Atmospheric deposition and mass balance of major and trace elements in two oceanic peat bog profiles, northern Scotland and the Shetland Islands

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A mass balance was performed on the major (Si, Al, Fe, Na, K, Mg, Ca) and some trace elements (Rb, Sr, Cl, Br, S) in peat cores from two oceanic blanket bogs in northern Scotland: Loch Laxford (LL) is 15 km east of the open ocean in the extreme northwest of the Scottish mainland; Fleck's Loch (FL) is on the small (15 km 2) island of Foula, 50 km west of the ‘mainland’ of Shetland. The supply of these elements to the peats can be explained in terms of two main atmospheric sources: mineral aerosols from weathering of crustal rocks, and sea salt spray. The average Al/Sc and Fe/Sc ratios of the peats at both sites are comparable to the corresponding ratios in soil-derived aerosols suggesting that crustal inputs account for essentially all of the Al and Fe in the profiles. The Si/Sc ratios of the peats from LL again correspond with the value for soil-derived aerosols, but at FL there is an excess of Si in the uppermost part of the profile mainly because of quartz from sources immediately around the bog. In contrast to Al, Fe, Si, the concentrations of Na, Mg, Ca, and Sr in the peats exceed the corresponding ratios in soil-derived aerosols by 5 to 30 times, reflecting the importance of marine aerosols for these elements. The conservative behaviour of Cl − allows the concentration of this species in the pore waters to be used as a quantitative indicator of the relative importance of sea salt inputs to the two bogs. In contrast, the concentrations of other important pore water species (SO 4 2−, Br −, Na +, K +, Mg 2+, and Ca 2+) do not simply reflect their concentrations in rainwater because of biological transformations (S, Br, K), ion exchange (Na, K, Mg, Ca), and possibly mineral dissolution (Na, Mg, Ca). The average pore water Cl − concentrations at FL (97.0 ± 15.2 mg/1) show that marine inputs are 4 times greater than at LL (24.0 ± 2.6 mg/1). The concentrations of other major and trace elements in the rainwater supplied to the bogs were calculated using the known abundances of the elements in seawater, and normalizing to the pore water Cl − concentrations at the two sites. These estimated rainwater compositions were then combined with rainfall data to calculate the annual inputs to the bogs. The peat core inventory of major and trace elements (corrected for crustal contributions) was compared with these inputs to determine the percentage of each element retained in the peat profiles. These findings indicate that 91.5 to 99.9% of the major elements (Cl, Na, Mg, K, Ca) and trace elements (Br, Sr, Rb) supplied to the bogs by rainwater are not retained by the peat. Thus, none of these elements in the solid phase (peat) can be used as a quantitative measure of sea salt inputs to the bogs.