210Pb Profiles Research Articles

Reconstruction of temporal variations of metal concentrations using radiochronology (239+240Pu and 137Cs) in sediments from Kizilirmak River, Turkey

Sediment cores retrieved from rivers, lakes, and coastal marine environment have been widely utilized to reconstruct historical variations of anthropogenic pollutants. A sediment core was collected in the Kizilirmak River, Turkey during 2014 and analyzed for a suite of metals (Cu, Pb, Zn, Ag, Ni, Co, Mn, As, Cd, Sb, V, Cr, Hg and Se) to reconstruct their temporal variations. Chronology was attempted using excess 210Pb (210Pbxs), 137Cs and 239,240Pu in the sediment cores. The vertical profile of excess 210Pb indicates that this core is not datable using excess 210Pb method. The 137Cs-based linear and mass apparent accumulation rates were estimated to be 0.84 cm year−1 and 0.93 g cm−2 year−1, respectively. These values are in agreement with the 239+240Pu peak-based linear (0.88 cm year−1) and mass apparent accumulation (0.97 g cm−2 year−1) rates. There is considerable broadening in the peaks of both 137Cs and 239,240Pu, likely due to post-depositional mixing processes. The measured sediment inventories of 210Pbxs, 137Cs and 239+240Pu were 38, 29, and 0.42 dpm cm−2, respectively, which were generally higher than the expected inventories based on local atmospheric depositional inventories. We attribute these enhanced inventories to potential sediment focusing and additional watershed erosional input. The Al-normalized enrichment factors (ANEF) were used to evaluate the contribution from natural versus anthropogenic sources. The ANEF of Ag, Ni, As, Cd and Se were > 1.5, which suggests a significant contribution of anthropogenic sources for these trace metals varied between 226% and 1163%. The geoaccumulation index (Igeo) values of the sediment column suggest that Cu, Pb, Zn, Co, Mn, Sb, V and Hg were in an unpolluted level and the others were in a polluted level. Besides, both the ANEF and Igeo values of Cu, Pb, Zn, Ag, As, Se, Cd and Hg showed a peak at 28–30 cm layer (corresponds to 1978–1981), which is attributed to maximum metal release during 1970s.

Sediment accumulation rates and vertical mixing of deep-sea sediments derived from14C and 210Pb in the southern Gulf of Mexico

Here we present new results on the sedimentation rates and vertical mixing in the deep-water region (966–3741 m) of the southern Gulf of Mexico, from a collection of 42 cores retrieved from the continental slopes and the abyssal plain. These cores were dated with the aid of accelerator mass spectrometry (AMS) 14C determinations on planktic foraminifera. We applied the Modified Mixing Box Model (MMBM) to evaluate the mean sedimentation rates (Sm), to estimate the age of surface sediments (T0) that result from mixing processes. Sm range between 5 and 9 cm kyr−1 in cores from the abyssal plain and the continental slopes, highest rates are consistently observed in specific locations on the slopes (17–24 cm kyr−1). T0 values range between 1000 and 2000 years, in cores from the abyssal plain and the continental slope sites, most probably as a result from the relatively low sedimentation rates and consequent mixing of older sediments. However, some cores from the continental slopes show consistently higher sedimentation rates, and T0 values that are close to zero age. Here we further show 210Pb profiles from 15 cores to evaluate the non-homogeneous mixing processes in surface sediments (0–10 cm). Biodiffusion coefficients (Db) derived from the solution of advection-diffusion model range between 14 and 51 cm2 kyr−1 on the abyssal plain and between 42 and 158 cm2 kyr−1 on the continental slopes.Fluxes of 210Pbex range between 60 and 111 Bq m−2 yr−1 on the abyssal plain, and higher values are consistently found on the continental slopes (158–247 Bq m−2 yr−1). We find the age of surface sediments (T0; derived from the MMBM of 14C) are linked with the transit time in the mixed layer through biodiffusion processes, derived from 210Pb profiles. One of the implications of these results is the relative low importance of benthic macrofauna in respect to the meio- and microfauna bioturbation processes to explain sediment mixing patterns in the deep-sea sediments of the Gulf of Mexico, most probably reflecting a low export of organic carbon to the sediments controlled by the oligotrophic conditions of its surface waters.

210Po and 210Pb distributions during a phytoplankton bloom in the North Atlantic: Implications for POC export

During the North Atlantic Bloom Experiment (NABE) of the Joint Global Ocean Flux Study (JGOFS), water column sampling for particulate and dissolved 210Po and 210Pb was performed four times (26 April and 4, 20, 30 May 1989) during a month-long Lagrangian time-series occupation of the NABE site, as well as one-time samplings at stations during transit to and from the site. There are few prior studies documenting short-term changes in 210Po and 210Pb profiles over the course of a phytoplankton bloom, and we interpret the profiles in terms of the classical “steady-state” (SS) approach used in most studies, as well as by using a non-steady state approach suggested by the temporal evolution of the profiles. Changes in 210Po profiles during a bloom are expectable as this radionuclide is scavenged and exported. During NABE, 210Pb profiles also displayed non-steady state, with significant increases in upper water column inventory occurring midway through the experiment. Export of 210Po from the upper 150 m using the classic “steady-state” model shows increases from 0.5 ± 8.5 dpm m−2 d−1 to 68.2 ± 4.2 dpm m−2 d−1 over the ~one-month occupation. Application of a non-steady state model, including changes in both 210Pb and 210Po profiles, gives higher 210Po export fluxes. Detailed depth profiles of particulate organic carbon (>0.8 μm) and particulate 210Po (>0.4 μm) are available from the 20 and 30 May samplings and show maxima in POC/Po at ~37 m. Applying the POC/210Po ratios at 150 m to the “steady state” 210Po fluxes yields POC export from the upper 150 m of 8.2 ± 1.5 mmol C m− 2 d−1 on 20 May and 6.0 ± 1.6 mmol C m−2 d−1 on 30 May. The non-steady state model applied to the interval 20 to 30 May yields POC export of 24.3 mmol C m−2 d−1. The non-steady state (NSS) 210Po-derived POC fluxes are comparable to, but somewhat less than, those estimated previously from 234Th/238U disequilibrium for the same time interval (37.3 and 45.0 mmol m−2 d−1, depending on the POC/Th ratio used). In comparison, POC fluxes measured with a floating sediment trap deployed at 150 m from 20 to 30 May were 11.6 mmol m−2 d−1. These results suggest that non-steady state Po-derived POC fluxes during the NABE agree well with those derived from 234Th/238U disequilibrium and agree with sediment trap fluxes within a factor of ~2. However, unlike the 234Th-POC flux proxy, non-steady stage changes in profiles of 210Pb, the precursor of 210Po, must be considered.

Morphological evolution of a macrotidal back‐barrier environment: The Amazon Coast

AbstractCoastal barriers provide sheltered, low‐energy settings for fine‐grained sediment deposition and retention, although the process of back‐barrier infilling and how tidal‐channel connectivity impacts this process is not well‐understood. Understanding how back‐barrier environments infill and evolve is necessary to predict how they will respond to future changes in sea‐level and sediment supply. With this motivation,in situobservations and sedimentary signatures from an Amazonian tidal‐channel system are interpreted to create a conceptual model of morphological evolution in a macrotidal back‐barrier environment that is rich in fine‐grained sediment, vegetated by mangroves and incised by tidal channels with multiple outlets. Results indicate that within a high‐connectivity back‐barrier channel, tidal processes dominate sedimentation and morphological development. Sediment cores (<60 cm) exhibited millimetre‐scale tidalites composed of sand and mud. High‐connectivity channels allow tidal propagation from multiple inlets, and in this case, the converging flood waves promote delivery of sediment fluxing through the system to the mangrove flats in the convergence zone. Sediment preferentially deposits in regions with adequate accommodation space and dense vegetation, and in these zones, sediment grain size is slightly finer than that transiting through the system. The greatest sediment‐accumulation rates (3 to 4 cm yr−1), calculated from steady‐state210Pb profiles, were found in the convergence zone near the mangrove‐channel edge. As tidal flats aggrade vertically and prograde into the channels, accommodation space diminishes. In effect, the channel’s narrowest stretch is expected to migrate along the path of net‐sediment flux towards regions with more accommodation space until it reaches the tidal‐convergence zone. The location of recent preferential infilling is evidenced by relatively rapid sediment‐accumulation rates, finer sediment and significant clustering of small secondary tidal channels. These findings shed light on how sediment transported through vegetated back‐barrier environments is ultimately preserved and how evidence preserved in surface morphology and the geological record can be interpreted.

Mixing depths and sediment accumulation rates on an arid tropical shelf based on fine-fraction 210Pb analysis

Continental shelves in arid tropical settings present particular challenges to 210Pb-based analysis of sedimentation rates and surface mixing owing to the combination of coarse sediment, deep and year-round bioturbation, and lower atmospheric flux of 210Pb. The modern continental shelf at the northern terminus of the hyper-arid Gulf of Eilat/Aqaba (GOE) receives flood-runoff of siliciclastics, producing a mixed seabed with ~20% carbonate that is dominated by larger benthic foraminifera and mollusks. Focusing analysis on only the fine-fraction of deep-penetrating cores (≥70 cm) from 15 to 40 m water depths yields reliable 210Pb profiles, in contrast to analysis of bulk sediment (graphical abstract). Sedimentation rates increase offshore five-fold, from 0.01–0.04 cm/y in 15 m water depth to 0.21–0.27 cm/y in 30–40 m depths, reflecting offshore redistribution of flood-delivered siliciclastic sediments away from the wadi mouth, a result also supported by an offshore increase in the inventory of excess 210Pb. In contrast, the thickness of the surface mixed layer (SML) decreases from >30 cm to ~20 cm with proximity to anthropogenic stressors (channelized flood runoff, historic release of sewage and operation of fish cages), which we attribute to the suppression of macrobenthic burrowers. The rate of sedimentation on the Gulf shelf –away from dynamic bypassing in the 15 m shoreface – is comparable to other tropical carbonate shelves rich in large benthic foraminifera, and is higher than rates documented on the adjacent slope, increasing confidence in this approach to 210Pb analysis using only the fine fraction. Analysis of the fine fraction rather than bulk sediment would be a useful adjustment to 210Pb methodology in any area with scarce fine-grained sediment.

Combined chronological and mineral magnetic approaches to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta

Delta deposits show large spatial heterogeneity in terms of depositional rate and age, which is critical to the study of delta erosion in response to the declining fluvial sediment load observed at many river mouths in the world. In this study, we show that the magnetic susceptibility (χ) can be an indicator to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta. Ages of three short sediment cores (<2 m) collected at 20–32 m water depth from the Yangtze River subaqueous delta were determined using 210Pb, 137Cs, and optically stimulated luminescence (OSL) dating. In addition, depth variation of χ, which is influenced by post-depositional diagenesis and hence age, was used to roughly infer sediment ages among the cores in a simple way. The profiles of 210Pb, 137Cs, and OSL dating results indicate the spatial variability of ages, ranging from the last 100 years to >1700 years. The cores at shallow water depths are younger than those from deeper sites. Modern deposits (i.e., <100 years old) occur primarily at water depths shallower than ca. 30 m. The core in the northern part of the subaqueous delta shows much older ages than the core at the southern site with similar water depth, which is caused by their longer distance relative to the mouth of active sediment discharge distributary. Profile of χ confirms such spatial variation of ages in terms of depth distribution pattern and χ value. Older sediments (>800 a) show lower and uniform χ values due to the reductive dissolution of ferrimagnetic minerals, while younger sediments (<350 a) show higher χ values in the top layer but they decline with increasing depth. Considering the quick way of magnetic measurement, stratigraphic correlation based on χ can be used first to screen for cores before they are subjected to more detailed dating. This study shows that the methodological approach of combining sediment dating with magnetic measurement has great potential in revealing heterogeneous deltaic deposits.

Recent sedimentation rates using 210Pb and 137Cs vertical profiles of core sediments at the Gulf of Corinth, Litochoro Coast and Marmara Sea

In this work, core sediment samples up to 50 cm have been collected from three different marine environments; Gulf of Corinth, Gulf of Thermaikos, specifically from Litochoro Coast and Lake Uluabat from the wider region of Black Sea. Natural (210Pb) and anthropogenic (137Cs) radionuclides in the samples were measured using an HPGe detector, calibrated for sediments geometry. The vertical distribution of 137Cs and 210Pb was used to determine the sedimentation rate. The results were 0.56 cm/year for the Gulf of Corinth and 0.54 cm/year for the coast of Litochoro. In the case of the lake Uluabat the sedimentation rate was 0.41 cm/year.

Accumulation rates and chronologies from depth profiles of 210Pbex and 137Cs in sediments of northern Beibu Gulf, South China sea

Beibu Gulf is a highly dynamic and complex coastal environment that is currently experiencing one of the largest rates of development and urbanization in west China. Little is known about the effects of this increased human activity on coastal sedimentation processes and on the rates of sediment accumulation and the variation of organic materials to the coast. In this study, four sediment cores were collected and applied the 210Pb dating method to reconstruct sedimentation rates and historical changes of materials to the northern Beibu Gulf over the past century. Depth profiles of excess 210Pb (210Pbex) showed highest activity values at the surface (28.4–104.0 Bq kg−1) followed by a linear or exponential decay with depth for all but one study site. 137Cs activity ranged between 0.236 and 2.034 Bq kg−1, and a distinct peak activity – representing the 1963 fallout maximum – was observed at all but one site. Sediment chronologies were determined using the Constant Rate of Supply (CRS) model. Calculated accumulation rates in the studied sites were the lowest in the late 1920s and early 1930s (mass accumulation rate (MAR): 0.06 ± 0.01 g cm−2 y−1; sediment accumulation rate (SAR): 0.08 ± 0.01 cm y−1) and increased gradually until reaching maximum values in the 2010s (MAR: 0.22 ± 0.09 g cm−2 y−1; SAR: 0.46 ± 0.32 cm y−1). Current accumulation rates are up to 800% higher than rates observed in the 1920s, with most of the increase happening after 1970, coinciding with the increasing rate of urbanization and development in the region. The highest increase in SAR over the last century (+877%) was observed in Sanniang Bay, with the lowest rate of increase (+283%) observed in Lianzhou Bay. TOC content in these sediments has also increased over the last 100 years. Current values (0.98–1.28%) are about 170% higher than historical concentrations (before 1970). The positive correlations between TOC and population density and GDP growth in major cities surrounding the gulf, provide further indication that human activities have significantly altered the sedimentary environment in recent decades along the northern Beibu Gulf coast.

A numerical investigation for dating 210Pbex and 137Cs vertical profiles in a coastal area: The Eastern Ligurian Sea, Italy

210Pb and 137Cs vertical profiles in coastal sediments are studied using a sedimentation-mixing model. The cores were sampled in a complex coastal area characterized by the presence of different impact sources and environmental uses (riverine inputs, commercial and military harbour, marine protected area) in the Eastern Ligurian Sea, Italy. The analysis of accumulation and dispersion processes is performed using a numerical advection and diffusion model, in terms of the independent variables - time and mass depth. The flux of 210Pb is considered in steady state while the time dependent input flux of 137Cs is estimated from the concentration of this radionuclide in seawater, starting from observed data from 1960 to date. Differently from the atmospheric fallout distribution, this input function contains, at least partly, the contribution that still continues to reach the sediment in the last 25 years as a result of coastal and riverine input. The analysis highlights some features obscured in experimental data, and allows comparison of the effects of different scenarios. The specific effect of a pulsed input is discussed by analysing the effects of the Chernobyl event. The effects of mass flux in non–steady state are also considered: we observe that since the pulsed inputs in 137Cs are now too old, a strong superficial mixing and a time-variable flux produce similar profiles for both radionuclides. Hence, the general environmental knowledge of the area remains the main instrument to fully define the active processes in some cores.

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.

Preliminary investigation of olimpi field, Mediterranean Sea, using in-situ and laboratory radio-tracing methods

In-situ and laboratory data of radioisotopes were acquired and interpreted for the Olimpi field, Mediterranean Sea. Standard oceanographic parameters and in-situ radon progeny profiles were analysed for four different sites, utilizing a state-of-the-art gamma-ray spectrometer optimized for deep sea research. In addition, a sediment core recovered from the Gelendzhik mud diapir plateau was analysed by means of high resolution laboratory gamma-ray methods providing vertical distribution of excess lead 210Pb, potassium 40K, and caesium 137Cs. Water column profiles of radon progenies revealed higher concentrations above mud volcano areas/structures in comparison to the selected reference site. This increase is likely attributed to higher emanation rates of radon gas in areas which are affected by the local diapirism. In all cases the concentration of radon progeny almost linearly decreased toward the surface due to radioactive decay and dilution processes, however differences in the concentration slopes between sites indicates a continuous release of fluids in the Moscow and Milano sites. The profile of the excess 210Pb in the Gelendzhik sediment core revealed two different accumulation rates. Using two local maxima of 137Cs, two dating points were used to suggest that an intense mud flow event took place during 1963–1986. Also, as the activity concentration of 40K increased by almost 30% during this period, and the chemical constituent of the accumulated material differed from the underneath layers, a material of different origin is presumed. Moreover, in the period 1986–2016 the accumulation rate was much lower indicating the absence of other, more recent, intense mud flow events in the neighbouring area of the core sampling.

Lead-210 profiles in radon barriers, Indicators of long-term Radon-222 transport

Conventional measurements of fluxes of Rn-222 from engineered earthen covers over uranium milling wastes use accumulation chambers over a 24-h period, making them sensitive to short-term environmental fluctuations. It is unlikely that such measurements represent a long-term average required for assessing cover performance. An alternative flux measurement approach is explored here based on the hypothesis that measurements of Pb-210 (a daughter of Rn-222) concentrations in the barrier material indicate where Rn-222 decayed in the barrier profile and thus indicate Rn transport within the barrier over long times. Samples of radon barrier materials from four mill tailings sites have been analyzed for Pb-210. In many cases, Pb-210 was detectable, was substantially above background values, and had higher concentrations at the bottom of the barrier adjacent to the underlying tailings. In some profiles, Pb-210 was found to decrease to background concentrations part way through the barrier, indicating that all Rn-222 had decayed before reaching the top. In other cases, Pb-210 concentrations were elevated near the top of the barrier, indicating radon migration through the entire radon barrier and possible long-term release of radon. These observations were generally confirmed by radon flux measurements at the top of the radon barrier. Some measured profiles of Pb-210 with depth in the radon barrier can be modeled by simple diffusion, suggesting that the diffusive transport mechanism, which was assumed for the original design of the barriers, is appropriate. In other cases, the Pb-210 profiles are not effectively modeled by diffusion, suggesting that at some locations the transport mechanism is complex and does not conform to the assumed process. Findings from this work indicate that Pb-210 concentrations measured near the top of radon barriers are promising as a monitoring approach for long-term Rn-222 radon fluxes. More detailed profiles of Pb-210 coupled with moisture and density information can be used to assess long-term Rn transport mechanisms.

Sedimentary signals of the upwelling along the Zhejiang coast, China

The muddy deposits of the Zhe–Min coastal area have attracted much recent research interest. However, the sedimentary processes that dominate along the Zhejiang coast remain controversial. Determining sedimentation rates is important for understanding sedimentary processes and depositional patterns. In the present study, 23 vibrocores were collected from the muddy area along the Zhejiang coast to analyse their sediment accumulation rates over the past 100 years using 210Pb geochronology. Three types of typical excess 210Pb profiles were identified in the sediment vibrocores. The spatial distribution of the sedimentation rates derived from the 23 vibrocores, as well as previously published data, demonstrated that the middle part of the study area around 29°N experienced relatively high sedimentation rates, and that the rate of deposition gradually decreased landwards and seawards. The location of high sedimentation rates is approximately consistent with that of the Holocene maximum thickness deposition, finest surface sediments and high concentration of chlorophyll, resulting from the existence of upwelling along the Zhejiang coast besides the fluvial inputs.

Reconstruction of sedimentation changes under anthropogenic influence in a medium-scale estuary based on a decadal chronological framework

The present study uses sedimentary records from the Yalu River Estuary (YRE) over the past five decades to establish a regional age framework and investigate the influence of anthropogenic activity. Ten sediment core samples collected from various locations in the estuary were analysed. Core-averaged sediment accumulation rates calculated based on the 210Pb and 137Cs profiles of these samples were in the range 1.09–2.83 and 0.94–3.11, respectively. Accordingly, a regional age framework with major environmental events was established assuming the deposition rate in a sub-section was constant. The age framework showed that most of the sediments in the YRE were deposited from the 1960s, and a grain size analysis also evidenced that the sedimentary environment experienced dramatic change after 1963. Changes in sedimentary environment in the estuary due to human activity (e.g. dam construction, deforestation and large mining activities) were also discussed: (1) the mean grain size at sites near the main channel became finer after 1963 due to the contemporaneous reduction in sediment discharge caused by upstream reservoir construction; (2) abnormal 137Cs signals in the records during the 1980s probably reflected a concurrent deforestation, and the 137Cs signals in the 1990s were related to mixing of sediments caused by mining activities. Results of this study demonstrate that sedimentation processes in a medium-scale estuary can be greatly changed under anthropogenic influence. The feasibility of using 137Cs time markers (especially auxiliary time markers in 1975 and 1986) in a dynamic estuary is confirmed in this study. Furthermore, the regional age framework established in this study can provide a chronological reference for future studies in the YRE and adjacent areas.

Lead (Pb) profiles in red coral skeletons as high resolution records of pollution in the Mediterranean Sea

Lead (Pb) concentrations in long-lived Corallium species of known age, from the Mediterranean Sea, Atlantic and Pacific Oceans, were determined by laser ablation, inductively coupled plasma mass spectrometry (LA-ICPMS). Lead concentrations in a 2000-year-old sub-fossil Mediterranean C. rubrum are ca 0.09 ± 0.03 μg/g. For the period 1894–1955, lead concentrations in C. rubrum skeletons from the Mediterranean are stable within the range 0.2–0.4 μg/g; concentrations increase to about 1–1.2 μg/g during the period 1960–1978, then decrease progressively to stabilize and reach values in the range 0.2–0.4 μg/g in present-day corals. These variations can be related to the lead gasoline pollution event that (1) started in the early 1950s with the increase of the numbers of cars in the world, and (2) was mitigated by the implementation of new regulations starting in 1975, leading to a return to pre-1950 levels in 2000. In the Pacific, lead concentrations in C. japonicum and C. konojoi are lower than in the Mediterranean C. rubrum, with values close to 0.17 ± 0.03 μg/g. The lowest lead concentrations in present-day samples (0.11 μg/g) are found in C. johnsoni and C. niobe from the Azores islands in the Atlantic, and in a Mediterranean C. rubrum from Montecristo Island, one of the least accessible and most protected areas in the Mediterranean Sea. Using lead concentrations in C. rubrum and in the Mediterranean seawaters, a partition coefficient Kd = [Pb/Ca]calcite/ [Pb/Ca]seawater of 13 ±3 is estimated; it allows calculating past and present lead contents in seawater in which corals grew. Application to Corallium species indicates that values endangering human health or threatening the preservation of aquatic ecosystem on long terms were nearly reached or exceeded in Mediterranean seawaters at the maximum of the lead gasoline pollution event in the 1980s. Measurements in C. rubrum from different places in the Mediterranean indicate that present-day seawater concentrations vary between 40 and 200 pmol/kg. As expected, the lowest concentrations come from protected areas insulated from human activities, while the highest come from places close to lead mining or processing sites.

Spatial and temporal patterns of sedimentation in an infilling reservoir

River dams are ubiquitous globally, altering the connection of up- and downstream environments. In particular, they store vast amounts of sediment in their upstream reservoirs, many of which are approaching storage capacity. There is much interest in sediment management for these systems, even though quantifying sedimentation rates remains somewhat challenging. This study focuses on one such system, Conowingo Reservoir, upstream of the last and largest dam (Conowingo Dam) on the Susquehanna River before it enters Chesapeake Bay. The trapping capacity of Conowingo Reservoir has decreased since its creation in 1928, potentially increasing fluvial loading to the Bay. In addition, large flood events can scour much more than the average annual sediment load from the Reservoir bottom, delivering exceptionally large sediment loads to the Bay within days. Predicting the downstream impacts of both Reservoir infill and scour events is essential for management; however, these predictions are limited by understanding of the complex spatial and temporal variability of sedimentation. To evaluate this variability, sedimentation rates are determined throughout the Reservoir over seasonal to decadal time scales with the naturally occurring radioisotopes 7Be and 210Pb (half-lives 53.3 days and 22.3 years, respectively), as well as anthropogenically produced 137Cs (half-life 30.7 y) as an independent geochronometer. These rates are placed within the context of sediment character (mud content) and fluvial sediment supply to the Reservoir, as well as its geomorphology. In general, seasonal sedimentation rates scale with fluvial loads and reflect deposition of watershed-derived sediment. Spatial patterns of seasonal sedimentation follow those of river deltas incised by channels, while decadal-scale sedimentation rates increase downstream. Episodic event sedimentation in channels is suggested by the character of 210Pb profiles, which indicate non-steady state sedimentation that makes application of sediment-age dating models challenging in this system. Although sediment mud content has clearly decreased as the Reservoir has filled, observed changes in sedimentation rates may reflect biases of these models rather than physical changes.

New coring study in Augusta Bay expands understanding of offshore tsunami deposits (Eastern Sicily, Italy)

AbstractTsunami deposits present an important archive for understanding tsunami histories and dynamics. Most research in this field has focused on onshore preserved remains, while the offshore deposits have received less attention. In 2009, during a coring campaign with the Italian Navy Magnaghi, four 1 m long gravity cores (MG cores) were sampled from the northern part of Augusta Bay, along a transect in 60 to 110 m water depth. These cores were taken in the same area where a core (MS06) was collected in 2007 about 2·3 km offshore Augusta at a water depth of 72 m below sea level. Core MS06 consisted of a 6·7 m long sequence that included 12 anomalous intervals interpreted as the primary effect of tsunami backwash waves in the last 4500 years. In this study, tsunami deposits were identified, based on sedimentology and displaced benthic foraminifera (as for core MS06) reinforced by X‐ray fluorescence data. Two erosional surfaces (L1 and L2) were recognized coupled with grain‐size increase, abundant Posidonia oceanica seagrass remains and a significant amount of Nubecularia lucifuga, an epiphytic sessile benthic foraminifera considered to be transported from the inner shelf. The occurrence of Ti/Ca and Ti/Sr increments, coinciding with peaks in organic matter (Mo incoherent/coherent) suggests terrestrial run‐off coupled with an input of organic matter. The L1 and L2 horizons were attributed to two distinct historical tsunamis (ad 1542 and ad 1693) by indirect age‐estimation methods using 210Pb profiles and the comparison of Volume Magnetic Susceptibility data between MG cores and MS06 cores. One most recent bioturbated horizon (Bh), despite not matching the above listed interpretative features, recorded an important palaeoenvironmental change that may correspond to the ad 1908 tsunami. These findings reinforce the value of offshore sediment records as an underutilized resource for the identification of past tsunamis.

Soil depth profile of 137Cs, 210Pb and 40K in Algeria

ABSTRACTThree soil depth profiles of 210Pb, 137Cs and 40K at different areas of Algeria were studied. The soil sampling areas are near the location where the French nuclear tests took place at 1960–1966. The two depth soil profiles were collected at Ghardaia region and the third one at Reggane region. The vertical distributions of radionuclides at the two soil depth profiles from Ghardaia region are different, probably due to the different soil composition. The soil depth profile from sandy soil show uniformity in the distribution of radionuclides without a clear maximum (peak). The soil depth profile that characterised by silty sand show a clear peak at 20–50 cm depth for all studied radioactive nuclides, while the observed activities are two times higher than the corresponding values in sandy soil samples. More specifically in Ghardaia region the 210Pb, 137Cs, 40K activity concentrations ranged between 27 and 50 Bq kg−1, 0.2 and 3 Bq kg−1 and 75 and 90 Bq kg−1 respectively in sandy soil type and between 37 and 75 Bq kg−1, 4 and 6 Bq kg−1 and 140 and 180 Bq kg−1 respectively in silty sand soil type. Finally, the third depth soil profile collected from Reggane site presents a completely different distribution of the studied radionuclides. The activity concentrations of 210Pb, 137Cs, 40K ranged between 37 and 51 Bq kg−1, 0.16 and 0.39 Bq kg−1 and 120 and 309 Bq kg−1 respectively. Three peaks in the 137Cs distribution revealed, were most probably, Chernobyl related 137Cs accounts for the surface peak, while the deeper peaks are connected to the weapons fallout.

Nuclear weapons fallout 137Cs in temperate and tropical pine forest soils, 50 years post-deposition

Following nuclear releases to the environment, 137Cs (half-life 30 years) is a long-term contaminant of many ecosystems, including forests. We recently sampled soils under pine forests in temperate and tropical climates to test the hypothesis that migration of 137Cs, 50 years after nuclear weapons fallout, is coupled with organic matter (OM) accumulation in these soils. Depth profiles of 137Cs, naturally-occurring 210Pb and weapons-derived 241Am were measured. After 50 years, migration of 137Cs into the temperate and tropical soils is limited to half-depths of 7–8 cm and 2–3 cm, respectively. At both locations, most 137Cs is associated with OM that accumulated from the early to mid-1960s. Illite, which immobilises radiocaesium, was undetectable by X-ray diffraction in the layer of peak 137Cs accumulation in the temperate forest soil, but apparent in the zone of peak concentration in the tropical soil. Data indicate that long-term (50 year) fate of 137Cs in organic-rich, temperate forest soil is coupled with OM accumulation; fixation of 137Cs by illite is more important in the tropical forest soil where OM is rapidly decomposed. Models of long-term radiocaesium migration in forest soils should explicitly account for the role of OM, especially when considering forests under contrasting climatic regimes.

Vertical distributions of mercury in marine sediment cores from central and southern part of Bohai Sea, China

In past decades, China has experienced substantial economic growth and industrialization. However, the effects of vast development of China on Hg input to the nearby oceans are still unclear. In this study, four representative 210Pb-dated sediment cores were collected in the central and southern part of Bohai sea to investigate vertical changes of Hg contents and explore the relationship between the Hg deposition and pollution history in this region utilizing a large amount of information available. The results indicated that Hg median concentrations of sediment core B62, B66, JQ17 and HZ24 were 0.043 mg/kg, 0.054 mg/kg, 0.033 mg/kg and 0.018 mg/kg respectively, among which, B66 in Yellow river estuary and B62 in central part of Bohai Sea had higher concentrations and HZ24 in Bohai Strait had a lower concentration. 210Pb profile appeared as a three segments model in the core B62, but as many steps of decay with depth in B66. For HZ24, 210Pb activity only fluctuated with depth without any discernible trend. Sedimentary rates of these cores decreased as follows: B66＞B62＞JQ17＞HZ24. Vertical distributions of Hg concentrations in sediment cores were totally different from each other. Hg concentrations in sediment core B62 experienced an initial fluctuation followed by a decreasing trend, while sediment core HZ24 almost showed the uniform decreasing trends from the surface to the bottom. There were three segments of variation in sediment cores JQ17: initial fluctuation followed by an obviously decreasing tendency and then a converted variation from surface to bottom. Hg contents changes in core B62 might reflect the additive effects from atmospheric deposition at a larger scale and the river-delivered sediment accumulation, while Hg vertical changes in B66 mainly had a close relationship with the input of Yellow river. The converted variation at the bottom section in sediment cores JQ17 was inferred to have some relationship with the Chengbei platform construction in the corresponding periods.