Isotopic Composition of Pb in Peat and Porewaters from Three Contrasting Ombrotrophic Bogs in Finland: Evidence of Chemical Diagenesis in Response to Acidification.

The isotopic composition of Pb was determined in Finnish peat bogs and their porewaters from Harjavalta (HAR, near a Cu-Ni smelter), Outokumpu (OUT, near a Cu-Ni mine), and Hietajärvi (HIJ, a background site). At HIJ and OUT, the porewaters yielded similar concentrations (0.1-0.7 µg/L) and isotopic composition ((206)Pb/(207)Pb = 1.154-1.164). In contrast, the peat profile from HAR yielded greater concentrations of Pb in the porewaters (average 2.4 µg/L), and the Pb is less radiogenic ((206)Pb/(207)Pb = 1.121-1.149). Acidification of the bog surface waters to pH 3.5 by SO2 emitted from smelting (compared to pH 4.0 at the control site) apparently promotes the dissolution of Pb-bearing aerosols, as well as desorption of metals from the surfaces of these particles and from the peat matrix. Despite this, the chronology of anthropogenic, atmospheric deposition for the past millenium recorded by the isotopic composition of Pb in all three peat bogs is remarkably similar. While the immobility of Pb in the peat cores may appear inconsistent with the elevated porewater Pb concentrations, Pb concentrations in the aqueous phase never amount to more than 0.01% of the total Pb at any given depth so that the potential for migration remains small. The low rates of vertical water movement in bogs generally combined with the size of the metal-containing particles in solution may be additional factors limiting Pb mobilization.

Porewater evidence of metal (Cu, Ni, Co, Zn, Cd) mobilization in an acidic, ombrotrophic bog impacted by a smelter, Harjavalta, Finland and comparison with reference sites.

Porewaters were collected from three Finnish peat bogs subjected to varying inputs of atmospheric trace metals: Hietajärvi (HIJ), a low-background site, Outokumpu (OUT), near a Cu-Ni mine, and Harjavalta (HAR), near a Cu-Ni smelter. Samples for metal analyses were collected at depths ranging from 10 to 70 cm using a purpose-built syringe-type sampler. Metal concentrations were determined using inductively coupled plasma-sector field-mass spectrometry (ICP-SF-MS). Porewater concentrations at HIJ and OUT (Cd <0.3 nM, Co <1.4 nM, Cu, Ni <8 nM, Zn <250 nM) are independent of metal concentrations in the solid phase (peat). At OUT there is a limited release of Ni to the porewaters, but concentrations in the aqueous phase are generally below 0.3% of the total concentration in any given peat sample. These data are consistent with the immobility of these metals after deposition from the air. In contrast, porewaters at HAR are enriched in trace metals compared to the other sites by a factor of 2 (Zn), 10 (Cd), 20 (Co), and 100 (Cu and Ni) with dissolved fractions of Cu and Ni accounting for ca. 20% of the metal inventories in the cores. The elevated release of metals from solid phases at HAR is consistent with the postdepositional migration of metals at this site and reflects the predominance of oxide phases supplied to the bog surface and the much lower pH values (<3.4). The elevated proton concentrations not only promote mineral dissolution but also compete with cation exchange processes and hinder the formation of metal complexes with organic ligands.

Comparison of atmospheric deposition of copper, nickel, cobalt, zinc, and cadmium recorded by Finnish peat cores with monitoring data and emission records.

This study aims to determine the extent to which the accumulation rates of Cu, Ni, Co, Zn, and Cd in peat cores agree with established histories of atmospheric emission from local pointsources. Metals accumulating in three Finnish peat cores with known metal deposition histories have been measured using inductively coupled plasma-sector field mass spectrometry. Samples were age-dated using both 210Pb and 14C (bomb pulse curve). At the Outokumpu (OUT) site as well as the low-background site Hietajärvi (HIJ), 210Pb age dates are in excellent agreement with the 14C bomb pulse curve method results, and the precision is between 1 and 10 years for most of the samples; at the Harjavalta (HAR) site, precision is > 6 years. Mean regional "background" concentrations have been calculated from deeper peat layers of the HIJ site (microg g(-1)): Cu, 1.3 +/- 0.2 (n = 62); Co, 0.25 +/- 0.04 (n = 71); Cd, 0.08 +/- 0.01 (n = 23); and Zn, 4 +/- 2 (n = 40). For layers accumulated within the past 100 years, accumulation rates (ARs) have been calculated. At sites with < 0.06 g m(-2) cumulative Ni inventory (HIJ and OUT), ARs of Cu and Co trace the known metal deposition histories very well. At HAR, where metal inventories are much greater, Cu and Co are mobile. ARs of Zn were between 3 and 30 mg m(-2) year(-1) and those of Cd between 24 and 140 microg m(-2) year(-1) at all sites and are independent of the chronology of their inputs from the atmosphere.

Suggested protocol for collecting, handling and preparing peat cores and peat samples for physical, chemical, mineralogical and isotopic analyses.

For detailed reconstructions of atmospheric metal deposition using peat cores from bogs, a comprehensive protocol for working with peat cores is proposed. The first step is to locate and determine suitable sampling sites in accordance with the principal goal of the study, the period of time of interest and the precision required. Using the state of the art procedures and field equipment, peat cores are collected in such a way as to provide high quality records for paleoenvironmental study. Pertinent field observations gathered during the fieldwork are recorded in a field report. Cores are kept frozen at -18 degree C until they can be prepared in the laboratory. Frozen peat cores are precisely cut into 1 cm slices using a stainless steel band saw with stainless steel blades. The outside edges of each slice are removed using a titanium knife to avoid any possible contamination which might have occurred during the sampling and handling stage. Each slice is split, with one-half kept frozen for future studies (archived), and the other half further subdivided for physical, chemical, and mineralogical analyses. Physical parameters such as ash and water contents, the bulk density and the degree of decomposition of the peat are determined using established methods. A subsample is dried overnight at 105 degree C in a drying oven and milled in a centrifugal mill with titanium sieve. Prior to any expensive and time consuming chemical procedures and analyses, the resulting powdered samples, after manual homogenisation, are measured for more than twenty-two major and trace elements using non-destructive X-Ray fluorescence (XRF) methods. This approach provides lots of valuable geochemical data which documents the natural geochemical processes which occur in the peat profiles and their possible effect on the trace metal profiles. The development, evaluation and use of peat cores from bogs as archives of high-resolution records of atmospheric deposition of mineral dust and trace elements have led to the development of many analytical procedures which now permit the measurement of a wide range of elements in peat samples such as lead and lead isotope ratios, mercury, arsenic, antimony, silver, molybdenum, thorium, uranium, rare earth elements. Radiometric methods (the carbon bomb pulse of (14)C, (210)Pb and conventional (14)C dating) are combined to allow reliable age-depth models to be reconstructed for each peat profile.

Identifying the sources and timing of ancient and medieval atmospheric lead pollution in England using a peat profile from Lindow bog, Manchester.

A peat core from Lindow bog near Manchester, England, was precisely cut into 2 cm slices to provide a high-resolution reconstruction of atmospheric Pb deposition. Radiocarbon and (210)Pb age dates show that the peat core represents the period ca. 2000 BC to AD 1800. Eleven radiocarbon age dates of bulk peat samples reveal a linear age-depth relationship with an average temporal resolution of 18.5 years per cm, or 37 years per sample. Using the Pb/Ti ratio to calculate the rates of anthropogenic, atmospheric Pb deposition, the profile reveals Pb contamination first appearing in peat samples dating from ca. 900 BC which clearly pre-date Roman mining activities. Using TIMS, MC-ICP-MS, and SF-ICP-MS to measure the isotopic composition of Pb, the (208)Pb/(206)Pb and (206)Pb/(207)Pb data indicate that English ores were the predominant sources during the pre-Roman, Roman, and Medieval Periods. The study shows that detailed studies of peat profiles from ombrotrophic bogs, using appropriate preparatory and analytical methods, can provide new insight into the timing, intensity, and predominant sources of atmospheric Pb contamination, even in samples dating from ancient times.