SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-7-1609-2016Stepwise drying of Lake Turkana at the end of the African Humid Period: a
forced regression modulated by solar activity variations?NutzAlexisnutz@geo.au.dkSchusterMathieuInstitut de Physique du Globe de Strasbourg (IPGS), UMR 7516, Centre
National de la Recherche Scientifique, Université de Strasbourg,
École et Observatoire des Sciences de la Terre, 1 Rue Blessig, 67084
Strasbourg, FranceAlexis Nutz (nutz@geo.au.dk)1December2016761609161829June201620July20161November201613November2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://se.copernicus.org/articles/7/1609/2016/se-7-1609-2016.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/7/1609/2016/se-7-1609-2016.pdf
Although the timing of the termination of the African Humid Period (AHP) is
now relatively well established, the modes and controlling factors of this
drying are still debated. Here, through a geomorphological approach, we
characterize the regression of Lake Turkana at the end of the AHP. We show
that lake level fall during this period was not continuous but rather
stepwise and consisted of five episodes of rapid lake level fall separated
by episodes marked by slower rates of lake level fall. Whereas the overall
regressive trend reflects a decrease in regional precipitations linked to
the gradual reduction in Northern Hemisphere summer insolation, itself
controlled by orbital precession, we focus discussion on the origin of the
five periods of accelerated lake level fall. We propose that these periods
are due to temporary reductions in rainfall across the Lake Turkana area
associated with repeated westward displacement of the Congo Air Boundary
(CAB) during solar activity minima.
Introduction
The African Humid Period (AHP), ca. 14.8 to 5.5 ka cal BP (kilo-annum
before present), is a major climate period that was paced by orbital
parameters (i.e. precession; deMenocal et al., 2000; deMenocal and Tierney,
2012; Bard, 2013; Shanahan et al., 2015) and that had a marked impact the
environment, ecosystems, and human occupation of Africa (Bard, 2013). An
increase in rainfall during this climate period led to the rise and highstand
of numerous African lakes (Street and Grove, 1976; Tierney et al., 2011). The
end of the AHP was characterized by the establishment of more arid
conditions, leading to dramatic lake level falls (Street-Perrott and Roberts,
1983; Kutzbach and Street-Perrott, 1985). This aridification forced Neolithic
populations to adapt to more limited resources (Kuper and Kröpelin, 2006)
and represents one of the most recent examples of major climate change. The
mid-Holocene termination of the AHP is thought to have been either abrupt
(deMenocal et al., 2000), gradual (Kröpelin et al., 2008), or
time-transgressive (Shanahan et al., 2015), depending on location. This
highlights the variable responses of proxies to dominant forcings and the
complex interactions among the multiple components of the local environment
(e.g. deMenocal et al., 2000; Renssen et al., 2006; Liu et al.,
2007; Tierney and deMenocal, 2013; Shanahan et al., 2015). However, drying
trends remain poorly constrained and, in consequence, the precise modes of
aridification are uncertain. A lack of continuous sedimentary archives has
led to the standard idea of a relative constant rate of lake level fall
during the regression of African lakes (e.g. Garcin et al., 2012; Forman et
al., 2014; Morrissey and Scholz, 2014; Junginger et al., 2014; Bloszies et
al., 2015). In this study, we investigate the drying trend of Lake Turkana at
the end of the AHP and, for the first time, present evidence that this final
regression was not continuous through time, revealing a more complex process
than the traditional idea of lake regression. Understanding the mode of
African lake regressions is particularly relevant in the context of
projecting future global climate change impacts on the African continent
(e.g. Patricola and Hook, 2011), especially in terms of evolution of water
resources of large lakes.
Location maps. (a) Lake Turkana basin in the East African Rift
system (EARS). (b) Digital elevation model (DEM) SRTM1 showing Lake Turkana
and the two investigated areas (Turkwel delta complex and the east side of
the Omo River valley). Dashed white line represents the maximum Holocene
lake level. All described geomorphological features are located between the
palaeolake limit and the modern lake shoreline.
Lake Turkana is one of the great lakes of the East African Rift system. It
is considered as a wind-driven body of water (Nutz et al., 2016) that developed
abundant wave-dominated coastal features along its shoreline. These coastal
features represent a valuable palaeohydrological archive that permits a
greater understanding of Lake Turkana evolution during the AHP (Butzer,
1980; Owen et al., 1982; Garcin et al., 2012; Forman et al., 2014; Bloszies
et al., 2015). However, the detailed and continuous evolution of lake level
over the course of the last forced regression (i.e. basinward migration of
the shoreline associated with a base level fall), marking the end of the
AHP, has not been clearly documented. Here, the delta complex of the Turkwel
River (Fig. 1), which developed during the last forced regression of Lake
Turkana, is examined using trajectory analysis (Helland-Hansen and Hampson,
2009). We highlight variations in the rate of lake level fall during this
ultimate regression. We then interpret these variations as markers that
reflect changes in precipitation during the crucial period corresponding to
the terminal phase of the AHP. Subsequently, we discuss potential forcings
responsible for the regressive pattern of Lake Turkana with a primary focus
on the role of the Sun and short-term variations in insolation.
Turkwel delta complex. For location, see Fig. 1b. (a) Raw
digital elevation model SRTM1 of the Turkwel delta. (b) Slope direction
shading applied to the DEM SRTM1 of the Turkwel delta to highlight the steps
separating the different plateaus. Markers display the correspondence
between (a) the DEM SRTM1 and (b) the slope direction shading. (c) SPOT5
satellite image of the Turkwel delta. (d) Interpretative geomorphological
map of the area showing five successive delta plains in addition to the
oldest plain associated with the late AHP highstand.
Methods
The dataset is comprised of satellite imagery and a digital elevation model
(DEM). A recently obtained SRTM1 dataset (Shuttle Radar Topography Mission;
Rabus et al., 2003) is available for the entire Lake Turkana area. This DEM
is produced by radar interferometry with 1 arcsec (approximately 30 m)
horizontal grid spacing and provides a maximum 5 m absolute vertical error
(Becek, 2008; Garcin et al., 2009). In addition, high-resolution (< 1 m) PLEIADES and (5 m) SPOT 5 images were used to focus on selected areas.
This dataset was processed using GIS software (Global Mapper 15;
www.globalmapper.com) to provide a high-resolution 3-D image of
geomorphological features. Topographic profiles, elevation differences, and
slope values were obtained using Global Mapper 15 software.
Geomorphological data for the Turkwel delta complex. For location,
see Fig. 1b. (a) SRTM1 images were processed to display a digital
elevation model of the Turkwel delta complex. Locations of the topographic
transects are presented. (b) Topographic transects P1, P2, and P3.
(c) Trajectory analyses show that the overall forced regressive trend (< 0.4∘)
is punctuated by five steeper slopes (1 to 3.8∘) revealing short-term increases in the rates of lake level
fall.
The trajectory analysis method is a recent development based on the
principles of sequence stratigraphy. This approach permits an estimate of
the palaeoevolution of sea or lake levels based on the analysis of lateral
and vertical migration of shore-dependent landforms (i.e. shelf, coastal
wedge, or deltaic systems). Four categories of shoreline trajectories exist:
ascending regressive, descending regressive, transgressive, and stationary.
These reflect normal regression, forced regression, transgression, and
stable trends, respectively. In terms of base-level evolution, normal
regression and transgression indicate a rise in sea or lake levels, while
forced regression reflects a water level decline. Here, we apply this method
to decipher the evolution of Lake Turkana levels at the end of the AHP.
Geomorphological analysis
The Turkwel delta complex is 35 km long, forming one of the major deltaic
systems of Lake Turkana (Fig. 1). It was developed as the shoreline migrated
basinward, lowering from 450 to 360 m a.s.l. (Fig. 2). From west to east, five
distinct progradational stages were identified (Fig. 2d). The first
progradational stage forms a lobe protruding out from the average
north–south palaeoshoreline, well defined by the 450 m a.s.l. elevation
contour (red line in Fig. 2d). According to regional age models (Garcin et
al., 2012; Forman et al., 2014; Bloszies et al., 2015), this first
progradational stage marks the last Holocene highstand before the end of the
AHP. Moving eastward, each of the three topographic profiles across the
Turkwel delta complex (Fig. 3) show five slightly inclined plateaus
separated by five abrupt 5 to 15 m high steps at ca. 445, 425, 410, 400, and
390 m a.s.l. (Fig. 4). Each plateau defines a different progradational stage.
The plateaus are 3–5 km wide, and correspond to successively abandoned
delta plains (Fig. 2d). To the north, these plateaus systematically end with
palaeospits that document ancient, northward-flowing alongshore currents.
The resulting landform reveals the Turkwel delta complex to be composed of
successive asymmetric wave-dominated deltas (Bhattacharya and Giosan, 2003;
Anthony, 2015) during most of its evolution, except in the early period
associated with the AHP highstand. None of the plateaus exhibit evidence of
significant erosion that would indicate reworking of the landforms
subsequent to their deposition, except for the fluvial incision by the
Turkwel River that progressively adjusted to base level fall. This supports
the idea that the Turkwel delta complex represents a primary depositional
landform displaying a continuous, comprehensive record of lake level
evolution. Trajectory analysis, performed for the three transects across the
Turkwel delta complex along its progradation axes (Fig. 3), reveals that the
plateaus are continuous, having slightly descending regressive trajectories
(< 0.4∘). The five abrupt steps that separate plateaus
have much higher slope gradients (1 to 3.8∘), and are
also defined as descending regressive trajectories. The trajectories reflect
a progradation associated with a general lake level fall that meets the
definition of a forced regression (Posamentier et al., 1992). The five
abrupt steps reflect recurrent, short-lived increases in the rate of lake
level fall that evidence a stepwise forced regression at the end of the AHP.
Landforms from the Turkwel delta. (a) Front view of a step grading
downward to a plateau. (b) Side view of the same step separating two
plateaus.
In the eastern Omo River valley (Fig. 1), topographic profiles along two
fossil spits (Fig. 5) confirm this interpretation. Both spits show
successive steps starting at elevations similar to those observed in the
Turkwel delta complex (ca. 445, 425, 410, and 400 m a.s.l.; Fig. 3). These
additional observations strongly support features in the Turkwel delta
complex that reflect a stepwise forced regression of Lake Turkana at the end
of the AHP.
Chronological framework
Humid conditions related to the AHP broadly prevailed over Africa from 14.8
to 5.5 ka cal BP (deMenocal et al., 2000; Shanahan et al., 2015). Several
lake level curves developed from Lake Turkana provide records of the
regional moisture history over the Holocene (Garcin et al., 2012; Forman et
al., 2014; Bloszies et al., 2015). Based on surveys of raised Holocene beach
ridges coupled with dated archaeological sites, these studies also provide a
relatively robust chronological framework for its regression at the end of
the AHP. Garcin et al. (2012) initially estimated the onset of the last lake
level fall in Lake Turkana at ca. 5.27 ± 0.36 ka cal BP based
on radiocarbon ages obtained from shells preserved in palaeoshorelines.
Subsequently, using a similar methodology, Forman et al. (2014) proposed
that the age of this last regression occurred between 5.5/5.0 and 4.6 ka cal BP associated to a lake level fall from 440 to 380 m a.s.l. Finally, Bloszies
et al. (2015) proposed an onset of the last regression of the AHP starting
at 5.18 ± 0.12 ka cal BP (shells at 90 m above the modern Lake
Turkana; sample SNU12-589) and finishing at 4.58 ± 0.25 ka BP
(optically stimulated luminescence (OSL) age reused from Forman et al., 2014; sample UIC2319) associated with a
lake level fall from 450 to 375 m a.s.l. Based on these published data, we
carried out minor complementary processing in order to refine the
chronology. First, we recalibrated sample SNU12-589, considered to provide
the age of the onset of the last regression. Using INTCAL13 (Reimer et al.,
2013), the onset of the last regression is now 5.14 ± 0.18 ka cal BP
(4.51 ± 0.06 ka 14C BP). Second, we converted the OSL age,
representing the end of the last regression of 4.58 ± 0.5 ka BP
(2σ) by Forman et al. (2014), to radiocarbon years. Forman et al. (2014) provide six samples that were dated by both OSL and radiocarbon
methods. Despite the limited number of samples, we ran a linear regression
to propose a statistical relationship between OSL and radiocarbon ages.
Based on this correlation
(age(OSL)= 0.98386063 × age(C(calibrated))14; b (the
intercept) has been forced to 0; r2= 0.9942), the age of
the end of the last regression is now estimated at 4.65 ± 0.51 ka cal BP (4.14 ± 0.17 ka 14C BP). As the investigated portion of the
Turkwel delta complex is located between 450 and 375 m a.s.l., the landforms
are considered to have developed between 5.14 ± 0.18 and 4.65 ± 0.51 ka cal BP.
Fossil spits along the eastern Omo River valley (for location see
Figure1b) from SRTM 1 (left panel) and from PLEIADES images (right panel).
The fossil spits are outlined by dashed white lines. They display plateaus
interrupted at similar elevations to those of the Turkwel delta.
Based on this time interval, the last regression of Lake Turkana would, at
the longest, span a period from 5.32 to 4.14 ka cal BP. Converting this
longest potential time interval as radiocarbon ages (i.e. the interval
between 4.57 and 3.97 ka 14C BP), a mean age of 4.27 ± 0.3 ka
14C BP is established to thereby allow calibration and provide a
probability curve. The probability curve reveals a ca. 43/44 % probability
that the last regression occurred precisely between 5.14 and 4.65 ka cal BP.
The red curve presents total solar irradiance (40-year moving
average) relative to the value of the PMOD composite during the solar cycle
minimum of the year 1986 (1365.57 W m-2; Steinhilber et al., 2009) for the period contemporaneous with AHP
regression of Lake Turkana. The shaded band (yellow) represents 1σ
uncertainty. The blue curve represents the precessional curve covering the
same time period (http://www.imcce.fr/Equipes/ASD/insola/earth/online/).
Grey stripes highlight solar activity minima.
DiscussionOrigin of Lake Turkana lake level evolution
Lake level fluctuations may result from changes in the quantity of water
supply to a lake, from altered evapotranspiration rates within the catchment
area, or from modifications in basin physiography. These changes may
originate from a number of potential external forcing processes, among which
the most commonly considered are tectonism and climate. Given the short
timescale considered in this study, abrupt falls in lake level cannot be
attributed to tectonism and any associated physiographic modification of the
Lake Turkana basin. Vertical crustal movements occur over much longer time
periods and the rate of subsidence in the basin is too low (i.e. 0.4 m ka-1 at the Eliye Spring well site; Morley et al., 1999)
to explain several lake level falls of > 5 m each occurring
within 1000 years. Moreover, vertical displacements at this scale would
require earthquakes having a magnitude > 9 (Pavlides and Caputo,
2004). Earthquakes of this magnitude are unknown in the area and are not
compatible with rift systems. Finally, volcanic activity is known to have
occurred during the Late Quaternary (Karson and Curtis, 1994), but its
timing is not well constrained. Repeated pulses of accelerated subsidence
related to successive emptying of a magma chamber are also inconsistent with
the limited amount of magma observed in the basin. Indeed, there is no
regional magmatic effusion observed that would have caused sudden
subsidence. Rather, magmatism corresponded to episodic, spatially limited
effusions that formed the north, central, and south islands. As such, it is
difficult to attribute the abrupt nature of the accelerated lake level falls
to tectonism and magmatism, thus rendering climate variability as the most
likely forcing mechanism.
During the Holocene, the overall climate pattern in East Africa was governed
by insolation patterns related to changes in precessional orbital parameters
of the Earth (Barker et al., 2004). Links between insolation and hydrology
are now well established for this region, with monsoonal rainfall intensity
being strongly correlated with summer insolation. In the early Holocene, an
increase in summer insolation produced wetter conditions over much of the
African continent leading to the establishment of the AHP and an expansion
of lakes (deMenocal et al., 2000; Shanahan et al., 2015). Subsequently, the
overall contraction of lakes at the end of the AHP is generally attributed
to decreased precipitation related to an orbitally controlled reduction in
summer insolation (deMenocal et al., 2000; deMenocal and Tierney, 2012;
Bard, 2013; Shanahan et al., 2015) Insolation changes drive modifications in
rainfall amounts through the strengthening or weakening of local climate
processes. In the Lake Turkana area, Junginger et al. (2014) suggest that
the increase in precipitation during the AHP is mainly a result of a
north-eastward shift of the Congo Air Boundary (CAB). The CAB is a
north-east–south-west-oriented convergence zone presently located west of
the Lake Turkana area. This convergence zone shifts eastward in response to
an enhanced atmospheric pressure gradient between India and East Africa
during Northern Hemisphere insolation maxima (Junginger and Trauth, 2013;
Junginger et al., 2014). When the CAB moves eastward over the Turkana area,
precipitation is expected to increase significantly. As the five abrupt
accelerations in lake level fall require short-term accentuated decreases in
precipitation, we propose that these five periods of significantly reduced
rainfall amounts are related to short-term decreases in insolation that
repeatedly displaced the CAB. In our opinion, at such decadal to centennial
timescales, variations in solar activity appear as the most likely
parameter to explain variations in insolation.
Linking solar activity and palaeohydrology
Establishing links between short-term (decadal-scale) solar activity and
climate change remains a point of debate. Periodicities in solar activity,
such as the 11-year sunspot cycle, the Gleissberg cycle (80–90 years; Peristykh and Damon, 2003) or the de Vries cycle (∼ 200 years; Raspopov et al., 2008) have been identified in Holocene
palaeoenvironmental records and indicate a possible forcing by short-term
solar activity on climate (Crowley, 2000; Bond et al., 2001; Gray
et al., 2013). In the Lake Turkana area, Halfman et al. (1994) unravelled the
expression of the 11-year sunspot cycle during the last 4 ka through a
time-series analysis of sediment cores. Several authors link more arid
periods inferred from lacustrine records with solar activity minima
(Verschuren et al., 2000; Stager et al., 2002; Junginger et al., 2014). The
ability of lakes to record changes in palaeohydrology attributed to
variations in solar activity may be enhanced for “amplifier lakes”
(Street, 1980). Indeed, relatively modest changes in climate are amplified
into significant lake level fluctuations due to their specific morphology.
As an amplifier lake, Lake Turkana should be sensitive to variations in
precipitation induced by small variations in insolation.
When we compare the proposed chronological framework with the solar activity
curve from Steinhilber et al. (2009), we observe between one and fourteen
solar activity minima during the minimum and maximum potential periods of
regression, respectively (Fig. 6). During the time period consistent with
the average duration of the regression – 490 years between 5.14 and 4.65 ka cal BP – five solar activity minima are observed. Given that the number of
these minima matches the number of abrupt lake level falls, this may suggest
a causal link between the short-term variability of solar activity and the
lake level changes in Lake Turkana at the end of the AHP. Even though robust
chronological correlations are not yet available between these short-term
accelerations of lake level fall and solar activity minima, we propose a
mechanism linking solar activity and lake level evolution. We suggest that
periods of solar activity maxima would be able to compensate for the
precession-induced reduction of insolation. The relatively limited reduction
of insolation would have led to a relatively stable position for the CAB
over the Lake Turkana area. As such, this would favour a reduced rate of
lake level fall due to slowly decreasing rates of precipitation amounts.
However, when short-term solar activity minima are coupled with the
precession-related insolation decrease, the CAB would have migrated rapidly
westward, resulting in a drastic reduction of rainfall and, as a consequence,
producing a rapid fall in lake level. As such, alternations of solar activity
maxima and minima could explain the geomorphological evidence for a
long-term forced regression interspersed by short-term accelerations in the
rate of lake level fall at the end of the AHP.
Conclusions
Geomorphic analysis (i.e. trajectory analysis) of the Turkwel delta complex
reveals, for the first time, a stepwise lake level fall of Lake Turkana
during its last forced regression at the end of the African Humid Period. Five rapid falls in lake level were identified, intercalated with
periods of slower lake level fall. These five rapid falls in lake level
reflect five short-term periods associated with drastic decreases in
precipitation. We propose that these abrupt, short-term decreases in
precipitation are associated with insolation minima altering the position of
the Congo Air Boundary, the large-scale circulation system responsible
for regional precipitation patterns over this region. Furthermore, we
propose that the short-term changes in insolation are caused by variations
in solar activity. The next research step would be to precisely date each
plateau and each step to a specific solar maximum and minimum, respectively.
Nevertheless, existing dating methods do not, however, provide precise
enough ages at such decadal to centennial timescales.
Data availability
SRTM digital elevation model is free to access using the website: earthexplorer.usgs.gov. PLEIADES and SPOT images
were bought thanks to the support of the CNES and are not freely
available.
Acknowledgements
This work is a contribution of the Rift Lake Sedimentology project (RiLakS)
funded by Total Oil Company. Satellite images (SPOT and PLEIADES) were
acquired thanks to the support of the CNES/ISIS program. Finally, we are
grateful to Murray Hay (Maxafeau Editing Services) for verifying the English
within the text.
Edited by: A. Stroeven
Reviewed by: two anonymous referees
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