Plio-Pleistocene reconstruction of East African and Arabian Sea palaeoclimate

301 Pages·2011·13.32 MB·English

Plio-Pleistocene Reconstruction of East African
and Arabian Sea Palaeoclimate

Thesis submitted for the degree of Doctor of Philosophy,
Department of Geography, UCL

Katy Elisabeth Wilson
I, Katy Elisabeth Wilson, confirm that the work presented in this thesis is my own.
Where information has been derived from other sources, I confirm that this has been
indicated in the thesis.

………………………………………… June 2011

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Abstract

Superimposed upon a long-term trend of aridification, the climate history of Africa was
punctuated by episodes of extreme climate variability, characterised by the precessionally-
forced appearance and disappearance of large lake systems within the East African Rift
Valley. In order to investigate the nature of low-latitude climate variability during the Late
Pliocene and Early Pleistocene, this thesis combines high-resolution analyses from one of
the lake phases in the Central Kenyan Rift with the reconstruction of long-term changes in
the transport of wind-borne terrigenous dust to the Arabian Sea. Climate in both regions is
strongly influenced by relative changes in the strength of the Indian Ocean monsoons, which
determine rainfall distribution in equatorial East Africa and generate the low-level winds
which transport dust offshore from the Arabian Peninsula. In the Baringo-Bogoria basin in
the Central Kenyan Rift, a well-dated package of fluvio-lacustrine sediments and diatomite
units documents a major humid phase between 2.7 and 2.55 million years ago (Ma),
coincident with the intensification of glaciation in the Northern Hemisphere. Stable oxygen
isotope measurements of diatom silica, combined with the XRF analysis of whole-sample
geochemistry, reveal that the deep lake phase was characterised by wet-dry cycles lasting,
on average, 1,400 years. Over longer timescales, variations in the aeolian delivery of
lithogenic matter to the Arabian Sea, reflected in normalised flux of titanium, show that
monsoonal circulation prior to 2.6 Ma was highly variable and primarily driven by orbitally-
forced changes in tropical summer insolation, modulated by the 400,000 year cycle of
orbital eccentricity. Millennial-scale fluctuations in the dust record also support the evidence
of abrupt wet-dry cycles in East Africa. Such high-resolution cycles are rarely found in
older records, thus giving a valuable insight to the nature of short-term fluctuations in Plio-
Pleistocene climate.

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Acknowledgements

There are a great many people to whom I owe thanks and gratitude for their support
throughout the completion of this Ph.D. thesis. First and foremost, I am immensely grateful
to my supervisor, Prof. Mark Maslin, for his unstinting belief, encouragement and guidance,
and for always having ‘a minute’ when I knock on the door. I am also greatly indebted to
my second supervisor, Dr. Anson Mackay, who has been an invaluable source of help and
advice on a huge range of subjects.

This project would not have been possible were it not for the ongoing work of the Baringo
Paleontological Research Program (led by Dr. Andrew Hill of Yale University), who have
been working in the Tugen Hills for more than forty years. Very special thanks must go to
my mentor, Dr. John Kingston, for his boundless enthusiasm, inspiring discussions about
diatomite sequences and for leading the bush-whacking adventures which resulted in my
legs being ripped to pieces by acacia for three years in a row. I am also grateful to Dr. Alan
Deino for all his work to provide dates for the Barsemoi sequence; to Dr. Robert Edgar for
counting endless numbers of diatom slides and to Boniface Kimeu, Julius, Brown, Bryce
Carlson and Suzy Cote for their assistance in the field. I would also like to thank Emily
Goble and Chris Gilbert for making the constant vehicle breakdowns bearable with never-
ending hilarity and bouts of rock-throwing.

I would also like to thank Professor Melanie Leng for her continuing guidance through the
highs and lows of diatom stable isotope analysis, to Dr. George Swann for all his advice and
for acting as a sounding board for some of my contamination theories, to Professor Jonathan
Holmes for his isotope expertise and finally, to Dr. Gavin Simpson who has helped me
enormously with the development of numerical models. I would also like to thank Professor
Martin Trauth of Universität Potsdam for his invaluable MATLAB® expertise and for
providing so much help with complicated lines of code whenever I couldn’t understand
them. I am also extremely grateful to Martin for inviting me to participate in the Suguta
Valley 2010 expedition and for giving me the chance to explore a different section of the
Kenyan Rift Valley with his team.

I am extremely grateful to Janet Hope for both personal and technical support over the last
few years and to Ian Patmore and Tula Maxted for assistance in the lab. I would also like to
thank Jim Davy for his help with SEM/EDX, Professor Juergen Thurow for training on the
use of the XRF scanner and Hilary Sloane for assistance at the NERC Isotope Geosciences
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Laboratory (NIGL), and particularly for taking two days to introduce me to the joys of the
fluorination line. A large part of this project would not have been possible were it not for the
help and support of Dr. Lallan Gupta of the Integrated Ocean Drilling Program (IODP)
repository in Kochi, Japan. By taking the unprecedented step of allowing the release of
archive core material to an external institution, Dr. Gupta ensured that I was able to
complete the analysis for this project on time. On a related note, I would also like to extend
my thanks to all the users of the basement cold store who tolerated the presence of the
‘coffin’ for 8 months!

There are a great many (past and present) Ph.D. students at UCL to whom I owe
considerable thanks for providing endless advice, entertainment and cups of tea and coffee.
In no particular order, these are: Alex Dickson, Ginnie Panizzo, George Swann, Jon Tyler,
Adam Young, Matt Owen, Charlie Hall, Emma Wiik, Jorge Salgado and Luca Marazzi –
thank you all for being great friends, conference partners and sources of constant
amusement and distraction.

On a more personal note I am hugely grateful to my family for their continued support and
encouragement but particularly to my parents who have always shown an interest in my
research, to Jo for being the best sister I could ever ask for, and to my brother Ed for being a
saint and managing to put up with me for the last five months. Finally, I would like to thank
Iain, for knowing what I wanted to do before I did and for giving me the courage and
conviction to follow my dreams. I couldn’t have done it without you.

This research was funded by a NERC Ph.D. studentship (NE/F008635/1) with support for
isotope analysis through NIGL facility grants (IP/1082/1108 and IP/1104/0509). Additional
funding was also received from the Quaternary Research Association Bill Bishop Award, a
UCL research bursary and the UCL Graduate School Conference Fund.

This thesis is dedicated to my niece Georgia, and to the memory of my late uncle Nick.

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Table of Contents

Title page ....................................................................................................................... 1
Abstract ....................................................................................................................... 3
Acknowledgements ................................................................................................................. 4
Contents ....................................................................................................................... 6
List of figures ..................................................................................................................... 10
List of tables ..................................................................................................................... 12

Chapter 1: Introduction and Palaeoclimate History ......................................... 13
1.1 Introduction ........................................................................................................... 13
1.2 Aims and Objectives ............................................................................................. 14
1.3 Thesis Structure .................................................................................................... 16
1.4 The Intensification of Northern Hemisphere Glaciation ....................................... 17
1.4.1 The Mid-Pliocene Warm Period ............................................................................... 19
1.4.2 The transition to cooler climates (iNHG) .................................................................. 21
1.4.3 Potential causal mechanisms for the iNHG............................................................... 21
1.4.4 Summary ................................................................................................................... 26
1.5 Evidence for Sub-Orbital Scale Climate Change during the Pliocene .................. 26
1.6 Low-Latitude Climate Change during the Plio-Pleistocene .................................. 28
1.6.1 The long-term aridification of Africa ........................................................................ 30
1.6.2 Possible causes of aridification ................................................................................. 33
1.6.3 Evidence for precessionally-forced climate change in the tropics ............................ 34
1.6.4 Episodes of extreme climate variability .................................................................... 35
1.7 Tropical Monsoonal Circulation ........................................................................... 39
1.7.1 Geological evolution of monsoonal circulation ........................................................ 40
1.7.2 Precessional forcing of monsoonal variations ........................................................... 41
1.7.3 Monsoon circulation and glaciation .......................................................................... 43
1.8 Summary ............................................................................................................... 44

Chapter 2: Theoretical Background - Stable Isotope Geochemistry ............... 45
2.1 Introduction ........................................................................................................... 45
2.2 Introduction to Stable Isotope Geochemistry ........................................................ 45
2.2.1 Equilibrium isotope exchange ................................................................................... 46
2.2.2 Kinetic isotope effects ............................................................................................... 46
2.2.3 Disequilibrium effects ............................................................................................... 47
2.3 The Application of Stable Isotopes as a Palaeoenvironmental Tool ..................... 49
2.4 Oxygen Isotope Systematics ................................................................................. 50
2.4.1 Isotopes in precipitation ............................................................................................ 51
2.4.2 Oxygen isotopes in lacustrine environments ............................................................. 54
2.4.3 Oxygen isotopes in marine environments ................................................................. 58
2.5 Carbon Isotope Systematics .................................................................................. 62
2.5.1 Factors affecting the δ13C of DIC in seawater........................................................... 63
2.5.2 Factors affecting the δ13C of foraminifera ................................................................ 64
2.5.3 Factors affecting the δ13C of bulk organic matter ..................................................... 65
2.6 Summary ............................................................................................................... 66

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Chapter 3: The Baringo-Bogoria Basin .............................................................. 67
3.1 Introduction ........................................................................................................... 67
3.2 Lake Baringo ......................................................................................................... 67
3.3 Geomorphlogy of the Baringo-Bogoria basin ....................................................... 69
3.4 Modern Climatic Setting ....................................................................................... 71
3.4.1 Atmospheric circulation ............................................................................................ 72
3.4.2 Rainfall Variability ................................................................................................... 74
3.4.3 Factors influencing East African climate .................................................................. 79
3.5 Regional Geological Setting ................................................................................. 82
3.5.1 The Muruyur Formation: 15.5 - 15 Ma ..................................................................... 83
3.5.2 The Ngorora Formation: 13 - 8.5 Ma ........................................................................ 84
3.5.3 Mpesida Beds: 7 - 6.2 Ma ......................................................................................... 85
3.5.4 Lukeino Formation: 6.2 - 5.6 Ma .............................................................................. 85
3.5.5 Chemeron Formation: 5.3 - 1.6 Ma ........................................................................... 86
3.5.6 Kapthurin Formation: 0.7 - 0.2 Ma ........................................................................... 86
3.6 The Barsemoi Diatomites: 2.68-2.55 Ma .............................................................. 87
3.6.1 Site description.......................................................................................................... 88
3.6.2 Chronology of the Barsemoi diatomites ................................................................... 92
3.7 Summary ............................................................................................................... 93

Chapter 4: Barsemoi Diatomites - Methodologies ............................................. 94
4.1 Introduction ........................................................................................................... 94
4.2 Characterisation of diatomite units ....................................................................... 94
4.2.1 Sediment stratigraphy ............................................................................................... 94
4.2.2 Variations in diatom assemblage .............................................................................. 94
4.3 Oxygen isotopes in biogenic silica ........................................................................ 96
4.3.1 Laboratory analysis techniques for δ18O ............................................................ 96
diatom
4.3.2 Comparison of analytical techniques ...................................................................... 101
4.3.3 Sample cleaning and preparation ............................................................................ 102
4.3.4 Isotope vital effects ................................................................................................. 106
4.4 Geochemistry: Contamination of the δ18O signal ........................................ 108
diatom
4.4.1 SEM/EDS microprobe analysis............................................................................... 109
4.4.2 Infrared spectroscopy .............................................................................................. 109
4.4.3 Whole-sample geochemistry by X-ray fluorescence spectrometry ......................... 110
4.5 Statistical and multivariate analyses ................................................................... 111
4.5.1 Ordination techniques ............................................................................................. 111
4.5.2 Mass-balance calculations ....................................................................................... 112
4.6 Summary ............................................................................................................. 114

Chapter 5: The Palaeoenvironmental Reconstruction of Climate Variability
and Lake Evolution at Palaeo-lake Baringo ................................. 116
5.1 Diatomite characterisation .................................................................................. 116
5.1.1 Meso-scale sedimentological stratigraphy .............................................................. 116
5.1.2 Variations in diatom assemblage ............................................................................ 118
5.2 Assessing contamination with the δ18O record ............................................ 123
diatom
5.2.1 Oxygen isotope analysis of biogenic silica ............................................................. 123
5.2.2 Qualitative assessment of contamination ................................................................ 125
5.2.3 Quantitative assessment of contamination .............................................................. 127
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5.2.4 Modelling of oxygen isotope signal ........................................................................ 131
5.3 Evaluation of δ18O changes recorded in the Barsemoi diatomite sequence ........ 142
5.4 Evolution of palaeo-lake Baringo ....................................................................... 147
5.5 Millennial-scale cyclicity in the Barsemoi diatomite sequence .......................... 151
5.5.1 The manifestation of millennial-scale cyclicity in African palaeoclimate records . 153
5.5.2 Mechanisms for millennial-scale cyclicity .............................................................. 156
5.5.3 Millennial-scale cyclicity during the Pliocene ........................................................ 158
5.6 Summary ............................................................................................................. 161

Chapter 6: The Arabian Sea .............................................................................. 163
6.1 Introduction ......................................................................................................... 163
6.2 The Owen Ridge – ODP Sites 721 and 722 ........................................................ 164
6.3 Modern Climatic Setting ..................................................................................... 167
6.3.1 Indian Ocean monsoonal circulation ....................................................................... 168
6.3.2 The aeolian dust flux ............................................................................................... 170
6.4 Oceanographic Setting ........................................................................................ 172
6.4.1 Indian Ocean circulation ......................................................................................... 172
6.4.2 Interannual variability ............................................................................................. 174
6.4.3 Arabian Sea circulation ........................................................................................... 176
6.5 Summary ............................................................................................................. 178

Chapter 7: Arabian Sea Project Methodologies .............................................. 180
7.1 Introduction ......................................................................................................... 180
7.2 Core Sampling .................................................................................................... 180
7.3 Foraminiferal Stable Isotopes ............................................................................. 181
7.3.1 Sample preparation and analysis ............................................................................. 182
7.4 Bulk Organics ..................................................................................................... 183
7.4.1 Sample preparation ................................................................................................. 183
7.5 X-Ray Fluorescence Elemental Analysis ............................................................ 184
7.5.1 Background information ......................................................................................... 184
7.5.2 Core preparation and measurement ......................................................................... 186
7.5.3 Calibration of XRF data .......................................................................................... 187
7.6 Core Chronology ................................................................................................. 187
7.6.1 Composite depth model .......................................................................................... 187
7.6.2 Orbital dating .......................................................................................................... 190
7.7 Data Analysis ...................................................................................................... 190
7.7.1 Generalised additive and linear modelling .............................................................. 191
7.7.2 Spectral analysis ...................................................................................................... 192
7.8 Summary ............................................................................................................. 193

Chapter 8: A 600-kyr Record of Dust Transport to the Arabian Sea ........... 194
8.1 Outline................................................................................................................. 194
8.2 Revised Chronology for ODP site 721/722 ........................................................ 194
8.2.1 Benthic foraminiferal isotopes ................................................................................ 196
8.2.2 Correlation to the LR04 global benthic stack .......................................................... 196
8.2.3 Sedimentation rates ................................................................................................. 197
8.2.4 Comparison to other age models ............................................................................. 199
8.2.5 Summary ................................................................................................................. 199
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8.3 Elemental Indicators of Terrigenous Dust Flux .................................................. 201
8.3.1 Calibration and normalisation of XRF data ............................................................ 201
8.3.2 The interpretation of geochemical trends as records of aeolian dust flux ............... 204
8.3.3 XRF elemental ratios .............................................................................................. 206
8.3.4 Summary ................................................................................................................. 210
8.4 Time-Series Analysis of Ti/Al data .................................................................... 211
8.4.1 Generalised additive models (GAM) ...................................................................... 212
8.4.2 Spectral analysis ...................................................................................................... 213
8.4.3 Summary ................................................................................................................. 221
8.5 Low-latitude Monsoonal Variability during the Pliocene ................................... 222
8.5.1 Sea surface temperatures ......................................................................................... 222
8.5.2 Tropical records of aeolian dust supply .................................................................. 226
8.5.3 Orbital changes in insolation ................................................................................... 236
8.5.4 Correlation to the Barsemoi Diatomites .................................................................. 239
8.6 Summary ............................................................................................................. 242

Chapter 9: Project Synthesis ............................................................................. 245
9.1 Summary of Results ............................................................................................ 245
9.1.1 Tropical climate fluctuations: A comparison to Barsemoi Diatomite #4 ................ 246
9.1.2 The impact of high-latitude changes upon tropical climate .................................... 249
9.1.3 Linking African palaeoclimate changes with early human evolution ..................... 250
9.1.4 Summary of findings ............................................................................................... 251
9.2 Directions for Future Research ........................................................................... 252

Chapter 10: References ........................................................................................ 256

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List of Figures

Chapter 1.

Figure 1.1: Map showing locations of study sites ..................................................................... 14
Figure 1.2: Plio-Pleistocene evolution of high-latitude climate.. .............................................. 18
Figure 1.3: Comparison of major transitions in Plio-Pleistocene climate. ................................ 29
Figure 1.4: Lacustrine and tectonic history of East Africa. ....................................................... 36

Chapter 2.

Figure 2.1: Global distribution of annual weighted δ18O values of precipitation ...................... 52
Figure 2.2: Controls of the δ18O composition of lacustrine diatom silica.. ................................ 56
Figure 2.3: Distribution of surface ocean δ13C values. .............................................................. 62
Figure 2.4: Controls on the δ13C composition of foraminifera. ................................................. 64

Chapter 3.

Figure 3.1: Satellite view of the Central Kenyan Rift Valley .................................................... 68
Figure 3.2: Geomorphology of the Kenyan Rift Valley and the Baringo region ....................... 70
Figure 3.3: Seasonal changes in atmospheric circulation over Africa.. ..................................... 73
Figure 3.4: Mean position of the mid-level African Easterly Jet. .............................................. 74
Figure 3.5: Mean annual precipitation distribution over Africa. ............................................... 75
Figure 3.6: Schematic monthly transect of East African rainfall ............................................... 77
Figure 3.7: Monthy precipitation δ18O values for Tanzania and Ethiopia.. ............................... 79
Figure 3.8: Site map of the Tugen Hills detailing exposures of the Chemeron formation. ....... 83
Figure 3.9: Stratigraphic framework of the Tugen Hills geological sequence. ......................... 84
Figure 3.10: Photographic plates depicting type section of the Barsemoi Diatomites ................ 89
Figure 3.11: General stratigraphic correlations between Barsemoi Diatomite exposures.. ......... 91
Figure 3.12: Interpolated 40Ar/39Ar ages for the Barsemoi Diatomites. .................................... 92

Chapter 4.

Figure 4.1: Variations in diatom assemblage throughout the Barsemoi Diatomites .................. 95
Figure 4.2: Schematic illustration of method used for cleaning of diatom samples.. .............. 103
Figure 4.3: Illustration of SPLITT separation. ........................................................................ 106

Chapter 5.

Figure 5.1: Sediment stratigraphic section of Barsemoi Diatomite type section ..................... 117
Figure 5.2: Variations in diatom assemblage for diatomite unit #4.. ....................................... 118
Figure 5.3: Scanning electron microscope images of Aulacoseira and Stephanodiscus. ........ 119
Figure 5.4: Raw δ18O isotope and XRF data from diatomite samples ..................................... 124
Figure 5.5: Scanning electron microscope images to assess diatom purity.. ........................... 126
Figure 5.6: Results of IR spectroscopy ................................................................................... 127
Figure 5.7: Results from contamination models tested to correct δ18O values .. .............. 134
diatom
Figure 5.8: Ordination biplot showing results of PCA performed on XRF results .................. 136
Figure 5.9: Attribute plots for elements indicative of tephra contamination. .......................... 137
Figure 5.10: Attribute plots for elements indicative of clay contamination .............................. 138
Figure 5.11: Conceptual model for three end-member contamination models.. ........................ 139
Figure 5.12: Ternary plot showing modelled distribution of δ18O values. ................................ 140
Figure 5.13: PCA axis 1 scores against modelled clay contamination proportions. .................. 142
Figure 5.14: Compilation of δ18O values, contamination and assemblage changes ......... 143
modelled
Figure 5.15: Mesoscale wind circulation changes in East Africa.. ............................................ 145
Figure 5.16: Setting of Barsemoi Diatomites against changes in insolation. ............................ 148
Figure 5.17: Conceptual models for development of Rift Valley lakes ..................................... 150
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