Searching for evidence of a global catastrophe in the East African Rift Basin: Did the Toba supereruption alter paleoflora at Gona, Ethiopia?

Approximately 75,500 (+/- 900) years ago, the largest supereruption of the Late Pleistocene occurred in Sumatra, Indonesia. This explosion introduced ~1015 grams of fine ash into the atmosphere, produced ~3000 km3 of magma, and pyroclastic flows carpeted a ~105 km2 radius around the epicenter of the eruption. All flora and fauna within a 350 km radius were annihilated, and the enormous amount of ash that was aerosolized into the atmosphere is thought to have increased the Earth’s average albedo, which is speculated to be a possible catalyst for a period of global cooling. The advent of the Toba supereruption coincides with a period of Anatomically Modern Human (AMH) migration out-of-Africa. While recent studies show that the effects from the “environmental catastrophe” observed in Sumatra likely did not significantly alter the environments of East Africa during this time, there are only two comprehensive studies of cryptotephra (microscopic ash) layers from the Toba volcano in the East African Rift System, only one of which focuses on the variability in vegetation pre- and post-eruption. Additionally, both of those studies are focused on localities in Lake Malawi, records of which are sourced from lacustrine cores. As AMH are terrestrial-based mammals, and only passively interact with lacustrine environments, the question then arises, How did the vegetation in terrestrial areas with AMH activity change pre- and post-Toba supereruption? The answer to this question may lie in the sediments and soils of Gona, Ethiopia, a significant paleoanthropological project area that contains an abundance of Early and Middle-to-Late Pleistocene archaeological and hominin fossil sites, fluvial sediments and soils, and a well-constrained chronostratigraphic record. Yaalu (11° 3\u27 51.55 N, 40° 25\u27 23.12 E), a paleoanthropological site in Gona, has strata that have been dated to 85-70ka. During the 2020 field season, an undergraduate assistant and I will scout the area for an outcrop or series of outcroppings that encompass the full strata. Once a suitable site is located, a step-trench will be dug down-section to reveal the underlying strata. Individual soil horizons will be characterized, and oriented samples that encompass the full scope of the trench will be extracted. Sediment and soil samples will then be examined for evidence of cryptotephra, and phytoliths, silicic imprints of cellular structures from flora, will be extracted and characterized. Phytoliths will be point-counted, and percent relative abundance calculations will be derived from total phytolith count, alongside the starting dry weight. Namely, the tree cover and aridity indexes will be utilized, to see the change in low elevation semi-deciduous forests, and the expansion or contraction of riparian grass communities in the area. This study will allow us to see examine the variability of flora (or lack thereof) at Gona during a critical period of early human development. If phytolith assemblages significantly change, or if the overall concentration of phytoliths decreases at Yaalu post-Toba, then one could infer that the Toba supereruption was a catalyst for, at the very least, minimal change within Gona’s ecosystem

Paleopedology associated with the rise and dispersal of Anatomically Modern Humans at Gona, Ethiopia

Geologists and paleoanthropologists continue to debate the onset, development, and rate of change of out-of-Africa dispersals by Anatomically Modern Humans (AMH). Climatic and environmental variability is often inferred to be the catalysts of these migrations, yet the precise context of these dispersals, including climate effects on local flora and fauna, remains unclear. This study addresses this uncertainty by examining a series of eleven fossilized soils (paleosols) that range in age from the Middle Pleistocene (~570 ka) to present at Gona, Ethiopia, a significant paleoanthropological area that has abundant archaeological and AMH fossil sites. Paleosols provide an ideal archive for reconstructing the localized changes in paleoenvironment and paleoclimate associated with Gona’s archaeological and fossil sites, as they are a reservoir of biogeochemical dynamics related to the surrounding environment. Initial morphological observations from the paleosols of the Yaalu (~80 ka) and Erole (12 ka) fossil sites show the presence of soil carbonate and shrink-swell features indicative of seasonal climate. Bulk geochemical data supports these observations, with the Yaalu paleosols yielding average mean annual precipitation (MAP) and temperature (MAT) values of 723 mm/yr (± 108) and 14.0°C (± 4.4), with Erole paleosols yielding average MAP and MAT values of 832 mm/yr (± 108) and 13.3°C (± 4.4), respectively. These data will continue to be expanded upon and will encompass eight archaeological and fossil localities in eleven different sites. Although these results are preliminary, this growing dataset will compliment more regional-scale paleoenvironmental and paleoclimate records when interpreting the forcings and responses of Out-of-Africa migrations

Evidence of Late Pleistocene and Holocene paleo-Critical Zones at Gona, Ethiopia

The African Humid Period (AHP), spanning a period of approximately 12-5 ka, resulted in Northern and Eastern Africa being wetter than today and had notable impacts on flora, fauna, and humans. Much of the work pertaining to the AHP across Eastern Africa utilizes lacustrine and marine proxies rather than fluvial. Gona, located in the Afar region of Ethiopia, is known for its extensive archaeological and fossil records in fluvial deposits. However, the paleoenvironments of the AHP at Gona have not been investigated. This study uses stratigraphy, geochronology, and paleopedology to reconstruct the Late Pleistocene and AHP paleoenvironments, i.e., paleo-Critical Zones. We examine two paleosols, the Odele and Erole paleosols, located in the Asbole study region of Gona. The Odele paleosol is between the Korina Tuff (\u3c39 ka) and the Kilaitoli Tuff (~25.7 ka) and weathered during late-stage MIS-3 and MIS-2. The Erole paleosol, a relict soil that weathered during the AHP, is ~15 m above the Kilaitoli Tuff and immediately above a calibrated 14C age of 12 ka. Both paleosols formed along paleo-tributaries of the ancestral Awash River, as only matrix-supported gravels are found. The Erole paleosol displays consistently darker Munsell values than the Odele paleosol. Average strain calculations using paleosol geochemistry show a volumetric collapse on the order of 34 ± 4% in the Erole paleosol and little to no dilation/collapse in the Odele paleosol, 0 ± 2%. Calculations of open-system mass transport of elements through the profiles (Tau) show an 18 ± 7% loss of SiO2 and a 69 ± 5% loss of CaO in the Erole paleosol, which are greater than the 2 ± 1% loss of SiO2 and 1 ± 3% loss of CaO in the Odele paleosol. These strain and tau results suggest more intense weathering and elemental loss in the Erole paleosol. These results are consistent with recent paleoclimate reconstructions, and we infer that the collapse and elemental loss in the Erole paleosol are due to a period of increased rainfall during the AHP than the preceding MIS-3 and MIS-2 tim

The persistence of salt-affected paleosols at Gona, Ethiopia: A sedimentary archive of Middle to Late Pleistocene soil salinity within a corridor of early human migration

The origin of Homo sapiens and their subsequent dispersal out of Africa during the Mid-to-Late Pleistocene are hallmark events in the evolution of humans and are recorded as snapshots within terrestrial sedimentary deposits in East Africa. This study uses paleosols to reconstruct paleoenvironments of those events at Gona, Ethiopia - an area with one of the most continuous records of East African Paleolithic and Neolithic archaeology. A preliminary survey of Gona paleosols from the past 250,000 years shows a wide range of paleosol types that resemble modern-day Entisols, Inceptisols and Vertisols. Approximate paleosol ages were estimated using nearby OSL, 14 C, U-series, 40 Ar/ 39 Ar and volcanic glass chemistry. The pH and EC of these paleosols were measured as they are weakly compacted and show few signs of diagenesis. Mean pH values are slightly alkaline (7.8 ±0.63) and mean EC values (9.8 ±7.9) are saline, suggesting salt-affected soil development. The high standard deviation of EC results from low-EC sodic soils found in some units. These pH and EC findings are consistent with bulk geochemical-based pedotransfer functions on paleo-Vertisols, which indicate the presence of saline and sodic conditions. Much like modern-day tributary and trunk-channel floodplains, these paleosols likely hosted halophytes like Tamarix, Vachellia and salt-tolerant grasses, flora which are commonly found at present-day Gona. Notably, some late Pleistocene salt-affected paleosols that coincide with Marine Isotope Stage 5 and the African Humid Period show signs of prolonged soil saturation and nearby standing water. High evapotranspiration due to Gona’s semi-arid climate likely resulted in low infiltration and subsurface drainage of water, insufficient to transport salt out of the system, resulting in abundant saline soil formation at the site. The high salinity of Mid-to-Late Pleistocene Gona paleosols, likely due to source material, climate, and drainage, limits the use of many bulk geochemical proxies that were developed using mostly normal, non-saline soils. Despite this, the results of this study shed light on the climate and environment of our ancestors at the pedon scale

Evidence of Late Pleistocene and Holocene paleo-Critical Zones at Gona, Ethiopia

The African Humid Period (AHP), spanning a period of approximately 14.5-5 ka, resulted in Northern and Eastern Africa being wetter than today and had notable impacts on flora, fauna, and humans. Much of the work pertaining to the AHP across Eastern Africa utilizes lacustrine and marine proxies rather than fluvial. Gona, located in the Afar region of Ethiopia, is known for its extensive archaeological and fossil records in fluvial deposits. However, the paleoenvironments of the AHP at Gona have not been investigated. This study uses stratigraphy, geochronology, and paleopedology to reconstruct the Late Pleistocene and AHP paleoenvironments, i.e., paleo-Critical Zones. We examine two paleosols, the Odele and Erole paleosols, located in the Asbole study region of Gona. The Odele paleosol is between the Korina Tuff (\u3c39 ka) and the Kilaitoli Tuff (~25.7 ka) and weathered during late-stage MIS-3 and MIS-2. The Erole paleosol, a relict soil that weathered during the AHP, is ~15 m above the Kilaitoli Tuff and immediately above a calibrated 14C age of 12 ka. Both paleosols formed along paleo-tributaries of the ancestral Awash River, as only matrix-supported gravels are found. The Erole paleosol displays consistently darker Munsell values than the Odele paleosol. Average strain calculations using paleosol geochemistry show a volumetric collapse on the order of 34 ± 4% in the Erole paleosol and little to no dilation/collapse in the Odele paleosol, 0 ± 2%. Calculations of open-system mass transport of elements through the profiles (Tau) show an 18 ± 7% loss of SiO2 and a 69 ± 5% loss of CaO in the Erole paleosol, which are greater than the 2 ± 1% loss of SiO2 and 1 ± 3% loss of CaO in the Odele paleosol. These strain and tau results suggest more intense weathering and elemental loss in the Erole paleosol. These results are consistent with recent paleoclimate reconstructions, and we infer that the collapse and elemental loss in the Erole paleosol are due to a period of increased rainfall during the AHP than the preceding MIS-3 and MIS-2 time

Geochemical analysis and physical characterization of fossilized soils during periods of early human activity and migration: A study of Gona, Ethiopia

Environmental change is often inferred to have driven dispersals of Anatomically Modern Humans (AMH) out of Africa, yet the precise landscape context of these migrations remains unclear. Furthermore, fluvial-based archives of paleoenvironment during periods of dispersal are scarce. Gona, an area in northeastern Ethiopia with one of the most continuous records of East African Paleolithic and Neolithic archaeology, contains abundant Middle to Late Pleistocene fluvial deposits interbedded with tephra. This study examines the physical and chemical changes of 11 fossilized soils (paleosols) extracted from Gona’s paleoanthropological sites that range in age from the Middle to the Late Pleistocene (~380-11ka). The paleosols from Gona provide valuable insight into the landscapes in which our earliest direct ancestors interacted, as they are a dynamic biogeochemical archive of weathering, which is related to the surrounding environment at the time of formation. We focus on paleo-Vertisols, or paleosols with vertic features that formed in distal floodplain settings to provide a control on landscape position. Colorimeter results show that the paleosols become darker from the Middle to Late Pleistocene and Early Holocene. We also see an increase in organic carbon (OC) content with notable peaks at 77.5, 50, and 11 ka. CALMAG-based estimates of mean annual precipitation (MAP) range from 405 to 527 mm/yr (+/- 108), with the exception of notable increases at 77.5 (688 mm/yr +/- 108) and 11ka (656 mm/yr +/- 108). These increases in OC and MAP coincide with previously documented episodes of wetter climates - MIS-5a and the African Humid Period. The paleo-Vertisols at Gona record evidence of wetter paleoclimates during certain periods of AMH migration and suggest that this terrestrial fluvial record is a unique source for paleoenvironmental data. This localized terrestrial dataset complements additional regional-scale paleoenvironmental records when interpreting the forcing and responses of Out-of-Africa migrations