Having been the first time series published of heavy metal pollution from an ice core, it was found that the climatic variability of these metals was strongly anti-correlated with those of the oxygen isotope, O18. As seen in the figure below, in the first half of the GRIP core, 0-80 kyr B.P., the high frequency observed for Cu, Pb, Cd and Zn was in contrast to the low and steady concentrations of O18. In the subsequent years, 80-160 kyr B.P., the concentrations of Cd, CU, Pb and Zn were very low but concentrations of O18 were very high and more variable. More O18 in the ice is a common indicator of a colder climate, this would allow one to conclude that heavy metals are more persistent and variable in the atmosphere when the climate is warmer, such as in the first half of the record and recent history.
Hong et al. (1006) look closely at the climatic record portrayed in their GRIP ice core before turning to analyse the sources of heavy metal pollution in the atmosphere. It was found that large scale pollution could be attributed to wind-bourne soil and rock particles, sea salt spray, volcanoes and wild fires. Cu and Pb were likely to be the product of soil and rock dust during the study time period (0-160 kyr B.P.). Hong discussed that the large variations depicted in the time series in the most recent inter-glacial, the holocene, was most likely the product of dramatic changes in environmental conditions. The retreat of northern ice sheets, such as the Eurasian and American, meant opening up and exposure of large areas of fresh rock and land. This would account for the variation in Pb and Cu which originated from rock and soil particles. Sea ice also retreating leaving more exposed ocean and coastal areas, thus more sea spray.
As far as cadmium goes, the variation is mainly accountable because of more dust and soil particle sources being exposed, which is a significant source, but also large contributions from the growing continental biosphere. The progressive expansion of vegetation and northward advance with retreating ice sheets meant that the Cd ratios increase observed between 13,000 to 9300 years ago was primarily a result of increasing plant matter on the continent. From reading my previous post one can presume this is because more plants means a larger Cd potential reservoir on the continent. Cadmium levels increased until 9300 kmyr B.P. but then decreased despite further advance of vegetation, this has been attributed to changes in atmospheric circulastio influencing the movement of air-bourne cadmium.
The paper by Hong et al. (1996) was another good indicator review of heavy metal pollution during the last glaciation and during deglaciation, they offered explanations for the reasons behind the variablitily to pollution. Yet the paper lacked a comprehensive discussion of the differnt sources of heavy metals into the atmosphere or how changes in atmospheric transport could influence heavy metal content in the ice core.
These results show that cadmium has indeed been an integral part of our environment for thousands of years and has been collecting in soils and plants before the industrial revolution. Although crops have only recently become a health threat it is important to recognise natural sources and past aptterns of cadmium pollution to better understand future fluctuations and their role in our health.
Reference: Hong, S., J-P. Candelone, C. Turetta and C.f. Boutron (1996) 'Changes in natural lead, copper, zinc and cadmium concetrations in central Greenland ice from 8250 to 149,100 years ago: their association with climatic changes and resultant variations of dominant source contributions,' Earth and planetary science letters 143: 233-244.
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