Late Quaternary climate variability in the source region of Homo sapiens. Dry-wet cycles in Chew Bahir, southern Ethiopia

Climate change, as a key topic in our society, has far reaching implications for many aspects of our lives, in the past present and evermore in the future. Climatic variability and a rapidly changing environment are considered to have had a significant influence on human evolution, migration and cultural and technological innovation. However, to evaluate the impact that climatic shifts on different timescales might have had on the living conditions of prehistoric humans, an understanding and continuous reconstruction of these climatic fluctuations and their underlying driving mechanisms are essential. This work presents results from such a high resolution (up to 3 years) lake-sediment record from the palaeo-lake Chew Bahir, a newly invested climate archive in a tectonic-bound basin in south- ern Ethiopia. The record was obtained from six 9–18.8 m long cores along a 17 km NW-SE transect across the basin, today an extensive saline mudflat. The objective of this work is to understand and reconstruct the sensitive patterns and expressions of East Africa‘s highly variable climate, as the climatic context for important cultural transitions in the source region of Homo sapiens. Now, the multi-proxy analyses and interpretation being the heart piece of this thesis, provide the climatic history of the past ~60 ka cal BP and show that Chew Bahir responded sensitively with pronounced shifts in moisture availability towards climatic fluctuations on millennial to centennial timescales, and to the precessional cycle. The first part of this work concentrates on a) the reconstruction of the velocity and character of these late-Quaternary wet-dry transitions on different time scales (orbital, millennial–centennial and decadal) and b) a basic proxy concept for Chew Bahir for the last two insolation controlled wet-dry cycles. This concept comprises besides the deciphering of major intra-basin dynamics and mechanisms controlling the way from source to sink, an initial understanding of site-specific proxies: especially potassium as a sensitive indicator for aridity and chlorine as a humidity proxy. The records are based on a set of geochemical, physical and biological indicators as well as a suite of AMS radiocarbon dates. The Chew Bahir cores document a highly non-linear response to the last insolation controlled dry-wet cycle, the so-called African Humid Period (~15–5 ka BP) with a pronounced abrupt onset of humid conditions within <500 yrs and a disproportionally gradual decline of moisture availability, as compared to the decrease in insolation. Feedback mechanisms and a complex interrelationship with the monsoon circulation and the diverse topography of the East African Rift have been suggested as possible key factors. The AHP frames a sharply defined arid phase, corresponding to the Younger Dryas chronozone (~12.8–11.6 ka BP). During the overall arid phase of MIS 3, several oscillations to wetter conditions have been recorded, that resemble the high latitude Dansgaard-Oeschger cycles. Heinrich-events are suggested to be expressed in several episodes of extreme aridity. The full humid conditions of the Holocene wet period [AHP], are punctuated by several abrupt droughts on a centennial to millennial time-scale, the termination of the AHP is though gradual, a textbook example for climatic instability during a transition. A series of 20–80 yr long droughts modulate the 1,500 yr long shift from full wet to arid conditions. In a broader spatio-temporal context this Mid Holocene wet-dry transition in Chew Bahir is evaluated together with two other examples of a change from stable to unstable environmental conditions: the MIS 5–4 transition in the Naivasha basin (central Kenya rift) and thirdly, the Mid Pleistocene Transition in the Olorgesaille basin (Southern Kenya Rift). The concept of hominin speciation, dispersal and cultural innovation being possibly influenced by this transition from stable to unstable environmental conditions is tested on the three different timescales provided by the three records. As a contribution towards a better understanding of human-climate interaction, we compared the last 20 ka of the paleo-climate record from Chew Bahir with the settlement history of adjacent possible refugia in the Ethiopian highlands and around lake margins. Shifts in and out of favourable living conditions are deducted from the climatic history, which shows besides orbitally driven long-term transitions several short abrupt climate events. These are expressed as shifts to pronounced aridity, suggesting phases of climatic stress. Comparing the frequency of archaeological findings as a parameter for human occupation in refugia to this close-by climate record, allows us to outline how complex the interplay between humans and environment during the last 20 ka really was. The results comprised in this work represent an important pre- requisite for the ICDP “Hominid Sites and Paleolakes Drilling Project” and for the CRC-806 programme “Our Way to Europe”, which aim to determine climatic and environmental context of hu- man evolution and dispersal. The potential of this deep terrestrial climate archive has been evaluated herein and proved that the sediment deposits are suitable to provide a longer climate history, to be precise to cover with a 400 m core the climatic history of >500,000 yrs in the source region of modern humans

Abrupt or gradual?:Change point analysis of the late Pleistocene–Holocene climate record from Chew Bahir, southern Ethiopia

We used a change point analysis on a late Pleistocene-Holocene lake-sediment record from the Chew Bahir basin in the southern Ethiopian Rift to determine the amplitude and duration of past climate transitions. The most dramatic changes occurred over 240 yr (from similar to 15,700 to 15,460 yr) during the onset of the African Humid Period (AHP), and over 990 yr (from similar to 4875 to 3885 yr) during its protracted termination. The AHP was interrupted by a distinct dry period coinciding with the high-latitude Younger Dryas stadial, which had an abrupt onset (less than similar to 100 yr) at similar to 13,260 yr and lasted until similar to 11,730 yr. Wet-dry-wet transitions prior to the AHP may reflect the high-latitude Dansgaard-Oeschger cycles, as indicated by cross-correlation of the potassium record with the NorthGRIP ice core record between similar to 45-20 ka. These findings may contribute to the debates regarding the amplitude, and duration and mechanisms of past climate transitions, and their possible influence on the development of early modern human cultures

Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL

The Ethiopian rift is known for its diverse landscape, ranging from arid and semi-arid savannahs to high and humid mountainous regions. Lacustrine sediments and paleo-shorelines indicate water availability fluctuated dramatically from deep fresh water lakes, to shallow highly alkaline lakes, to completely desiccated lakes. To investigate the role lakes have played through time as readily available water sources to humans, an enhanced knowledge of the pace, character and magnitude of these changes is essential. Hydro-balance models are used to calculate paleo-precipitation rates and the potential pace of lake level changes. However, previous models did not consider changes in hydrological connectivity during humid periods in the rift system, which may have led to an overestimation of paleo-precipitation rates. Here we present a comprehensive hydro-balance modeling approach that simulates multiple rift lakes from the southern Ethiopian Rift (lakes Abaya, Chamo, and paleo-lake Chew Bahir) simultaneously, considering their temporal hydrological connectivity during high stands of the African Humid Period (AHP, ~15–5 ka). We further used the Surface Energy Balance Algorithm for Land (SEBAL) to calculate the evaporation of paleo-lake Chew Bahir's catchment. We also considered the possibility of an additional rainy season during the AHP as previously suggested by numerous studies. The results suggest that an increase in precipitation of 20–30% throughout the southern Ethiopian Rift is necessary to fill paleo-lake Chew Bahir to its overflow level. Furthermore, it was demonstrated that paleo-lake Chew Bahir was highly dependent on the water supply from the upper lakes Abaya and Chamo and dries out within ~40 years if the hydrological connection is cut off and the precipitation amount decreases to present day conditions. Several of such rapid lake level fluctuations, from a freshwater to a saline lake, might have occurred during the termination of the AHP, when humid conditions were less stable. Fast changes in fresh water availability requires high adaptability for humans living in the area and might have exerted severe environmental stress on humans in a sub-generational timescale

Multiband Wavelet Age Modeling for a ∼293 m (∼600 kyr) Sediment Core From Chew Bahir Basin, Southern Ethiopian Rift

The use of cyclostratigraphy to reconstruct the timing of deposition of lacustrine deposits requires sophisticated tuning techniques that can accommodate continuous long-term changes in sedimentation rates. However, most tuning methods use stationary filters that are unable to take into account such long-term variations in accumulation rates. To overcome this problem we present herein a new multiband wavelet age modeling (MUBAWA) technique that is particularly suitable for such situations and demonstrate its use on a 293 m composite core from the Chew Bahir basin, southern Ethiopian rift. In contrast to traditional tuning methods, which use a single, defined bandpass filter, the new method uses an adaptive bandpass filter that adapts to changes in continuous spatial frequency evolution paths in a wavelet power spectrum, within which the wavelength varies considerably along the length of the core due to continuous changes in long-term sedimentation rates. We first applied the MUBAWA technique to a synthetic data set before then using it to establish an age model for the approximately 293 m long composite core from the Chew Bahir basin. For this we used the 2nd principal component of color reflectance values from the sediment, which showed distinct cycles with wavelengths of 10–15 and of ∼40 m that were probably a result of the influence of orbital cycles. We used six independent 40Ar/39Ar ages from volcanic ash layers within the core to determine an approximate spatial frequency range for the orbital signal. Our results demonstrate that the new wavelet-based age modeling technique can significantly increase the accuracy of tuned age models

Advanced Hyperspectral Analysis of Sediment Core Samples from the Chew Bahir Basin, Ethiopian Rift, in the Spectral Range from 0.25 to 17 µm:Support for Climate Proxy Interpretation

Establishing robust environmental proxies at newly investigated terrestrial sedimentary archives is a challenge, because straightforward climate reconstructions can be hampered by the complex relationship between climate parameters and sediment composition, proxy preservation or (in)sufficient sample material. We present a minimally invasive hyperspectral bidirectional reflectance analysis on discrete samples in the wavelength range from 0.25 to 17 mu m on 35 lacustrine sediment core samples from the Chew Bahir Basin, southern Ethiopia for climate proxy studies. We identified and used absorption bands at 2.2 mu m (Al-OH), at 2.3 mu m (Mg-OH), at 1.16 mu m (analcime), and at 3.98 mu m (calcite) for quantitative spectral analysis. The band depth ratios at 2.3/2.2 mu m in the spectra correlate with variations in the potassium content of the sediment samples, which also reflect periods of increased Al-to-Mg substitution in clay minerals during drier climatic episodes. During these episodes of drier conditions, absorption bands diagnostic of the presence of analcime and calcite support this interpretation, with analcime indicating the driest conditions. These results could be compared to qualitative analysis of other characteristic spectral properties in the spectral range between 0.25 and 17 mu m. The results of the hyperspectral measurements complement previous sedimentological and geochemical analyses, allowing us in particular to resolve more finely the processes of weathering in the catchment and low-temperature authigenic processes in the sediment. This enables us to better understand environmental changes in the habitat of early humans

A phytolith supported biosphere-hydrosphere predictive model for Southern Ethiopia:Insights into paleoenvironmental changes and human landscape preferences since the last glacial maximum

During the past 25 ka, southern Ethiopia has undergone tremendous climatic changes, from dry and relatively cold during the Last Glacial Maximum (LGM, 25–18 ka) to the African Humid Period (AHP, 15–5 ka), and back to present-day dry conditions. As a contribution to better understand the effects of climate change on vegetation and lakes, we here present a new Predictive Vegetation Model that is linked with a Lake Balance Model and available vegetation-proxy records from southern Ethiopia including a new phytolith record from the Chew Bahir basin. We constructed a detailed paleo-landcover map of southern Ethiopia during the LGM, AHP (with and without influence of the Congo Air Boundary) and the modern-day potential natural landcover. Compared to today, we observe a 15–20% reduction in moisture availability during the LGM with widespread open landscapes and only few remaining forest refugia. We identify 25–40% increased moisture availability during the AHP with prevailing forests in the mid-altitudes and indications that modern anthropogenic landcover change has affected the water balance. In comparison with existing archaeological records, we find that human occupations tend to correspond with open landscapes during the late Pleistocene and Holocene in southern Ethiopia

ICDP workshop on the Deep Drilling in the Turkana Basin Project:Exploring the link between environmental factors and hominin evolution over the past 4 Myr

Scientific drill cores provide unique windows into the processes of the past and present. In the dynamic tectonic, environmental, climatic, and ecological setting that is eastern Africa, records recovered through scientific drilling enable us to look at change through time in unprecedented ways. Cores from the East African Rift System can provide valuable information about the context in which hominins evolved in one of the key regions of hominin evolution over the past 4 Myr. The Deep Drilling in the Turkana Basin (DDTB) project seeks to explore the impact of several types of evolution (tectonic, climatic, biological) on ecosystems and environments. This includes addressing questions regarding the region’s complex and interrelated rifting and magmatic history, as well as understanding processes of sedimentation and associated hydrothermal systems within the East African Rift System. We seek to determine the relative impacts of tectonic and climatic evolution on eastern African ecosystems. We ask, what role (if any) did climate change play in the evolution of hominins? How can our understanding of past environmental change guide our planning for a future shaped by anthropogenic climate change? To organize the scientific community’s goals for deep coring in the Turkana Basin, we hosted a 4-day ICDP supported workshop in Nairobi, Kenya in July 2022. The team focused on how a 4 Myr sedimentary core from the Turkana Basin will uniquely address key scientific research objectives related to basin evolution, paleoclimate, paleoenvironment, and modern resources. Participants also discussed how DDTB could collaborate with community partners in the Turkana Basin, particularly around the themes of access to water and education. The team concluded that collecting the proposed Pliocene to modern record is best accomplished through a 2-phase drilling project with a land-based transect of four cores spanning the interval from 4 Ma to Middle/Late Pleistocene (&lt;0.7 Ma) and a lake-based core targeting the interval from ~1 Ma to present. The second phase, while logistically more challenging due to the lack of drilling infrastructure currently on Lake Turkana, would revolutionize our understanding of a significant interval in the evolution and migration of Homo sapiens for a time period not currently accessible from the Kenyan part of the Turkana Basin. Collectively, the DDTB project will provide exceptional tectonic and climatic data directly associated with one of the world’s richest hominin fossil localities

ICDP workshop on the Deep Drilling in the Turkana Basin Project:Exploring the link between environmental factors and hominin evolution over the past 4 Myr

Scientific drill cores provide unique windows into the processes of the past and present. In the dynamic tectonic, environmental, climatic, and ecological setting that is eastern Africa, records recovered through scientific drilling enable us to look at change through time in unprecedented ways. Cores from the East African Rift System can provide valuable information about the context in which hominins evolved in one of the key regions of hominin evolution over the past 4 Myr. The Deep Drilling in the Turkana Basin (DDTB) project seeks to explore the impact of several types of evolution (tectonic, climatic, biological) on ecosystems and environments. This includes addressing questions regarding the region’s complex and interrelated rifting and magmatic history, as well as understanding processes of sedimentation and associated hydrothermal systems within the East African Rift System. We seek to determine the relative impacts of tectonic and climatic evolution on eastern African ecosystems. We ask, what role (if any) did climate change play in the evolution of hominins? How can our understanding of past environmental change guide our planning for a future shaped by anthropogenic climate change? To organize the scientific community’s goals for deep coring in the Turkana Basin, we hosted a 4-day ICDP supported workshop in Nairobi, Kenya in July 2022. The team focused on how a 4 Myr sedimentary core from the Turkana Basin will uniquely address key scientific research objectives related to basin evolution, paleoclimate, paleoenvironment, and modern resources. Participants also discussed how DDTB could collaborate with community partners in the Turkana Basin, particularly around the themes of access to water and education. The team concluded that collecting the proposed Pliocene to modern record is best accomplished through a 2-phase drilling project with a land-based transect of four cores spanning the interval from 4 Ma to Middle/Late Pleistocene (&lt;0.7 Ma) and a lake-based core targeting the interval from ~1 Ma to present. The second phase, while logistically more challenging due to the lack of drilling infrastructure currently on Lake Turkana, would revolutionize our understanding of a significant interval in the evolution and migration of Homo sapiens for a time period not currently accessible from the Kenyan part of the Turkana Basin. Collectively, the DDTB project will provide exceptional tectonic and climatic data directly associated with one of the world’s richest hominin fossil localities

Modern Sedimentation and Authigenic Mineral Formation in the Chew Bahir Basin, Southern Ethiopia:Implications for Interpretation of Late Quaternary Paleoclimate Records

We present new mineralogical and geochemical data from modern sediments in the Chew Bahir basin and catchment, Ethiopia. Our goal is to better understand the role of modern sedimentary processes in chemical proxy formation in the Chew Bahir paleolake, a newly investigated paleoclimatic archive, to provide environmental context for human evolution and dispersal. Modern sediment outside the currently dry playa lake floor have higher SiO2 and Al2O3 (50-70 wt.%) content compared to mudflat samples. On average, mudflat sediment samples are enriched in elements such as Mg, Ca, Ce, Nd, and Na, indicating possible enrichment during chemical weathering (e.g., clay formation). Thermodynamic modeling of evaporating water in upstream Lake Chamo is shown to produce an authigenic mineral assemblage of calcite, analcime, and Mg-enriched authigenic illitic clay minerals, consistent with the prevalence of environments of enhanced evaporative concentration in the Chew Bahir basin. A comparison with samples from the sediment cores of Chew Bahir based on whole-rock MgO/Al2O3, Ba/Sr and authigenic clay mineral delta O-18 values shows the following: modern sediments deposited in the saline mudflats of the Chew Bahir dried out lake bed resemble paleosediments deposited during dry periods, such as during times of the Last Glacial Maximum and Younger Dryas stadial. Sediments from modern detrital upstream sources are more similar to sediments deposited during wetter periods, such as the early Holocene African Humid Period

Using multiple chronometers to establish a long, directly-dated lacustrine record:Constraining &gt;600,000 years of environmental change at Chew Bahir, Ethiopia

Despite eastern Africa being a key location in the emergence of Homo sapiens and their subsequent dispersal out of Africa, there is a paucity of long, well-dated climate records in the region to contextualize this history. To address this issue, we dated a ∼293 m long composite sediment core from Chew Bahir, south Ethiopia, using three independent chronometers (radiocarbon, 40Ar/39Ar, and optically stimulated luminescence) combined with geochemical correlation to a known-age tephra. The site is located in a climatically sensitive region, and is close to Omo Kibish, the earliest documented Homo sapiens fossil site in eastern Africa, and to the proposed dispersal routes for H. sapiens out of Africa. The 30 ages generated by the various techniques are internally consistent, stratigraphically coherent, and span the full range of the core depth. A Bayesian age-depth model developed using these ages results in a chronology that forms one of the longest independently dated, high-resolution lacustrine sediment records from eastern Africa. The chronology illustrates that any record of environmental change preserved in the composite sediment core from Chew Bahir would span the entire timescale of modern human evolution and dispersal, encompassing the time period of the transition from Acheulean to Middle Stone Age (MSA), and subsequently to Later Stone Age (LSA) technology, making the core well-placed to address questions regarding environmental change and hominin evolutionary adaptation. The benefits to such studies of direct dating and the use of multiple independent chronometers are discussed. Highlights • Four independent dating methods applied to ∼293 m lake core from southern Ethiopia. • Reveals 620 ka high-resolution sedimentary record near key fossil hominin sites. • Mean accumulation rate of 0.47 mm/a comparable to other African lacustrine sediments. • Accumulation rate fell to 0.1 mm/a during MIS 2, likely due to reduced sediment supply. • Use of multiple independent chronometers is a powerful approach in lake settings