Polar ice caps conserve within them exceptional archives that can be worked upon to reveal important climatic and environmental information. The layers of snow that accumulate year after year contain diverse substances that are transported by the wind in the form of fine particles to remote areas. These are then incorporated into the snow mantle during precipitation and via post-depositional processes overtime. The dust encapsulated in the snow can be of anthropogenic origin, from manmade industrial emissions, or from natural sources, deriving from phenomena such as wind erosion or even from space.
The exceptional purity of the snow/ice matrix means it's not only possible to identify potential origins of anthropogenic pollution, that show themselves via anomalous enrichments in heavy metals, but it is also possible to discriminate between individual sources. In particular, in the case of cosmic dust, it is possible to identify its presence by determining the levels of platinum and iridium in snow and ice. These elements are amongst the least abundant in the earths crust, but conversely are heavily enriched in cosmic material by factors of ~ 9500 and 2500 respectively for iridium and platinum compared to their crustal levels. The dimensions of the particles considered are so small they are called "meteoric smoke particles"; and are composed of particles of quasi-atomic dimensions. They form in the mesosphere at an altitude of 90-100 km following the impact of meteorites with the Earths atmosphere. Once formed, they fall to earth very slowly via sedimentation type processes and consequently the platinum and iridium in them is re-distributed over the entire surface of the planet.
In this way, the concentrations of platinum and iridium in ice cores can be used to evaluate the flux of "meteoric smoke particles" onto our planet. Ice cores have three main advantages for this kind of study compared to other environmental archives; a) the Ir and Pt remains essentially unaltered in the relatively chemically inert snow and ice; b) the layers of snow can be dated with good precision; c) it is possible to know the snow accumulation rate with good precision.
In the course of this study we have analysed 35 samples of ice dated from between circa 13000 and 128000 years ago. The results show that the values remained more of less constant during the Holocene, while higher levels were found during the last ice age, a testament to the increased crustal dust levels found in the snow and ice. The ratio between the fluxes of Ir and Pt calculated during the Holocene (~ 0.49 ± 0.2) is a value similar to that for cosmic material (0.49) thus isolating unequivocally the origin of the elevated concentrations of Pt and Ir.
Looking at the data for the fluxes of Ir and Pt calculated in snow, and considering the mean concentrations of such elements in cosmic material, it's therefore possible to calculate thanks to a model of the distribution of cosmic material in the mesosphere, the global flux of "meteoric smoke particles" onto the earths surface. The calculated flux (14 ± 5 kt/y) is in agreement with values found using complex models or expensive instruments installed on satellites, and confirms the efficacy of investigating ice cores to study the biogeochemical cycles of elements on a global scale.
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