Excess 210Pb Inventories Research Articles

Geochronology of sediments in the Sabine-Neches estuary, Texas, U.S.A.

The accumulation rate of sediments in the Sabine-Neches estuary, near Beaumont, Texas, was studied in four sediment cores using radioisotopes 210Pb and 239,240Pu. Due to very low and variable activities of unsupported 210Pb in the sediments, the sedimentation rates could not be reliably established using the 210Pb dating method. However, 239,240Pu profiles showed distinct peaks, corresponding to the maximum fallout in 1963, and using these Pu peaks the average sedimentation rates were estimated at 4–5 mm yr −1 in the upper and lower estuary. Various assumptions that are commonly used in the 210Pb dating method were tested. It was found that the 226Ra activities were in equilibrium with 238U, and the possible loss of 222Rn was judged to be insignificant. The lack of an exponential profile of excess 210Pb with depth did not appear to be a direct result of sediment mixing, as the post-depositional mixing rates estimated using a numerical mixing model were low compared to other coastal areas. A significant positive correlation between the amount of < 63-μm fraction sediments and the total 210Pb activity was observed in the surface sediments. While the hydraulic residence time was relativelt short in this estuary (∼ 10 days), the average total residence time of 210Pb was ∼ 10 days. Between 50% and 94% of the total 210Pb in the water column was found in the < 0.5-μm fraction, resulting in low K d-values. The measured inventories of excess 210Pb and 239,240Pu in estuarine sediments were only ∼ 10–34% and ∼ 19–50%, respectively, of the total expected inventories. It appears that the estuarine retention and sediment inventory of these radionuclides, and by analogy, other particle reactive metals, are a function of the hydraulic residence time, grain size, and possibly complexation of these nuclides with DOC.

210Pb balance and implications for particle transport on the continental shelf, U.S. Middle Atlantic Bight

Supply of 210Pb to the continental shelf off the northeastern United States is dominated by the deposition from the atmosphere, the rate of which is reliably known from previously published work. Excess 210Pb inventories in the shelf sediments show accumulations that are nearly in balance with the supply, even in areas of relict sands where it is believed that no net accumulation of sediment presently occurs. The 210Pb distributions in shelf and slope water indicate that the two-way fluid exchange at the shelf-slope front and the net transport in the alongshore flow make comparatively small contributions to the shelf 210Pb budget. The near balance between supply and decay of 210Pb on the shelf implies a limit to the particle export flux. It is concluded that the export of particulate organic carbon does not exceed 60 g m−2 y−1 (∼25% of primary production) and is probably lower. The hypothesis is advanced that fine particulate matter introduced to the continental shelf is detained in its transit of the shelf because of bioturbational trapping in the sediment due to benthic animals. Distributions of 210Pb in suspended particulate matter and in the fine fraction of shelf sediments suggest that the average fine particle must undergo several cycles of deposition-bioturbation-resuspension-redeposition and requires a number of decades for its transit and ultimate export from the shelf. Thus, only the most refractory organic matter is likely to be exported.

Sources of sediment to the Neuse River estuary, North Carolina

The Neuse River estuary is part of the large Albemarle/Pamlico estuarine/lagoonal system (APES) of North Carolina. Exchange between the APES and the open ocean is restricted to inlets in the Outer Banks barrier islands. Freshwater discharge of the Neuse River is low in relation to the size of its estuary, so that the estuary is normally brackish over most of the area which accumulates fine-grained sediments. Mud (silt + clay) accumulation rates are low (⩽ 6 mm year −1). From the mud distribution and the dynamics of suspended sediment in the estuary it has been inferred that the Neuse retains fines efficiently, with probable episodic (high discharge) losses seaward, to Pamlico Sound. As part of a study of particle transport, deposition, and accumulation in the Neuse estuary, we have collected radiochemical data (10 cores) and chemical data (seven cores) on sediment cores from the main channel of the estuary between New Bern, North Carolina and the estuary mouth. K/Al increases in the lower estuary, consistent with earlier reports of increasing illite in the clay fraction. Landward transport of sediment from Pamlico Sound is the most plausible explanation for the increasing K/Al (illite). A marine sediment source may, therefore, be important for the lower estuary. The distribution of sediment inventories (quantities per cm 2 of sediment column) of particle-reactive radionuclides is consistent with the hypothesis of landward sediment transport. Inventories of excess 210Pb, 137Cs, and 239,240Pu are coherent through the estuary and show the effects of particle redistribution processes on regionally uniform inputs. To obtain excess (anthropogenic) Zn and Cu, we use metal/Al normalization to estimate natural backgrounds. Anthropogenic metal inputs are concentrated at the head of the estuary, and sediment inventories of excess Zn and Cu generally decrease seaward. Normalization of the excess Zn and excess Cu inventories to excess 210Pb inventories corrects the raw metal inventories for effects of lithology and sediment redistribution. The normalized excess Zn inventory decreases smoothly seaward, while the normalized excess Cu inventory shows an unexplained mid-estuary maximum. Low normalized inventories of anthropogenic metals at the estuary mouth indicate that little modern riverine sediment is stored there. Increasing K/Al (illite) and decreasing anthropogenic Zn and Cu in the lower estuary are thus both consistent with a predominantly marine sediment source. We conclude that landward transport of muds from Pamlico Sound has contributed significantly to the sediment balance of the lower Neuse estuary.

Sediment dispersal and accumulation on the continental margin of the Gulf of Lions: sedimentary budget

An integrated study of particle transport, deposition and storage was carried out over the continental margin of the Gulf of Lions (northwestern Mediterranean Sea). The dispersal system of fine-grained sediment is discussed and a sedimentary budget of sediment transfer and accumulation is established. Sediment cores were collected from the continental shelf and slope, along the ECOMARGE—I transect. Accumulation rates were measured using radiochemical techniques. Sediment traps were deployed, from 1985 to 1986, at two sites: on the continental shelf (27 m) and in the Lacaze-Duthiers Canyon (645 m). This study revealed that the most important sedimentary processes on this margin are high seasonal variability of lithogenic particle fluxes and lateral advection. The major fluctuations are related to storm events, to an increase in river discharge and the intensity of the regional current. Flux gradients increase exponentially toward the bottom, in shallow and deep water. However, in the deeper traps at the two sites annual lithogenic fluxes decrease seaward, from 25 to 3.5 g m −2 day −1. Across this margin, sediment accumulation rates ranged from 2.9 g m −2 day −1, on the middle of the prodeltaic area, to 0.16 g m −2 day −1 at 645 m in the canyon. Smectite was found to be a natural tracer for characterizing an annual cycle of sediment input, deposition, resuspension and shelf-slope exchange processes. At the shelf site, two intra-annual cycles of sedimentation and erosion of the surface sediments were recognized. Comparison between trap deposition rates and bottom sediment accumulation rates implies that the prodelta area acts as a zone of storage (11%) for the silty material and as a temporary sink for fine-grained sediment, that is eroded from the area in winter. In the canyon, variations in deposition rates, smectite content, and grain size are related to morphological features and the various modes of transport that control sedimentation. The discontinuity in the flux gradient between surface and deep layers, and the discrepancy between long-term accumulation rates based on 14C dating and trap deposition rates suggest that fluxes in the deeper layer in the canyon are in fact essentially controlled by settling from the intermediate nepheloid layers detached from the shelf break and by along-bottom transport; also that resuspension has a longterm impact on the entire canyon maintenance. On the other hand, comparing near-bottom trap 210Pb fluxes to the fluxes required to support the excess 210Pb inventories in the sediments leads to the conclusion that much (70%) of the particulate flux passes through the canyon to the deep sea on this short time scale (< 100 years).

210Pb scavenging in the North Atlantic and North Pacific Oceans

The radionuclide 210Pb shows significant geographic variations in the extent of its removal from the open ocean water column. This “texture of scavenging” is defined by mapping: (1) the integrated deficiency of 210Pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved 226Ra, and (2) inventories of excess 210Pb in deep-sea sediments. The ratio of 210Pb deficiency to its supply, termed the scavenging effectiveness, is ∼ 20% in the North Equatorial Pacific and ∼ 50% in the North Atlantic. This variation is related to the combined effects of uptake of 210Pb onto sinking particles and lateral transport of 210Pb to areas of more intense removal. Sediment inventories of excess 210Pb, normalized to the 210Pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the 210Pb removed from the water column is present in the underlying sediments but in the mid-latitude North Atlantic, the sediments comprise only about 50% of the 210Pb removed. The deficiencies of 210Pb in the mid-latitude North Atlantic sediments south of 50°N are qualitatively offset by surpluses in high-latitude sediments north of 50°N. Higher primary productivity and new production in the surface waters of the high-latitude North Atlantic and North Equatorial Pacific, relative to the oligotrophic central North Atlantic, may account for the greater fluxes of 210Pb to bottom sediments in those areas.

The behaviour of 210Pb and 226Ra in the eastern Irish Sea

The sedimentary inventory of excess 210Pb in the eastern Irish Sea is higher than can be supported by atmospheric deposition plus its in-situ production from 226Ra in the overlying water column. The depositional fluxes (30–500 Bq m −2 y −1) necessary to maintain the inventories increase towards the coast. Enhanced concentrations of dissolved 226Ra found in this area cannot account for the excess 210Pb production but evidence suggests that both these nuclides may have a common local source. Similarly, very high concentrations of unsupported 230Th are found in the surface sediments (1–60 Bq kg −1), the distribution of which mirrors that of 210Pb. The source of these natural radionuclides appears to be liquid waste discharged from a chemical factory involved in phosphate production. Annual variations of 226Ra and unsupported 210Pb concentrations in sediments over the period 1959–1986 were derived from cores, of known chronology, collected from a nearby harbour (Maryport). The temporal records of these nuclides correlate well with discharge data provided by the chemical plant.

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.

Correlation of 210Pb removal with organic carbon fluxes in the Pacific Ocean

The flux of particulate organic carbon reaching the deep sea is fundamental to our understanding of the global carbon cycle. Measurements of carbon accumulation in sediments do not quantify this flux because carbon is rapidly oxidized during settling and prior to burial. Using data from material collected by particle traps, we report here a strong correlation between the fraction of 210Pb removed from the ocean by particle settling and the flux of particulate organic carbon. The fraction of 210Pb removed from the water column may be determined by measurements of the inventory of excess 210Pb in bottom sediments as well as by measurements in sediment traps. Thus, the excess 210Pb inventory in deep-sea sediments is a proxy for the amount of particulate organic carbon delivered to the sediments over the past 100 years.

Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements

The effects of suspended-sediment concentration on the biological uptake of silica and the chemical behavior of particle-reactive elements are examined for the continental shelf waters near the mouth of the Amazon River. Dissolved silica and suspended-sediment concentrations measured during periods of high and low discharge indicate that uptake of dissolved silica begins where the suspended-sediment concentration falls below 10–20 mg l 1. Near the mouth of the Amazon River the 10 mg l 1 turbidity contour and the region of initial silica uptake both occur 20–40 km farther offshore during periods of high discharge (May and June) than during periods of low discharge (October and November). The location of initial silica uptake along an across-shelf transect near the river mouth varies by as much as 20 km on a time scale of days to weeks, which is comparable to the seasonal variation. Silica uptake, chlorophyll a concentration, and suspended-sediment concentration are poorly correlated along this transect (correlation coefficients range from +0.03 to −0.3). The poor correlations may be a result of grazing by zooplankton. Phaeopigment:chlorophyll a ratios increase offshore from 0 to 0.7, which supports the hypothesis that zooplankton grazing may be responsible for the low chlorophyll concentrations in the low-turbidity offshore waters exhibiting extensive silica removal. Excess 210Pb inventories for box cores collected from an across-shelf transect just south of the river mouth range from 21 to 124 dpm cm 2, which are 2–10 times greater than the inventory supported by atmospheric supply and in-situ production in the overlying water column. Riverine supply and lateral transport from offshore waters are the main sources of excess 210Pb to the Amazon continental shelf. 210Pb and 234Th data indicate that the high particulate flux from the Amazon River scavenges particle-reactive species not only from river water and continental shelf water but also scavenges these elements from offshore open-ocean water which flows onto the shelf.

239,240Pu and excess 210Pb inventories along the shelf and slope of the northeast U.S.A.

Sedimentary Pu and excess 210Pb inventory data are examined from a large number of sites along the U.S. North Atlantic shelf and slope. From these data, an estimate can be made of the magnitude and location of Pu and 210Pb accumulation in this region. A major feature of the data is a decrease in the Pu and 210Pb inventories with water depth, which appears to reflect a decrease in the net scavenging of these elements off-shore. When Pu and 210Pb inventories are summed over water depths less than 4000 m in the North Atlantic shelf and slope region, the sediments can account for 24 ± 8% of the expected Pu and 83 ± 15% of the expected 210Pb inputs. At water depths greater than 4000 m in this region, we find a greater deficiency in the measured inventories of both 210Pb and Pu, which cannot be accounted for in the shallower sediments. This suggests that the northwest Atlantic continental margin is not a major site of Pu or 210Pb removal and storage, when compared to atmospheric, in situ, and lateral inputs of these radionuclides.

Radionuclides as indicators of sedimentary processes in abyssal Caribbean sediments

Concentrations of 210Pb and 239,240Pu were measured in triplicate sub-cores from three different sedimentary environments of the Venezuela Basin. 239,240Pu profiles gave bioturbation rate coefficients from 500 to 900 cm 2 ka −1. Bioturbation rate coefficients determined from 239,240Pu distributions were not consistent with those derived from a two-layer mixing model applied to the 210Pb concentration data. A four-layer mixing model established that, at two of the sites, mixing was most rapid from 2 to 7 cm, indicating that macrofauna at these depths are more effective bioturbators. These findings are discussed with reference to the animals found at each site. Approximately 20% of the 239,240Pu that has fallen to the surface of Caribbean Sea is present in the sediments. Excess 210Pb inventories varied from 30 to 110% of the estimated 210Pb in the overlying water column. Sediments from the hemipelagic, turbidite, and pelagic carbonate sites contained 121, 108, and 40% of the estimated flux of 210Pb to the sea floor. Higher 210Pb inventories at the hemipelagic and turbidite sites could have resulted from the recent incorporation of shelf sediments rich in 210Pb. Alternatively, high inventories may indicate that 210Pb, produced in deep waters, is effectively scavenged by the cycling of reduced metals near the sediment—seawater interface. Iron and manganese are found in reduced forms near the sediment—seawater interface at these sites.

Plutonium and americium in anoxic marine sediments: Evidence against remobitization

239 + 240Pu activities of 100–450dpm/kg are found down to 15–18 cm in anoxic Saanich Inlet sediments, with a subsurface maximum in undisturbed deposits. Integrated 239 + 240Pu inventories which overlap delivery estimates are present both in two cores of anoxic sediments from Saanich Inlet and in one core of oxic sediments 65 km away in Dabob Bay, Washington. 241Am 239 + 240Pu ratios in Saanich Inlet sediments overlap ratios in unfractionated midnorthern latitude fallout, in oxic sediments from the Washington continental shelf, and in anoxic sediments from two basins off southern California and Mexico. The 239 + 240Pu 137Cs ratios in three intervals of Saanich Inlet sediments are also in agreement with ratios previously reported for oxic coastal marine sediments. The Pu inventories, the Am Pu and Pu Cs ratios, and the Saanich Inlet Dabob Bay comparison all argue that Pu is not rapidly remobilized in anoxic sediments. The subsurface 239 + 240Pu activity maximum is not in agreement with the historical record of peak Pu fallout in 1963–1964 unless our 210Pb-derived sedimentation rates are incorrectly high. However, they are in good agreement with previous 210Pb and varve chronologies in Saanich Inlet, and also give reasonable dates for times when 239 + 240Pu and SNAP-9A supplied 238Pu first appear in the sediments. We conclude they properly date the maximum in sedimentary 239 + 240Pu activity at 1970–1973, and seek explanations for the 7–10yr time lag after peak fallout. 239 + 240Pu inventories in one core from the eastern basin of the Cariaco Trench and in two cores from Golfo Dulce. an anoxic basin off the Pacific coast of Costa Rica, are also in reasonable agreement with fallout delivery to these latitudes when excess 210Pb inventories and fluxes are used to verify recovery of at least a major fraction of the most recently deposited sediments.

Geochemical Evidence for Modern Sediment Accumulation on the Continental Shelf off Southern New England

ABSTRACT An area of fine-grained sediment approximately 170 km 74 km in size, located in water depths between 60 m and 150 m, south of Martha's Vineyard, Mass., is a site of modern sediment deposition. The 14C ages systematically increase with sediment depth from about 1,300 years B.P. at the surface to 8,000-10,000 years B.P. at the depth of maximum core penetration. The old age for the surface sediments probably results from a combination of deposition of old carbon and faunal mixing. In the finest sediments, the sedimentation rates were approximately 130 cm/1,000 yrs when deposition began and have decreased to about 25 cm/1,000 yrs. The decreasing sedimentation rate reflects a diminishing source of fine sediments, which presuma ly came from the Georges Bank and Nantucket Shoals area. Inventories of excess 210Pb in undisturbed cores average 70 dpm/cm2 (disintegrations per minute per square centimeter), more than two times higher than the flux of 210Pb from the atmosphere and from 226Ra decay in the overlying water. This additional influx of 210Pb either must be with new fine-grained sediment material or from solutions that are stripped of their 210Pb by particulates in the bottom nepheloid layer. Stable Pb concentrations in surface sediments are about 28 ppm, as much as two times higher than concentrations at depth. The high accumulation rates, 210Pb inventories, and trace-metal profiles imply that this area is a modern sink for fine-grained sediments and for pollutants associated with particulate matter in the water column. To our knowledge, this is the only site of present-day natural deposition on the Continental Shelf off the eastern United States, exclusive of the Gulf of Maine. Because the net currents on the outer half of this Continental Shelf flow from northeast to southwest, this fine-grained deposit may receive its sediments and possible contaminants from the Nantucket Shoals and Georges Bank regions.

210Pb balance in Long Island Sound

A material balance is constructed for excess 210Pb (relative to 226Ra) as a test of the retentivity of Long Island Sound for a reactive heavy metal. Excess 210Pb is supplied to Long Island Sound chiefly by direct atmospheric deposition [1 ± 0.2(dis·min −1) cm −2· yr −1]. Rivers supply less than 20% of the atmospheric flux, and other inputs, from open ocean waters, 226Ra decay, groundwater seepage, and sewage discharge, appear to be negligible. The total input of excess 210Pb represents approximately the flux required to maintain the inventory of excess 210Pb measured in sediment cores from central Long Island Sound; that is, excess 210Pb is lost from Long Island Sound chiefly by radioactive decay. The retention of excess 210Pb within Long Island Sound is achieved in two steps: a rapid removal of soluble 210Pb onto suspended particles and the ongoing entrapment of particles in the basin by the residual bottom-water influx from the east.