) ................................................................................................................................................... 126
Introduction of Anthropogenic Forcing .................................................................................................................... 129
6.4. CONCLUSIONS ................................................................................................................................... 130
CHAPTER 7. CAPE FLATS AND ITS ASSOCIATED LATE HOLOCENE PALAEOENVIRONMENT ............................ 132
7.1. REGIONAL SETTING ............................................................................................................................ 132
7.1.1. Regional Climate ....................................................................................................................... 135
7.1.2. Hydrology.................................................................................................................................. 137
7.1.3. Vegetation ................................................................................................................................ 142
7.1.4. Land-use ................................................................................................................................... 143
7.2. RESULTS: PRINCESSVLEI ..................................................................................................................... 144
7.2.1. Chronology: ............................................................................................................................... 145
7.2.2. Assemblage Zonation ................................................................................................................ 147
2600 – 1300 cal yrs BP [Zone PV11.3a] ..................................................................................................................... 147
~1300 – (920) – 610 cal yrs BP [Zone PV11.3b] ......................................................................................................... 147
~620 – 450 cal yrs BP [Zone PV11.3d] ...................................................................................................................... 148
~320 – 240 cal yrs BP [Zone PV11.3d] ...................................................................................................................... 148
7.2.3. Statistical Analysis ..................................................................................................................... 151
7.2.4. Indice de Polluosensibilité .......................................................................................................... 157
7.2.5. Isotopic Analysis ........................................................................................................................ 158
7.2.6. Conclusions ............................................................................................................................... 159
7.3. A 2600 YEAR PALAEOENVIRONMENTAL RECORD FROM PRINCESSVLEI, CAPE FLATS: ............................ 160
7.3.1. 2600 – 1300 cal yrs BP ............................................................................................................... 160
7.3.2. Medieval Climate Anomaly (MCA) ............................................................................................. 166
7.3.3. Little Ice Age (LIA) ...................................................................................................................... 167
7.3.4. The Neumann et al. (2011) pollen record ................................................................................... 171
7.3.5. Isotopic comparisons ................................................................................................................. 173
7.3.6. Conclusion: ................................................................................................................................ 175
CH A PT ER 8 . REGIONAL TO GLOBAL CLIMATE DYNAMICS .................................................................... 177
8.1. REGIONAL MOISTURE BUDGET ........................................................................................................... 179
8.2. ATMOSPHERIC VARIABILITY ............................................................................................................... 184
8.3. ENVIRONMENTAL SUMMARY ............................................................................................................. 190
8.4. CONCLUSION ..................................................................................................................................... 192

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CHAPTER 9. CONCLUDING REMARKS ........................................................................................................... 193
9.1. CONSERVATION EFFORTS ................................................................................................................... 193
9.2. FUTURE TRENDS AND OUTLOOK ......................................................................................................... 196
9.3. CLOSING REMARKS: ........................................................................................................................... 197
REFERENCES ....................................................................................................................................................... 199
APPENDICES....................................................................................................................................................... 226
APPENDIX ONE ............................................................................................................................................... 226
APPENDIX TWO .............................................................................................................................................. 232
APPENDIX THREE ............................................................................................................................................ 309

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LIST OF FIGURES

Figure 1.1: Selected southern African palaeosites across the three rainfall regimes, from west – east: winter rainfall
zone (WRZ), year-round rainfall zone (YRZ) and summer rainfall zone (SRZ) also indicating the southern coast of
South Africa (source: see Appendix One) ................................................................................................................. 3

Figure 2.1: The major biomes of the southern Cape coastal plain of South Africa bounded to the north by the Cape
Fold Mountains (Source: Appendix One) .................................................................................................................. 9

Figure 2.2: Generalised schematic of major atmospheric (white) and oceanic circulation (warm currents = red, cold
currents = blue) across southern Africa. AnC = Angola Current, SEC = South Equatorial Current, MC = Mozambique
Current, MCE = Mozambique Current Eddies, EMC = East Madagascar Current, AC = Agulhas Current, ARC = Agulhas
Return Current, SIC = South Indian Ocean Current, STF = Subtropical Front, ACC = Antarctic Circumpolar Current,
SAC = South Atlantic Current, BOC = Benguela Ocean Current, BCC = Benguela Coastal Current, SB = southern
boundary, ABF = Angola-Benguela Front (Source: see Appendix One) .................................................................... 12

Figure 2.3: A conceptual model of the response of oceanic circulation and coastal sea surface temperatures (SST) to
atmospheric circulation anomalies responsible for extended wet spells (A) and extended dry spells (B) in South
Africa (after Cohen and Tyson, 1995). ................................................................................................................... 18

Figure 2.4: Distribution of palaeo-sites across southern Africa discussed in text with respect to rainfall regimes, the
demarcation of the rainfall zones are based on the percentage rainfall received over the course of the year. Winter
rainfall = shades of pink, Year-round rainfall = yellow, Summer rainfall zone = shades of green (sources: see
Appendix One). ..................................................................................................................................................... 19

Figure 2.5: A 1400 year record of diatom abundance at Verlorenvlei (data source: Stager et al., 2012) .................. 23

Figure 2.6: late Holocene temperature curve with standard deviation derived from the Cango Cave δ18O stalagmite
sequence (Source: Talma and Vogel, 1992, Pg 208) ............................................................................................... 25

Figure 2.7: late Holocene δ18O stalagmite sequence from Cold Air Cave; horizontal line indicates the average for the
entire 6300 yr record (Source: Lee-Thorp et al., 2001, Pg 4908) ............................................................................. 29

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