Isotopic composition of Pb in tree rings of Norway spruce (Picea abies) from the Fensfjorden area, western Norway

The annual growth of tree rings makes the trees as possible archive of the past atmospheric pollution. However, several aspects addressed in literature such as post-depositional mobility of Pb, intra-ring heterogeneity and possible detoxification mechanisms can make the interpretation on past atmospheric pollution, difficult. Annual rings of two Norway spruce (Picea Abies) trees 43 and 42 years old located 10.1 km north-northwest and 17.8 km east-southeast of Mongstad refinery, respectively, were analysed by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) for lead isotopic composition, and 63Cu/12C, 66Zn/12C and 44Ca/24Mg elemental ratios. These two trees, recorded 206Pb/207Pb ratios between 1.13 and 1.32, and 1.12 and 1.29, respectively, but showed no systematic trend towards higher or lower ratios from the innermost to the outermost tree ring. For all ratios, only the 208Pb/12C (for tree #1) and 44Ca/24Mg (for tree #1 and #2) ratios showed pronounced trends. The 208Pb/12C ratio of tree #1 increased from 1968 and peaked in the year 1979, followed by a steady decrease toward the present (2010), and showed significant correlation with the lead content and the pH of precipitation in the area. The 44Ca/24Mg of tree #1 and #2 demonstrated showed a systematically decrease from 1968-2010 and showed significant correlation with the pH of the precipitation. Comparison of the lead isotopic composition of the tree rings with that recorded by mosses in the same area during the time period 1975-2000 showed significant differences. Most of the tree rings plotted have higher 206Pb/207Pb and 208Pb/207Pb ratios relative to the Norwegian peat bogs (1691-1991), and also show some overlap with the relatively higher recorded ratios of a Greenlandic ice core (7313 BC to 1523 AD). The tree rings have also higher Pb isotopic ratios compared to the European aerosols. Comparisons with the 206Pb/207Pb ratios recorded by the Scottish peat bogs and mosses covering the same time period as the trees did not reveal any similarity. The onset of production at Mongstad refinery in 1975 and the subsequent expansion in the refinery production and occasional accidents in the factory did not have any significant impact on lead isotopic composition of the tree rings, nor on the 208Pb/12C, 63Cu/12C, 66Zn/12C or the 44Ca/24Mg ratios. Pollution associated with the Mongstad refinery may be minor compared to the long range pollution by atmospheric aerosols. In order to differentiate between pollution sources and natural sources, local soil and bedrock measurements and a chemical and isotopic analysis of Pb in oil that is being refined at Mongstad, should be carried out.

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