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| Titel |
Isotopic chemical weathering behaviour of Pb derived from a high-Alpine Holocene lake-sediment record |
| VerfasserIn |
Marcus Gutjahr, Finn Süfke, Adrian Gilli, Flavio Anselmetti, Lukas Glur, Anton Eisenhauer |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149087
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| Publikation (Nr.) |
 EGU/EGU2017-13403.pdf |
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| Zusammenfassung |
| Several studies assessing the chemical weathering systematics of Pb isotopes provided
evidence for the incongruent release of Pb from source rocks during early stages of
chemical weathering, resulting in runoff compositions more radiogenic (higher)
than the bulk source-rock composition [e.g. 1]. Deep NW Atlantic seawater Pb
isotope records covering the last glacial-interglacial transition further support these
findings. Clear excursions towards highly radiogenic Pb isotopic input in the deep NW
Atlantic seen during the early Holocene, hence after the large-scale retreat of the
Laurentide Ice Sheet in North America, are interpreted to be controlled by preferential
release of radiogenic Pb from U- and Th-rich mineral phases during early stages
of chemical weathering that are less resistant to chemical dissolution than other
rock-forming mineral phases [2-4]. To date, however, no terrestrial Pb isotope record
exists that could corroborate the evidence from deep marine sites for efficient late
deglacial weathering and washout of radiogenic Pb. We present a high-resolution
adsorbed Pb isotope record from a sediment core retrieved from Alpine Lake Grimsel
(1908 m.a.s.l.) in Switzerland, consisting of 117 Pb compositions over the past
10 kyr. This high-Alpine study area is ideally located for incipient and prolonged
chemical weathering studies. The method used to extract the adsorbed lake Pb isotope
signal is identical to previous marine approaches targeting the authigenic Fe-Mn
oxyhydroxides fraction within the lake sediments [5, 6]. The Pb isotope compositions are
further accompanied by various elemental ratios derived from the same samples
that equally trace climatic boundary conditions in the Grimsel Lake area. The Pb
isotopic composition recorded in Lake Grimsel is remarkably constant throughout the
majority of the Holocene until ∼2.5 ka BP, despite variable sediment composition and
–age, and isotopically relatively close to the signature of the granitic source rock. In
contrast, adsorbed Th and U concentrations (given in concentrations of ng/g of
sediment) are indeed significantly elevated during the earliest part of the record, while
other adsorbed metals such as Al and Ti display highest adsorbed concentrations
during the mid-Holocene. Elements such as Nd display fairly constant normalised
concentrations throughout the record. Hence, while our Pb isotopic record appears
remarkably insensitive towards climatic perturbations seen during the Holocene, the
various elemental records display a striking sensititivity towards the overall climate
evolution of the Holocene. Finally, the rise and fall of the Roman Empire as well
as the onset of the industrial revolution are clearly resolvable in our Pb isotopic
records.
References
[1] Harlavan, Y. Y. et al. (1998), GCA 62, 33-46; [2] Gutjahr, M. et al. (2009), EPSL 286,
546-555; [3] Kurweil, F. et al. (2010), EPSL 299, 458-465; [4] Crocket, K.C. et al. (2012)
QSR 38, 89-99; [5] Gutjahr et al. (2007) Chem. Geol. 242, 351-370; [6] Blaser, P. et al.
(2016) Chem. Geol. 439, 189-204. |
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