Mineralogical, chemical, and isotopic (Sr, Pb) composition of atmospheric mineral dusts in an ombrotrophic peat bog, southern South America [Elektronische Ressource] / vorgelegt von Atindra Sapkota

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2006
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 20 Mo

Extrait

Mineralogical, Chemical, and Isotopic (Sr, Pb) Composition of
Atmospheric Mineral Dusts in an Ombrotrophic Peat Bog,
Southern South America
INAUGURAL – DISSERTATION
Zur
Erlangung der Doktorwürde
Der
Naturwissenschaftlich-Mathematischen
Gesamtfakultät
Der
Ruprecht-Karls-Universität
Heidelberg
Vorgelegt von
M.Sc. Atindra Sapkota
aus Nepal
Tag der mündlichen Prüfung:
08.12.2006Mineralogical, Chemical, and Isotopic (Sr, Pb) Composition of
Atmospheric Mineral Dusts in an Ombrotrophic Peat Bog,
Southern South America
Gutachter: Prof. Dr. William Shotyk
Institute für Umwelt-Geochemie
Ruprechts-Karls-Universität Heidelberg
Im Neuenheimer Feld 236
D-69120 Heidelberg

Prof. Dr. Heinz Friedrich Scholer-Table of contents-
Summary 3
Zusammenfassung 5
Acknowledgements 7
Chapter 1
1-1 Introduction and background 11
Importance of mineral dusts 14
1-2 Tracing the sources of atmospheric dusts 15
1-2.1 Mineralogy 16
1-2.2 Trace elements 17
1-2.3 Radiogenic isotopes (Sr, Nd, Pb) 19
1-3 Archives of atmospheric dusts 21
1-3.1 Polar snow and ice 21
1-3.2 Marine sediments 23
1-3.3 Peat bogs 23
1-3.3.1 Peatlands 24
- Definitions 24
- Formation 25
- Bog environments 26
1-3.3.2 Peat as an archive of environmental change 27
- Geochemistry 27
- Mineralogy 28
- Composition of peat ash 30
- Peat bogs as archives of atmospheric dusts 31
1-4 Atmospheric deposition of Pb in southern Hemisphere 32
1-5 Aims of the study 33
1-6 Materials and methods 34
1-6.1 Study site 34
1-6.2 Sampling procedure 35
1-6.3 Sample handling and analytical methods 36
1-6.3.1 Modified sample handling and analytical methods 36
- Peat slice and volume/density calculation 36
- Ash and AIA contents 37
1-6.3.2 Summary of sample handling and analytical methods 38
1-7 Results 40
1-8 Conclusions and perspectives 40
Chapter 2
Six millennia of atmospheric dust deposition in southern South America (Isla Navarino,
Chile). (Submitted to The Holocene, 2006) 51
Chapter 3
Characterization of atmospheric mineral dusts preserved in an ombrotrophic peat bog from
southernmost Southern Hemisphere, Oreste Bog, Tierra del Fuego, Chile. (Submitted to
Quaternary Research, 2006) 71
1Chapter 4
87 86Holocene record of Sr/ Sr ratios in dust components of an ombrotrophic bog at the
southernmost edge of South America: volcanic inputs, atmospheric soil dust, and
comparison with ice core dust of the Eastern Antarctic Plateau. (Submitted to Earth and
Planetary Science Letters, 2006) 93
Chapter 5
The Paradigm of natural Pb in the atmosphere: 6,000 years of soil dust deposition recorded
by the Oreste peat bog, southern Patagonia. (Submitted to Global Biogeochemical Cycles,
2006) 113
Appendix I
Suggested protocol for collecting, handling and preparing peat cores and peat samples for
physical, chemical, mineralogical and isotopic analysis. (Journal of Environmental
Monitoring, 6 (2004) 481-492) 129
Appendix II
Analytical procedures for the determination of selected major (Al, Ca, Fe, K, Mg, Na and
Ti) and trace (Li, Mn, Sr, and Zn) elements in peat and plant samples using inductively
coupled plasma optical emission spectrometry. (Analytica Chimica Acta, 540 (2005) 247-
256) 149
Appendix III
Atmospheric deposition of trace elements (Cr, Cu, Zn, and Rb) in southern South America
(a brief overview). 163
Appendix IV
Cleaning of Acid Insoluble Ash (AIA) for Sr isotope measurements. 169
Appendix V
Titanium in an ombrotrophic peat profile from Valle de Andorra (Ushuaia, Tierra del Fuego)
as an indicator of atmospheric deposition of mineral dust for the last ca. 1300 years. 173
2Summary
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
14Oreste bog began at ~ 11160 C 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),
87 86manganese (Mn) and strontium (Sr) in bulk peat, and Sr/ Sr 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
-2 -1predominantly atmospheric and effectively constant (0.43 ± 0.12 g m yr ) 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
87 86within a narrow range of 0.7087 < Sr/ Sr < 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 K O/SiO ratio), and the published 2 2
volcanic eruption records in southern South America collectively indicated four episodes of tephra
87 86input from the Mt. Burney volcano ( Sr/ Sr = 0.7042) to the Oreste bog. For the rest of the profile,
3the 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
87 86( Sr/ Sr = 0.70884 ± 0.00014) were identical to the Eastern Antarctic ice core dust (ICD)
87 86deposited during the Last Glacial Maximum (LGM) and Glacial Stage 2 ( Sr/ Sr = 0.7088 ±
0.0002). Therefore, various Anrgentinian sediment (southern South Ameri