About this title - Sedimentary Dynamics in the Albertine Rift Valley, Equatorial East Africa

Appendices A1–A3 and Acknowledgements for GSL Memoir 61, Sedimentary Dynamics in the Albertine Rift Valley, Equatorial East Africa .

Convectively coupled equatorial Kelvin waves (CCEKWs) are those types of equatorially trapped disturbances that propagate eastward and are among the most common intra-seasonal oscillations in the tropics. There exists two-way feedback between the inter-tropical convergence zone (ITCZ) and these equatorially trapped disturbances. Outgoing Longwave Radiation (OLR) was utilized as a proxy for deep convection. For CCEKWs, the modes are located over the West Atlantic, equatorial West Africa, and the Indian Ocean. The influence of other circulations and climate dynamics is studied for finding other drivers of climate within East Africa. The results show a positive relationship between Indian and Atlantic Oceans Sea Surface Temperatures and March-May rainfall over equatorial East Africa over the period of 1980 to 2010. This influence is driven by the Walker circulation and anomalous moisture influx enhanced by winds. Composite analysis reveals strong lower-tropospheric westerlies during the active phase of the CCKWs activities over Equatorial East Africa. The winds are in the opposite direction with the upper-tropospheric winds, which are easterlies. Singular Value Decomposition shows a strong coupling interaction between rainfall over equatorial East Africa and CCKWs. This study concludes that Kelvin waves are not the main factors that influence rainfall during the rainy season. Previous studies show that the main influencing factors are ITCZ, El-Nino Southern Oscillation (ENSO), and tropical anticyclones that borders the African continent. However, CCKWs are a significant factor during the dry seasons.

Continental rifting is a fundamental geological process that has been responsible for breaking supercontinents apart and opening new ocean basins throughout Earth's history and the East African Rift System (EARS) is the best active example of this happening in the world today. The Albertine Rift lies at the northern end of the Western Arm of the EARS, consisting primarily of the Lake Albert and Lake Edward basins as they pass along the border between Uganda and the Democratic Republic of Congo. This Memoir describes the key geomorphological features found in the Albertine Rift Valley at the present day, identifying the sedimentary dynamic elements that dovetail together to characterize the dominant depositional environments in these early continental rift basins. These are then used to interpret the Pliocene–Holocene sedimentary sequences, documented during geological field surveys over an eight-year period, exposed around the onshore parts of the Lake Albert and Lake Edward rift valleys. Ultimately, the sedimentary dynamics of the Albertine Rift, and the stacking of its 3D stratigraphic architecture, is shown to beat to the rhythm of oscillating global glacial–interglacial climatic cyclicity and how this affects the Equatorial Tropics of Africa.

In this study, the performance of dynamical seasonal forecast systems is evaluated for the prediction of short rain anomalies over equatorial East Africa. The evaluation is based on observational datasets and the Asia-Pacific Climate Center (APCC) Ocean–Atmosphere coupled multi-model ensemble (MME) retrospective forecasts (hindcasts). These forecast systems have different hindcast periods; here, we have selected common years from 1982 to 2005. The ensembles of individual models and their MME mean are evaluated. Hindcasts initialized on the 1st of August from each year alone are considered, as these are the most relevant to short rain predictions. The coupled climate model ensemble reproduces the spatial distribution of mean September-October-November (SON) rainfall and seasonal climate variations over equatorial East Africa with further improvement in MME mean. Individual coupled models and MME mean also show statistically significant skill in forecasting sea surface temperatures anomalies (SSTAs) over the western and eastern parts of the equatorial Indian Ocean, giving significant correlation at 99 % confidence level for Indian Ocean dipole (IOD). Moreover, five out of ten coupled models and MME mean show statistically significant skill in predicting equatorial East Africa short rains. The fidelity of hindcasts is further measured by anomaly correlation coefficient (ACC) and four models as well as MME mean show significant skill over East Africa. It is shown that the reproduction of the observed variability in the East African region is mainly due to a realistic relationship of East African rainfall with the Indian Ocean dipole. Overall, the skill of the dynamical models is attributed to the fact that slowly evolving SSTs are the primary source of predictability and to the fact that coupled climate models produce skillful predictions of SON SST anomalies over the tropical Indian Ocean. This information opens the possibility of using readily available seasonal forecasts as skillful predictions of equatorial East Africa short rains.

Knowledge of natural long-term rainfall variability is essential for water-resource and land-use management in sub-humid regions of the world. In tropical Africa, data relevant to determining this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard high-resolution proxy records such as tree rings and ice cores. Here we present a decade-scale reconstruction of rainfall and drought in equatorial east Africa over the past 1,100 years, based on lake-level and salinity fluctuations of Lake Naivasha (Kenya) inferred from three different palaeolimnological proxies: sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. Our data indicate that, over the past millennium, equatorial east Africa has alternated between contrasting climate conditions, with significantly drier climate than today during the 'Medieval Warm Period' (approximately AD 1000-1270) and a relatively wet climate during the 'Little Ice Age' (approximately AD 1270-1850) which was interrupted by three prolonged dry episodes. We also find strong chronological links between the reconstructed history of natural long-term rainfall variation and the pre-colonial cultural history of east Africa, highlighting the importance of a detailed knowledge of natural long-term rainfall fluctuations for sustainable socio-economic development.

The Plio-Pleistocene of East Africa marks a crucial yet poorly understood period for hominin evolution. To better understand hominin activities from this time, the environmental substrate on which they resided must first be understood, as changes in the environment most likely influenced hominin evolutionary developments. Detailed palaeoenvironmental reconstructions throughout East Africa have been implemented using multiproxy approaches to address this. Such records thus far are spatially and temporally limited. This thesis aims to rectify this by producing a multiproxy palaeoenvironmental reconstruction of two sites on the Homa Peninsula, western Kenya – a novel palaeoenvironmental setting which hosts Plio-Pleistocene sedimentary sequences containing traces of hominin activity. These sites include Nyayanga (~ 2.6 Ma) and Sare River (~ 1.77 Ma). The multi-proxy approach implemented in this research encompasses analyses of stratigraphy, particle size and phytoliths to reconstruct site sedimentary dynamics, depositional environment and palaeovegetation. End-member mixing analysis is additionally utilised to ‘unmix’ multimodal particle size distributions and provide more detailed information on sedimentary dynamics. Nyayanga is interpreted as an alluvial plain environment upon which deposition occurred via episodic hyperconcentrated flows and intermittent unconfined fluvial flows. During flow hiatuses, secondary processes including fluvial runoff and aeolian deposition occurred, as well as stable land surface development. Higher energy hyperconcentrated flows became absent from the record throughout time, whilst unconfined fluvial activity became more infrequent. This was likely caused by a migration of the active sector of the alluvial plain. Bushy grasslands and grassy bushlands with infrequent sedges and woodland characterised the landscape during this time. An alluvial plain is also identified as the depositional environment for sediments at Sare River. Intermittent unconfined fluvial activity deposited sediments on gentle slopes. This activity became more infrequent throughout time and flow hiatuses, characterised by the occurrence of secondary processes and stable land surface development, became more frequent. This could be attributed to a migration of the ii active sector of the alluvial plain, or to aridification at the site. Bushy grasslands with infrequent woodland characterised the landscape here. Cosmogenic nuclide dating suggests sediments here could be younger than previously thought. An interplay of both regional and local tectonics as well as climate drove palaeoenvironmental change at both sites. The development of the East African Rift System created extensive space for the accumulation of sediment, whilst also altering base-level through the formation and destruction of palaeolakes. A variable climate regime influenced variations in deposition at Nyayanga. Both sites experienced an overall trend towards greater aridity throughout deposition. Changes in palaeoclimate at Nyayanga are attributed to the intensification of the Northern Hemisphere Glaciation, whilst the development of an intensified Walker Circulation is identified as the cause of aridification at Sare River. Environmental preferences of Paranthropus at Nyayanga ~ 2.6 Ma are similar to those presented at other East African sites. This suggests these hominins inhabited or frequented similar depositional settings in open landscapes characterised by bushy grasslands with infrequent wooded vegetation. Evidence from Nyayanga provides support for the pulsed-climate hypothesis linking environmental change to hominin evolution, which suggests that the long term drying trend observed in East Africa was punctuated by periods of extreme climate variability in which large lakes appeared and disappeared. During these periods evolutionary changes in hominins are suggested to have occurred. Sediments from Sare River provide support that hominin activity ~ 1.5 Ma thrived in open environments characterised by alluvial/fluvial deposition much like other East African sites. A trend towards greater aridity and an increase in hominin activity throughout the sediments at this site is interpreted. This suggests that hominin activity here might provide support for the aridity hypothesis linking palaeoenvironmental change to hominin evolution, which suggests progressive aridity across Africa initiated grassland expansion and the novel adaptations associated with these environments.

<p indent=0mm>Coastal sediment transport and geomorphological evolution are important scientific focuses in the research field of land-ocean interaction. The feedback between various factors such as hydrodynamics, extreme climate conditions, biological effects, human activities results in complex ecological and environmental characteristics and poses a great challenge to the development and protection of coastal areas. Closely related to the evolution of coastal geomorphology, the stability of coastal sediment is mainly affected by sediment properties and associated dynamics. Traditionally, it is considered that the change of sediment characteristics and dynamics is greatly driven by physico-chemical factors such as particle size distribution, water content and mineral composition and so on. However, the impact of various active biological factors is generally neglected. Over the past decades, with the deepening understanding of coastal ecological environment, more attention has been paid to the critical roles of biological factors in coastal systems. A large amount of field observation data and laboratory-controlled experiments demonstrated that organisms inhabiting coastal areas contribute a lot to sediment erosion, transportation, deposition, consolidation processes in direct or indirect ways. For example, the cohesion between sediment particles could be increased by microorganisms through secreting extracellular polymeric substances (EPS) so as to enhance sediment stability; salt-tolerant vegetation has an ability to improve the stability of the sediment bed by capacity of sediment retention and reducing hydrodynamic force; the structure of sediment bed could be destroyed by coastal benthos through feeding and burrowing, which would lead to the failure of the bed. On the one hand, these multiple biological effects endow sediment particles with “biological characteristics”, gradually modifying sediment characteristics and sediment dynamics; on the other hand, they try to strengthen or weaken external erosive forces by remolding sedimentary or dynamic environment. Finally, a more stable or unstable coastal biological sedimentological system different from the traditional physico-chemical sedimentological system is formed as a result of the interaction between these diverse biological factors. This enhances the cognition of traditional coastal sediment dynamics, and relevant research is gradually becoming a research hotspot. Understanding the roles of biological effects in coastal sediment dynamics and evolution of coastal geomorphology is helpful for enriching theory of coastal sediment dynamics, promoting the intersection of coastal dynamics, sediment dynamics and ecological environmentology, and promoting the development of the research field “biomorphodynamics”. Focusing on the role of biological factors in the coastal sediment dynamics, this paper reviews the state-of-the-art research on the distribution characteristics of salt-tolerant vegetation, benthos and microorganisms in the coastal area, their contribution to coastal sediment movement as well as the underlying mechanisms. At present, the research on the influence of biological factors on the coastal sediment dynamics is still in the initial stage, and many of the research conclusions are relatively loose, mostly qualitative at macro-level, and a complete theoretical framework has not yet been established. The mechanism of the interaction between coastal sediment and biology, as well as the establishment of simulation models of sediment dynamics and morphological evolution will be the focus of future research.

Tropospheric ozone climatology in eastern Equatorial Africa has been at the core of this study. Seasonal and annual tropospheric ozone distribution and variation have been investigated using SHADOZ network data from Nairobi for the period 1998-2013. Meteorological parameters including air temperature, relative humidity, atmospheric pressure as well as ozone partial pressure have permitted to provide the first comprehensible tropospheric ozone climatology over this region. Mean seasonal tropospheric ozone distribution displays two distinct peaks occurring in winter with 43 DU (July) and 46.8 DU in spring (October). Comparison of mean seasonal ozone partial pressure with relative humidity profiles shows a logarithmic trend with strong regression coefficient for ozone partial pressure (0.81<R2<0.92). Conversely relative humidity variation displays a linear trend with a weak seasonal regression (0.57<R2<0.74). Seasonal vertical tropospheric ozone variation displays two ozone peaks of 121 ppbv in JJA and 126 ppbv in SON at 100 hPa respectively. A minimum photochemical source contribution from local and neighboring countries has been noted at surface to mid latitude. Investigation of individual profiles chosen as case studies for JJA (25/07/2001) and SON (09/10/2002), to assess the origin of high tropospheric ozone concentration has been performed by using back trajectory HYSPLIT model. The strong role played by the Indian Ocean in the long range transport of easterly air mass into eastern Africa at mid and upper troposphere in both seasons has been confirmed. However mean monthly NCEP/NCAR Reanalysis model used to assess the contribution of STE (Stratospheric Tropospheric Exchange), shows a positive mean eddy divergence values during the months of June to July 2001, with evidence of high vorticity 1 sigma values occurring over the same period. In summer higher positive 1 sigma divergence as well as 1 sigma vorticity values have been noted showing stronger STE activity occurring in SON period. These parameters suggest that STE contribution to ozone enhancement in equatorial east Africa is stronger during SON than in JJA. Given the complexity of climate patterns over the equatorial region, in which Nairobi is located, the influence of ENSO (El Nino-Southern Oscillation) and QBO (Quasi Biennial Oscillation) in the STE occurrence cannot be excluded as noted by previous studies.

The effect of subaquatic volcanism on the structure of Lake Kivu in the Albertine Rift; East Africa

Lowland forest collapse and early human impacts at the end of the African Humid Period at Lake Edward, equatorial East Africa

Understanding precipitation variability and extremes in Equatorial East Africa is vital for ensuring water and food security and mitigating the socioeconomic consequences of extreme events. Previous research has shown that sub-seasonal precipitation variability in this region is closely related to the wind direction, with precipitation more probable on days where the wind blows anomalously from the west, advecting moisture from the Congo basin. However, the exact nature of the westerly circulation and the conditions under which it forms are not fully understood. Here, we present a multi-decadal analysis of East African westerly winds. We use methods developed from studies of atmospheric rivers to objectively identify “westerly moisture transport events” (WMTEs), facilitating new insights into the seasonal distribution and importance of these westerlies, the regions within Eastern Africa where they occur, and the role of both the Madden-Julian Oscillation and tropical cyclones in their development. Finally, we also investigate the role of WMTEs as drivers of regional sub-seasonal precipitation variability.

Highland forest dynamics across equatorial East Africa during the end of the African humid period

The genus Myosorex has a classic relict distribution within sub-Saharan Africa. Montane populations in eastern and western equatorial Africa are separated by ca. 2900 km. Until this study, the closest known populations in southern Africa were separated by nearly 2000 km from the closest populations in the Albertine Rift Valley. Here we document previously unknown populations of Myosorex, representing two new endemic taxa from montane forests adjacent to the Albertine Rift. In conjunction with additional data from Malawi, we fill in major gaps in our knowledge of the biodiversity and distribution of this genus in the areas of the Albertine and Malawi Rift Valleys. We demonstrate that this gap is an artefact of survey effort and collecting serendipity. The two new species described herein, as well as other species of Myosorex from north of the Zambezi River, exhibit limited distributions and are confined to montane habitats, typically above 1000 m. Our new species of Myosorex from Kahuzi-Biega NP (...

The seismic-stratigraphic record of lake-level fluctuations in Lake Challa: Hydrological stability and change in equatorial East Africa over the last 140 kyr

Equatorial East Africa suffered severe drought during its 2005 “short rains,” centered on October–November. The circulation mechanisms of such precipitation anomalies are examined, using long-term upper-air and surface datasets, and based on diagnostic findings from earlier empirical investigations. The steep eastward pressure gradient is conducive to fast westerlies over the central-equatorial Indian Ocean, surface manifestation of a powerful zonal circulation cell with subsidence over East Africa, and ascending motion over Indonesia. With fast westerlies, rainfall in East Africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial Indian Ocean Basin, without any seesaw between these domains. In October–November 2005, pressure in the west was anomalously high, entailing a steep eastward pressure gradient along the equator, conducive to fast westerlies and, further symptomatic of the zonal circulation cell, subsidence in the west and ascending motion in the east were enhanced. Overall, the chain of causalities can be traced to anomalously high pressure in the west.