フィッション・トラック

ニュースレター第35号

27 - 29 2022年

210-lead dating of lacustrine sediment:

Tsagaan Lake in Valley of the Gobi Lakes in Mongolia
Shuukhaaz Ganbat*, Noriko Hasebe**, Davaadorj Davaasuren***,Uyangaa Udaanjargal* and Yuma Shibuya** Department of Earth Sciences, Kanazawa University** Institute of Nature and Environmental Technology, Kanazawa University*** School of Arts and Sciences, National University of Mongolia

as subtracting the supported 210Pb from total
210
Pb. According to the radio decay law, the210
Pbxs concentration is exponentially decreased
along the sediment profile when sediment input
and 210Pbx input is constant through the time.
Based on the exponentially decreasing patternof 210Pbxs profile, it is possible to establish thelast ~150 years of sediment chronology. We
retrieved a short core sample (~130 cm depth,
CoreID - 18TS1) from the Tsagaan lake in2018. Total 26 samples were prepared by 1 cm
and 2 cm intervals for radioisotope
measurement. The activity concentration ofeach prepared sample was measured by a highpurity Ge-detector in the RI laboratory,
Kanazawa University. To interpret the 210Pbxs
profile, we used the Constant Rate of Supply(CRS), Constant Initial Concentration (CIC),and Constant Flux and Constant Supply(CFCS) models (Appleby and Oldfield, 1978;
Krishnaswamy et al., 1971; Robbins andEdgington, 1975) to establish the relative ageof the lacustrine sequences.

Introduction
Mongolia is a landlocked country in NorthCentral Asia in transition zone between
Siberian taiga and arid region of Central Asia
through dry steppes. Climate is extremely
variable relative to same latitude’s countries, as
it is controlled by the Westerlies, East Asian
Summer Monsoon (EASM), and Siberian
anticyclone. The climate change intensively
impacts its environment and life of local people(IPCC, 2014). Paleoenvironment study isimportant to understand the intensity of climate
fluctuation and related environmental issues,and to implement the environmentalmanagement. Past environmental information
is often reconstructed by analyses ofsedimentary archives. A reliable age dating
method is a crucial to explain the climate
oscillation in the past. Thus, we aim to conduct
the 210Pb dating method for the recent sedimentcollected from Tsagaan lake in Mongolia.
A total 210Pb activity in the sediment iscomprised of the unsupported (210Pbxs) andsupported 210Pb. A fraction of the 222Rn atoms
produced by the decay of 226Ra in soils escape
into the atmosphere where they decay througha series of short-lived radionuclides to 210Pb.
Whereas, supported 210Pb radionuclide iscontinuously produced by the in-situ decay of226
Ra in bottom sediments. 210Pbxs is expressed

Results and Discussions
A concentration of 210Pbxs was 132 Bq/kg on
SWI (Sediment Water Interface), thendecreased between 0 and 26 cm depth (Figure1-A). However, the profile does not show
monotonic decreasing trend, showing that the27assumption of constant sediment and 210Pbxs
supply is not realistic in this lake. Based on theconcentration of 210Pbxs, CRS, CIC, and CFCS
models were applied (Figure 1-B). According
to the comparative estimation of different
models, the CRS model can reveal the reliableage estimation for the down core profile
relative to other models, and this model
estimated 24 cm sediment deposited during
87 years. Mass accumulation rate is calculatedbased on the CRS age and mass depth profile,and it varies through time (Figure 1-C) . The
average is approximately 1.001 g/ cm2 year.
The low accumulation rate is associated with
the increasing concentration of 210Pb xs
obtained at 15 cm depth and 20 cm depth(Figure 1-A, C). Whereas the CIC model
results are not sequenced with depth profile.
Because this model assumes the constant rateof sedimentation, the increasing concentrationof 210Pbxs in 10-14 cm and 18-20 cm depths isimpossible to explain. The CFCS model ages,
based on both the sedimentation rate andatmospheric flux of 210Pb are constant, wererelatively older than the CIC and CRS age
model. This model calculated 139 years at28 cm depth. The age estimation range of theCFCS model is wider than the CRS and CIC
models. But the uncertainty of age estimation
was higher than in the CRS and CIC models.

age dating results reveal relatively different
sedimentation rate for the 18TS1 core basedon distinct assumptions in age estimate. Ageestimation by the CIC model reports shorter
sedimentation time, whereas the CFCS model
reveals a longer age estimation, the uncertainty
was wider than the CIC and CRS models’ age
uncertainty. The CRS model can prove a
reliable age with less uncertainty and indicatesthe variable sediment input rate..
References
Appleby, P.G., Oldfield, F., 1978. The
calculation of lead-210 dates assuming a
constant rate of supply of unsupported
210Pb to the sediment. CATENA 5,
1–8.https://doi.org/10.1016/
S0341-8162(78)80002-2
IPCC (Ed.), 2014. Detection and Attribution of
Climate Change: from Global to Regional,
in: Climate Change 2013 - The PhysicalScience Basis. Cambridge UniversityPress, Cambridge, 867–952. https://doi.org/
10.1017/CBO9781107415324.022
Krishnaswamy, S., Lal, D., Martin, J.M.,
Meybeck, M., 1971. Earth and Planetary
Science Letters 11 (1971) 407-414.
Robbins, J.A., Edgington, D.N., 1975.
Determination of recent sedimentation
rates in Lake Michigan using Pb-210 andCs-137. Geochimica et Cosmochimica
Acta 39, 285–304. https://doi.org/
10.1016/0016-7037(75)90198-2

SummaryThe concentration of 210Pbxs is measured fromthe top sediment until 26 cm depth. The 210Pb28

Figure 1. (A) Down core proﬁle of ²¹⁰Pbxs (Unsupported ²¹⁰Pb). (B) Chronology Results
obtained by 3 diﬀerent models, Constant Rate of Supply (CRS), Constant Initial Concentration(CIC), and Constant Flux and Constant Supply (CFCS) models. (C) Mass Accumulation Rate
based on CRS model result, red bars indicate that the mass accumulation rate associates tothe increasing pattern of ²¹⁰Pbxs concentration.

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