Abstract

Using cosmogenic isotopes and solute load analysis, we quantify chemical weathering (solutional erosion) and denudation rates over variable time scales in a tectonically stable, moderate-relief, carbonate terrain (Soreq drainage, Judea Hills, Israel), located in a semihumid Mediterranean climate. Long-term (>104 yr) denudation rates were calculated from 36Cl concentrations in 51 bedrock and sediment samples. Bedrock samples range in elevation (340–850 m), hillslope gradient (0°–30°), and mean annual precipitation (MAP; 500–630 mm) and vary in soil cover thickness (0–75 cm), Mg/Ca ratio (0.0–1.0 mol), clay mineral contents (0–6 wt%), and mechanical strength (41–58 Schmidt hammer rebound units). Soil pCO2 values at a single location during the course of 1 yr, range between 0.4 and 9.0 mmol mol–1. Average long-term denudation rate of exposed bedrock samples is 21 ± 7 mm k.y.–1. Field observations and 36Cl measurements indicate that soil pockets undergo cycles in the rate of deepening, and that over 105 yr time scale, average denudation rates beneath soil pockets are similar to those of exposed bedrock. Sediment samples yield even higher denudation rates, which are probably anthropogenically induced, but could also indicate that the sediment source is soil pockets. Long-term denudation rates are decoupled from hillslope gradient, elevation, and rock strength. Denudation rates show a positive correlation with present-day MAP values, exhibit a complex relation with rock Mg content, and show a weak correlation with clay content. Annual chemical weathering rates were calculated from modern-day solute load measured in waters of perched springs and the regional carbonate aquifer. Our results indicate that on annual, decadal, and 104 yr time scales, chemical weathering and denudation are controlled by carbonate dissolution, while mechanical processes are far less significant. Overlap between the distributions of HCO3 concentrations measured in runoff, springs, and the regional aquifer water suggests that chemical weathering focuses at the bedrock surface and therefore is comparable with solutional denudation. This result is in contrast to the features of ancient fluvial and colluvial activity (steep nonconcave hillslopes and stream profiles and knickzones in the streams) preserved in the present landscape. Such features were formed in response to mid-Pleistocene uplift and could have been preserved due to a decrease in stream power following the formation of subsurface drainage and the lowering in abrasive clast supply that followed the stabilization of hillslopes in the drainage. Long-term denudation rates calculated from exposed bedrock samples are higher by factor of 1.4 relative to annual, contemporary chemical weathering rates. Increased precipitation by a similar factor, averaged over the last glacial and present interglacial, can explain this difference.

You do not currently have access to this article.