During the past four decades it has been recognized that natural disruption of the U and Th decay chains, resulting in the separation of parent and daughter nuclides, occurs as a consequence of geochemical processes acting in the near-surface environment. This realization has lead to a rapid growth in the application of the naturally occurring radioactive disequilibria diversifying broadly to touch upon a wide spectrum of problems in earth sciences.This paper is a brief review of the subject and addresses a number of issues concerning (1) physico-chemical processes responsible for fractionation of radionuclides in the decay series, (2) the range of observed radioactive disequilibrium in nature, (3) implications in terms of relative mobilities of U, Th, Ra and their daughters in the geosphere, and (4) examples of the application of these concepts to studies of paleoclimate, dating of old groundwaters, and rock/water interaction processes.This review concludes that the concept of radioactive disequilibrium in naturally occurring decay series is a powerful isotopic tool with applications in both terrestrial and marine environments over timescales of up to one million years.

No abstract

No abstract

A high-resolution, niultiproxy palaeolininological record frorn the Manga Grasslands, northeastern Nigeria, spanning the last 5500 calendar years, reveals the episodic deterioration in Sahelian climate as significant biogeophysical thresholes were crossed.. Desert-dust deposition began to increase -4700 cal. BP. Raintall during the AtLmmer-monsoon season declined permanently after 4100 cal. BP. A further significant change in atmospheric circulation, giving rise to multidecadal to centenniall-scale droughts and enhanced dust deposition.

This paper presents a synthesis of results from investigations into palaeolimnology and dune chronology in the Manga Grasslands and adjacent areas of NE Nigeria, in order to reconstruct the evolution of this semi-arid landscape since the late glacial. A marked wet phase gave way to a fall in lake levels during the late glacial. Dune emplacement was active at this time to the south of the Manga Grasslands, but seems to have been absent from the grasslands themselves. Wet conditions prevailed during much of the early to mid-Holocene, accompanied by periods of significant barchanoid dune reactivation. A marked deterioration in climate and vegetation commenced around 4.1 ka BP, leading to the formation of the present-day semi-arid landscape. The data suggest that there have been major changes in humidity in this part of Subsaharan West Africa during the late glacial to Holocene and that the relationship between changes in precipitation and aeolian activity is complex.