Calculations of Background Beta-Gamma Radiation Dose Through Geologic Time

Abstract

Life on earth is exposed to a background level of ionizing radiation from a number of sources, including beta and gamma radiation from geologic and biologic materials. Radiation dose from geologic emitters has changed because of the chemical evolution of the continental crust, changes in the relative abundances of 235U and 238U, and the radioactive decay of uranium, thorium, and 40K with time. The radiation dose from internal 40K has decreased by a factor of about eight because of changes in the activity concentration of 40K in potassium over the past 4 billion years. Radiation exposure from geologic materials has decreased from about 1.6 mGy y(-1) to 0.66 mGy y(-1) over the past 4 billion years, and radiation exposure to an organism with a potassium concentration of 250 mmol L(-1) has decreased from about 5.5 to about 0.70 mGy y(-1). Accordingly, background radiation exposure from these two sources has dropped from about 7.0 to 1.35 mGy y(-1) during the time life has existed on Earth. The conservative nature of mutation repair mechanisms in modern organisms suggest that these mechanisms may have evolved in the distant past and that organisms may retain some of the capability of efficiently repairing damage from higher radiation levels than exist at present.

Radiation-hormesis phenotypes, the related mechanisms and implications for disease prevention and therapy

“…The radiation exposures involved both low linearenergy-transfer (LET) (e.g., beta and gamma radiations) and high-LET (e.g., alpha radiation) sources. The level of natural background radiation exposure of life forms during that era is estimated to have been five-fold larger than for recent times (Karam and Leslie 1999). Mammals later emerged, and in doing so, survived via adapting to the harsh radiation and also oxygen environments.…”

An Extended Dose–Response Model for Microbial Responses to Ionizing Radiation

“…The effects of IR are, therefore, a product not just of total dose but also of dose rate. As regards the second assumption, on the early Earth the background β/γ radiation was several times higher than it is at present (Karam and Leslie, 1999), and before the formation of the ozone layer the effects of UV-B and UV-C (which at its shortest wavelengths is weakly ionizing) were significant. Mean global current background ionizing radiation dose rates are approximately 3 mGy y −1 , which is the lowest background dose rate to which life has ever been exposed.…”

Ionizing Radiation and Life

“…Ionizing radiation can induce mutations and act as a selective filter for survival, and so it can potentially influence both of these drivers of natural selection and evolution of life (Committee on the Origins and Evolution of Life, 2005). Except for transient extreme ionization events (see Section 8), Earth's surface has been well shielded from CR throughout its history by a thick atmosphere (Svensmark, 2006, estimated variation in CR flux over the planet's history; discussed here in Section 8.8), and although organism exposure from environmental radionuclides has decreased by a factor of around 5 since primordial Earth, this has largely been counteracted by rising oxygen levels and consequent enhancement of radiation damage (Karam and Leslie, 1999;Karam et al, 2001). Surface radiation contributes to mutagenesis rates along with a variety of other causes, including replication errors and DNA damage from UV, chemical oxidants, and desiccation (Makarova et al, 2001).…”

“…In addition to this role in habitability, the intrinsic radiation environment of a planet or moon, generated by radioisotope decay as opposed to CR impinging onto an atmosphere and biosphere, is also a critical factor in habitability and the origin of life. On Earth, ambient radiation levels from radionuclide decay peaked about four billion years ago, at a level approximately 7 times higher than today, and have been steadily decreasing since (Karam and Leslie, 1999). However, the rise in free oxygen over geological time and its enhancing effect on DNA damage through increasing reactive oxygen species generated by radiolysis may have served to steady mutation rates over the history of life (Karam et al, 2001).…”