Excess 210Pb Inventories Research Articles

Heterogeneous tidal marsh soil organic carbon accumulation among and within temperate estuaries in Australia

The scarcity of data on tidal marsh soil accumulation rates (SAR) and soil organic carbon accumulation rates (CAR) globally precludes a comprehensive assessment of the role of tidal marshes in climate change mitigation and adaptation. Particularly few data exist from the southern hemisphere and for Australia in particular, which contains ~24% of globally recognised tidal marsh extent. Here we estimate SAR and CAR over the last 70 years using 210Pb-based geochronologies in temperate estuarine tidal marsh ecosystems in southern Western Australia (WA). Specifically, we assessed tidal marsh ecosystems situated in two geomorphic settings (marine vs. fluvial deltas) within 10 wave-dominated, barrier estuaries. Overall, average SAR (1.1 ± 0.3 mm yr−1) and CAR (32 ± 9 g m−2 yr−1) estimates were 5-fold lower than global mean estimates. Furthermore, we showed that hotspots of soil organic carbon stocks are not indicative of current hotspots for CAR. The lack of significant differences (P > 0.05) in SAR, CAR, and excess 210Pb inventories between marine and fluvial settings can be explained by the high heterogeneity among and within estuaries throughout the region. The relative stability of recent and Holocene relative sea-levels in WA likely explains the limited CAR potential in tidal marshes under relatively stable sea-level conditions. However, further research exploring interactions among biotic and abiotic factors within estuaries is required to shed more light on the small spatial-scale variability in SAR and CAR across tidal marsh ecosystems in WA and elsewhere. This study provides baseline estimates for the inclusion of tidal marshes in national carbon inventories, identifies hotspots for the development of blue carbon projects, and supports the use of site-specific assessments opposed to regional means for estimating blue carbon resources.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

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.

Evaluating the historical sedimentation patterns in two different Mediterranean deep environments (Sardinia and Sicily Channels)

The sediment accumulation rate in the Sardinia and Sicily channels in the central part of the Mediterranean Sea was studied by using short-lived radionuclides (210Pb and 137Cs) in two deep sediment cores. Different sedimentation regimes were identified indicating substantial differences in accumulation rates and historical patterns. The 210Pb-derived mean accumulation rate found in the Strait of Sardinia was 0.05 g.cm-2.y-1, lower than that in Sicily Channel (0.1 g.cm-2.y-1) suggesting an inverse correlation with water depth. Excess 210Pb inventories were 24 ± 1 and 6.0 ± 0.4 kBq.m-2, while the fluxes to the sediment were 745 ± 31 and 188 ± 11 Bq.m-2.y-1 in Sicily and Sardinia channels, respectively. 137Cs failed to use for the validation of the established chronologies, while its inventories found 450 Bq.m-2 and 355 Bq.m-2 in the Sicily and Sardinia channel, respectively.

Sediment geochronology for bar-built estuaries subject to flood deposition and erosion: A robust multiproxy approach across an estuarine zone

This paper presents a contiguous late Anthropocene 210Pb sediment geochronology and a late Anthropocene flood/fire event geochronology for an estuary subject to flood deposition and erosion. The procedure used multiple proxies to identify and evaluate flood facies overlying regions of erosion. Reconstructions of the original baseline sediments were undertaken for the major 210Pb geochronological methods. Only the lower estuary core site, which was deficient in inventory of excess 210Pb of baseline sediments, over the expected atmospheric supply for, was chosen to calculate the total amount of erosion. Erosion, as both cumulative mass and thickness, was then proportioned across flood events, relative to the size of the Rp index flood signal. The Rp index is the product of a simple stepwise thermogravimetric procedure. The flood geochronology was constructed by correlating the relative magnitudes of pivotal floods from an existing 100-year rainfall/river flow model and correlations in peak sediment char content outside of the flood facies with contemporary fire records. Overall, it was determined that 210Pb sediment isotope tomography gave the closest convergence with flood and fire events over those of the constant rate of supply and constant initial concentration methods. The agreement signified that around 40% of the original baseline accretion (25 cm) had been eroded by floods over the last 83 years and that there was possibly considerably more erosion at the upper site, which led to the loss of two flood facies.

Determining rates of sediment accumulation on the Mekong shelf: Timescales, steady-state assumptions, and radiochemical tracers

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess 210Pb distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3m, and stations sampled topset, foreset, and bottomset beds (water depths 7–21m). Apparent excess 210Pb sediment accumulation rates ranged from > 10cm/y (no down-core decrease of excess activity over 300cm core length) near the Song Hau river mouth, to 1–3cm/y in topset and foreset beds within 20–50km of the river mouth, to rates as low as 0.4cm/y in cores from bottomset beds. The 210Pb sediment accumulation rates yield an overall sediment burial rate of 6.1 × 1013g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 × 1014g/y; based on our 210Pb and seismic data and 210Pb data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3–1.6 × 1014g/y) from the literature. The inventory of excess 210Pb in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon 14C ages were measured on 10 cores from the proximal Mekong delta and compared to 210Pb sediment accumulation rates in the same core. The 210Pb accumulation rates in all 10 cores were considered to be more robust and accurate than the 14C geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the δ13C value of the seabed organic carbon.

Impact of seagrass loss and subsequent revegetation on carbon sequestration and stocks

Summary Seagrass meadows are sites of high rates of carbon sequestration and they potentially support ‘blue carbon’ strategies to mitigate anthropogenic CO2 emissions. Current uncertainties on the fate of carbon stocks following the loss or revegetation of seagrass meadows prevent the deployment of ‘blue carbon’ strategies. Here, we reconstruct the trajectories of carbon stocks associated with one of the longest monitored seagrass restoration projects globally. We demonstrate that sediment carbon stocks erode following seagrass loss and that revegetation projects effectively restore seagrass carbon sequestration capacity. We combine carbon chronosequences with 210Pb dating of seagrass sediments in a meadow that experienced losses until the end of 1980s and subsequent serial revegetation efforts. Inventories of excess 210Pb in seagrass sediments revealed that its accumulation, and thus sediments, coincided with the presence of seagrass vegetation. They also showed that the upper sediments eroded in areas that remained devoid of vegetation after seagrass loss. Seagrass revegetation enhanced autochthonous and allochthonous carbon deposition and burial. Carbon burial rates increased with the age of the restored sites, and 18 years after planting, they were similar to that in continuously vegetated meadows (26.4 ± 0.8 gCorg m−2 year−1). Synthesis. The results presented here demonstrate that loss of seagrass triggers the erosion of historic carbon deposits and that revegetation effectively restores seagrass carbon sequestration capacity. Thus, conservation and restoration of seagrass meadows are effective strategies for climate change mitigation.

Fluxes and inventories of 210Pb and 137Cs in lake sediments from East Asia and their relationships to sedimentary environment

The activities, fluxes and inventories of 210Pb and 137Cs in sediments collected from freshwater lakes, brackish-water lakes and seabeds in the East Asia are studied to elucidate their geochemical behaviors and characteristics concerning sedimentation environments. The surface concentrations varied widely by location. The average flux of excess 210Pb increased from freshwater lakes to brackish-water lakes to seawater, and the average flux of 137Cs showed an inverse trend. The inventories of excess 210Pb and 137Cs were in good correlation in samples from the same lake or adjacent areas. The inventory ratio (137Cs/excess 210Pb) was in the order of samples from seawater, increasing in brackish-water, further increasing in freshwater, and samples from Chinese lakes showed higher values. It is inferred that the inventory ratio may become a kind of index of sedimentation environment, geological, and/or radiochemical events.

Decadal-scale sediment dynamics and environmental change in the Albemarle Estuarine System, North Carolina

During the summer of 2001, several short cores (<50 cm) were collected in the Albemarle Estuarine System (AES). Down-core measurements for radiochemical tracers ( 210Pb, 137Cs) and organic matter signatures (δ 13C, δ 15N, C:N ratio, and LOI) have been used to elucidate potential temporal changes in fluxes and cycles of organic matter in the AES. Pb-210 geochronology indicates temporal and spatial variations in sediment deposition rates (0.08–0.57 cm y −1) with highest rates near the AES western limit relative to the rest of the estuary. Low accumulation rates, deficient excess 210Pb inventories, and near linear 137Cs profiles throughout the AES suggest that sediments are resuspended by wind-generated waves and currents and flushed from the system by river discharge and wind-tides, probably to Pamlico Sound to the south. Sediments in the AES are accumulating at rates less than the current rate of relative sea-level rise for this region except in protected portions of the estuary. Thus sediment accumulation in the AES is controlled in the short term by storm wave-base and in the long term by the creation of accommodation space by basin subsidence and sea-level rise. The geochemical and sedimentological data characterize the evolution of the Albemarle Sound and associated tributaries over the past 200–300 years. The majority of cores collected throughout the system show a significant decrease in 13C and increase in 15N isotopic signatures up-core. Thus, the estuarine system of eastern North Carolina has changed from a marine-influenced, high brackish environment to the modern-day system, which is a highly variable, terrestrially influenced, low brackish environment.

Recent sedimentation rates derived from 210Pb and 137Cs methods in Ise Bay, Japan

Ten sediment cores were collected from Ise Bay, Japan, and excess 210Pb and 137Cs were measured gamma-spectrometrically. Using the constant flux of lead-210 (CFL) model, the temporal and spatial variations of sedimentation rates in Ise Bay were determined. A generally increasing trend in sedimentation rate over the last century, especially since the mid-20th century, was found at all of the locations. The increase in sedimentation rate since the mid-20th century is consistent with the rapid economic growth and corresponding intensification of environmental alteration. The spatial distribution of sedimentation rates is time-independent. The high sedimentation rates were found near the Kiso Rivers mouth, indicating the river input is a main source of sediment. The sedimentation rates in the eastern half of the bay are higher than those in the western half, reflecting the transport pathway of suspended matter. The mean sedimentation rates over the last 40 years derived from both 210Pb and 137Cs methods were roughly consistent. The inventories of excess 210Pb and 137Cs in sediment are accounted for by the direct atmospheric fluxes, except one core near the river mouth.

Accumulation rates of major constituents of hemipelagic sediments in the deep Alboran Sea: a centennial perspective of sedimentary dynamics

The accumulation rates of sediment and major constituents in three different geographic areas of the Alboran Sea have been assessed by means of 210Pb and 137Cs concentration profiles. Mixing is present in the top layer of the sedimentary column, with mixing coefficients estimated to range from 0.2 to 15 cm 2 yr −1. Overall, apparent sedimentation rates for the last 100 years range from 0.014 to 0.182 g cm −2 yr −1 and show a tendency to decrease with distance to the coast and water column depth. However, specific characteristics of the sea floor for each physiographic environment govern the actual patterns of sediment accumulation. Particularly noticeable are the feeding role of submarine canyons, the trapping effect of slope terraces and the isolation caused by ridges. Sediment accumulation induced by turbiditic flows is also observed north of the Almeria–Oran area. Excess 210Pb inventories and surface concentrations reveal a net effect of sediment focussing and point to the role of bottom nepheloid layers in supplying a significant fraction of sediments to the deep areas. This is also evidenced by comparison of bottom sediment with the sediment trap data available from the Malaga area. We estimate that the particle advective input below 30 m above the bottom accounts for as much as 50–70% of the material that is ultimately deposited onto the sea floor. The greater abundance of lithogenic material in the Malaga zone (∼80%) reflects its larger input in the western Alboran Sea. Conversely, carbonate contents increase from less than 20% in this area to about 30% in the Almeria–Oran zone, reflecting the lesser importance of the dilution by lithogenic components. Biogenic silica was only detected in some surface samples, and no significant preservation was observed below the E h boundary. Abundance and accumulation rates of organic matter are in accordance with the spatial patterns of primary production in the Alboran Sea: higher in the western part, due to the presence of the Western Alboran Gyre. However, near-bottom redistribution leads to the homogenisation of organic matter concentrations in bottom sediments of any given area. Finally, from comparisons with sediment trap data, the degradation of organic matter has been estimated to be approximately 30–40% of what reaches the bottom.

Analysis of 210Pb in sediment trap samples and sediments from the northern Arabian Sea: evidence for boundary scavenging

The concentrations of 210Pb were measured in particles intercepted by sediment traps deployed at 1024 m and 3024 m in the northern Arabian Sea for a period of 1 year from November 1986 to October 1987. The 210Pb activity in the biweekly samples varied from 11 to 108 dpm g −1 at 1024 m and 58 dpm g −1 to 588 dpm g −1 at 3024 m. The mean annual 210Pb flux increased from 0.27 dpm cm −2 yr −1 in the 1024 m trap to 0.65 dpm cm −2 yr −1 in the 3024 m trap. The 210Pb flux exhibited a strong seasonal pattern associated with variations in the sediment mass flux and organic carbon (C org) flux except during early southwest monsoon in the 3024 m trap. This could be due to enhanced scavenging of 210Pb on inorganic phases consisting mostly of Fe–Mn oxides. 210Pb concentrations were dependent on particle composition, indicating that 210Pb scavenging from the water column is directly coupled to the type of settling particles. Organic matter does not appear to be the dominant carrier phase for 210Pb as shown by the poor 210Pb–C org correlation. The fraction of 210Pb removed from the ocean by settling particles F/ P ( 210Pb Flux / 210Pb production) cannot be used as a proxy to determine the C org flux in this region. The excess 210Pb inventories in 13 sediment cores in the Arabian Sea ranged from 11 to 211 dpm cm −2. Such highly variable spatial distribution of the excess 210Pb inventory reflects a non-uniform deposition of 210Pb in this region. The measured excess 210Pb inventories (124–211 dpm cm −2) for the sea floor sediments below the Western Arabian Sea Trap and from other locations in the vicinity of the sediment trap mooring lend support to boundary scavenging within the northwestern Arabian Sea as a sink for 210Pb.

What controls the mixed‐layer depth in deep‐sea sediments? The importance of POC flux

The depth of biogenic particle mixing, i.e., the mixed‐layer depth (L), is fundamental in models of organic‐matter recycling and paleoreconstructions for deep‐sea sediments. Factors postulated to control L in the oxygenated deep sea include particulate organic carbon (POC) flux, oxygen penetration into the sediment, and a balance between the downward mixing and decay of labile POC. We explore the dependence of L on biogeochemical characteristics by compiling, from 36 sites in three oceans, an internally consistent set of deep‐sea estimates of L, POC flux, biogenic mixing intensity (Db), and POC reactivity. We use excess 210Pb as a tracer for L and Db to avoid the confounding effects of tracer‐dependent mixing. We find that L, estimated from the penetration depth of excess 210Pb, varies systematically with POC flux, with an asymptotic function explaining 88% of the variance in L. Stepwise multiple regression suggests that the penetration depth of excess 210Pb (and estimated L) is much more likely to be controlled by POC flux than by (1) the sediment inventory of excess 210Pb or (2) biogenic mixing intensity (Db). In addition, L is negatively related to oxygen penetration into the sediment (r = ‐0.629) and not significantly related to predictions of L from a recent mixing/POC‐decay model. We conclude that in the food‐poor deep sea, POC flux substantially controls the size and activities of the sediment‐mixing benthos and, in turn, the thickness of the biogenic mixed layer. Thus, in contrast to previous suggestions, average mixed‐layer depth is not environmentally invariant but rather responds predictably to ecologically important parameters such as POC flux.

Atmospheric Deposition of Metals to Coastal Waters (Long Island Sound, New York U.S.A.): Evidence from Saltmarsh Deposits

Deposition chronologies of six high saltmarsh cores collected in New York City and Long Island, New York, determined using excess210Pb, show that accretion rates range from 0·1 to 0·4cm year−1. On average the marshes are accreting within about a factor of two of the rise in mean sea level for the area. Excess210Pb inventories in the marshes average 31±14dpm cm−2, comparable to values expected from atmospheric deposition of210Pb. Chronologies of fluxes of137Cs and excess Pb, Cu, Zn and Cd (defined as values in excess of the pre-anthropogenic background at depth in a core) indicate significant migration of137Cs and Cd in the marsh cores. Lead shows maximum fluxes between 1970 and 1980, consistent with the historical record of consumption of leaded gasoline in the U.S.A. Copper shows recent decreases in flux at four of the six sites and Zn shows variable patterns. Fluxes of Pb, Cu and Zn are generally less than or equal to directly measured atmospheric fluxes of these metals, indicating the importance of atmospheric supply of the metals to the marsh sites. Assuming that the atmospheric pathway is dominant for both metal and210Pb supply to the marshes, the regional pattern of atmospheric metal fluxes and the relative importance of the atmosphere in contributing metals to adjacent Long Island Sound can be determined. The marsh data indicate the atmospheric fluxes of Pb, Cu and Zn are greatest in New York City and decrease eastward along the north shore of Long Island. Comparison of the ratios of the inventories of excess metals to excess210Pb in the marshes and in the subtidal sediments of adjacent Long Island Sound suggests that atmospheric supply can contribute large fractions of Pb (at most 70–90% of integrated input). The atmospheric contributions of Cu and Zn are <50%, indicating the dominance of non-atmospheric sources for these metals. For all the metals, lower contributions of atmospheric input are evident in western Long Island Sound, suggesting that non-atmospheric sources of metals (point and non-point) are of greater importance in this portion of the estuary.

An attempt to evaluate dry deposition velocity of airborne210Pb in a forest ecosystem

Atmospheric concentration and total (wet plus dry) and dry deposition of210Pb and7Be have been measured at our laboratory in Ishikawa Prefecture in Japan which faced the Sea of Japan during 1993–1994. At the same time inventories of excess210Pb in soil samples were measured from a mixed pine forest. On the basis of the data obtained, an attempt was made to evaluate the dry deposition velocity as a result of the interception by the canopies of airborne210Pb in a pine stand The dry deposition velocity of210Pb in the pine stand was roughly estimated to be 0.9–0.8 cm/s, higher by a factor of about 10 than the value (mean: 0.12±0.09 cm/s; range: 0.06–0.28 cm/s) in the open area measured on the roof of our laboratory.

210Pb and 239,240Pu in the Northeast Water Polynya, Greenland: particle dynamics and sediment mixing rates

Abstract Distributions of the radionuclides 210Pb and 239,240Pu in sediment cores from the Northeast Water Polynya, Greenland, showed that these nuclides reached depths of 5–15 cm by particle mixing and sediment accumulation. End-member average values of the particle mixing coefficient and sediment accumulation rate were 0.13 cm2 y−1 and 0.06 cm y−1, obtained from the 210Pb profiles by assuming that each process is dominant relative to the other. Both 210Pb and 239,240Pu were measured on four cores; using the Pu data to constrain mixing rates produced corrected sediment accumulation rates that were 20–80% of the values calculated by neglecting mixing. Organic carbon burial in the polynya sediments was ≤0.4 mmol m−2 d−1, based on measured POC values at depth in the sediments and sediment accumulation rates corrected for mixing. This value is about 1% of the independently measured POC flux leaving the euphotic zone and compares with benthic carbon remineralization rates of 7% calculated by others from O2 uptake in the sediments. The inventories of excess 210Pb in the sediments ranged from 6 to 28 dpm cm−2. Relative to the atmospheric input of 210Pb and in situ production from decay of 226Ra, approximately 5 dpm cm−2 of 210Pb was being removed from the water column. The difference between the removal from the water column and sediment inventories suggests a net import of 210Pb to the polynya. This may occur by input of dissolved 210Pb from offshore waters or by input of 210Pb carried by sea ice. Particulate matter in land-derived fast ice adjacent to the polynya contained 330 ± 14 dpm of excess 210Pb g−1. If particles transported in sea ice are comparable to those extracted from fast ice, then sea ice transport into the polynya followed by melting may be an important source of excess 210Pb to the area. Fast ice also may contribute 210Pb if portions break off and melt within the polynya, as occurred in 1993.

Natural radionuclides and plutonium in sediments from the western Arctic Ocean: sedimentation rates and pathways of radionuclides

Sediment cores collected during R. V. Polar Sea AOS94 expedition from the Chukchi Shelf to the North Pole were analyzed for several decay-series natural radionuclides and Pu isotopes to study sedimentation rates and pathways of radionuclides in the western Arctic Ocean. The measured sedimentation rates vary by more than three orders of magnitude along the transect, from 210Pb-based rates of 200–700 cm kyr−1 over the Chukchi Shelf and 89 cm kyr−1 at the Chukchi Slope to 230Th-based rates of 0.02–0.3 cm kyr−1 at various settings in the deep basin. 230Thex profiles in the central western Arctic Basin are characterized by a cyclic pattern and a pronounced sub-surface maximum superimposed on an overall decrease with depth. Sediment inventories of excess 210Pb and 230Th in the deep basin as a whole cannot account for their in situ production and 210Pb fall-out. The opposite is true at the slope and shallower waters. We contend that, as with other ocean basins, boundary scavenging also exists in the Arctic Ocean. The broad continental shelves and the slope region may have the potential of removing all or most of the particle-reactive radionuclides unaccounted for in the deep basin. The Pu isotope data are consistent with the notion of boundary scavenging. Sediment inventories and concentrations of Pu decrease rapidly offshore. Isotopic composition of Pu suggests mixing of fall-out Pu, which decreases with increasing latitudes, and fuels reprocessing Pu derived from the Russian and Atlantic sides of the Arctic Ocean. Although fuel reprocessing Pu has impinged on the Chukchi Slope, its existence over the Chukchi Shelf is not evident and probably overshadowed by fall-out Pu.

Lead-210 and plutonium fallout in Taiwan as recorded at a subalpine lake

A sediment core collected from the Sun Moon Lake in middle Taiwan was investigated for 210Pb and 239,240Pu. Based on the decrease of excess 210Pb with depth, the mean sedimentation rate in the lake during the past half century is about 0.9 cm yr −1, which is unequivocally supported by the appearance of the 1963 plutonium fallout maximum at 26–28 cm. The short-term sedimentation rate is one order of magnitude greater than 14C-based long-term sedimentation rates in the lake reported previously. This may be attributed to the deterioration of soil conservation associated with the construction of a dam five decades ago. Total downcore inventories of excess 210Pb and 239,240Pu are 54.5 dpm cm −2 and 0.21 dpm cm −2, respectively. This 210Pb inventory requires a flux of 1.7 dpm cm −2 yr −1 at steady state, which is similar to 210Pb fluxes observed at other sites in the western Pacific rim. The inventory of 239,240Pu is in excellent agreement with documented cumulative 239,240Pu from global fallout (0.96 ± 0.07 mCi km −2) at the 20–30°N latitudinal band. Our data suggest that the Sun Moon Lake is an ideal setting for monitoring atmospheric fallout and environmental changes in this region.

The behavior of particle-reactive tracers in a high turbidity environment: 234Th and 210Pb on the Amazon continental shelf

Excess 234Th and 210Pb seabed inventories were measured in cores collected from the Amazon continental shelf to examine particle scavenging and seabed dynamics. Typical excess 210Pb inventories range from 100 to 300 dpm cm −2, and the total excess 210Pb inventory for the Amazon shelf was determined to be 2.7 × 10 17 dpm. The 210Pb measurements indicate that particle-reactive species are scavenged not only from the Amazon River but also from the lateral advection of offshore water. In order to sustain the 210Pb inventories, the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the Amazon River. This lateral advection supplies about 67% of the total excess 210Pb to the Amazon continental shelf with relatively small contributions from riverine input (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). The 234Th inventories were measured on four cruises, which occurred during periods of differing river discharge, wind stress, and flow rates of the North Brazil Current. The 234Th excess seabed inventories show large spatial and seasonal variability, with a range from 0 to 22 dpm cm −2, Ratios of predicted 234Th inventories over observed 234Th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments. The 234Th excess seabed inventories are controlled by resuspension, cycling the inventories between the water column and the seabed. 210Pb and 234Th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations. This approach indicates that for most of the shelf, the inventories of the shorter-term tracer 234Th are less than predicted by the inventories of the longer-term tracer ( 210Pb). There are two explanations for this trend. The first is that a larger portion of the 234Th inventory occurs in the fluid muds or the water column relative to 210Pb. The second is that the supply of offshore water, scavenging efficiency, and/or deposition have been lower over the two year study period relative to the last one hundred years.

Highly variable sediment distribution in shallow, wind-stressed lakes: a case for sediment-mapping surveys in paleolimnological studies

We examine sediment distribution patterns in seven Florida lakes and discuss implications for paleolimnological studies of shallow, subtropical lakes. The study lakes are highly productive and should exhibit thick organic sediment deposits, but organic sediments are often grossly lacking because basins are shallow, and frequent mixing, lack of stratification, and warm temperatures lead to breakdown of organic material. Organic sediment distribution patterns are highly variable. We observe three types of distribution patterns. When organic sediments are abundant, there may be (1) uniform sediment distribution. In lakes lacking organic sediments, there are (2) distribution to deeper areas if present, or (3) distribution to peripheral areas and embayments when deep waters are absent. We advocate the use of systematic mapping surveys to locate optimal coring sites for paleolimnological studies of shallow, wind-stressed lakes. Because numerous factors affect diagenesis and sediment redistribution, sediment abundance and location are not predictable. Sediment chronologies may be discontinuous and disturbed even in accumulation zones. The extent to which sedimentary records are discontinuous or disturbed is not quantifiable in any practical manner. 210Pb and 137Cs radioisotopic profiles provide qualitative evidence of the degree of stratigraphic disturbance. Total excess 210Pb inventories show that sediments are focused into depositional zones when sediment distribution is uneven. Excess 210Pb inventories are not informative about the completeness of sedimentary profiles unless small inventories suggest discontinuous sedimentation or erosional events. We present examples of disturbed and undisturbed profiles, and discuss how we use radioisotopic and geochemical evidence, and multiple cores to assess stratigraphic continuity.