Evidence of Late Pleistocene and Holocene paleo-critical zones archived in paleosols at Gona, Ethiopia

Abstract

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 (<39 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.