Mineralogical, Chemical, and Isotopic (Sr, Pb) Composition of Atmospheric Mineral Dusts in an Ombrotrophic Peat Bog, Southern South America
Peat bogs are excellent environmental archives of atmospheric deposition of many major and trace elements and of mineral dusts. Geochemical studies of major and trace elements, mineralogy of atmospheric dust and their radiogenic isotope compositions in the peat, provide important information about the mineralogical host phases of major and trace elements transported through the atmosphere, grain size fractionation during transport, and their possible source areas (PSAs). To date, no peat bog records of atmospheric dust depositions in southern South America have been reported. Therefore, the major and trace elements, and mineralogical and radiogenic isotopic compositions of inorganic fraction in a peat bog from southern South America were investigated. The main goal was to characterize the atmospheric dust deposition in southern South America, and to identify their PSA. For this purpose, a 542 cm long core from the Oreste bog in southern Chile, Isla Navarino (55º13’13’’S, 67º37’28’’W), was used. The peat formation at the Oreste bog began at ~ 11160 14C yr before present (BP) (13 ka cal. BP). Because of the limited amount of mineral material available from thinly sliced cores (~ 2 – 3 mg from a 2 cm slice), an analytical method was developed to extract, isolate, and chemically characterize atmospheric dusts from ombrotrophic peat. About 2 g of dried peat was combusted overnight at 550 °C that on ashing yielded ca. 20 mg. 1M HCl was allowed to react for 15 min to dissolve soluble minerals such as carbonates, sulphates, and oxides which were mainly formed during combustion. Acid insoluble ash (AIA) was then separated from the solution (acid soluble ash: ASA) by using polycarbonate filters of pore size 0.2 µm. The analytical procedure effectively extracted and isolated the AIA fraction which was composed of silicates and refractory oxides derived from crustal weathering, as well as volcanic ash and glass particles. The residue obtained was analyzed directly on the filters using the TITAN and EMMA XRF spectrometers for major (Al, Ca, Fe, K, Si, and Ti) and trace (Cr, Mn, Pb, Sr, and Zr) elements. Although the AIA amounts to no more than a thin layer (ca 2-3 mg) of atmospheric dust, excellent XRF spectra were obtained. Afterwards, Scanning Electron Microscopy (SEM) was used to identify the predominant minerals in selected AIA samples. Similarly, radiogenic isotopes of Pb and Sr compositions in selected AIA and ASA were measured by Multicollector Thermal Ionization Mass Spectrometer (MC-TIMS). The concentration of titanium (Ti) was used as a surrogate of mineral input in the peat because of its association with accessory minerals which are generally resistant to chemical weathering. The mineral accumulation rate (MAR) was calculated by using Ti concentration in bulk peat, dry bulk density, and long term peat accumulation rate. The distribution of calcium (Ca), manganese (Mn) and strontium (Sr) in bulk peat, and 87Sr/86Sr compositions in the ASA indicate that the peat was truly ombrotrophic above ~ 300 cm. The distribution of Ca, Mn, Sr, and Ti in the bulk peat were used to show that the mineral dust accumulation for the last ca. 6000 yrs were predominantly atmospheric and effectively constant (0.43 ± 0.12 g m-2 yr-1) except at ca. 4200 cal yr BP. This was further supported by the Ti and zirconium (Zr) concentrations in the acid insoluble ash (AIA). At ca. 4200 cal yr BP where the dust abundance was elevated, Zr was enriched relative to Ti. The SEM micrographs showed that most of the minerals above ca. 400 cm of the peat profile were fine (10 – 20 µm or less) and rounded, except at ~ 300 cm where volcanic tephra grains (> 20 – 40 µm) were predominant. Combined together with the relatively stable amount of AIA, they reflect long-range transport of atmospheric mineral dust and the climate stability for the past ca. 6000 yrs. The Sr isotope compositions of AIA for the last six millennia (above ~ 420 cm) varied within a narrow range of 0.7087 < 87Sr/86Sr < 0.7090 (average: 0.70884 ± 0.00014; n = 9), except at ca. 50 – 60 cm, 160 cm, and 300 cm. At ~ 300 cm, where the tephra grains were predominant and at the other two exceptional points, the Sr isotope compositions deviated towards less radiogenic composition. Similar deviation was also observed in the fen peat before six millennia (at ~ 490 cm). The available age dates, Sr isotope compositions of AIA, elemental concentration of silica (Si) and potassium (K) (expressed in the oxide form and used as a K2O/SiO2 ratio), and the published volcanic eruption records in southern South America collectively indicated four episodes of tephra input from the Mt. Burney volcano (87Sr/86Sr = 0.7042) to the Oreste bog. For the rest of the profile, the Sr isotope compositions indicate that the atmospheric dusts were derived from the Patagonian belt (southern South America). In general, they collectively supported the previously held view that the atmospheric mineral dusts were well mixed and relatively stable for the last ca. 6000 yrs. In addition, the observed average value of Sr isotope compositions in the Oreste bog (87Sr/86Sr = 0.70884 ± 0.00014) were identical to the Eastern Antarctic ice core dust (ICD) deposited during the Last Glacial Maximum (LGM) and Glacial Stage 2 (87Sr/86Sr = 0.7088 ± 0.0002). Therefore, various Anrgentinian sediment (southern South America) deposits accounted for the dust deposition in the Eastern Antarctic ice at least since the Late Pleistocene.
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