The principle of 210Pb dating of sediments.
Abstract
The principles of 210Pb dating have been extensively outlined by Appleby and Oldfield (1992). In summary, the isotope 210Pb occurs as part of the radioactive decay chain of 238U. Uranium 238 is present in small quantities in the materials of the Earth's crust. It decays through a series of non volatile intermediates to 226Ra, a solid with a half life of 1622 years. This decays in turn to 222Rn, an inert gas with a half life of 3.825 days. A proportion of the 222Rn escapes into the atmosphere from the crust, surface waters and airborne dust, and is distributed globally (Figure 1). It decays via a series of short lived daughters to 210Pb, a solid with a half life of 22.26 years. As a result of the change in phase, radon is isolated from its precursor, 226Ra, and its initial secular equilibrium is thereby destroyed. Radon and its decay products then reach a new secular equilibrium in the atmosphere, with 210Pb becoming the predominant radionuclide. The lead becomes attached to aerosols, which reach the Earth's surface either by dry fallout, or by being washed out of the atmosphere in precipitation. Lead-210 has a mean atmospheric residence time of the order of 5 10 days. Deposition therefore takes place on a time scale much shorter than the half life of the isotope. The aerosol 210Pb which settles into lake waters is adsorbed by suspended sediment and subsequently incorporated in the lake sediments.
... Pertanggalan 210 Pb pertama kali diperkenalkan pada tahun 1963 oleh Golberg (5,6) dan telah dikembangkan secara luas oleh Appleby dan Oldfield pada tahun 1979. Metode pertanggalan ini didasarkan pada proses alami yaitu terbentuknya 210 Pb jatuhan, hasil peluruhan 222 Rn yang merupakan radionuklida yang dapat teradsorpsi kuat pada mineral/partikel tanah atau sedimen, sehingga dapat dijadikan sebagai perunut (tracer) untuk mempelajari pergerakan atau perpindahan tanah/sedimen. ...
... Pertanggalan 210 Pb adalah metode pertanggalan yang didasarkan pada pengukuran aktivitas 210 Pb. Pertanggalan (6) ini khusus diperuntukkan untuk cuplikan sedimen, dan batasan umurnya maksimum adalah 150 tahun. Metode ini telah berhasil diaplikasikan pada danau muara dan lingkungan sekitar pantai dan diperoleh hasil laju sedimentasi berkisar dari satuan milimeter hingga sentimeter per tahun. ...
PERAN METODE PERTANGGALAN RADIOMETRIS DI BIDANG ARKEOLOGI DAN GEOLOGI. Telah dilakukankajian tentang peranan metode pertanggalan radiokarbon pada bidang geologi dan arkeologi serta metodepertanggalan 210Pb di bidang geologi. Metode pertanggalan radiokarbon adalah metode pertanggalan dengan batasanumur maksimumnya adalah 45.000 tahun. Dalam kajian ini lebih ditinjau pada metode pertanggalan yangmenggunakan proses sintesis karbon radioaktif (14C) yang terkandung dalam cuplikan diubah menjadi benzena(14C6H6) lalu dicacah dengan pencacah kelip cair, dan pertanggalan 210Pb yaitu metode pertanggalan yang didasarkanpada pengukuran aktivitas 210Pb. Pertanggalan ini khusus diperuntukkan bagi cuplikan sedimen, dan batasan umurmaksimumnya adalah 150 tahun. Beberapa cuplikan lingkungan di sekitar situs arkeologi maupun geologi telahberhasil dilakukan analisis nilai umurnya dengan metode pertanggalan radiokarbon. Pertanggalan radiokarbon dapatjuga memberikan kontribusi sebagai data dukung penelitian tentang paleotsunami, sehingga dapat digunakan untukmemperkirakan kemungkinan kapan terjadinya tsunami di masa mendatang, sedangkan untuk metode pertanggalan210Pb telah dilakukan penelitian mengenai laju sedimentasi serta penelitian tentang keberadaan alga berbahaya(harmful algal bloom, HAB) di dalam cuplikan sedimen yang sangat membahayakan lingkungan dan telah diteliti padadaerah-daerah tertentu. Dari hasil kajian menunjukkan bahwa pertanggalan radiokarbon yang didasarkan pencacahan14C, pada masa yang akan datang para ahli pertanggalan radiokarbon kecenderungannya lebih memilih pertanggalanradiokarbon modern atau pertanggalan radiokarbon yang menggunakan alat spektrometer massa berbasis siklotron,disamping karena cuplikan yang dibutuhkan relatif jauh lebih kecil, juga hasil simpangan bakunya juga relatif lebihkecil.
... Only the excess 210 Pb component is used to derive sedimentation rates (and thus core chronologies) because once incorporated into the sediments, it decays exponentially with time in accordance with its half-life of 22.26 years. We used the Brugam model, a variant of the constant initial concentration (CIC) model, to derive sedimentation rates (Appleby & Oldfield 1978; Heijnis 2001). The CIC model is most appropriate when input of excess 210 Pb occurs predominantly via the erosion of catchment surfaces , as in this case, whereas the alternative constant rate of supply model (CRS) should be applied when input of excess 210 Pb to the sediments occurs predominantly by direct atmospheric input (Turner & Delorme 1996 ). ...
... Because the CIC model assumes a constant sedimentation rate, a plot of the log of excess 210 Pb versus depth should yield a straight line indicating sedimentation rate. Sedimentation rates, however, were nonlinear for many of our sites; for this reason, we applied the Brugam model, in which the profile is divided into linear segments and separate sedimentation rates are calculated for each segment (Heijnis 2001). Sedimentation rates are assumed to change instantaneously at points of inflection. ...
We used historical patterns of deposition of mollusc shells to infer changes to inshore benthic assemblages in the southeastern Tasmanian region over the past 120 years. We identified and counted shells in slices embedded within 1m long 210Pb-dated sediment cores were collected at 13 sites in water depths of 8-16 m. Declines in mollusc species richness and shell production occurred during the past century at all sites studied, with a mean decline per 5-cm sediment slice from 21 species in 1890 to 7 species in 1990 and in shell abundance from 150 to 30 individuals over the same period. The time course of decline notably corresponded with the history of the scallop dredge fishery, presumably either because scallop dredging caused general declines in populations of mollusc species or because other factors caused a catastrophic regional decline in molluscs that included scallops. As a consequence, the fishery was forced to close. Of major concern is that losses had not previously been recognized but extended throughout the 100-km coastal span of the study. Given that fishing and other anthropogenic impacts, as well as a lack of observational data, are virtually ubiquitous for the coastal zone, major recent losses in mollusc biodiversity may be globally widespread but have gone unnoticed.
... Mass accumulation and linear sedimentation rates in Norrviken estimated by the CIC and CRS models are similar (Fig. 3), as is expected under conditions of constant sediment accumulation rate (Turner and Delorme 1996). The CIC model is recommended if sedimentation rate is constant and 210 Pb ex is measurable in downcore sediments (Heijnis 2001;Appleby 2001). Lake Norrviken is located in tectonically stable granitic terrain and hence, large-scale variation in sediment flux (e.g. from landslides or accelerated chemical weathering) is unexpected, especially over short (decadal-centennial) timescales. ...
Human-induced perturbations in the Lake Norrviken catchment, Sweden, over the last 100+years have left distinctive geochemical
imprints in the sediments. Disposal of sewage, industrial, and agricultural run-off into the lake since the end of the nineteenth
century changed the trophic status from eutrophic to hyper-eutrophic. The primary organic matter (OM) source in the lake is
in situ algal material. Total organic carbon (TOC) concentrations increased near the mid-section of a short sediment core
collected from the deepest part of the lake, reflecting elevated epilimnetic productivity and consequent hypolimnetic anoxia.
Accompanying shifts to lighter stable organic C and total N isotopic compositions suggest that cyanobacterial productivity
increased during this period. The distribution of pigments in the core indicates a shift in the phytoplankton community to
a cyanobacteria-dominated system. Moreover, pigments confirm that N2-fixing versus non-N2-fixing phytoplankton varied depending upon the external inputs of N and P. Conditions in the lake improved after sewage input
was diverted and the lake is currently mesotrophic.
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