DENUDATION OF CARBONATE TERRAINS IN THE MEDITERRANEAN CLIMATE: INSIGHTS FROM 36Cl MEASUREMENTS IN THE SOREQ DRAINAGE, JUDEA HILLS, ISRAEL

Denudation of carbonate terrains is frequently measured over short time scales by calculation of spring dissolved load, micro erosion meters, or weight loss measurements using standard tablets. While measuring the above is straightforward, the results are sensitive to episodic or local environmental conditions. ³⁶Cl concentrations in carbonate samples yield denudation rates averaged over 10⁴-10⁵ yr time scale. Our study focuses on the Soreq drainage, central Israel. Incising ~400 m into the carbonate highland of the Judea Hills, the Soreq is a well-developed fluvial system. With steep linear hillslopes and non-concave tributary profiles, it may be interpreted to be adjusting to a subsiding base level. Nevertheless, a recent study suggests that the present streambed had been stable since mid-Pleistocene. We measured ³⁶Cl concentrations in 54 bedrock and sediment samples, collected along interfluve and hillslope profiles in the Soreq drainage. Sampling location range in elevation between 340 and 850 m), hillslope gradient (0-30°), and Mean Annual Precipitation (MAP) (500-630 mm/yr). Bedrock samples also vary in soil cover thickness (0 to 75 cm), MgO/(MgO+CaO) ratio (0-0.46 wt), clay mineral contents (0-6%wt), and mechanical strength (41-58 Schmidt hammer rebound units). Average calculated denudation rate from soil-covered samples (40 (σ1=13) mm/ka) is ~60% higher than that of the exposed bedrock samples (27 (σ1=14) mm/ka). Such an increase is expected beneath soil cover, where pCO₂ is higher. Denudation rates are decoupled from hillslope gradient, elevation, bedrock strength and clay content. Denudation rates show a positive correlation with present day MAP and a complex relation with Mg content. Precipitation also correlates with denudation on an annual time scale; this is shown using discharge and solute load of springs and of the regional aquifer. The above relations suggest a dominance of carbonate dissolution processes rather than mechanical erosion over denudation. This interpretation stems from the entire bedrock sample population as well from single hillslope and interfluve profiles. The absence of a spatial trend of denudation rates along slope profiles suggests parallel hillslope retreat at about the same rate that interfluves are being lowered, leading to a steady state landscape.