Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmenta... more Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions. Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods. Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO 2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.
Hawks et al. argue that our analysis of Australopithecus sediba mandibles is flawed and that spec... more Hawks et al. argue that our analysis of Australopithecus sediba mandibles is flawed and that specimen LD 350-1 cannot be distinguished from this, or any other, Australopithecus species. Our reexamination of the evidence confirms that LD 350-1 falls outside of the pattern that A. sediba shares with Australopithecus and thus is reasonably assigned to the genus Homo.
The role that climate and environmental history may have played in influencing human evolution ha... more The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered. We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012–2014 HSPDP coring campaign.
To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnet... more To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnetostratigraphy was applied to Pliocene lacustrine sediments from the eastern part of the Hadar Basin (Ledi-Geraru research area). Characteristic geochemistry and lithologic features of three tuffaceous horizons at the bottom, middle and top of the stratigraphy are recognized as the Sidi Hakoma Tuff (SHT), Triple Tuff-4 (TT-4) and the
The Geology of Early Humans in the Horn of Africa, Geological Society of America Special Paper
"To date and characterize depositional environments of the hominin-bearing Hadar Formation, lacus... more "To date and characterize depositional environments of the hominin-bearing Hadar Formation, lacustrine sediments from the eastern part of the Hadar Basin (Ledi- Geraru research area) were studied using tephrostratigraphy and magnetostratigraphy.
The Sidi Hakoma Tuff, Triple Tuff-4, and the Kada Hadar Tuff, previously dated by 40Ar/39Ar in other parts of the basin, were identifi ed using characteristic geochemical composition and lithologic features. Paleomagnetic samples were collected every 0.5 m along an ~230-m-thick composite section between the Sidi Hakoma Tuff and the Kada Hadar Tuff. A primary detrital remanent magnetization mostly carried by (titano-) magnetites of basaltic origin was recognized. Consistent with existing data of the Hadar Basin, paleomagnetic directions show a postdepositional counterclockwise vertical-axis tectonic rotation (~5°–10°) and shallowing of paleomagnetic inclination (~5°–10°) related to sedimentation and compaction. Two normal-polarity intervals (chrons 2An.3n and 2An.2n) are recorded bracketing a reversed interval identifi ed as the Mammoth event (chron 2An.2r). Resulting sediment accumulation rates (~90 cm/k.y.) are high compared to existing accumulation-rate estimates from the more western part of the Hadar Basin. The resulting eastward increasing trend suggests that deposition took place in an eastward-tilting basin. Sediment accumulations were constant throughout the basin from ca. 3.4 to 3.2 Ma. At 3.2 Ma, a regional and relatively short-lived event is indicated by signifi cant change in depositional conditions and a large increase in accumulation
rate. This disruption may have been related to increased climate variability due to astronomical climate forcing. It provides a possible explanation for changes in the Hadar faunal community and Australopithecus afarensis in particular."
Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmenta... more Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions. Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods. Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO 2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.
Hawks et al. argue that our analysis of Australopithecus sediba mandibles is flawed and that spec... more Hawks et al. argue that our analysis of Australopithecus sediba mandibles is flawed and that specimen LD 350-1 cannot be distinguished from this, or any other, Australopithecus species. Our reexamination of the evidence confirms that LD 350-1 falls outside of the pattern that A. sediba shares with Australopithecus and thus is reasonably assigned to the genus Homo.
The role that climate and environmental history may have played in influencing human evolution ha... more The role that climate and environmental history may have played in influencing human evolution has been the focus of considerable interest and controversy among paleoanthropologists for decades. Prior attempts to understand the environmental history side of this equation have centered around the study of outcrop sediments and fossils adjacent to where fossil hominins (ancestors or close relatives of modern humans) are found, or from the study of deep sea drill cores. However, outcrop sediments are often highly weathered and thus are unsuitable for some types of paleoclimatic records, and deep sea core records come from long distances away from the actual fossil and stone tool remains. The Hominin Sites and Paleolakes Drilling Project (HSPDP) was developed to address these issues. The project has focused its efforts on the eastern African Rift Valley, where much of the evidence for early hominins has been recovered. We have collected about 2 km of sediment drill core from six basins in Kenya and Ethiopia, in lake deposits immediately adjacent to important fossil hominin and archaeological sites. Collectively these cores cover in time many of the key transitions and critical intervals in human evolutionary history over the last 4 Ma, such as the earliest stone tools, the origin of our own genus Homo, and the earliest anatomically modern Homo sapiens. Here we document the initial field, physical property, and core description results of the 2012–2014 HSPDP coring campaign.
To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnet... more To date and characterize depositional environments of the hominin-bearing Hadar Formation, magnetostratigraphy was applied to Pliocene lacustrine sediments from the eastern part of the Hadar Basin (Ledi-Geraru research area). Characteristic geochemistry and lithologic features of three tuffaceous horizons at the bottom, middle and top of the stratigraphy are recognized as the Sidi Hakoma Tuff (SHT), Triple Tuff-4 (TT-4) and the
The Geology of Early Humans in the Horn of Africa, Geological Society of America Special Paper
"To date and characterize depositional environments of the hominin-bearing Hadar Formation, lacus... more "To date and characterize depositional environments of the hominin-bearing Hadar Formation, lacustrine sediments from the eastern part of the Hadar Basin (Ledi- Geraru research area) were studied using tephrostratigraphy and magnetostratigraphy.
The Sidi Hakoma Tuff, Triple Tuff-4, and the Kada Hadar Tuff, previously dated by 40Ar/39Ar in other parts of the basin, were identifi ed using characteristic geochemical composition and lithologic features. Paleomagnetic samples were collected every 0.5 m along an ~230-m-thick composite section between the Sidi Hakoma Tuff and the Kada Hadar Tuff. A primary detrital remanent magnetization mostly carried by (titano-) magnetites of basaltic origin was recognized. Consistent with existing data of the Hadar Basin, paleomagnetic directions show a postdepositional counterclockwise vertical-axis tectonic rotation (~5°–10°) and shallowing of paleomagnetic inclination (~5°–10°) related to sedimentation and compaction. Two normal-polarity intervals (chrons 2An.3n and 2An.2n) are recorded bracketing a reversed interval identifi ed as the Mammoth event (chron 2An.2r). Resulting sediment accumulation rates (~90 cm/k.y.) are high compared to existing accumulation-rate estimates from the more western part of the Hadar Basin. The resulting eastward increasing trend suggests that deposition took place in an eastward-tilting basin. Sediment accumulations were constant throughout the basin from ca. 3.4 to 3.2 Ma. At 3.2 Ma, a regional and relatively short-lived event is indicated by signifi cant change in depositional conditions and a large increase in accumulation
rate. This disruption may have been related to increased climate variability due to astronomical climate forcing. It provides a possible explanation for changes in the Hadar faunal community and Australopithecus afarensis in particular."
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The Sidi Hakoma Tuff, Triple Tuff-4, and the Kada Hadar Tuff, previously dated by 40Ar/39Ar in other parts of the basin, were identifi ed using characteristic geochemical composition and lithologic features. Paleomagnetic samples were collected every 0.5 m along an ~230-m-thick composite section between the Sidi Hakoma Tuff and the Kada Hadar Tuff. A primary detrital remanent magnetization mostly carried by (titano-) magnetites of basaltic origin was recognized. Consistent with existing data of the Hadar Basin, paleomagnetic directions show a postdepositional counterclockwise vertical-axis tectonic rotation (~5°–10°) and shallowing of paleomagnetic inclination (~5°–10°) related to sedimentation and compaction. Two normal-polarity intervals (chrons 2An.3n and 2An.2n) are recorded bracketing a reversed interval identifi ed as the Mammoth event (chron 2An.2r). Resulting sediment accumulation rates (~90 cm/k.y.) are high compared to existing accumulation-rate estimates from the more western part of the Hadar Basin. The resulting eastward increasing trend suggests that deposition took place in an eastward-tilting basin. Sediment accumulations were constant throughout the basin from ca. 3.4 to 3.2 Ma. At 3.2 Ma, a regional and relatively short-lived event is indicated by signifi cant change in depositional conditions and a large increase in accumulation
rate. This disruption may have been related to increased climate variability due to astronomical climate forcing. It provides a possible explanation for changes in the Hadar faunal community and Australopithecus afarensis in particular."
The Sidi Hakoma Tuff, Triple Tuff-4, and the Kada Hadar Tuff, previously dated by 40Ar/39Ar in other parts of the basin, were identifi ed using characteristic geochemical composition and lithologic features. Paleomagnetic samples were collected every 0.5 m along an ~230-m-thick composite section between the Sidi Hakoma Tuff and the Kada Hadar Tuff. A primary detrital remanent magnetization mostly carried by (titano-) magnetites of basaltic origin was recognized. Consistent with existing data of the Hadar Basin, paleomagnetic directions show a postdepositional counterclockwise vertical-axis tectonic rotation (~5°–10°) and shallowing of paleomagnetic inclination (~5°–10°) related to sedimentation and compaction. Two normal-polarity intervals (chrons 2An.3n and 2An.2n) are recorded bracketing a reversed interval identifi ed as the Mammoth event (chron 2An.2r). Resulting sediment accumulation rates (~90 cm/k.y.) are high compared to existing accumulation-rate estimates from the more western part of the Hadar Basin. The resulting eastward increasing trend suggests that deposition took place in an eastward-tilting basin. Sediment accumulations were constant throughout the basin from ca. 3.4 to 3.2 Ma. At 3.2 Ma, a regional and relatively short-lived event is indicated by signifi cant change in depositional conditions and a large increase in accumulation
rate. This disruption may have been related to increased climate variability due to astronomical climate forcing. It provides a possible explanation for changes in the Hadar faunal community and Australopithecus afarensis in particular."