Mass Depth Profiles Research Articles

210Pb-dating of recent sediments with the χ-mapping CF and CSAR models. On the attractors

Conventional 210Pb-dating models use assumptions on sedimentary conditions that allow for analytical formulations. The novel χ-mapping models use numerical methods to generate and test a large number (∼106) of potential solvers. Empirical data (excess 210Pb vs. mass depth profile) serve to attract the solver that minimizes the χ function (the attractor), and it has been assumed that it also defines the most likely chronology. This work aims to test this assumption in a deep way. In synthetic and varved sediments, the performance of each solver can be quantified through a parameter ξa accounting for the deviation of the model and the true ages. This work studies the complex relationships between χ and ξa using the constant flux (χ-CF) and the constant sediment accumulation rate (CSAR) models, which operate in a parametric 3D space. The full mapping of the 3D χ function serves to find the absolute minimum, for the graphical representation of the complex topology of the attractors, which is model-specific, and for plotting clouds of chronological lines from solvers with varying χ values. The minimum value of ξa (the best chronology) is achieved for a wide range of χ values, including the region of the absolute minimum. In complex cases, tiny changes in χ can result in quite different chronologies. Alternative attractors that include a reference date and an objective function are studied. The results provide guidelines for strengthening the 210Pb-based chronologies.

Environmental radioactivity in a sediment core from Algiers Bay: Radioecological assessment, radiometric dating and pollution records

Abstract The sedimentary system of the Algiers Bay has been impacted in the last decades by the exponential increases in population, urbanism and industrial activities of the city of Algiers, and by episodic floods, mudflows, earthquakes and tsunamis. Activity concentrations of environmental gamma-emitters, namely 210Pb, 226Ra, 40K, 228Ra, 228Th and 137Cs, were measured in a sediment core sampled in 2015 at the central zone of this bay, at 73 m depth from the surface water. The 210Pb-based radiometric dating was challenging due to the incomplete recovery of the inventory, the absence of 137Cs peaks, and the failure of classical models. The analysis of clusters in the logarithmic plot of the excess 210Pb versus mass depth profile allowed for establishing a reliable chronology. Fluxes of excess 210Pb and sediment accumulation rates (SAR) have both varied over the last five decades. Recent SAR values are of 0.73 ± 0.21 g cm−2y−1. Major and trace elements were determined by X-Ray Fluorescence in some selected sediment slices. Moderate enrichments have been observed for Mn, Zn, Br, Rb, Sr, Y and Pb in the 0–1 cm sediment layer (when using Al as normalization element), being governed by Fe-rich carriers. The major impact is observed in the carbon content, which increased by two orders of magnitude (from their basal values) for three decades since early 70's. The dataset also allowed for a radioecological assessment in the area, performed using the ERICA software.

Multimodal-TERESA, a 210Pb-based radiometric dating model for recent sediments under largely varying rates of supply

Abstract Lead-210 from natural atmospheric fallout is widely used in multidisciplinary studies for dating recent sediments. In anthropogenically-impacted and/or high energy systems the 210Pb flux onto the sediments may show non-random temporal variability, leading to the failure of classical dating models. The problem of how identifying and dating such cases remained unsolved, and it is the goal of the present work. Empirical evidences from varved sediments prove that initial activity concentrations of excess 210Pb (210Pbexc) and sediment accumulation rates (SARs) show large and independent temporal variability. M-TERESA model describes such variability by using multimodal frequency distributions and decodes the chronology from the 210Pbexc versus mass depth profile. The new model can solve scenarios with largely varying rates of supply, which fall beyond the limits of the piecewise versions of the classical models. Its use is demonstrated with some complex 210Pbexc profiles from varved sediments and synthetic cores for which an alternative and complete reconstruction of palaeofluxes and SARs is possible. The paper is supported by a wide set of supplementary material, including numerical codes for applying TERESA. The proposed methods are also useful for improving the reliability of routinely applications of the 210Pb-based radiometric dating of recent sediments.

Radiometric dating of recent sediments: On the performance of 210Pb-based CRS chronologies under varying rates of supply

Abstract Radiometric dating was a revolutionary contribution to the study of sedimentary processes. Empirical data from varved sediments show that unsupported 210Pb (210Pbexc) fluxes vary over time while they statistically correlate with sediment accumulation rates (SAR). This contradicts the basic assumption of the Constant Rate of Supply (CRS) model, which is the most widely used technique for recent sediments. This work is aimed at assessing the CRS model-errors by quantifying the effects on chronologies of various patterns of temporal variability of fluxes. Results are discussed through applications with synthetic and real cores for which and independent varve chronology or a set of reference dates are available. Periodic harmonic and random fluctuations in fluxes tend to cancel out positive and negative deviations in CRS ages, but they produce spurious variability in SARs. Persistent changes in fluxes lead to cumulated and unacceptable deviations of the CRS ages, which cannot be satisfactorily compensated by the use of a piecewise CRS model. The raw and piecewise CRS models can still be powerful tools of widespread use when the conditions for their applicability are well understood. The paper shows how the analysis of clusters in the 210Pbexc vs mass depth profile, along with the estimation of equivalent constant fluxes pre and post-dating a known reference date, are powerful methods to prevent misapplications of the CRS model.

A 210Pb-based chronological model for recent sediments with random entries of mass and activities: Model development

Unsupported 210Pb (210Pbexc) vs. mass depth profiles do not contain enough information as to extract a unique chronology when both, 210Pbexc fluxes and mass sediment accumulation rates (SAR) independently vary with time. Restrictive assumptions are needed to develop a suitable dating tool. A statistical correlation between fluxes and SAR seems to be a quite general rule. This paper builds up a new 210Pb-based dating tool by using such a statistical correlation. It operates with SAR and initial activities that closely follow normal distributions, what leads to the expected correlation between fluxes and SAR. An intelligent algorithm solves their best arrangement downcore to fit the experimental 210Pbexc vs. mass depth profile, generating then solutions for the chronological line, and for the histories of SAR and fluxes. Parametric maps of a χ-function serve to find out the solution and to support error estimates. Optionally, the model's answers can be better constrained through the use of time markers. The performance of the model is illustrated with a synthetic core, and with real cases using published data for varved sediment cores.

Measurement of Dissolved Reactive Phosphorus Using the Diffusive Gradients in Thin Films Technique with a High-Capacity Binding Phase

Measurement of dissolved reactive phosphorus (DRP) by the diffusive gradients in thin films (DGT) technique was investigated using a new binding phase. Half-dried amorphous zirconium oxide (with 50 ± 5% of water content) was mixed with acrylamide solution for the preparation of the new binding phase. The resulting binding gel had a high binding capacity (223 μg P cm(-2)) for phosphate. The solution of NaOH (1 M) was used for elution of phosphate from the gel, and an elution efficiency of 0.95 was obtained. A test of DGT uptake with this gel showed its dependence on temperature, and there was no influence of pH (3 to 10) and ionic strength (10 nM to 0.1 M). Its capacity for DGT response exceeded 100 μg P cm(-2), corresponding to a DRP concentration of more than 20 mg L(-1) for a 24 h deployment with a standard DGT device at 25 °C, which was at least 50 times of the Fe-oxide gel commonly used in the present DGT technique. Measurements with this high-capacity DGT technique in a laboratory microcosm of homogeneously mixed sediments gave smooth and reproducible mass-depth profiles. This technique was well demonstrated by in situ measurements in algal- and macrophyte-dominated regions of Lake Taihu. The DGT-measured concentrations of DRP were on average 20% and 40% of the DRP concentrations in pore waters, respectively, indicating a partial resupply of the sediments to the pore waters with DRP.

Chemical durability of alkali-borosilicate glasses studied by analytical SEM, IBA, isotopic-tracing and SIMS

Simple and complex alkali-borosilicate glasses were submitted to aqueous corrosion at room temperature, 60 and 90°C in solutions with pH ranging between 0 and 12. Analytical scanning electron microscopy (SEM), ion beam analysis (IBA) techniques, isotopic tracing and secondary ion mass-depth profiling (SIMS) have been used to investigate the variations of the surface composition of glass. In acidic medium, the glass surface is generally covered by a thick hydrated silica layer, mobile elements like Li, Na and B and transition elements (Fe, Zr, Mo, etc.) are strongly depleted. Near pH 7, relative enrichments of aluminium, iron and rare earths are shown together with strong Li, Na and B depletions. In basic medium, the glass surface exhibits relative enrichments of the major part of transition metals (from Cr to U) whereas mobile elements seem to be kept close to their nominal concentration level at the glass surface and Si is severely impoverished. Hydrogen incorporated at the glass surface after leaching is much more immobile in neutral and basic media than in acid medium.

Difficulties in interpreting fast mixing in the radiometric dating of sediments using210Pb and137Cs

210Pb geochronologies should be validated with independent tracers such as 137Cs. In the cases with constant 210Pb activity in the topmost sediments, the presence of a distinct 137Cs peak within the 210Pb plateau has been used as a definitive demonstration of acceleration (increase in the sedimentation rate in recent years) versus fast mixing. Nevertheless, some limitations can be identified in the use of semiquantitative arguments, and a global understanding of the whole 137Cs activity profile is then required. Particularly, the incomplete mixing within the top sediment zone (described through the Incomplete Mixing Zone model) can explain quantitatively and simultaneously the 137Cs peak and the flattening in the 210Pb activity profile. This is demonstrated using selected examples from literature data. Thus, measured constant 210Pb activities in the top 6 cm of a sediment core from Lake Zurich. Nevertheless, they found 7Be only in the uppermost layer, the distinct 137Cs maximum at 6 cm depth, and undisturbed varves. The fast mixing seemed then opposed to common sense. The constant rate of supply model shows acceleration and it adequately matches the position of the 1963 137Cs peak. Nevertheless it fails to explain the whole 137Cs profile when handling time series of 137Cs atmospheric deposition. Finally, it is shown how the incomplete mixing of the activity (through the pore water) over a certain mass depth at the top sediment, with a finite value of the mixing coefficient, can quantitatively explain the whole activity versus mass depth profiles of 137Cs and 210Pb, and the presence of 7Be only in the uppermost sediment layer. A further validation of these ideas is presented from other literature data.