Assessment of 7Be content in precipitation in a South American semi-arid environment

Abstract

There are two naturally occurring radiogenic isotopes of beryllium, 7Be and 10Be. These are produced when cosmic radiation interacts with oxygen and nitrogen in the atmosphere. After production, these radionuclides are input to ecosystems through wet and dry deposition. In recent years 7Be and 10Be have proved to be powerful tools for studying dynamic processes that occur on the surface of the earth. We measured the 7Be content in precipitation at a semiarid location in central Argentina. From November 2006 to March 2009, 68 precipitation events were collected. Measured 7Be content ranged from 0.7±0.4BqL-1 to 3.2±0.7BqL-1, with a mean of 1.7BqL-1±0.6BqL-1. Beryllium-7 content of rainfall did not show clear relationships with amount of rainfall (mm), mean intensity (mmh-1) or duration (h-1), or elapsed time between events (day). The general results indicate that for the typical range of precipitation there was no atmospheric washout and that the reload of the atmosphere is not a relevant factor, but when the amount of precipitation is very high washout may occur. On the other hand, when the 7Be content was measured during single rain events, a high content of this radionuclide was found to be associated with very low rainfall intensity (≈3mmh-1), this suggests that rain intensity could affect the 7Be content. Using all data, a good linear relationship between 7Be deposition and rain magnitude was obtained (r2=0.82, p<0.0001). Because of this, the slope of this linear regression equation may be applied as a tool for tracing environmental processes that affect the surface of the earth. We can do this by directly estimating erosion/sedimentation processes using 7Be or by estimating the input of 10Be in the environment with the aim to evaluate land degradation phenomena. © 2012 Elsevier B.V.