Sedimentation rates in flow-restricted and restored salt marshes in Long Island Sound

Many salt marshes in densely populated areas have been subjected to a reduction in tidal flow. In order to assess the impact of tidal flow restriction on marsh sedimentation processes, sediment cores were collected from flow-restricted restricted salt marshes along the Connecticut coast of Long Island Sound. Cores were also collected from unrestricted reference marshes and from a marsh that had been previously restricted but was restored to fuller tidal flushing in the 1970's. High bulk densities and low C and N concentrations were found at depth in the restricted marsh cores, which we attribute to a period of organic matter oxidation, sediment compaction, and marsh surface subsidence upon installation of flow restrictions (between 100 and 200 years before the present, depending on the marsh). Recent sedimentation rates at the restricted marshes (as determined by¹³⁷Cs and²¹⁰Pb dating) were positive and averaged 78% (¹³⁷Cs) and 50% (²¹⁰Pb) of reference marsh sedimentation rates. The accumulation of inorganic sediment was similar at the restricted and reference marshes, perhaps because of the seasonal operation of the tide gates, while organic sediment accretion (and pore space) was significantly lower in the restricted marshes, perhaps because of higher decomposition rates. Sedimentation rates at the restored marsh were significantly higher than at the reference marshes. This marsh has responded to the higher water levels resulting from restoration by a rapid increase in marsh surface elevation.     

Balanced Sediment Fluxes in Southern California’s Mediterranean-Climate Zone Salt Marshes

Salt marsh elevation and geomorphic stability depends on mineral sedimentation. Many Mediterranean-climate salt marshes along southern California, USA coast import sediment during El Niño storm events, but sediment fluxes and mechanisms during dry weather are potentially important for marsh stability. We calculated tidal creek sediment fluxes within a highly modified, sediment-starved, 1.5-km² salt marsh (Seal Beach) and a less modified 1-km² marsh (Mugu) with fluvial sediment supply. We measured salt marsh plain suspended sediment concentration and vertical accretion using single stage samplers and marker horizons. At Seal Beach, a 2014 storm yielded 39 and 28 g/s mean sediment fluxes and imported 12,000 and 8800 kg in a western and eastern channel. Western channel storm imports offset 8700 kg exported during 2 months of dry weather, while eastern channel storm imports augmented 9200 kg imported during dry weather. During the storm at Mugu, suspended sediment concentrations on the marsh plain increased by a factor of four; accretion was 1–2 mm near creek levees. An exceptionally high tide sequence yielded 4.4 g/s mean sediment flux, importing 1700 kg: 20 % of Mugu’s dry weather fluxes. Overall, low sediment fluxes were observed, suggesting that these salt marshes are geomorphically stable during dry weather conditions. Results suggest storms and high lunar tides may play large roles, importing sediment and maintaining dry weather sediment flux balances for southern California salt marshes. However, under future climate change and sea level rise scenarios, results suggest that balanced sediment fluxes lead to marsh elevational instability based on estimated mineral sediment deficits.     

Accretion rates and sediment accumulation in Rhode Island salt marshes

In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,²¹⁰Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr^?1 (0.25 to 0.60 cm yr^?1) based on the constant flux model, 0.40±0.15 cm yr^?1 (0.15 to 0.58 cm yr^?1) based on the constant activity model, and 0.44±0.11 cm yr^?1 (0.30 to 0.59 cm yr^?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess²¹⁰Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr^?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.     

Mixing and cycling of uranium,thorium and 210Pb in Puget Sound sediments

Activity profiles of excess ²³⁴Th, excess ²¹⁰Pb, ²³²Th, ²³⁰Th, ²³⁴U and ²³⁸U, and ^228/232Th ratios determined in eight box cores of sediment from six sites in central Puget Sound provide new insights into the dynamic nature of solid phase mixing in surface sediments, the exchange of ²²⁸Ra and other soluble species across the sediment-water interface, and the cycling of U, Th and ²¹⁰Pb in this coastal zone.Comparison of excess ²³⁴Th inventories in sediments with its production rate in the overlying water column indicates a mean residence time of at most 14 days for particles in the central Puget Sound water column.Surface sediment horizons with excess ²³⁴Th have no excess ²²⁸Th which might be used to ascertain sediment accumulation rates over the past decade. Instead, deficiencies of ²²⁸Th due to loss of soluble ²²⁸Ra from pore water to the overlying water persist to 20–30 cm, revealing that exchange of soluble chemicals between pore and overlying waters reaches these depths in the extensively bioturbated sediments of Puget Sound.Solid phase U isotope concentrations tend to increase by up to a factor of two with depth in sediments, as a result of dissolved U being biologically pumped down into sediments where it is partially removed when conditions become mildly reducing. ²³²Th and ²³⁰Th activities and ^230/232Th ratios are constant with depth in sediments, indicating constant detrital phase compositions and essentially no authigenic ²³⁰Th. Steady state ²¹⁰Pb depositional activities in and fluxes to Puget Sound sediments average only about onehalf those for sediments of the open Washington coast north of the Columbia River mouth, primarily because of a much lower supply of dissolved ²¹⁰Pb in sea waters adverting into Puget Sound.Excess ²³⁴Th profiles in sediments reveal much more detail about the depth dependency, dynamic nature and recent history of solid phase mixing processes than excess ²¹⁰Pb profiles. At least six of eight ²³⁴Th profiles show that mixing within the ²¹⁰Pb-defined surface mixed layer is depth dependent. In three profiles, ²³⁴Th-derived mixing rates are fastest several centimeters below the sediment-water interface, indicating greater macro-benthic activity at these depths. Depth dependent mixing coefficients derived from the best fit of a four layer, advection-diffusion-decay model to the ²³⁴Th data are consistent with ²¹⁰Pb profiles determined for the same sediments, strongly suggesting that ²³⁴Th and ²¹⁰Pb are mixed equivalently and in a multilayered manner.     

Role of Spartina alterniflora on sediment dynamics of coastal salt marshes-case study from central Jiangsu and middle Fujian coasts

Coastal salt marshes represent an important coastal wetland system. In order to protect coastlines from erosion and rapid increase in accumulation rate, Spartina alterniflora (S. alterniflora) was introduced into the Chinese coast. Two study areas (Wanggang and Quanzhou Bay) were selected that represent the plain type and embayment type of the coastal salt marshes. In situ measurements show that the tidal current velocities are stronger on the intertidal mudflat without S. alterniflora than that with S. alterniflora, and the velocity above the canopy surface is larger than that in the salt marsh canopy. The existence of S. alterniflora also influences the velocity structure above the bare flat during ebb tide. With the decrease in current flow velocity when seawater enters into the S. alterniflora marsh, suspended sediments are largely entrapped on the marsh surface, leading to increase in sedimentation rates and change in physical evolution processes of the coastal salt marshes. The highly developed root systemof S. alterniflora induces sediment mixing and exchange between subsurface sediment strata and affects the vertical sediment distribution remarkably. The sedimentation rate of S. alterniflora marsh at the Wanggang area is much higher than the relative sea level rise rate, where rapid progradation of theWanggang saltmarshes that is protecting the coast from sea erosion is observed.     

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (²¹⁰Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.     

Suspended sediment sources and tributary effects in the lower reaches of a coastal plain stream as indicated by radionuclides, Loco Bayou, Texas

Characterizing the dynamics of fluvial sediment sources over space and time is often critical in identifying human impacts on fluvial systems. Upland interfluve and subsoil sources of suspended sediment at Loco Bayou, Texas, were distinguished using ²²⁶Ra/²³²Th, ²²⁶Ra/²³⁰Th and, ²²⁸Ra/²³²Th. Source contributions were apportioned at three stations during within-bank and flood flows. ¹³⁷Cs and ²¹⁰Pb_xs (excess ²¹⁰Pb) were used to determine floodplain sedimentation; suspended sediment ²¹⁰Pb_xs/¹³⁷Cs data mirrored results of Ra/Th, showing dominance of subsoil sources during within-bank flows, changing to interfluve sources during flood. This trend corresponds spatially to influx of sediment from ephemeral tributaries, reflecting mobilization of stored interfluvial sediments during flood stage. Upper basin sedimentation was similar but markedly less at the lowermost station. These results indicate (1) modified ephemeral tributaries store sediment derived from sheet wash, discharging them during flood, and (2) southernmost Loco Bayou is episodically re-worked, resulting in significantly reduced local rates of sedimentation.     

The Declining Role of Organic Matter in New England Salt Marshes

The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (²¹⁰Pb and ¹³⁷Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year^?1) compared to historical averages (0.24–0.60 cm year^?1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year^?1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41?±?0.07 cm year^?1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.     

Patterns of sediment accumulation in the tidal marshes of Maine

One year’s measurements of surficial sedimentation rates (1986–1987) for 26 Maine marsh sites were made over marker horizons of brick dust. Observed sediment accumulation rates, from 0 to 13 mm yr^?1, were compared with marsh morphology, local relative sea-level rise rate, mean tidal range, and ice rafting activity. Marshes with four different morphologies (back-barrier, fluvial, bluff-toe, and transitional) showed distinctly different sediment accumulation rates. In general, back-barrier marshes had the highest accumulation rates and blufftoe marshes had the lowest rates, with intermediate values for transitional and fluvial marshes. No causal relationship between modern marsh sediment accumulation rate and relative sea-level rise rate (from tide gauge records) was observed. Marsh accretionary balance (sediment accumulation rate minus relative sea-level rise rate) did not correlate with mean tidal range for this meso- to macro-tidal area. Estimates of ice-rafted debris on marsh sites ranged from 0% to >100% of measured surficial sedimentation rates, indicating that ice transport of sediment may make a significant contribution to surficial sedimentation on Maine salt marshes.     

Hydrographic surveys and sedimentation in Deep Bay,Hong Kong

Three hydrographic surveys were carried out in Deep Bay, which is located in the eastern part of Pearl estuary between Shenzhen, the most successful special economic zone of PRC, and Hong Kong. Data on current, size distributions of bottom sediment, suspended solids, and some water quality parameters were obtained. This information is of value for mathematical modeling of tidal circulation and sediment transport in the bay, and also useful in the planning of further development in this area. The sedimentation rate in Deep Bay was estimated by two different approaches, viz. comparison of historical navigation maps and²¹⁰Pb dating. Information obtained from the maps indicated that average sedimentation rate between 1898 and 1949 was about 8 mm/yr, while a 15 mm/yr figure was obtained from the²¹⁰Pb dating for recent years.     

Influence of Grasshopper Herbivory on Nitrogen Cycling in Northern Gulf of Mexico Black Needlerush Salt Marshes

Herbivory is a common process in salt marshes. However, the direct impact of marsh herbivory on nutrient cycling in this ecosystem is poorly understood. Using a ¹⁵N enrichment mesocosm study, we quantified nitrogen (N) cycling in sediment and plants of black needlerush (Juncus roemerianus) salt marshes, facilitated by litter decomposition and litter plus grasshopper feces decomposition. We found 15 times more ¹⁵N recovery in sediment with grasshopper herbivory compared to sediment with no grasshopper herbivory. In plants, even though we found three times and a half larger ¹⁵N recovery with grasshopper herbivory, we did not find significant differences. Thus, herbivory can enhance N cycling in black needlerush salt marshes sediments and elevate the role of these salt marshes as nutrient sinks.     

Metal accumulation in surface salt marsh sediments of the Bay of Fundy,Canada

We measured the amount of arsenic, chromium, copper, lead, nickel, vanadium, and zinc accumulated over a five-year period from 1997 to 2002 in surface sediments of seven salt marshes along the New Brunswick coast of the Bay of Fundy, Canada. Study sites extended from outer to inner Bay, spanning a gradient in tidal range (6–12 m) and mean sediment deposition rate (0.27–1.76 cm yr⁻¹). In each study site, metal concentrations were measured in low and high marsh areas. Concentrations of chromium, nickel, and zinc appear to be within their natural range, while arsenic, lead, and vanadium are enriched in some sites. Calculated sediment metal loadings rates showed variability among marsh sites that closely followed sediment deposition patterns, suggesting sediment deposition rate is the driving factor of short-term metal accumulation in Fundy marshes. The value of salt marshes as a sink for metals may be enhanced by high sedimentation rates.     

Influence of Wind-Driven Inundation and Coastal Geomorphology on Sedimentation in Two Microtidal Marshes, Pamlico River Estuary, NC

Marsh sediment accumulation is predominately a combination of in situ organic accumulation and mineral sediment input during inundation. Within the Pamlico River Estuary (PRE), marsh inundation is dependent upon event (e.g., storms) and seasonal wind patterns due to minimal astronomical tides (<10 cm). A better understanding of the processes controlling sediment deposition and, ultimately, marsh accretion is needed to forecast marsh sustainability with changing land usage, climate, and sea level rise. This study examines marsh topography, inundation depth, duration of inundation, and wind velocity to identify relationships between short-term deposition (tile-based) and long-term accumulation (²¹⁰Pb and ¹³⁷Cs) recorded within and adjacent to the PRE. The results of this study indicate (1) similar sedimentation patterns between the interior marsh and shore-side marsh at different sites regardless of elevation, (2) increased sedimentation (one to two orders of magnitude, 0.04–4.54 g m^?2 day^?1) within the interior marsh when the water levels exceeded the adjacent topography (e.g., storm berm), and (3) that short-term sea level changes can have direct effects on sediment delivery to interior marshes in wind-driven estuarine systems.     

U- and Th-decay series constraints on the timescales of crystal fractionation to produce the phonolite erupted in 2004 near Tristan da Cunha, South Atlantic Ocean

Phonolite pumice found floating offshore of Tristan da Cunha following intense seismic activity southeast of the island July 29-30, 2004 was analyzed for ²³⁸U- and ²³²Th-series nuclides to determine initial ²³⁰Th, ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, ²²⁸Ra, and ²²⁸Th activities. The initial (²¹⁰Po/²¹⁰Pb) value of 0.15 for the phonolite shows that, like most subaerial lavas, this subaqueous tephra degassed most of its ²¹⁰Po upon eruption. The (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) values for the phonolite are similar to those of the trachyandesites erupted in 1961 from Tristan da Cunha. However, the relative activities of ²¹⁰Pb, ²²⁶Ra, and ²³⁰Th in the phonolite contrast with those of the trachyandesites, in that ²¹⁰Pb and ²³⁰Th are both strongly enriched with respect to ²²⁶Ra. In addition, the phonolite had a small deficit in ²²⁸Ra with respect to ²³²Th. The Ra deficits likely resulted from partitioning into feldspars and hornblende in a time frame that extended over several decades to a century. These disequilibria can be explained by crystal fractionation at a decreasing rate through time at an average of 3-5 × 10⁻³ year⁻¹. The calculated crystallization rate is about an order of magnitude faster than has been calculated for most other phonolites and trachytes, and about half that calculated for crystallization of the Makaopuhi lava lake. These data imply that the 2004 magma was not the differentiated cap of a much larger body that remained at depth. Instead, it was likely the residue of a relatively small body of more mafic magma that was injected into the crust southeast Tristan and underwent extensive and rapid crystal fractionation before it erupted.     

Distribution and abundance of tidal marshes along the coast of maine

Planimetry studies of coastal geology maps prepared by the Maine Geological Survey show that there is more than an order of magnitude more tidal marsh area in the state of Maine than documented in previously published estimates. The highly convoluted coast of Maine, which is approximately 5,970 km long, contains almost 79 km² of salt marsh, far more than any other New England state, New York, or the Bay of Fundy region. Reasonable estimates for the per-unit primary productivity of salt marshes lead to projections of total marsh productivity on the order of 10¹⁰ g dry weight yr^?1 for the Maine coast and 10¹¹ g dry weight yr^?1 for the Gulf of Maine as a whole. Distribution of tidal marsh area is strongly controlled by coastal geomorphology, which varies considerably along the coast of Maine. The salt marsh area is concentrated in the southwestern coastal region of arcuate bays, where marshes have developed behind sandy beaches. A series of long islands and bedrock peninsulas in the south-central portion of the coast also provides sheltered areas where large marshes occur. Northeast of Penobscot Bay salt marshes become more numerous and smaller in average areal extent. A lack of protection from waves, along with limited sources of glacio-fluvial and glacio-marine sediments, restricts the occurrence of salt marshes in that region to the frignes of coves and tidal rivers.     

The relationship of soil 210Po and 210Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

The relationship of soil Po and Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

Radionuclide partitioning across great lakes natural interfaces

Several water and surface microlayer samples from Lake St. Clair, the Niagara River, and the North Shore of Lake Ontario collected during 1983–1986 have been assayed for a variety of radionuclides. In addition, the foam accumulating in the pool just below Niagara Falls was also analyzed and found to be the most efficient aqueous phase collector of¹³⁷Cs,²¹⁰Pb, and²²⁶Ra.The order of radioisotope specific activities from highest to lowest is: Lake Ontario sediment, Niagara River suspended solids, Niagara River foam, surface microlayer water, and subsurface water. Radiological dose rates to the sediments from¹³⁷Cs,²²⁶Ra, and²²⁸Th total about 5 mGy/y.     

Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of particle scavenging processes

Vertical and temporal variations in the activities of²³⁴Th,²¹⁰Po and²¹⁰Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these two seas having distinct biogeochemical properties. A common feature of the²³⁴Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity of²³⁴Th is deficient in the surface 200 m with respect to its parent,²³⁸U. This gross deficiency is attributed to the preferential removal of²³⁴Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved²³⁴Th are comparable in these two basins. The temporal variations in the²³⁴Th-²³⁸U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m), the scavenging residence time of²³⁴Th ranges from 30 to 100 days. The surface and deep waters of both the seas show an enhanced deficiency of dissolved²¹⁰Po relative to²¹⁰Pb and that of²¹⁰Pb relative to²²⁶Ra. The deficiencies of both²¹⁰Po and²¹⁰Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these nuclides from the water column. The scavenging rates of²¹⁰Po and²¹⁰Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved²¹⁰Po/²¹⁰Pb and²¹⁰Pb/²²⁶Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the three isotope pairs used in this study.     

Seasonal variations of 210Pb and 210Po concentrations in an oligotrophic lake

The vertical distribution of ²¹⁰Pb and ²¹⁰Po in the dissolved (<0.4 μm) and the paniculate (>0.4 μm) phases was measured in Crystal Lake, Wisconsin, to examine the spatial and temporal variability during the seasonal cycle of this oligotrophic lake. The concentration of unsupported ²¹⁰Pb in the water column is maintained principally by atmospheric input. However, most of the ²¹⁰Po in Crystal Lake is produced in situ from radioactive decay of ²¹⁰Pb.Mass balance considerations indicated that the removal rates of ²¹⁰Pb and ²¹⁰Po from the water column to the sediment varied temporally by nearly an order of magnitude. During transient periods of high biological productivity, a large net flux of these nuclides into the sediment occurred. In addition, ²¹⁰Pb was rapidly stripped from the water column during fall turnover. It was during these short-lived events that most of the annual net removal of ²¹⁰Pb and ²¹⁰Po occurred. The mean removal residence time was estimated to be 0.095 yr for ²¹⁰Pb and 0.26 yr for ²¹⁰Po. These residence times suggest that there is a difference between ²¹⁰Pb and ²¹⁰Po in the extent of their recycling in the water column. Calculations indicated that there was a cyclic response of the water column ²¹⁰Po inventory corresponding to successive time periods where there was a net loss or net gain. This cycling is attributed to rapid biological removal and subsequent release from the sediment of freshly deposited ²¹⁰Po. For ²¹⁰Pb, replenishment of the water column appeared to occur mainly from atmospheric input.