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Ryb U.,The Fredy and Nadine Herrmann Institute of Earth science | Matmon A.,The Fredy and Nadine Herrmann Institute of Earth science | Erel Y.,The Fredy and Nadine Herrmann Institute of Earth science | Haviv I.,Ben - Gurion University of the Negev | And 2 more authors.
Earth and Planetary Science Letters | Year: 2014

Carbonate minerals, unlike silicates, have the potential to dissolve almost completely and with high efficiency. Thus, in carbonate terrains denudation rate and style (the governing process of denudation, mechanical or chemical) should be more sensitive to climatic forcing. Using 36Cl measurements in 39 carbonate bedrock and sediment samples, we calculate long-term denudation rates across a sharp climatic gradient from Mediterranean to hyper-arid conditions. Our samples were collected along the Arugot watershed, which drains the eastern flank of the Judea Range (central Israel) to the Dead Sea and is characterized by a pronounced rain shadow. Denudation rates of flat-lying bedrock outcrops sampled along interfluves differ by an order of magnitude from ~20 mm ka-1 in the Mediterranean zone to 1-3 mm ka-1 in the hyper-arid zone. These rates are strongly correlated with precipitation, and thus reflect the importance of carbonate mineral dissolution in the overall denudation process. In contrast, denudation rates of steep bedrock surfaces depend on the hillslope gradient, but only in the hyper-arid climate zone, indicating that mechanical processes dominate the overall hillslope denudation within this zone. The dominance of slope-dependent mechanical erosion in the hyper-arid zone is also reflected by an increase in spatially-average denudation rates from 17-19 mm ka-1 in the Mediterranean-semi-arid zones to 21-25 mm ka-1 in the hyper-arid zone. These higher rates are attributed to clast contribution from steep slopes under arid climate. This suggests an increased importance of mechanical processes to the overall denudation in the hyper-arid zone.We demonstrate that the transition between chemically-dominated denudation to mechanically-dominated denudation occurs between 100 and 200 mm of mean annual precipitation. Long-term denudation rates across the Judea Range indicate that between Mediterranean and hyper-arid climates, chemical weathering rates are limited by precipitation. Nevertheless, in more humid climates, chemical weathering rates are apparently limited by the rates of carbonate mineral dissolution. This study demonstrates that carbonate terrains have the capacity to shift between mechanically and chemically dominated denudation in response to changes in precipitation. Similar transitions in response to changes in temperature or the level of tectonic activity have been previously reported. We suggest that the abrupt nature of such transitions can be primarily attributed to the efficiency of carbonate dissolution processes and the competition between surface and subsurface drainage systems in carbonate terrains. © 2014 Elsevier B.V.

Ryb U.,The Fredy and Nadine Herrmann Institute of Earth science | Matmon A.,The Fredy and Nadine Herrmann Institute of Earth science | Erel Y.,The Fredy and Nadine Herrmann Institute of Earth science | Haviv I.,Ben - Gurion University of the Negev | And 7 more authors.
Bulletin of the Geological Society of America | Year: 2014

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. © 2014 Geological Society of America.

Wurgaft E.,The Fredy and Nadine Herrmann Institute of Earth science | Shamir O.,The Fredy and Nadine Herrmann Institute of Earth science | Barkan E.,The Fredy and Nadine Herrmann Institute of Earth science | Paldor N.,The Fredy and Nadine Herrmann Institute of Earth science | Luz B.,The Fredy and Nadine Herrmann Institute of Earth science
Limnology and Oceanography | Year: 2013

A time series of the 17O excess (17Δ) was measured in the Gulf of Elat (Aqaba) between May 2007 and August 2009.17Δ is unaffected by respiration; thus it is a unique, conservative tracer that preserves the signature acquired in the photic zone, the source region for deep-water formation. In this study we used 17Δ to assess the ratio of photosynthetic O2 to atmospheric O2 in deep water. We observed an increase of 17Δ by 20 per meg in the water residing below 300 m over a period of 3 months, followed by a decrease of 60 per meg over the next 7 months. These changes indicated penetration of photosynthetic O2, followed by penetration of atmospheric O2 into the deep water. To test whether vertical mixing could explain the observed variations in 17Δ, we compared our results with simulated values obtained from a one-dimensional hydrodynamic model, which was extended to include dissolved O2 isotopes. Although successfully reproducing the observed temperatures, salinities, and dissolved O2 concentrations in the gulf, the model could not reproduce the observed variations in 17Δ in the deep water. This discrepancy shows that horizontal mixing processes have an important role in the interaction between deep and surface water in the gulf. We suggest that for the most part, these processes occur along the coastal boundaries of the gulf. © 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.

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