Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift

Abstract

The African Humid Period (AHP) between ∼15 and 5.5 cal. kyr BP caused major environmental change in East Africa, including filling of the Suguta Valley in the northern Kenya Rift with an extensive (∼2150 km 2 ), deep (∼300 m) lake. Interfingering fluvio-lacustrine deposits of the Baragoi paleo-delta provide insights into the lake-level history and how erosion rates changed during this time, as revealed by delta-volume estimates and the concentration of cosmogenic 10 Be in fluvial sand. Erosion rates derived from delta-volume estimates range from 0.019 to 0.03 mm yr −1 . 10 Be-derived paleo-erosion rates at ∼11.8 cal. kyr BP ranged from 0.035 to 0.086 mm yr −1 , and were 2.7 to 6.6 times faster than at present. In contrast, at ∼8.7 cal. kyr BP, erosion rates were only 1.8 times faster than at present. Because 10 Bederived erosion rates integrate over several millennia, we modeled the erosion-rate history that best explains the 10 Be data using established non-linear equations that describe in situ cosmogenic isotope production and decay. Two models with different temporal constraints (15-6.7 and 12-6.7 kyr) suggest erosion rates that were ∼25 to ∼300 times higher than the initial erosion rate (pre-delta formation). That pulse of high erosion rates was short (∼4 kyr or less) and must have been followed by a rapid decrease in rates while climate remained humid to reach the modern 10 Be-based erosion rate of ∼0.013 mm yr −1 .Our simulations also flag the two highest 10 Be-derived erosion rates at ∼11.8 kyr BP related to nonuniform catchment erosion. These changes in erosion rates and processes during the AHP may reflect a strong increase in precipitation, runoff, and erosivity at the arid-to-humid transition either at ∼15 or ∼12 cal. kyr BP, before the landscape stabilized again, possibly due to increased soil production and denser vegetation.

Mechanical weathering and rock erosion by climate‐dependent subcritical cracking

“…Similarly, as relatively unweathered rock that contains longer cracks (joints) or denser flaws (foliation or bedding) is exhumed, weathering rates might initially be large but then slow down as the rock adjusts—through subcritical cracking along these inherent weaknesses—to its new stress‐loading conditions. We hypothesize that such a relationship might provide a mechanistic explanation for observations of faster regolith production rates in faster eroding landscapes [ Heimsath et al ., 2012], or initially fast, followed by slower, erosion after a climate change [ Garcin et al ., ].…”

Alluvial channel response to environmental perturbations: fill-terrace formation and sediment-signal disruption

“…Physical experiments provide an alternative approach to studying the dynamics of the transfer zone, including terrace formation (Baynes et al, 2018;Frankel et al, 2007;Gardner, 1983;Lewis, 1944;Mizutani, 1998;Schumm and Parker, 1973;Wohl and Ikeda, 1997) and the evolution of Q s,out (Bonnet and Crave, 2003;van den Berg van Saparoea and Postma 2008). Most experimental studies have tested the cutting of terraces due to base-level fall (BLF; Frankel et al, 2007;Gardner, 1983;Schumm and Parker, 1973) or explained their cutting through autogenic processes (Lewis, 1944;Mizutani, 1998). Only one experimental study by Baynes et al (2018) investigated terrace formation as a response to changes in Q s,in or Q w , but this study focused on vertical incision into bedrock and strath-terrace cutting.…”

Ecohydrology Controls the Geomorphic Response to Climate Change

“…(2) Strong positive response: observed in regional studies that have shown an increase in long-term erosion rates with precipitation and runoff, sometimes mediated by relief under high uplift rates (e.g., Bookhagen & Strecker, 2012;Garcin et al, 2017;Henck et al, 2011). (3) Strong but complex response: Watershed studies of sediment yield over decadal time scales (Jeffery et al, 2014;Langbein & Schumm, 1958;Wilson, 1973) and evidence from recent millennial-scale hillslope erosion rate estimates (Schaller et al, 2018;Torres Acosta et al, 2015) have shown a hump-shaped response of erosion to an increase in precipitation, sometimes with multiple peaks (Walling & Webb, 1983). In this humped response, a positive dependence was largely limited to arid and semiarid climates; but a further increase in precipitation led to a downturn in sediment yield and to lower erosion rates.…”