Fine scale distributions of porosity and particulate excessPb, organic carbon and CaCO3 in surface sediments of the deep equatorial Pacific

Sediment samples were recovered from the central equatorial Pacific Ocean, sectioned at 1-mm intervals, and analyzed for porosity, organic carbon, excess²¹⁰Pb and CaCO₃. Steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface. Similarly, particulate organic carbon contents are highest at the sediment surface, decrease sharply in the upper 1 cm, and are relatively constant between 1 and 5 cm. CaCO₃ values, on the other hand, are lowest at the sediment surface and increase to a constant value below 5–10 mm depth. At the carbonate ooze sites, excess²¹⁰Pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates. However, extremely high excess²¹⁰Pb activities (> 100 dpm/g) are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing. This apparent contradiction along with the major features of the CaCO₃ and particulate organic carbon profiles can be explained by a particle-selective feeding mechanism in which organic carbon, excess²¹⁰Pb-enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms.     

Effects of biological sediment mixing on the210Pb chronology and trace metal distribution in a Long Island Sound sediment core

An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:²¹⁰Pb and²²⁶Ra;^{239, 240}Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess²¹⁰Pb (relative to²²⁶Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.^{239, 240}Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess²¹⁰Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess²¹⁰Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments.The bioturbation rate in the top several centimeters at this station has been determined previously using²³⁴Th (24-day half-life). The distribution of^{239, 240}Pu can be used to estimate a bioturbation rate for the underlying layer (to ～10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess²¹⁰Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.     

Sedimentary processes in the inner New York Bight: Evidence from excess210Pb and239,240Pu

Particle-reactive radionuclides were determined in sediments from the inner New York Bight to trace transport and storage of fine-grained sediments and associated reactive materials. Seven sediment ? cores 20–50 cm in length were analyzed for water content, loss on ignition (LOI) and excess²¹⁰Pb; three of these were also analyzed for^239,240Pu. Excepting some depth horizons in a core from a dredge-spoil dumpsite, every sample analyzed contained excess²¹⁰Pb. Variations in the concentration of excess²¹⁰Pb with depth in the sediment at all stations correlated strongly with LOI, which apparently traces that fraction of the sediment which is active in removing reactive elements from the water column. In the cores analyzed for^239,240Pu, every sample contained finite Pu, and Pu concentrations correlated strongly with excess²¹⁰Pb.The radionuclide distributions may be simply viewed as products of steady-state sediment accumulation or of mixing. Geochemically reasonable accumulation rates are very high (0.5–2.6 g/cm² y) and could probably only be sustained by offshore transport of dumped materials. At the other extreme the relationships between excess²¹⁰Pb and LOI are compatible with rapid mixing of a²¹⁰Pb carrier phase (traced by LOI) into the pre-existing substrate with little or no actual accumulation. Other non-steady-state processes, such as sediment gravity flow, could also explain the observed distributions.Measured sediment inventories (dpm/cm²) of excess²¹⁰Pb and Pu at these stations are greatly in excess of those supportable by direct atmospheric deposition: lateral supply is required. Incorporation of sedimentary fines into the sand substrate could make the inner New York Bight an important repository of reactive materials.     

Estimates of particle flux and reworking at the deep-sea floor using234Th/238U disequilibrium

Because of high specific activities of excess²³⁴Th (t_1/2 = 24.1 days) on suspended particles in the deep sea, this nuclide is potentially an extremely sensitive indicator of particle inputs and dynamics at the seafloor. Measurements were made at two deep-sea sites in order to examine this potential. Inventories of excess²³⁴Th at a low-current hemipelagic mud site (3990 m) in the Panama Basin were～ 1.5 (September, ′81) and～ 2.5 (June, ′82) dpm/cm². The steady state fluxes to the seafloor calculated from these inventories are in rough agreement with radionuclide fluxes measured in sediment traps. Small-scale (～ 100m) spatial variability in inventories implies biologically significant heterogeneity in particle inputs. Sediment from the continental rise site in the northwest Atlantic (2800 m), a site with higher current velocities than the Panama Basin, had an inventory of～ 1.9dpm/cm². This inventory is also in rough agreement with predictions made on the basis of nearby sediment trap data. Particle mixing coefficients of～ 30cm²/yr calculated at the Pacific and Atlantic sites are similar to those in shallow water deposits but could reflect disturbance during handling. Based on²¹⁰Pb data from the Panama Basin, sediment from below～ 6cm is mixed at a rate～ 10 × slower than the near-surface sediment to a depth of at least 20 cm. Agreement between²³⁴Th predicted mixing rates at the Panama Basin site with²¹⁰Pb profiles and in-situ experiments with glass bead tracers implies that these rates are real. Although the diffusion of dissolved²³⁴Th into deep-sea sediments complicates interpretations,²³⁴Th_xs distributions in bottom sediments offer a useful adjunct to sediment traps for investigation of particle dynamics near the deep-sea floor.     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: Evidence from7Be,210Pb,and239,240Pu

Cosmogenic⁷Be(t_1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess²¹⁰Pb and/or^239,240Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale.In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water)⁷Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10^?7 cm²/s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess²³⁴Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact.Measurements of total (wet + dry) atmospheric deposition of⁷Be in New Haven give an average flux of 0.07 dpm/cm² day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm² in a perfect collector. Sediment cores from Long Island Sound contain about half this⁷Be inventory, consistent with either a mean residence time for⁷Be in the water column of about one half-life or with post-depositional loss of⁷Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm², much nearer the atmospheric delivery. A higher inventory of⁷Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for⁷Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess²¹⁰Pb and^239,240Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.     

Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean

The distribution of “ash” (the non-combustible fraction of marine suspended matter) and concentrations of particulate Al, Ca, Fe, Cr, Ni, Cu, Sr and²³⁴Th in surface waters and of²¹⁰Pb,²³⁰Th and²³⁴Th in two vertical profiles (385–4400 m) of the Indian Ocean are reported.The ash concentrations in surface waters follow the primary productivity pattern, with higher abundances in samples south of 40°S and lower concentrations in the equatorial and subtropical regions. Opaline silica and CaCO₃ are the dominant components of the ash in samples from >40°S and from 7°N to 39°S, respectively. Aluminosilicates are only a minor constituent of the surface particulate matter. The metal/Al ratios in the surface particles are significantly higher compared to their corresponding crustal ratios for all the metals analyzed in this work. Comparison of enrichment factors between marine aerosols, plankton and surface oceanic particles, seem to indicate that this high metal/Al ratio in surface particles most likely arises from their involvement in marine biogeochemical cycles. Particulate²³⁴Th activity in surface waters parallels the ash abundance implying that its scavenging efficiency from surface waters depends on the particulate concentration.The particulate²³⁰Th and²¹⁰Pb concentration profiles increase monotonously with depth. It is difficult to ascribe this increase to a process other than the in-situ vertical scavenging of²³⁰Th and²¹⁰Pb from the water column by settling particles. The mean settling velocities of particles calculated from the particulate²³⁰Th data using a one-dimensional settling model is about 2 × 10^?3 cm/s. The settling velocity computed from the particulate²³⁰Th profiles does not appear to be compatible with the particulate²¹⁰Pb depth profiles; one possible explanation to account for the disparity would be that²³⁰Th and²¹⁰Pb are scavenged by different size populations of particles.On the whole, the geographic distribution of particulate matter, their composition and settling velocities in the Atlantic, Pacific and Indian Oceans are similar indicating that they are controlled by quite similar processes in the marine hydrosphere.     

210Pb/226Ra disequilibrium in the Santa Barbara Basin

The vertical distributions of²¹⁰Pb and²²⁶Ra in the Santa Barbara Basin have been measured. The²¹⁰Pb/²²⁶Ra activity ratio is close to unity in surface water, but ranges from 0.2 to 0.6 in deep water with a mean value of 0.3 (d > 250m), suggesting rapid removal of²¹⁰Pb from the water column. The²¹⁰Pb concentrations in the particulate phase at different water depths indicate that the removal of²¹⁰Pb is due to adsorption on settling particles.It is estimated that the particulate²¹⁰Pb contributes about 50–70% of the total²¹⁰Pb measured on unfiltered water samples of the Santa Barbara Basin. The fate of²¹⁰Pb (and Pb) in the water column is thus strongly controlled by the settling particles, which have a mean residence time of one year or less in the basin. Material balance calculation for²¹⁰Pb in the basin suggests that there is an external source supplying about 70–80% of the²¹⁰Pb observed in particulate material or sediments. This excess²¹⁰Pb is most likely provided by particles entering the basin loaded already with²¹⁰Pb.     

Pb and Th in settling particles in the western Northwest Pacific Ocean: Particle flux and scavenging

A mooring of three conical time-series sediment traps was deployed at two sites in the western Northwest Pacific Ocean for 9 months. Total mass fluxes and activities of ²¹⁰Pb and ²³⁰Th were determined for the settling particles to elucidate their scavenging and transport processes. Sediment samples also were analyzed for ²¹⁰Pb activities. Total mass fluxes, ²¹⁰Pb fluxes and ²³⁰Th fluxes showed large seasonal variations and their weighted mean fluxes tended to increase with depth, with an especially large increase near-bottom. The ratios of the observed ²¹⁰Pb fluxes to the ²¹⁰Pb deficiency fluxes in the upper traps at the two sites were only 0.02 and 0.12, and were attributable to advective export of ²¹⁰Pb from the surface waters. Those ratios in the near-bottom traps ranged between 1.22 and 2.63. This suggests that these high ratios are due to effective particle scavenging, large lateral ²¹⁰Pb import and input of resuspended particles that have not become incorporated into the sediments. The mean total ²³⁰Th fluxes at the near-bottom traps were 4.2–6.7 times higher than that expected from production in the overlying water column. The ²¹⁰Pb activities in the surficial sediments were much lower than those in the near-bottom traps. The ²¹⁰Pb accumulation rates estimated from the excess ²¹⁰Pb inventory in the sediment column were 40–70% higher than the mean ²¹⁰Pb fluxes at the near-bottom traps. The ratios of the ²¹⁰Pb accumulation rates to the ²¹⁰Pb deficiency fluxes at the near-bottom traps ranged between 2.0 and 3.7. The high fluxes of particulate ²¹⁰Pb and ²³⁰Th at the near-bottom traps reflected a combination of enhanced scavenging of dissolved nuclides and the lateral redistribution of particulate matter by downslope and alongshore transports. However, it was not possible to discriminate among the various processes contributing to high nuclide fluxes.     

Radioactivity in sediments of the Great Lakes: Post-depositional redistribution by deposit-feeding organisms

At two locations in southern Lake Huron (U.S.A.), twelve 35.5-cm diameter cores of fine-grained sediments were taken for comparison of the vertical distributions of²¹⁰Pb and falllout¹³⁷Cs with the distributions of benthic macroinvertebrates, mainly oligochaete worms (Tubificidae) and the amphipod,Pontoporeia affinis. Locations were selected on the basis of²¹⁰Pb distributions measured a year earlier which indicated contrasting depths of mixing of surface sediments. At one location the activity of²¹⁰Pb is uniform down to about 6 cm and 95% of total invertebrates occur within this zone; at the other location the zone of constant activity is only 3 cm deep but 90% of the invertebrates occur within it. Comparison of published tubificid defecation rates with sediment accumulation rates based on²¹⁰Pb shows that oligochaetes alone can account for mixing in one case while the effects of amphipods may be required in the case of shallower mixing. If mixing is represented as a diffusional process, eddy diffusion coefficients are at least 5.8 and 3.3 cm² yr^?1 at respective locations. In comparison with bioturbation, molecular diffusion is of minor importance in the post-depositional mobility of¹³⁷Cs. The necessity for introducing a diffusion coefficient varying continuously with depth is indicated by characteristics of the distribution of¹³⁷Cs. Biological reworking of near-surface sediments is an important process affecting radioactivity and chemical profiles in profundal deposits of this and probably other Great Lakes.     

Particulate and soluble210Pb activities in the deep sea

Particulate and soluble,²¹⁰Pb activities have been measured by filtration of large-volume water samples at two stations in the South Atlantic. Particulate phase²¹⁰Pb (caught by a 0.4-μm filter) varies from 0.3% of total²¹⁰Pb in equatorial surface water to 15% in the bottom water. The “absolute activity” of²¹⁰Pb per unit mass of particulate matter is about 10⁷ times the activity of soluble²¹⁰Pb per unit mass of water, but because the mass ratio of particulate matter to water is about 10^?8, the particulate phase carries only about 10% of the total activity. In Antarctic surface water the particulate phase carries 40% of the total²¹⁰Pb activity; the absolute activity of this material is about the same as in other water masses and the higher fraction is due to the much larger concentration of suspended matter in surface water in this region.In the equatorial Atlantic the particulate phase²¹⁰Pb activity increases with depth, by a factor of 40 from surface to bottom, and by a factor of 4 from the Antarctic Intermediate Water core to the Antarctic Bottom Water. This increase with depth is predicted by our previously proposed particulate scavenging model which indicated a scavenging residence time of 50 years for²¹⁰Pb in the deep sea. A scavenging experiment showed that red clay sediment removes all the²¹⁰Pb from seawater in less than a week. The Antarctic particulate profile shows little or no evidence of scavenging in this region, which may be due to the siliceous nature of the particulate phase in circumpolar waters. Our previous observation that the²¹⁰Pb/²²⁶Ra activity ratio is of the order of 0.5 in the deep water is further confirmed by the two South Atlantic profiles analyzed in the present work.     

Carbon and nitrogen cycling in the Zhubi coral reef lagoon of the South China Sea as revealed by Po and Pb

The radionuclides ²¹⁰Po and ²¹⁰Pb were examined to trace the cycling of particulate organic carbon (POC) and particulate organic nitrogen (PON) in the Zhubi coral reef lagoon. The net export flux of POC to the open sea is 14 mgC m⁻² d⁻¹. However, the net exchange of PON has not yet been observed. On average, the vertical export fluxes in the lagoon of POC and PON, as derived from ²¹⁰Po/²¹⁰Pb disequilibria, are 43 mgC m⁻² d⁻¹ and 13.8 mgN m⁻² d⁻¹, respectively. The deficit of ²¹⁰Po relative to ²¹⁰Pb in particulate matter provides evidence for the degradation of particulate organic matter. According to the mass balance budgets, 310 mgC m⁻² d⁻¹ and 121 mgN m⁻² d⁻¹ were recycled into dissolved fractions. Based on a first-order kinetics model, the degradation rate constants of POC and PON are 0.28 and 0.30 m⁻¹, respectively. Thus, ²¹⁰Po and ²¹⁰Pb can quantify the cycling of carbon and nitrogen in this coral lagoon.     

Migration of Lead-210 and Stable Load in Surface Waters Impacted by Uranium Mine Wastes

Water and sediment samples from two surface water systems were analysed for ²¹⁰Pb and stable lead in order to elucidate the environmental importance and fate of ²¹⁰Pb released from uranium mining. In one system mine drainage waters treated by precipitation of barium sulfate contained a considerable activity of ²¹⁰Pb in particulate form. Spoil piles were the main source of ²¹⁰Pb in the second system, with the dissolved ²¹⁰Pb being the predominant migration form. In both systems the concentration of ²¹⁰Pb rapidly decreased with distance from the source indicating that waterborne ²¹⁰Pb did not represent an environmental problem. Uranium mining did not add significant amounts of stable lead into the surface water systems. It has been found that bottom sediments are a sensitive indicator of the pollution of surface waters with ²¹⁰Pb.     

Distribution and source of heavy metals in the surface sediments from the near-shore area,north Jiangsu Province,China

Samples of surface sediment and vibrocore were collected in the near-shore area of north Jiangsu Province for grain size, elements, ²¹⁰Pb_excess and ¹³⁷Cs analyses. In our study area, the diversity of metal concentration was controlled not by the grain size, but by the source. The element content of the old Yellow River Delta was used as baseline for our study area. Geoaccumulation indexes indicate that no station was polluted by Cu, Pb, Zn and As, but the I_geo values of As were close to zero in some stations. Slight pollution caused by Cd was observed in some stations. Correlation and enrichment factors suggest that Cu, Pb and Zn are lithogenic in origin, while As and Cd are mixed in origin. Especially, in some polluted stations Cd was obviously anthropogenic in origin.     

Decadal record of sediment export to the deep sea via Eel Canyon

An active upper-canyon system, Eel Canyon, was studied to determine its role as a conduit and/or sink for terrigenous material over decadal timescales and to assess the sedimentary record preserved by transport processes. These data are used to (1) link seasonal fluctuations in sediment transport and deposition to preserved stratigraphic signatures, and (2) assess sediment storage and removal in the upper Eel Canyon (100–850 m water depth) over decadal timescales. Previous research has shown that upper thalwegs commonly experience gravity-driven flows during winter (November–March), due to increased sediment supply from Eel River flooding and intense storms that produce energetic wave/current conditions. Thick winter deposits composed of recently discharged fluvial sediment are formed in upper thalwegs, with distinct short- and long-lived radioisotopic and textural signatures (detectable ⁷Be and ²³⁴Th_xs, lowered ²¹⁰Pb activity, elevated clay content, and physical structures). Box and kasten cores were collected in the upper canyon (thalwegs and walls) to measure these signatures in recent and preserved winter deposits, and to calculate 100-yr accumulation rates. Non-bioturbated deposits (that have signatures indicative of rapid accretion by gravity-driven flows during the winter) are common in the upper canyon thalwegs. Short-lived radioisotopes (⁷Be and ²³⁴Th) show that sediment delivery to the upper thalweg varies temporally, sometimes beginning at the onset of river flooding, and at other times beginning during fall/early winter dry-storm events. In contrast, bioturbated deposits (which do not have signatures indicative of rapid deposition) are found on canyon walls.Non-bioturbated winter deposits are easily identified in the decadal record of thalwegs by decreases in ²¹⁰Pb activity and increases in clay content. Stacking of multiple years of winter deposits (∼10 cm preserved per winter) results in non-steady-state ²¹⁰Pb profiles and high decadal accumulation rates. However, down-core changes in ²¹⁰Pb profiles show that slope failures are actively redistributing these winter deposits. Partial or total removal of multiple winter deposits appears to happen periodically (every ∼13 yr), which will inhibit preservation of the longer decadal record. 100-yr accumulation rates were calculated in the thalwegs from the resulting ²¹⁰Pb profiles (i.e., the result of winter accretion and decadal removal by failures). Accumulation rates are much higher in thalwegs (1–6 cm/yr) than walls (0.1–0.8 cm/yr), which is likely the result of differing sediment delivery processes (via gravity-driven flows and nepheloid layers, respectively). At least 2.6±1.4% of the Eel River sediment budget is accumulating in the upper canyon over 100-yr timescales. However, this value greatly underestimates the total amount entering the canyon system because minimum accumulation rates were used in many areas (due to limited core length) and slope failures are moving sediment out of the budget area.     

210Pb in the Pacific: the GEOSECS measurements of particulate and dissolved concentrations

We report here on particulate and dissolved²¹⁰Pb profiles at 16 stations, and on total²¹⁰Pb profiles at 3 stations, all occupied during the Pacific GEOSECS expedition. Comparison with measurements at Yale on GEOSECS library samples indicates that during separation of particulate lead from dissolved lead, our filtered water samples suffered some loss of²¹⁰Pb in the filtration system; this effect appears to have reduced the dissolved²¹⁰Pb activities by ～ 20% in stations where the water was filtered. However, for these first Pacific data on the²¹⁰Pb distribution between the two phases, this effect does not significantly interfere with our recognition of the major features of both particulate and dissolved²¹⁰Pb distributions.The dissolved²¹⁰Pb profiles in general vary geographically, following the²²⁶Ra profiles. In deep water,²²⁶Ra increases northward and eastward from the southwest Pacific, from ～ 22dpm/100kg, to over 40 dpm/100 kg in the northeast Pacific. Our dissolved²¹⁰Pb profiles show a similar increase in deep water, varying from about 10 to 20 dpm/100 kg along this line, and are commonly characterized by a mid-depth maximum. This²¹⁰Pb maximum reflects the mid-depth²²⁶Ra maximum of the Pacific Deep Water observed along the western boundary current.In surface water at low latitudes there is a significant²¹⁰Pb flux from the atmosphere, which produces a²¹⁰Pb/²²⁶Ra activity ratio generally greater than unity. This flux penetrates as deep as 600 m, as indicated by an “induced”²¹⁰Pb minimum caused by the surface maximum. The surface water²¹⁰Pb excess decreases toward high southern latitudes and vanishes in the Circumpolar region.The particulate²¹⁰Pb profiles show a general increase with depth, from ～ 0.3dpm/100kg in subsurface water to ～ 1.5dpm/100kg in bottom water, with or without a mid-depth maximum that reflects the²²⁶Ra or dissolved²¹⁰Pb maximum. The particulate²¹⁰Pb normally comprises about 2% of the total²¹⁰Pb in subsurface water, and this fraction increases to about 10% near the bottom. As the filtration loss is not taken into account, the fraction of particulate²¹⁰Pb quoted here is an upper limit. Since the particulate matter concentrations are quite uniform in the water column below a few hundred meters, the²¹⁰Pb activity of the particulate matter also increases with depth. The particulate matter has a²¹⁰Pb concentration of ～ 100dpm/g in subsurface water, but the concentration increases to ～ 500dpm/g or more toward the bottom. This indicates that there is a cumulative adsorption of Pb onto the suspended particles as they are sinking through the water column.     

Chemical composition of short sediment cores from Thermaikos Gulf (Eastern Mediterranean): Sediment accumulation rates, trawling and winnowing effects

Four cores recovered within the framework of the INTERPOL Project have been analysed for their grain size and geochemistry; sediment accumulation rates (SARs) were also determined from ²¹⁰Pb and ¹³⁷Cs profiles. Two cores are representative of the Axios and Aliakmon Rivers depositional environment, whilst the third core represents the Pinios River province; the fourth core represents an environment of outer shelf relict sands. Apparent SARs ranged between 0.667 g cm⁻² yr⁻¹ (Axios and Aliakmon Rivers) and 0.414 g cm⁻² yr⁻¹ (Pinios River). Trawling activities and biomixing are critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess ²¹⁰Pb profiles. The thickness of the surface mixed layer was 4.5 cm in the vicinity of Axios and Aliakmon Rivers and in the area of Pinios River, 3.75 cm on the outer shelf and 1 cm in the area where no trawling was observed. Sediment accumulation appeared to be regulated by variations in the riverine discharge, shelf transport pathways and winnowing processes. Major element variations, such as Si, Al, Ti, V and Ni, were dominated by terrigenous supply as aluminosilicate minerals and quartz, whereas most Ca and Sr were biogenic. Si/Al and Ca/Al ratios have been used to express changes in sediment accumulation and winnowing. Redox processes were depicted by Mn, which showed an increase in the depth of its redoxcline, from 1 cm in inshore stations to 2 cm on the outer shelf. Si/Al ratios follow the Ca/Al ratios and can be used to assess percentage winnowing in the sediment. Increases in these ratios indicate a decrease in sediment input rates and are seen in the upper parts of most of the cores. Anthropogenic or ‘excess’ metal contents have been calculated from Zn/V and Pb/V ratios. Their distributions in the cores showed that by far the highest contamination is associated with the Axios River output, whilst sediments influenced by the Pinios River were relatively uncontaminated.     

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

²¹⁰Pb- and¹³⁷Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with²¹⁰Pb/²¹⁰Po and revealed sediment accumulation rates of (0.055±0.015) g·cm^?2·y^?1 and (0.09±0.03) g·cm^?2·y^?1, respectively. A comparable rate of (0.07±0.01) g·cm^?2·y^?1 has been obtained from¹³⁷Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm^?2·y^?1. Constant²¹⁰Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct¹³⁷Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of⁷Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant²¹⁰Pb activities. Remobilization may also be the reason for an incomplete²¹⁰Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the²¹⁰Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.     

Disequilibria betweenRa, Pb and Po in the Arctic Ocean and the implications for chemical modification of the Pacific water inflow

Measurements have been made of²²⁶Ra and both dissolved and particulate forms of²¹⁰Pb and²¹⁰Po in a vertical profile at 85°50′N, 108°50′W in the Arctic Ocean.In the upper water column²²⁶Ra shows a concentration maximum that is coincident with one in the nutrients, silicate, phosphate, and nitrate, while at the same depth, dissolved and particulate²¹⁰Pb and²¹⁰Po all show minimum concentrations. It is suggested that the concentration maxima are partly due to sources of the respective elements in the continental shelf sediments, the shelf waters being subsequently advected into the Arctic Ocean basins. The²¹⁰Pb and²¹⁰Po minima have similarly been explained by interaction between the shelf sediments and overlying waters. An estimate is made of the possible contributions of shelf sediments to the layer of silica-rich water which covers the Canada Basin at a depth of 100–150 m.Residence times have been calculated for dissolved²¹⁰Pb and²¹⁰Po at various depths in the water column. Surface water residence times of dissolved and particulate forms of these radionuclides are longer than in surface Atlantic waters, probably due to lower biological activity in the surface waters of the Canada Basin. An estimatee has been made of the average sinking velocity of particulate material.