PALEOSOLS REVEAL CLIMATE-DRIVEN FLUCTUATIONS OF WEATHERING IN THE MIDDLE TO LATE PLEISTOCENE PALEO-CRITICAL ZONES OF GONA, ETHIOPIA

Understanding the Critical Zones (CZs) Anatomically Modern Humans interacted with as they dispersed out of Africa is vital towards gaining insight into what drove these migrations. Records of these Middle to Late Pleistocene CZs are abundant and well-preserved at Gona, Ethiopia in the form of fossil soils, or paleosols; well-constrained by a detailed chronostratigraphic record and accompanied by one of the most comprehensive East African archaeological records. To reconstruct the paleo-CZs of Gona, ten paleosol profiles that formed within a distal floodplain near hominin fossil and artifact sites were sampled and described, with landscape position providing a control for this study. These ten fine-grained paleosols weathered mixed alluvium and have vertic features that range in age from 380 to 11 ka. Characterization revealed two paleosol types (i.e., pedotypes): Alsa (“Moon” in the Afar language), and Ayro (“Sun” in Afar), primarily differentiated by color. The darker-colored Alsa paleosols had a lower electrical conductivity (EC) of 4.9 dS/m with mean soil organic carbon (SOC) of 0.92 wt %, whereas Ayro paleosols were lighter-colored, with higher mean EC (12.1 dS/m) and lower mean SOC (0.14 %) content. Mass-balance geochemistry of total Na showed greater mean loss in the Alsa (-0.19) opposed to Ayro (-0.15). Total Ca loss was also greater in the Alsa (-66 %) than Ayro (-42 %). Consistent with this Ca loss, mean CaCO₃ content of the <2 mm fraction is lower in Alsa (0.22 wt. %) than Ayro (0.63 wt. %). Total soil P shows >100 % gain in the darker Alsa than lighter Ayro paleosols. Although EC suggests that all paleosols were salt-affected, the mass-balance geochemistry shows that Alsa lost more essential plant nutrients than Ayro over the duration of soil formation. We attribute this to paleoclimate, where paleorainfall estimates using the geochemistry yielded a higher mean rainfall for Alsa (666 +/- 108 mm/yr) than Ayro paleosols (475 +/- 108 mm/yr). Additionally, chronostratigraphic data suggest that Alsa paleosols weathered during known pluvials (e.g., MIS-5, MIS-3 and AHP), which is consistent with our rainfall estimates. This study suggests that climate drove biogeochemical cycling of nutrients on land, affecting plant productivity on these paleo-CZs of Middle to Late Pleistocene floodplains at Gona.