Hugh D. Livingston

Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria

Abstract The disequilibrium between the particle-reactive tracer 234 Th ( t 1 2 = 24.1 days) and its soluble parent, 238 U, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April–30 May 1989) at ∼47°N, 20°W. Four profiles of dissolved and particulate 234 Th in the upper 300 m and a non-steady box model were used to quantify dissolved 234 Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/ 234 Th and PON/ 234 Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m −2 day −1 and 0.9–6.5 mmol N m −2 day −1 from the 0–35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.

Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions, largely from GEOSECS stations

Abstract From GEOSECS stations, largely, the 1974 distributions of Pu and of 137 Cs are described in the Pacific Ocean north of about 20°S latitude. Changes in some of these distributions are described from 1978 cruises by the authors. The Pacific exhibited, everywhere, a shallow subsurface layer of Pu-rich water with its concentration maximum at about 465 m in 1974; over a large portion of the central North Pacific a second layer of Pu-labelled water, less concentrated than the shallow layer, lay just above the bottom. Similar features were not observed in the case of 137 Cs. The inventories of both Pu and 137 Cs in the water column at most 1974 stations are substantially greater than those to be expected from world-wide fallout alone; these inventory excesses appear to be attributable to close-in fallout, but only if the ratio Pu/ 137 Cs in this source was much higher than in world-wide fallout. The North Pacific mean ratio of the inventories is 2.2 times that observed in world-wide fallout. Resolubilization of Pu both from sinking particles and from sediments explains peculiarities of its depth distributions. There is little evidence for tracer movement by sliding downward along density surfaces; 137 Cs appears to have moved to depth by downmixing at the edge of the Kuroshio, and then moved horizontally and upward alongσ t contours. The shallow Pu-rich layer shows no coordination with density, salinity or O 2 isopleths. The deep Pu-rich layer is restricted to a narrow range of O 2 concentrations that confirm its origin in the Aleutian Trench and rapid spread southward and laterally. Near-bottom circulation processes have been much more active than here-to-fore described.

A comparison of doses from 137Cs and 210Po in marine food: A major international study

Radioactivity levels of natural 210Po and anthropogenic 137Cs in sea water and biota (fish and shellfish) have been estimated for the FAO fishing areas on the basis of measurements carried out in recent years. Collective doses resulting from seafood consumption are calculated for each FAO area using radioactivity data for water and biota. Good agreement is observed between the results calculated by these two methods, with the exception of the doses from 210Po via shellfish consumption. The collective effective dose commitment from 137Cs in marine food in 1990 has been estimated at 160 man Sv with an uncertainty of 50%. The corresponding dose from 210Po is 30000 man Sv with an estimated uncertainty of a factor of 5. The results confirm that the dominant contribution to doses derives from natural 210Po in fish and shellfish and that the contribution from anthropogenic 137Cs (mainly originating from nuclear weapons tests) is negligible.

Determination of thorium isotopes in seawater by nondestructive and radiochemical procedures

Abstract Procedures have been developed for the analyses of dissolved and particulate 234Th, 228Th, 230Th and 232Th in seawater. Large volume samples (>1000 1) are collected using in situ pumps. Seawater is pumped sequentially through a filter cartridge and two MnO2 adsorbers for the collection of particulate and dissolved Th, respectively. Both filters adsorbers are analysed for 234Th using a simple gamma counting technique. This newly developed 234Th procedure can be conducted at sea, and thus provides an easy and efficient method for 234Th analyses on large volume samples. Subsequent radiochemical purification procedures and low-level alpha counting techniques are used in the laboratory for the analyses of 228Th, 230Th and 232Th on these same samples.

210Pb scavenging in the North Atlantic and North Pacific Oceans

The radionuclide210Pb 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 of210Pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved226Ra, and (2) inventories of excess210Pb in deep-sea sediments. The ratio of210Pb 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 of210Pb onto sinking particles and lateral transport of210Pb to areas of more intense removal. Sediment inventories of excess210Pb, normalized to the210Pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the210Pb 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 the210Pb removed. The deficiencies of210Pb 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 of210Pb to bottom sediments in those areas.

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides137Cs,239,240Pu and241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and241Am are supplied to the deep water by advection. The depth distributions of the naturally occurring radionuclides232Th,228Th and230Th reflect their sources to the oceans.232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of232Th decrease with depth to values ⩽ 0.01 dpm/1000 l.228Th shows high activities in near surface and near bottom water, due to the distribution of its parent,228Ra. Dissolved230Th, produced throughout the water column from234U decay, increases with depth to ∼ 3000 m. Values in the deep water (> 3000 m) are nearly constant (∼ 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and241Am data. The ratio of particulate to dissolved activity for both230Th and228Th is ∼ 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths228Th tends to have a higher particulate/dissolved ratio than230Th, suggesting uptake and release of230Th and228Th by different processes. This could occur if228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.230Th,241Am and239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with ∼ 15% of the230Th on particles compared with ∼ 7% for Am and ∼ 1% for Pu. Distribution coefficients (Kd) are 1.3–1.6 × 107 for Th, 5–6 × 106 for Am and 7–10 × 105 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show ∼ 85% oxidized (V + VI) Pu. However, theKd value for Pu may be an upper limit because Pu, like228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.

Pu and137Cs in coastal sediments

Abstract Analyses are presented of 137 Cs, 238 Pu, and 239,240 Pu, in relation to depth in sediment, in 21 gravity cores. These cores span the ranges of times 1964–1975, and of water depths 12–2000 m; they come from three distinct sedimentation areas off the northeast coast of the United States. Although the ranges of total sediment inventories of 239,240 Pu and of 137 Cs from the various areas hardly overlap, the range of ratios of the inventories of these two nuclides is probably the same in all the areas. In the shallow-water cores the 239,240 Pu/ 137 Cs ratio regularly diminishes with depth in the core, and a tendency is seen for curves of this function to have similar slopes in each area; ratios of 238 Pu/ 239,240 Pu show no change with depth in these shallow-water cores. In the deeper-water cores, the 239,240 Pu/ 137 Cs ratio shows no systematic change with depth, but sometimes the 238 Pu/ 239,240 Pu ratio shows a minimum at the sediment surface, and is much higher deeper in the cores. We believe that these phenomena can be explained in terms of a complicated bioturbational process moving the nuclides, together, down into the sediments, of chemical resolubilization, at depth, of plutonium only, and of its subsequent upward translocation in the interstitial solution. Some re-immobilization of plutonium near the sediment surface is implied, and a mechanism is suggested for this, based on displacement of plutonium from organic complexes by the increasing concentrations, in upper layers of the sediment, of re-oxidized dissolved iron.

Natural and anthropogenic radionuclide distributions in the Nansen Basin, Artic Ocean: Scavenging rates and circulation timescales

Abstract Determination of the naturally occurring radionuclides 232 Th, 230 Th, 228 Th and 210 Pb, and the anthropogenic radionuclides 241 Am, 239,240 Pu, 134 Cs and 137 Cs in water samples collected across the Nansen Basin from the Barents Sea slope to the Gakkel Ridge provides tracers with which to characterize both scavenging rates and circulation timescales in this portion of the Arctic Ocean. Large volume water samples (∼ 15001) were filtered in situ to separate particulate (> 0.5 μm) and dissolved Th isotopes and 241 Am. Thorium-230 displays increases in both particulate and dissolved activities with depth, with dissolved 230 Th greater and particulate 230 Th lower in the deep central Nansen Basin than at the Barents Sea slope. Dissolved 228 Th activities also are greater relative to 228 Ra, in the central basin. Residence times for Th relative to removal from solution onto particles are ∼1 year in surface water, ∼10 years in deep water adjacent to the Barents Sea slope, and ∼20 years in the Eurasian Basin Deep Water. Lead-210 in the central basin deep water also has a residence time of ∼20 years with respect to its removal from the water column. This texture of scavenging is reflected in distributions of the particle-reactive anthropogenic radionuclide 241 Am, which shows higher activities relative to Pu in the central Nansen Basin than at the Barents Sea slope. Distributions Of 137 Cs show more rapid mixing at the basin margins (Barents Sea slope in the south, Gakkel Ridge in the north) than in the basin interior. Cesium-137 is mixed throughout the water column adjacent to the Barents Sea slope and is present in low but detectable activities in the Eurasian Basin Deep Water in the central basin. At the time of sampling (1987) the surface water at all stations had been labeled with 134 Cs released in the 1986 accident at the Chernobyl nuclear power station. In the ∼1 year since the introduction of Chernobyl 134 Cs to the Nansen Basin, it had been mixed to depths of ∼800 m at the Barents Sea Slope and to ∼300 m in the central basin. “PreChernobyl” inventories of 137 Cs (as well as 239,240 Pu) are 10 times those expected from global atmospheric fallout from nuclear weapons testing and are derived principally from releases from the Sellafield, U.K., nuclear fuel reprocessing facility on the Irish Sea. Based on the sources Of 137 Cs to the Nansen Basin, mixing time scales are 9–18 years for the upper water column (to 1500 m) and ∼40 years for the deep water. These mixing time scales, combined with more rapid scavenging at the basin margin relative to the central basin, produce residence times of particle-reactive radionuclides in the Nansen Basin comparable to other open ocean areas (e.g. north-west Atlantic) despite the presence of permanent ice cover and long periods of low-light levels that limit productivity in the Arctic.

Thorium isotopes as indicators of particle dynamics in the upper ocean: results from the JGOFS North Atlantic Bloom experiment

Abstract Measurement of 234 Th and 228 Th in suspended and sinking particles made during the 1989 JGOFS North Atlantic Bloom Experiment permit estimation of the rates of particle cycling. Using a simple model of thorium-particle interactions applied to water column and floating sediment trap data at 150 and 300 m, the rate constant, β 2 , for aggregation of small suspended particles into large rapidly sinking (∼150 m day −1 ) particles increases from ∼0 to ∼30 y −1 over the course of the bloom. The rate constant for disaggregation of sinking particles, β −2 , similarly increases from ∼100 to ∼300 y −1 over the same period. These suggest that small particle residence times (relative to packaging or aggregation) decreasesto ∼15 days and that large particle residence times (relative to disaggregation) decrease to ∼1 day as the bloom progresses. Late in the bloom, particles are cycled such that aggregation of suspended particles (∼2 μ g 1 −1 day − ) is comparable to particle break-up (∼3 μg 1 −1 day − ). Errors on the rate constants, calculated by propagating estimated errors on the individual terms in the model, are large and arise principally from uncertainty in the gradient in activity and mass fluxes between the two trap depths. However, the values calculated independently from the two tracers ( 234 Th and 238 Th) generally agree to within 30%. The 234 Th balance for the upper water column ( Buesseler et al., Deep-Sea Research , 39 , 1115–1137, 1992) suggests that a substantial portion of the thorium and mass flux is not recorded by the traps. If it is assumed that this flux is carried on more slowly sinking particles (∼50 m day −1 ) that are not trapped efficiently, and these particles directly interact with the suspended particles pool in the same fashion as the trapped sinking particles, calculation of aggregation and disaggregation rate constants late in the bloom shows a higher value for β 2 but a comparable value for β −2 relative to the values determined for the trapped particles. This suggests that the slowly sinking material (e.g. marine snow) is more effective at aggregating small, suspended particles than are the rapidly sinking particles. Temporal increases in β 2 and β −2 for the trapped particles are matched by increases in the rate constants for decomposition of particulate organic carbon and nitrogen (2–35 y −1 for C; 4–40 y −1 for N), suggesting that increases in microbial activity are directly reflected in rates of particle aggregation and disaggregation.

Strontium and uranium concentrations in aragonite precipitated by some modern corals

Abstract Ahermatypic corals, and hermatypic corals without zooxanthellae, have Sr and U concentrations that are greater than those of reef corals containing zooxanthellae. The Sr and U concentrations in the corals are independent of water temperature, in that respect differing from those in aragonite experimentally precipitated from sea-water in the laboratory. The deposition of Sr and U in scleractinian coral aragonite may be related to coral metabolic processes and to the influence of algal photosynthesis on calcification rates.