Geochemistry of the peat bog at Etang de la Gruere, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 C-14 yr BP
W. Shotyk et al., Geochemistry of the peat bog at Etang de la Gruere, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 C-14 yr BP, GEOCH COS A, 65(14), 2001, pp. 2337-2360
A 650 cm core from a Swiss bog represents 12,370 C-14 years of peat accumul
ation and provides the first complete record of atmospheric Pb deposition f
or the entire Holocene. Excess, non-atmospheric Sr in the peat was calculat
ed by normalizing Sr/Sc to crustal abundance; this was used to differentiat
e between the ombrogenic section of the bog in which inorganic solids are s
upplied exclusively by atmospheric deposition and the minerogenic zone wher
e mineral-water interactions contribute metals to the peat. While sediment
dissolution contributes significantly to the Sr inventory of the minerogeni
c section of the peat profile, there is no measurable effect of this proces
s on the Ph burden. Isotopic analyses (Pb-204, Pb-206, Pb-207, Pb-208) show
that effectively all of the Pb in the peat profile was supplied exclusivel
y from the atmosphere. To separate natural and anthropogenic Pb, Sc was sel
ected over Ti, Y, Zr, Hf and REE as a conservative, reference element which
is supplied by soil dust aerosols derived from rock weathering. Enrichment
factors (EF) were calculated using the Pb/Sc ratio in the peat samples, no
rmalized to the "natural, background" Pb/Sc which is found in peats dating
from 8030 to 5320 C-14 yr BP. The results show that anthropogenic source ha
ve dominated the supply of atmospheric Pb to the peat core continuously sin
ce 3000 C-14 yr BP. The aerosols supplied to the bog can be divided into 3
classes: a) Pre-Anthropogenic (older than 3000 C-14 yr BP with Pb EF < 2 an
d Pb-206/Pb-207 > 1.194); b) Pre-Industrial (dating from 3000-240 C-14 yr B
P, with Pb EF greater than or equal to 2 but < 20 and Pb-206/Pb-207 1.193-1
.179); c) Industrial (more recent than 240 C-14 yr BP, with Pb EF > 20 and
Pb-206/Pb-207 < 1.179). Elevated soil dust fluxes are observed at 5320, 823
0 and 10,590 C-14 yr BP; the latter corresponds to the Younger Dryas. Alumi
num, Zr, Hf, and REE/Sc ratios also are elevated at the same depths, sugges
ting differences in particle size, wind strength, or source regions. Pre-An
thropogenic aerosols deposited since 8230 C-14 yr BP reveal Pb/Sc which is
significantly higher, and Pb-206/Pb-207 which is less radiogenic, than duri
ng the early Holocene. While the trend toward increasing Pb/Sc could be due
to chemical weathering and soil development, this could not explain the sh
ift in Pb isotopic composition. The changes which took place at 8230 C-14 B
P, therefore, may be related to a large scale climatic reorganization which
, at present, is poorly understood. Copyright (C) 2001 Elsevier Science Ltd
.