Beryllium-7 as a natural tracer for short-term downwash in peat

Several factors can affect the integrity of natural archives such as peat records, e.g., decomposition and nutrient cycling, and it has also been hypothesized that some rapid downward transport of atmospherically derived elements may occur. We test this hypothesis by analyzing the short-lived, natural tracer beryllium-7 (tA1/2A = 53.4 days) in five cores from two peatlands. In triplicate hummock cores from a raised bog in southern Sweden, Be-7 could be measured to 20, 18 and 8 cm depth, and in a nutrient-poor mire in northern Sweden to a depth of 16 cm in a Sphagnum lawn core, but only 4 cm in the dominant, more-decomposed fen peat, indicating some spatial variability both within and between sites. Total Be-7 inventories were 320-450 Bq m(-2) in the bog, and 150 Bq m(-2) (lawn) and 240 Bq m(-2) (fen peat) in the mire. 25-79 % of the total inventory of Be-7 was located in the upper 2-cm layer. To further test downwashing, in the laboratory we applied a CuBr-solution to two cores and a Cu-solution to one core taken from the mire Sphagnum lawn, all with low water table conditions. About 50 % of the added Cu and similar to 35 % of the added Br were retained in the surface (2 cm) layer; 1-3 % of the Cu was found at 8-12 cm depth and similar to 1 % of the Br was measured in the lowest level (20-22 cm). Based on our novel approach using Be-7 and experimental work we show that short-term downwashing can occur in peatlands and we suggest the depth of this will depend on the properties of the peat, e.g., bulk density and decomposition, as well as hydrology.