Isotope sediment dating
Cosmogenic production in the upper atmosphere Be (half-life of 53 days) are continuously produced in the atmosphere by the high-energy proton component in cosmic radiation, causing spallation of atmospheric oxygen and nitrogen atoms.
Beryllium is rapidly washed from the atmosphere by precipitation, and is subsequently incorporated in continental and marine sediments.
Mercury (Hg) concentrations and Hg isotopic composition were investigated in three sediment cores in Lake Michigan (LM).
Two cores were collected from Green Bay, a region heavily impacted by Hg contamination and one core from an offshore region of LM absent of direct point source Hg.
Landform-evolution studies look at the build-up of long-lived radioisotopes produced on the surface of the earth (i.e., Be has also been measured to determine erosion rates and fluvial transport mechanisms (Dominik et al., 1987) and to determine direct contribution of rainfall to terrestrial waters (Cooper et al., 1991). de Vries, Environmental Isotopes in the Hydrological Cycle: Principles and Applications, vol. Hovius, Quantification of Continental Erosion Processes.
Recently deposited sediment reflecting more anthropogenic influence shows similar δHg values (-0.02 to 0.27‰) was observed from inner Green Bay to the offshore of LM, which may indicate increased input of atmospheric Hg and decreased watershed inputs along this transect.
It is an excellent way of directly dating glaciated regions.
It is particularly useful in Antarctica, because of a number of factors: Cosmogenic nuclide dating is effective over short to long timescales (1,000-10,000,000 years), depending on which isotope you are dating.
Overall, this study suggests that sedimentary Hg isotopes maybe a useful tracer in understanding Hg sources and history of Hg contamination in large lakes. Blum; Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, Michigan, United States Guest Editor: Robert Mason; University of Connecticut, United States).
Upon entering aquatic ecosystems, Hg may undergo complex reactions and transformations including oxidation, reduction, evasion to the atmosphere as gaseous Hg(0), binding to organic matter in the water column, and adsorbing to particles and depositing to sediments ().
Be, the only stable isotope of beryllium, occurs naturally on earth.