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[en] The naturally-occurring particle-reactive radionuclides protactinium-231 (231Pa) and thorium-230 (230Th) are used as tracers of a variety of oceanic processes, both at present and in the past. Most notably, the sediment 231Pa/230Th ratio has been used to infer changes in the Atlantic Meridional Overturning Circulation over the last (de)glaciation. However, recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about 231Pa and 230Th behaviour in the ocean: (i) a sharp decrease in dissolved 231Pa and 230Th activities with depth below 2000-4000 m and (ii) very large particulate 231Pa and 230Th activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of particulate matter concentration and potential temperature showed that both features are associated with the benthic nepheloid layer (BNL) and the bottom mixed layer (BML) that are present at these stations. Here we develop and apply a simplified model of the exchange of particles, 231Pa, and 230Th between the BNL and the upper sediment, to explore the extent to which the radionuclide anomalies observed near the bottom at a number of GA03 stations can be explained by local sediment resuspension. We find that the model can broadly reproduce the observed anomalies at two stations where samples for radionuclide analyses were collected near the seafloor. Sensitivity tests with the model show that the 231Pa/230Th ratio of particles in the BML and the sediment varies by a factor of 3 as the sediment resuspension rate fluctuates within a range consistent with observational estimates. The modelled variability is comparable to the spatial variability of 231Pa/230Th of suspended particles in the modern North Atlantic and to the variability of Atlantic sediment 231Pa/230Th records across the last (de)glacial period. Two factors are found to contribute to the modelled sensitivity of the sediment 231Pa/230Th to sediment resuspension rate: the vertical turbulent mixing in the BML and the differential scavenging intensity of Pa and Th due to variation in particle concentration. Overall, our study indicates that the exchange of material between the BNL and the upper sediment can affect the particulate 231Pa/230Th ratio in the bottom water and the sediment, which may complicate the use of sediment 231Pa/230Th as a palaeoceanographic tracer.

[en] The total deposition fluxes of ²¹⁰Pb and ⁷Be were determined at New Haven, Connecticut, and Bermuda over approximately the same annual period in 1977-1978. The ²¹⁰Pb flux has remained virtually constant at New Haven from 1973 to 1978, the flux in the 1977--1978 period being 1.2 dpm/cm²/y. The ²¹⁰Pb flux at Bermuda is 0.69 dpm/cm²/y. This lower flux than expected from model calculations is due to the establishment of a blocking high pressure cell during the summer which deflects continental air. The ⁷Be fluxes at New Haven and Bermuda are 22.7 and 17.1 dpm/cm²/y, values consistent with western North Atlantic oceanic standing crop measurements, but higher than some other estimates. Where the difference cannot be attributed to differences in sampling it is ascribable to regional differences compatible with the oceanic data

[en] Petroleum residues (pelagic tar) have been reported from beaches all over the world since the 1960s, and have been quantitatively measured at a few locations. At the south-facing open beaches of Bermuda, rapid exchange of tar with the sea makes it possible to use the quantity of beach tar as a measure of open-ocean petroleum pollution. Brief surveys conducted as part of a summer course from 1989-96 are reported here and compared with previously published results. A correlation between beach tar on Bermuda and the estimated input to the North Atlantic from accidents and operations is presented. We recommend continued systematic monitoring of ocean-facing beaches for petroleum residues as a simple and inexpensive way of verifying changes in pollution input from shipping and the effectiveness of international laws such as MARPOL 73/78. (author)

[en] Heinrich event 1 (H1) is a climate event resulting from the release into the North Atlantic of a huge volume of sea ice and icebergs from the northern hemisphere ice sheets. We present here high-resolution diatom records from the Bermuda Rise (Sargasso Sea) and the Laurentian Fan (South of Newfoundland) to assess its impacts on North Atlantic surface circulation and its timing. The event is composed of three phases: the two first correspond to major pulses of Ice Rafted Debris (IRD) and the third phase relates to its immediate aftermath. At both sites, diatom abundances start to rise at 16.8 ka. This increase is marked by diatom species thriving in cold environments over the Laurentian Fan, while brackish and fresh water diatom species characterize this increase over the Bermuda Rise. This last record implies icebergs migration to subtropical latitudes and nutrient-rich meltwater to support such diatom productivity. During the second phase of the event, both sites record maximum diatom abundances, when sea ice conditions are dominant over the Laurentian Fan and the contribution of brackish-fresh water diatoms culminates over the Bermuda Rise. The persistence of lower salinity related diatom species over the Bermuda Rise suggests a continuous injection of cold fresh water by cold-core rings, as observed nowadays. The last phase witnesses the persistence of low salinity water over the Laurentian Fan, while a decrease towards the disappearance of diatoms in the sediment occurs over the Bermuda Rise. Regarding the relationship between IRD and diatom abundance, it appears that a critical amount of icebergs is necessary to stimulate diatom productivity. The disturbances induced by H1 appear to end ∼14.6 ka over the Bermuda Rise, while over the Laurentian Fan, the high diatom production persists until 14.1 ka and the salinity anomaly until 13.8 ka.

[en] Particle-reactive, naturally occurring radionuclides are useful tracers of the sinking flux of organic matter from the surface to the deep ocean. Since the Joint Global Ocean Flux Study (JGOFS) began in 1987, the disequilibrium between ²³⁴Th and its parent ²³⁸U has become widely used as a technique to measure particle export fluxes from surface ocean waters. Another radionuclide pair, ²¹⁰Po and ²¹⁰Pb, can be used for the same purpose but has not been as widely adopted due to difficulty with accurately constraining the ²¹⁰Po/²¹⁰Pb radiochemical balance in the ocean and because of the more time-consuming radiochemical procedures. Direct comparison of particle flux estimated in different ocean regions using these short-lived radionuclides is important in evaluating their utility and accuracy as tracers of particle flux. In this paper, we present paired ²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb data from oligotrophic surface waters of the subtropical Northwest Atlantic and discuss their advantages and limitations. Vertical profiles of total and particle size-fractionated ²¹⁰Po and ²³⁴Th activities, together with particulate organic carbon (POC) concentrations, were measured during three seasons at the Bermuda Atlantic Time-series Study (BATS) site. Both ²¹⁰Po and ²³⁴Th reasonably predict sinking POC flux caught in sediment traps, and each tracer provides unique information about the magnitude and efficiency of the ocean's biological pump.

[en] The land-sea contrast in turbulent surface fluxes on small flat islands offers a real-world scenario that regularly results in organised convection. This contrast results in a thermally driven circulation that tilts downwind of the island, leaving a warm plume in the wake of the island. Convection that forms in the warmed wake of the island is organised into a band aligned parallel to the flow. These thermally forced convective bands, or ‘cloud trails’, have been observed to extend more than 100 km downwind of islands in a wide range of environments. For example, Bermuda in the Subtropical Atlantic and Nauru in the Tropical Pacific. Cloud trails are found to be controlled by a combination of low-level humidity which controls the height of the lifting condensation level, and the land-sea contrast in turbulent surface fluxes which controls the strength of thermal circulation (i.e. lift) downwind of the island. Convective precipitation within a cloud trail, or in the background environment, results in cold pools that can act against the organisation due to the thermally forced island circulation. In the present study, idealised simulations at high (100 m) resolution are performed. Analysis aimed at a deeper exploration into the lifecycle of cloud trails and the processes involved in the initiation of convection and the maintenance of convective organisation are presented. (author)

[en] Continuous measurements of atmospheric ²²²Rn were made for the past three years by the Environmental Measurements Laboratory (EML) at the Atmosphere/Ocean Chemistry Experiment site located at Tudor Hill, Bermuda and at the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Mauna Loa Observatory. The ²²²Rn analyzer is based on the two filter tube method. At the Bermuda site, monthly median ²²²Rn concentrations range from ∼50 to 700 mBq m^-3, and the concentrations very seasonally, with a maximum in the winter and a minimum in the summer. The concentrations are significantly elevated when local winds traverse the island. At MLO, monthly median ²²²Rn range from ∼100 mBq m^-3 during the fall months to ∼400 mBq m^-3 during the spring months. The springtime maximum values correspond to periods of rapid transport in the free troposphere from the Asian continent, corroborated by backward air mass trajectories. The ²²²Rn data are also used to help decipher local daily upslope/downslope conditions present at MLO. This study illustrates the value of atmospheric ²²²Rn monitoring, when screened through the filter of local meteorology, in studying the transport of continental air to the oceans, as well as the dynamics of local meteorological effects. (author) 11 refs.; 10 figs

[en] Recent observations of abundant, nonliving, submicrometre particles in the upper ocean and new measurements of 'dissolved' organic carbon have fuelled speculation concerning the role of colloidal matter in ocean chemistry and biology. Colloids may act as reactive intermediates in the marine geochemistry of trace metals, and a biologically labile pool of colloidal matter would affect models of ocean carbon cycling. Here we report the use of naturally occurring ²³⁴Th as an in situ tracer to estimate the residence time of colloidal matter in the surface waters near Bermuda. The ²³⁴Th activity of colloidal matter (size range 10,000 nominal molecular weight to 0.2 μm) is similar to that of small particles (0.2-53 μm). Modelling of our results indicates a mean residence time of colloidal ²³⁴Th with respect to aggregation into small particles of 10 days, which is roughly the same as for small-particle ²³⁴Th, yet a factor of ∼6 less than for the dissolved pool. These results suggest that, more generally, macromolecular colloidal matter has a short residence time and hence a rapid turnover rate in the upper open ocean. (author)

[en] As part of an international measurement intercomparison of instruments used to measure atmospheric ²²²Rn, four participating laboratories made nearly simultaneous measurements of ²²²Rn activity concentration in commonly sampled, ambient air over approximately a 2 week period, and three of these four laboratories participated in the measurement comparison of 14 introduced samples with known, but undisclosed (blind) ²²²Rn activity concentration. The exercise was conducted in Bermuda in October 1991. The ²²²Rn activity concentrations in ambient Bermudian air over the course of the intercomparison ranged from a few hundredths of a Bq · m^-3 to about 2 Bq · m^-3, while the standardized sample additions covered a range from approximately 2.5 Bq · m^-3 to 35 Bq · m^-3. The overall uncertainty in the latter concentrations was in the general range of 10%, approximating a 3 standard deviation uncertainty interval. The results of the intercomparison indicated that two of the laboratories were within very good agreement with the standard additions, and almost within expected statistical variations. These same two laboratories, however, at lower ambient concentrations, exhibited a systematic difference with an averaged offset of roughly 0.3 Bq · m^-3. The third laboratory participating in the measurement of standardized sample additions was systematically low by about 65% to 70%, with respect to the standard addition which was also confirmed in their ambient air concentration measurements. The fourth laboratory, participating in only the ambient measurement part of the intercomparison, was also systematically low by at least 40% with respect to the first two laboratories