Many authors have used lakes as indicators for climate and landscape changes in the Quaternary [1
]. Lakes associated with the East African Rift System (EARS) respond in a quite sensitive way to tectonic and climatic changes [1
]. Different studies therefore deal with the paleo-climate and the paleo-environment of this focus region [1
]. Moreover, the direct and indirect influences of some of the EARS lakes on human evolution are intensively discussed. These lakes functioned as migration corridors or mixing barriers, and therefore contributed considerably to the development of biodiversity in the region [10
The Lake Manyara Basin has been frequented by early hominins since the Early Middle Pleistocene. Hominin fragments of Homo heidelbergensis
(see Figure 1
), Acheulian material and a rich vertebrate fauna indicate the paleoanthropological importance of the region [11
]. In addition, the vicinity to the hominin sites at Olduvai Gorge highlights the relevance of the Lake Manyara area in terms of paleo-landscape ecology and paleo-landscape development. Findings between Lake Manyara and the Engaruka Basin and in the vicinity of Makuyuni (see Figure 1
) show that archaeological evidence is closely related to different paleolake stages [12
]. One example is evidence from the Late Stone Age [14
] (with radiocarbon dated findings from 9280 ± 60 y BP), which is related to a high lake level between 12,700 to 10,000 y BP [15
The Lake Manyara area is characterized by different lake levels that are related to specific paleo-shorelines, which appear mainly as terraces and beaches. These features and forms have been investigated only partly by detailed mapping and radiometric dating methods [17
]. To the knowledge of the authors, a synoptic investigation of the spatial distribution of the paleolake shorelines, paleolake terraces and related features is not available. Thus, the objective of this study is to contribute to the understanding of the complex landscape evolution in the Lake Manyara area by delineating paleo-shorelines. The elevation of the shorelines in relation to the lowest outlet of the closed basin is also a focus point of the study. Because a delineation of the mentioned paleolake features was not feasible with optical remote sensing methods, backscatter intensity information from TerraSAR-X StripMap and ALOS PALSAR images were used. The morphological structures of shorelines and terraces east of Lake Manyara are characterized by high backscatter values, due to their geometric structure and texture. Linear features representing the paleo-shorelines were extracted from the intensity information of Synthetic Aperture Radar (SAR) images with a Canny edge detector and mapped in the study area. The results were linked to other studies of the Paleolake Manyara. A methodological approach in a similar context is not documented in the literature, to the knowledge of the authors.
In the last decades airborne and satellite remote sensing methodologies have been utilized increasingly for related tasks to detect paleo-landscape pattern, features and forms: Paleolakes on the Sinai Peninsula [20
] were detected with IKONOS high resolution satellite imagery in combination with topographic analysis. Remote Sensing and topographical analysis were also used to delineate a paleolake in northern Darfur [21
]. With Radarsat-1, Landsat ETM+ and a Shuttle Radar Topography Mission (SRTM), digital elevation model (DEM) paleo-shorelines and the paleolake highstand were identified in a megalake in Sudan [22
]. SAR data was successfully applied in different studies for the mapping of subsurface geology and paleo-landscape [23
]. Abdelsalam et al.
] provide a thorough overview about the use of this methodology in arid regions. The integrated application of optical and microwave remote sensing and geospatial analysis also led to the successful delineation of buried paleo-drainages in Egypt, Sudan and Libya [26
]. Single optical multispectral (e.g., ASTER) or high resolution (e.g., QuickBird) remote sensing data only allow the detection of the most prominent landscape features because of the spectral similarity of the covering material with its surroundings. Thus, in this study we utilize the TerraSAR-X sensor with its high resolution X-band images. This sensor offers the possibility to delineate distinct morphological structures due to the relation of backscatter intensity and geometry, texture and surface roughness [29
Extracting line features is necessary for the delineation of paleo-shorelines. A general overview about line extraction methods is provided by Quackenbush [31
]. Hellwich et al.
] used a Markov Random Model and Bayesian classification for the extraction of linear objects from interferometric SAR data. Chanussot et al.
] utilized a fuzzy fusion technique and a morphological line detector to extract a road network from multitemporal SAR images. Different Lee filters have been applied to derive linear features like coastlines from RADARSAT-1 and ERS-1 SAR data [34
]. Using a Canny algorithm, Marghany [36
] successfully delineated shoreline erosion from multitemporal SAR images.
2. Study Area and Paleolake Evidence
Lake Manyara (954 m a.s.l.) is located in an endorheic basin in the eastern branch of the EARS in northern Tanzania (Figure 1
). The Manyara Basin is an asymmetrically shaped half graben, with a 200 to 600 m high escarpment in the west and a west dipping monocline in the east. The morphology of the landscape is strongly related to Quaternary volcanism and tectonic activity, which is still ongoing [36
]. The basin is affected by minor en echelon
step faults (NNE and E), whereas the lake sediments and the shorelines between Makuyuni and the lake are not disturbed by faulting [37
]. The water supply originates from perennial springs and streams of the rift escarpment and from several seasonal drainages, of which the Makuyuni River and the Tarangire River are the largest. Today Lake Manyara is a shallow alkaline lake covering a varying area of up to approximately 550 km2
, with a maximum depth of 1.18 m, and can episodically dry out nearly completely [40
]. The lowest possible outlet of the basin is located in the north of the lake along the main rift system, 78 to 80 m above today’s lake level, and drains into the Engaruka Basin (Figure 1
). The Tropical Rainfall Measurement Mission (TRMM) monthly Rainfall Estimate product 3B43 (V7) shows a bimodal rainfall pattern for the study area for the years 2000 to 2012, with a wet season from November to January and a second more intense wet season between March and May. The average annual precipitation ranges from 1200 mm at the escarpment to 700 mm at the plain east of the lake [41
]. This results in a tropical semihumid vegetation cover with highland forests on the elevated areas west of the lake, and in a semiarid environment with bushed grassland east of the lake.
The oldest lacustrine strata within the Manyara Basin are known as the Manyara Beds and were first mentioned in 1942 [42
]. They appear in the east of Lake Manyara and, based on sediments, define a maximum paleolake extent, approximately 140 m above today’s lake surface. The Manyara Beds can be subdivided into a lacustrine grayish lower member (mudstones, siltstones, diatomite, marls and tuff) that was deposited between 1.3 to 0.98 Ma and 0.633 Ma, and a fluvial and terrestrial, up to 13 m thick, reddish-brown upper member (siltstones, mudstones, conglomerates and breccias) deposited between 0.633 Ma and 0.44 to 0.27 Ma. In most sections, the transition between both members is marked by a distinct tephra layer which was used for Ar40
]. The sections are best exposed close to the town of Makuyuni, where the sediments are partly overlain by a thin layer of Holocene soils and caliche and where big gully systems eroded into the savanna landscape.
The predominantly N-S aligned shorelines west of Lake Manyara vary from small beaches with a local relief of about 1 m to terraces that are several meters high. These terraces consist of up to three steps, indicating fluctuations of the distinct paleolake levels (Figure 2a
). The terraces consist of conglomerates with different grain sizes up to 30 cm in diameter. The scarps are covered by coarse carbonates. The treads are sometimes vegetated by densely growing shrubs and small trees. Stromatolites and oncolites can be found along most of the ridges (Figure 2c
). The areas between the shorelines are mostly covered by young carbonate rich soils, few are covered by carbonate gravel.
Several authors mention younger evidence of different paleolake phases than the lower member of the Manyara Beds. At the beginning of the 20th century, former shorelines and beaches up to 40 m above today’s lake level were detected [44
]. A complete series of beach terraces was found and identified as different shorelines of pluvial periods [46
]. A first systematic documentation of shorelines was published in 1975 for the Lake Manyara and the Engaruka Basin [17
]. The authors found eight main terraces and several beaches in the Lake Manyara Basin and mapped them without taking elevation information in the vicinity of the road leading from Makuyuni to the southern part of the lake. Another study identified four different lake levels about 6 m, 21 m, 43 to 52 m and 82 m above the current level, respectively, during field visits. The highest level also marks the threshold of a lowest possible outlet to the Engaruka and Natron-Magadi Basin [19
]. These terraces have not been mapped for the whole basin yet, and the description of their location remains vague.
Other research has focused on oncolites and stromatolites, which are closely connected to flat tidal environments and therefore, have been documented as evidence for paleo-shorelines [18
]. In an analysis of stromatolites from the northwestern area of today’s lake extent close to the town of Mto Wa Mbu (Figure 1
], late Pleistocene and Holocene stromatolites collected on a distinct level of 20 m above today’s lake level were dated by 14
C and Th/U series. Results showed humid periods with high lake levels for 22,000 y BP (increased water residence time, nutrient enrichment), 27,000 to 23,000 y BP (stable hydrological conditions with diluted fresh water), 35,000 to 32,000 y BP, 90,000 y BP, and an uncertain age of about 140,000 y BP [18
]. Other research groups dated the humid period between 27,500 and 26,000 y BP, and an even younger high stand between 12,700 to 10,000 y BP by diatom analysis of two drilling cores from Lake Manyara [15
]. All radiocarbon ages corroborate other studies of lake level fluctuations along the EARS. For the Nakuru-Elmenteita and the Naivasha Basin high lake level stands were postulated for the periods 146,000 to 73,300 y BP and 15,200 to 9600 y BP [1
]. Trauth et al.
] studied the nearby Ol Njorowa Gorge where they detected high level stands at 146,000 to 141,000 y BP and ca. 93,000 to 89,000 y BP (amongst other time intervals for the Upper Pleistocene). Garcin et al.
] suggested high paleolake levels for Lake Suguta for the periods 16,500 to 15,000 y BP, a high stand at 12,800 y BP and a high lake level from 11,800 y BP to 8500 y BP. For the Lake Natron-Magadi Basin north of Lake Manyara ages for stromatolites and related shorelines (ca. 240,000 y BP, 135,000 ± 10,000 y BP and from 12,000 to 10,000 y BP) have been found to differ from the ages of stromatolites of the Lake Manyara Basin, but agree with the ages of the other lake systems mentioned before, and the ages of the drilling cores of Lake Manyara [3
]. The comparison of stromatolite ages in East African lake systems may involve a methodological problem, because the encrusting benthic microbial communities react very sensitively to changes in water chemistry and hydrological conditions. Therefore, stromatolites do not necessarily indicate all paleolake levels and radiometric dating and interpretation should not focus solely on these materials [18
]. Trauth et al.
] provide a synoptic view on East African climate change and lake systems for the last 175,000 y.
4. Results and Discussion
Different sources were used for the validation of the results. Two studies are applicable as reference from the published literature. Eight main terraces and several beaches were found and mapped up to an elevation of 1,115 m a.s.l. within the vicinity of the road leading from Makuyuni in SE direction by Keller et al.
]. Four different lake levels were identified in the field but not mapped by Somi [19
]. As further reference for this study, 25 paleo-shorelines and the occurrence of stromatolites (14) east of Lake Manyara were mapped with GPS points in the field during surveys in 2012 and 2013 (Figure 4
A dense occurrence of shorelines can be seen northeast of the lake. Distinct levels of paleo-shorelines start from about 6 m to 80 m above today’s lake level, some features even a few meters higher. The most frequent elevation for the occurrence of shorelines is between 1002 m and 1008 m a.s.l. (48 to 54 m above today’s lake level, Figures 5
). The large number of linear lacustrine features across the entire range of elevations is indicative of nearly-continuous lake level evidence by transgression and regression periods. The Makuyuni River, draining from Makuyuni into the Lake Manyara, later eroded some of the paleo-shorelines and disturbed their original distribution. The morphology of ancient shorelines is also eroded or disturbed by the Tarangire River, draining the southeastern part of the Lake Manyara Basin. The proposed method identified 24 of 25 paleo-shorelines marked by GPS points during field survey. Further, the paleolake level 2 to 6 described by Keller et al.
] were delineated. The geometry of paleolake stage level 1 closest to the lake is not well enough defined for high backscatter signals. The paleolake level 7 and 8 [17
] are located on elevations up to 1115 m a.s.l. and far above the lowest possible outlet into the Engaruka Basin. They coincide with the Manyara Beds, but do not agree well with findings of other studies [11
No faulting for the identified shorelines can be stated, which coincides with [37
]. Since the shorelines are in a parallel sequence and only interrupted by fluvial processes tectonic activity has not, or only marginally, affected the absolute elevations of the detected paleo-shorelines since the formation of these structures. This can be maintained at least for the shorelines in the east of the central lake and to the northeast and north of the lake. In the southeast of the lake, insufficient shorelines remain to make a corresponding statement.
Some structures in the southeast of Lake Manyara that were mapped as smaller “beach ridges” [17
] and are marked with “D” (Figure 4
) could not be identified by TSX1 analysis. Because the beach ridges are not described by Keller et al.
] and no field observations are available for this region we can only assume that they are too small to be detected by our analysis, or that their geometry is not well defined. Paleo-shorelines north and west of the inselbergs “Pyramids” were recognized.
Some prominent shorelines at the level of 1030 m to 1036 m a.s.l., reported by Somi [19
], could not be identified by our method or during field surveys. Because their position is not described clearly and the structures are not mapped, they might be located outside the area covered by the SAR images. Alternatively, the structures might be hidden by dense trees and shrub vegetation of the Tarangire National Park so that backscatter signals fail to delineate distinct features. In addition, the PALSAR L-Band scenes could not identify these shorelines. In contrast, structures mentioned by Somi [19
] close to Mto Wa Mbu and in the northeast of the Lake were clearly identified.
Linear features other than paleo-shorelines were also identified by the implemented method. These had to be reclassified using existing maps and field reference data. The structures are nephelenitic ridges north of Essimingor and west of Makuyuni, which resulted from volcanic activity, and some quartzite inselbergs southeast of Lake Manyara (Figure 4
). The location marked with “A” (Figure 4
) shows a section incised by the Makuyuni River, where stromatolites were found during field survey. Corresponding paleo-shorelines could not be identified in the field or by image analysis. It is likely that they belong to the older lake sediments of the Lower Manyara Beds (“C”, Figure 4
) close to Makuyuni, which have a radiometric age older than 0.633 Ma [11
]. The Lower Manyara Beds are not the focus of this study, because they do not form distinct shorelines on the surface, because their age exceeds the radiometric age of the paleo-shorelines, and because they have been found so far only in the surroundings of Makuyuni.
As mentioned before, we identified structures located up to 80 m above today’s lake level. This is interesting because the threshold of the first possible outlet of the Manyara Basin is at about 1032 m and leads into the Engaruka and to the Natron-Magadi Basin in the north. Some authors assume that the lake level did not reach this elevation [18
], others assume an even higher level above 1050 m a.s.l. [15
], and other authors expect the maximum level of the shorelines to be around the level of the lowest possible outlet [19
]. With the results from this study and the evidence of the maximum shoreline heights from field survey conducted in this study, we agree with the latter authors. The location marked with a “B” (Figure 4
) illustrates the abundance of stromatolites on linear features. In this unique situation, stromatolites and oncolites are sparsely distributed on a mafic ridge and not on a well-defined shoreline. They occur four meters above the threshold for today’s outflow, which can be explained by tectonic processes or an incision of the outlet from the Manyara Basin to the Engaruka Basin. By tracing these maximum lacustrine sediments to the south along the slopes of the Essimingor volcano, linear structures continue until a reference point confirms a shoreline. Directly beneath the slopes of Essimingor the paleo-shoreline might be covered by material from the talus of the volcano, like it is the case for the shorelines west of the lake at the escarpment [19
]. The ridges to the east of “B” (Figure 4
) are absent of lacustrine carbonate.
The histogram of the paleo-shoreline height levels (Figure 5
) shows a peak at the current lake level, followed by a gap and then a steady occurrence from 960 m a.s.l. to 1040 m a.s.l. The continuous distribution of shorelines within this range indicates fluctuating lake levels. The peaks in the histogram at 970 m, 978 m, 1002 m to 1008 m, 1018 m and 1030 m depict more steady paleo-environmental conditions. For the paleolake stages marked with yellow bars in Figure 5
(24 m, 51 m, 64 m and 78 m above the current lake level) the lake extent is illustrated in Figure 6
. For those paleolake levels the lake volume and the corresponding surface area were calculated (Table 2
The lake volume and the lake surface area increase drastically with higher lake levels (Figure 6
, Table 2
), which is caused by the flat plain in the north, south and west of the lake. Due to the strong increase in surface, the Paleolake Manyara must have been very sensitive to climatic changes in this high temperature environment. Changes in precipitation and temperature are strongly correlated with evapotranspiration rates [17
]. On one hand, this explains the amount of paleo-shorelines, and on the other hand, it suggests that Lake Manyara passed through several transgression and regression phases.
Further paleolake evidence can be confirmed for Minjingu Hill, a small inselberg five kilometers east of Lake Manyara (Figure 6
). The topographic analysis shows that, even with a high stand of the lake at 1030 m (close to the height of the outlet), the hill is still not covered by water, and that very shallow waters emerge to the east of it. The hill served as a platform for cormorants which produced large quantities of guano; today, phosphate is mined in this area [17
Overall, the detection of paleo-shorelines using the intensity information of TSX1 data was successful. The parallel orientation of the paleo-shorelines in the Lake Manyara Basin relative to the flight direction of the satellite causes a strong backscatter signal. The proposed workflow of speckle filtering to remove noise, the Canny algorithm to identify linear features, and morphological operators to extract the linear structures from the SAR images did perform well for the purpose of this study. The mapping approach is semi-automated because contextual information was necessary to remove or identify other linear features like roads, river beds and volcanic ridges. The transferability of the methodological approach is therefore limited by the availability of additional sources about other linear structures in the respective study area. Assessing the accuracy of the approach is challenging because the collection of reference points in remote areas like some parts of the Manyara Basin is difficult. Single reference points allow verifying whether a paleo-shoreline could be detected by the proposed method. To assess the completeness of the morphology of the detected paleo-shorelines, the mapping of distinct sections with multiple GPS points or kinematic measurements in the field would have been an advantage. With the field reference and literature sources [17
], we can provide a qualitative assessment that our approach yielded good mapping results of the paleo-shorelines in the Lake Manyara Basin.