Marine geochronology with210Pb
Abstract
The application of210Pb dating to an inshore marine deposit off the California coast has been successful. The amounts of unsupported210Pb annually accumulated in the Santa Barbara sediments are much greater than those from lake and glacial deposits and than the amount produced through the decay of radium in the overlying water column.
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Cited by (318)
Excess 210Pb (210Pbex) has been widely applied as a proxy indicator for establishing the chronology of sediments in lakes where the sedimentation processes comply with the assumptions for using 210Pbex dating models. However, little is known regarding the applicability of 210Pbex to elucidate contemporary seasonal sedimentary dynamics in reservoir water-level fluctuation zones (WLFZs). In the present study, sedimentary core and bulk sediments were collected from the WLFZ of the Three Gorges Reservoir (TGR) in China, and the physical and chemical properties were determined. Results for 210Pbex activity, total organic carbon, and total nitrogen of bulk sediments indicate pronounced elevation-dependent spatial variation (p < 0.05), but 137Cs activity and particle composition show no systematic trends, allowing 210Pbex activity to be used to differentiate seasonal stratigraphy. For the sedimentary core, therefore, sediment layers with lower 210Pbex activity were inferred to represent sedimentation during wet seasons, when the 210Pbex concentration was diluted by a higher suspended sediment load, whereas sediment layers characterised by higher 210Pbex activity were ascribed to sedimentation during dry seasons, when the tranquil water favoured 210Pbex fallout to be absorbed by suspended sediments and then deposited in the WLFZ. Hereupon, temporal variation is observed in annual sedimentary thickness, with relatively low sedimentation rates during the initial stage (2006–2008) of the TGR impoundment, followed by relatively high sedimentation rates from 2009 to 2013. Over the period from 2014 to 2017, sedimentation rates were low because the suspended sediment was intercepted by upstream cascade reservoirs. Subsequently, sedimentation rates were higher during 2018–2019 as a consequence of an increase in extreme rainfall events. The findings herein highlight that 210Pbex-based seasonal stratification discrimination is effective approach to establish high-resolution sedimentary chronosequences and that net sedimentation rates in WLFZ are controlled by inflow suspended load of the TGR.
Sedimentation on the Siberian Arctic Shelf as an indicator of the arctic hydrological cycle
2023, AnthropoceneSiberia occupies a significant part of the Eurasian continent and environmental changes in this region can have an important impact on the climate system of the Northern Hemisphere. The sediment flux of Siberian rivers is sensitive to changes in physical, chemical, and biological processes taking place on the continent, and these changes can be recorded in marine sediments on the Siberian Arctic Shelf. This paper presents data on grain-size distribution of the sediments, the sedimentation rates, and the mass accumulation rates on the shelf over the past 100 years. Age models are based on the decay rate of excess lead isotope 210Pb, taking into account the sorption capacity of the marine sediments, and the presence of cesium isotope 137Cs peaks in the sediment cores. The highest sedimentation and mass accumulation rates were observed prior to the 1920–40 and coincide with larger particle sizes, indicate a period of active sediment-laden sea ice and iceberg melt. Systematic decrease in the sedimentation and mass accumulation rates against the background of an increase in the proportion of silt fractions in the shelf sediments in the second half of the 20th century can be explained by an acceleration of the arctic hydrological cycle.
Revisiting <sup>228</sup>Th as a tool for determining sedimentation and mass accumulation rates
2022, Chemical GeologyThe use of 228Th has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound 228Th using a radium delayed coincidence counting system (RaDeCC). Herein we review the 228Th cycle in the marine environment and revisit the historical use of 228Th as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from 228Th and 210Pb are presented for a micro-tidal salt marsh and a marginal sea environment. 228Th and 210Pb have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that 228Th (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from 210Pb, reflecting different integration periods. Use of 228Th is well-suited for shallow (<15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of 228Th and 210Pb can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment 228Th.
How the reconstruction of faunal communities in a marine protected area (Columbretes Reserve, western Mediterranean) evidence human and natural impacts on fauna
2022, Ecological IndicatorsReconstruction of marine communities in search of baseline (pristine) conditions is a crucial first step for their future restoration. A recent reconstruction (last century) of the sandy-muddy bottom fauna on the continental shelf of a marine protected area (MPA) was performed in the Columbretes Reserve, including periods after and before the Reserve declaration. The dating of sediments and identification of faunal remains (e.g., shells of benthic bivalves and gastropods and pelagic pteropods) were performed in a core (MC2) at a depth of 87 m in 2018. Radiometric data identified sediments older (below 11 cm) and younger (from the top of the core to 11 cm) than ca. 110 years. Mercury analyses validated the 210Pbxs data at 5–7 cm (1967–1989), with a significant Hg peak that coincided with a period of military activities occurring until 1982 in the Columbretes Islands. Both human and climatic variables affected benthic and pelagic communities. Among the human impacts, the cessation of trawling activity after the declaration of the MPA (1980s) influenced the most dominant benthos (bivalves and gastropods) by i) increases in their abundance and ii) changes in the feeding guilds, with a return to baseline conditions by the increase in filter feeders after trawling cessation vs a high abundance of detritus feeders occurring under high trawling activity. Human activities apparently did not affect diversity levels. In parallel, we also identified some recolonization by Octocorallia since the 1980s. Finally, the increase in the pelagic pteropod Creseis acicula since 1995 at the MC2 station probably indicates the result of warming of surface waters in recent decades. Our study based on core reconstructions provides for the first time an historical perspective of the impact of trawling on marine benthos and the positive effect of conservation measures in marine protected areas.
The influence of anthropogenic activities on heavy metal pollution of estuary sediment from the coastal East China Sea in the past nearly 50 years
2022, Marine Pollution BulletinHistorical records of eight heavy metals (HMs: Cu, Zn, Cr, Ni, Pb, As, Hg and Cd) were analyzed in two dated sediment cores collected from the coastal East China Sea (Meishan Bay, MSB) to investigate the influence of anthropogenic activities on estuary sediment from 1972 to 2019. The sedimentary records of HMs in the two cores reflected the development of economy and change of energy consumption in China's east coastal areas. The contents of most HMs in sediments had no change or showed a downward trend before the 1980s. After the 1980s, the contents of HMs showed an increasing trend, mainly because of the rapid development of aquaculture, industry, and agriculture. The distribution of HMs outside the bay (OB) was mainly affected by industry, shipping, and agriculture (vegetable planting), while HMs in sediments inside the bay (IB) was affected by shipping, agriculture, and aquaculture. Principle component analysis (PCA) showed that Cd, Cr, and Ni could be attributed to industrial discharge, while As, Cu, Pb, and Zn were associated with shipping, agriculture, and aquaculture. Sediments were slightly polluted with Cd (Igeo:0.10–0.71, CF:1.90–7.74) and Ni (Igeo:0.08–0.92, CF:1.25–12.55), and seriously polluted with Hg (Igeo:0.95–1.76, CF:13.43–85.65). This study provides insights for the local governments to raise awareness of HM pollution in the coastal East China Sea and formulate corresponding pollution control measures.
The 210Pbex accumulation-decay model is widely applied for sediment dating while not applicable for dating sediments derived from debris flows because the sources of excess lead-210 (210Pbex) are diverse and changeable. In this study, we developed a new model () for dating recent silting soils of debris flow deposits using changes in 210Pbex concentration in surface soil layers, where C(n) is the 210Pbex concentration in the top layer of the silting soil after n years since final sedimentation (Bq/kg), Cr is the 210Pbex concentration in the top layer at a reference site (no erosion or sedimentation in the long term), and coefficient λ = 0.03114 year−1 is the decay constant of 210Pbex. The model was validated with data obtained from the Hunshui Gully watershed in southwestern China. Six sampling sites with different sedimentation stages were selected in the Hunshui Gully watershed. The 210Pbex concentration increased from 51.20 ± 4.61 for the site with debris flow deposits in 2003 to 112.56 ± 7.88 Bq/kg for those in 1965. The derived time of the new model fits the actual value well, and the root-mean-square error of this new model was 3.23 years. Our results confirm that the fallout radionuclide 210Pbex can be applied for dating recent silting soils in debris flow deposits. However, further studies are needed to verify the model and determine the coefficients for various environments.