Abstract
Ombrotrophic peatlands are recognized archives of past atmospheric mineral dust deposition. Net dust deposition rates, grain size, mineral hosts and source areas are typically inferred from down-core elemental data. Although elemental analysis can be time efficient and data rich, there are some inherent limitations. X–ray diffraction (XRD) analysis allows direct identification of mineral phases in environmental samples but few studies have applied this method to peat samples and a well–developed protocol for extracting the inorganic fraction of highly organic samples (>95%) is lacking. We tested and compared different levels of pre–treatment: no pre–treatment, thermal combustion (300, 350, 400, 450, 500 and 550 °C) and chemical oxidation (H2O2 and Na2S2O8) using a homogenised highly organic (>98%) composite peat sample. Subsequently, minerals were identified by XRD. The results show that combustion is preferred to chemical oxidation because it most efficiently removes organic matter (OM), an important pre–requisite for identifying mineral phases by XRD analysis. Thermally induced phase transitions can be anticipated when temperature is the only factor to take into consideration. Based on the data required in this study the recommended combustion temperature is 500 °C which efficiently removes OM while preserving a majority of common dust minerals.
Highlights
During the last three decades records from ombrotrophic peat bogs have been increasingly used to reconstruct past changes in mineral dust deposition, most commonly based on multi–element datasets from XRF and ICP–MS analyses (e.g., Shotyk et al 2001; Marx et al 2009; Le Roux et al 2012; Kylander et al 2016)
There are, methodological limitations to using elemental data alone to examine net dust deposition rates and mineral composition: (1) chemically similar minerals are difficult to distinguish; (2) dust events can vary widely in their mineralogical composition, leading to an under– or over– estimation of dust variability if a single element is used as a dust proxy (Kylander et al 2016); and (3) minerogenic and biogenic minerals cannot be separated based on elemental data alone
To evaluate what minerals can be detected in bulk peat, four untreated freeze–dried samples from the Store Mosse (SM) peat sequence were crushed in a mortar and packed for X–ray diffraction (XRD) analysis (Smieja–Król et al 2010)
Summary
During the last three decades records from ombrotrophic (rain–fed) peat bogs have been increasingly used to reconstruct past changes in mineral dust deposition (paleodust), most commonly based on multi–element datasets from XRF and ICP–MS analyses (e.g., Shotyk et al 2001; Marx et al 2009; Le Roux et al 2012; Kylander et al 2016). A conservative lithogenic element such as Ti or Al, is typically taken as a proxy for mineral dust deposition and used to calculate elemental mass accumulation rates (MAR). We are limited by that fact that (4) some methods (e.g., ICP– MS) are destructive, leading to sample loss; and (5) some elements are lost during the pre–treatments (e.g., Si in HF digestions), complicating mineral predictions.
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