干旱区生态环境知识资源中心

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its delta O-18 and tritium content, dissolved inorganic carbon (DIC) and its delta C-13 (parts per thousand) and Delta C-14 (parts per thousand) content, and delta C-13 (parts per thousand) and Delta C-14 (parts per thousand) in the solid sediment. The profiles of moisture and delta O-18 in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr(-1)). The source of carbon in the DIC is CO2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The delta C-13 (parts per thousand) profiles show clearly the chemical transition from dissolved CO2 (delta C-13 = -22 parts per thousand) to bicarbonate (delta C-13 = -14 parts per thousand). At greater depth (-11.3 parts per thousand), the delta C-13 becomes similar to the delta C-13 in the aquifer below (-12.5 parts per thousand). The effect of secondary processes is evident in the profile of Delta C-14 in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10 parts per thousand yr(-1). Precipitation of carbon from the DIC on the sediment is 1.1 mg C L-sed(-1) yr(-1), negligible compared to the 28 g C in 1 L-sed. In the solid sediment, there is a gradient in Delta C-14(carb) at the top meter. The net precipitation of C-14 from the DIC on the sediment (0.25 to 1.1 parts per thousand yr(-1)), corrected for decay, cannot be observed in the deeper sediment. The presence of C-14 in the top 1 m of the sediment is explained by two possible processes: accumulation of C-14-tagged dust (similar to 0.05 mm yr(-1)) and/or long-term cumulative precipitation from the DIC.