James F. Todd

The distributions of uranium, radium and thorium isotopes in two anoxic fjords: Framvaren Fjord (Norway) and Saanich Inlet (British Columbia)

Abstract Depth profiles of the naturally-occurring radionuclides 238 U, 234 U, 226 Ra, 228 Ra and 228 Th were obtained in two diverse anoxic marine environments; the permanently anoxic Framvaren Fjord in southern Norway and the intermittently anoxic Saanich Inlet in British Columbia. Concentrations of total H 2 S were over three orders of magnitude greater in the anoxic bottom waters of Framvaren Fjord compared to those in Saanich Inlet. In Framvaren Fjord, the O 2 /H 2 S interface was located at 17 m. While dissolved 238 U behaved conservatively throughout the oxic and anoxic water columns, concentrations based on the 238 U/salinity ratio in oxic oceanic waters were almost 30% lower. Dissolved 226 Ra displayed a sharp maximum just below the O 2 /H 2 S interface, coinciding with dissolved Mn (II) and Fe (II) maxima in this zone. It is suggested that reductive dissolution of Fe-Mn oxyhydroxides remobilizes 226 Ra in this region. In Saanich Inlet, the O 2 /H 2 S interface was located at 175 m. Dissolved 238 U displayed a strongly nonconservative distribution. The depth profiles of dissolved 226 Ra and 228 Th correlated well with the distribution of dissolved Mn (II) in the suboxic waters above the O 2 /H 2 S interface, suggesting that reduction of particulate Mn regulates the behavior of 226 Ra and 228 Th in this region. Removal residence times for dissolved 228 Th in the surface oxic waters of both systems are longer than those generally reported for particle-reactive radionuclides in coastal marine environments. In the anoxic waters of Framvaren Fjord and Saanich Inlet, however, the dissolved 228 Th removal residence times are quite similar to values reported for dissolved 210 Pb in the anoxic waters of the Cariaco Trench and the Orca Basin. This implies that the geochemistries of Th and Pb may be similar in anoxic marine waters.

Isotopic fractionation of dissolved ammonium at the oxygen‐hydrogen sulfide interface in anoxic waters

The δ15N of dissolved ammonium was determined in three anoxic marine basins: Black Sea, Saanich Inlet, B.C., Canada, and Framvaren Fjord, Norway. In each basin, the δ15N-NH4+ was greatest near the 02/H2S interface, with δ15N as high as +21‰. The depth distributions of NH4+ and δ15N-NH4+ for Black Sea and Framvaren Fjord were examined with a one-dimensional, steady-state, vertical advection-diffusion model to calculate the isotope fractionation during the consumption of NH4+ by bacteria. Isotope enrichments, ϵ, for Black Sea were between 5 and 15 ‰, whereas in Framvaren Fjord ϵ ranged from 20 to 30 ‰. These differences are related mainly to the ambient concentration of NH4+. Biosynthetic uptake of NH4+ rather than nitrification was responsible for the fractionation. The δ15N-NH4+ in Saanich Inlet appears related to in-situ regeneration of NH4+ with little isotopic fractionation between dissolved and particulate nitrogen (PN).

A new method for the rapid measurement of 224Ra in natural waters

We report a new method for the measurement of 24Ra (T12 = 3.6 days) in natural waters. 224Ra preconcentrated from water samples by adsorption onto manganese-dioxide-impregnated acrylic fiber (Mn-fiber). The amount of 224Ra adsorbed on the fiber is determined by stripping its daughter, 220Rn, directly from the fiber bundle by circulating a stream of air through it and measuring the 220Rn activity concurrently. This method is simple, fast, sensitive, accurate and inexpensive. The counting system is portable, and is easily adapted for shipboard use in oceanographic investigations. The same equipment may also be used for measuring 228Th (T12 = 1.9 years) and 226Ra (T12 = 1600 years). 224Ra activities in water samples collected from continental shelf and estuarine waters of the eastern U.S. coast are presented.

Uranium biogeochemistry across the redox transition zone of a permanently stratified fjord: Framvaren, Norway

During August 1995, the vertical concentration profile of dissolved and particulate uranium exhibited strong non-conservative characteristics in the upper 30 m of Framvaren Fjord. There was a pronounced peak in both particulate (> 0.2 μm; 1.09 nM) and dissolved (< 0.2 μm; 17.06 nM) uranium in the finely stratified waters at the O 2 /H 2 S interface which is positioned well within the euphotic zone at about 20-21 m. Such concentration maxima at the redox boundary are also observed for dissolved organic carbon (DOC), Sr and Ba. Dissolved U levels seen in the water column from 18 m down to 30 m exceeded the high salinity (salinity = 35) U concentrations (13.63 ± 0.84 nM; Chen, J.H., Edwards, R.L., Wasserburg, G.L., 1986. 238 U, 234 U and 232 Th in seawater. Earth Planet Sci. Lett. 80, 241-251.) observed uniformly in the open ocean. A prolific population of S microbes (e.g., Chromatium, Chlorobium sp.) flourishes at the O 2 /H 2 S interface. The source of elevated U at the redox boundary must be due to microbial uptake and subsequent release processes rather than dilution from oceanic uranium. Uranium oxidation state determinations in waters from 1, 22 and 30 m depth reveal that reduced U(IV) is not present in significant abundance, and that the chemical and/or biological reduction of hexavalent uranium is largely inhibited. Our results suggest that U and other trace constituents such as DOC, Sr, Ba, Fe(II), Mn(II) are greatly modified by direct and indirect microbial transformation reactions which are most concentrated across the redox transition zone in Framvaren Fjord.

210Pb and 210Po, manganese and iron cycling across the O2/H2S interface of a permanently anoxic fjord : Framvaren, Norway

Abstract Vertical profiles of dissolved and particulate 210 Po and 210 Pb were measured across the redox transition zone at Station F1 in Framvaren Fjord, Norway. In this fjord, a sharp decrease in pH above the O 2 /H 2 S interface facilitates the aerobic dissolution of MnO 2 . In contrast, Fe(II) concentrations begin to increase only at the O 2 /H 2 S interface depth. Activity profiles reveal that dissolved 210 Po and 210 Pb are sequestered efficiently by particulates in surface waters. As polonium-210 and lead-210 activities descend down into the aerobic manganese reduction (AMR) zone, they are remobilized during the reductive dissolution of the carrier phase oxyhydroxides. Both 210 Po and 210 Pb are highly enriched at the O 2 /H 2 S interface where an active community of microbes, such as anoxygenic phototrophs (e.g., Chromatium , Chlorobium sp.), thrives. The coincident peaks in 210 Po , 210 Pb and microbial biomass suggest a strong biological influence on the behavior of these radionuclides. There is a strong covariance between the vertical distribution of Mn and Pb, indicating that their redox cycling is closely coupled and is likely microbially mediated.

Excess 222Rn above the Cleft segment of the Juan de Fuca Ridge

The distributions of excess 222Rn above the North Cleft segment of the Juan de Fuca Ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment. Based on the inventories of excess 222Rn, the hydrothermal heat flux was estimated to be 2.7±1.3 GW in 1990. The large uncertainty results from errors in calculating the standing crop of 222Rn, the difficulty in determining the 222Rn to heat ratio, and the problem of entrainment of low-temperature waters into the rising plume. The high-resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume, while to the west the plume appears to be younger. This plume age distribution could indicate a current flow to the northwest during the time this transect was sampled. The mean 222Rn/heat ratios for the high- and low-temperature vent fluids were 0.0017 disintegrations per minute (dpm) g−1 °C−1 and 0.0044 dpm g−1 °C−1, respectively. The extrapolated end-member concentrations of excess 222Rn in low-temperature vent fluids are a factor of 6 greater than the high-temperature fluids, based on the regressions of excess 222Rn activity and Mg concentration in North Cleft segment vent fluid samples.

A simplified method for 226Ra determinations in natural waters

Abstract We have developed a simple method for 226 Ra determinations in natural waters that employs the pre-concentration of water samples onto Mn-fiber and alpha scintillation counting techniques. By directly transferring a known fraction of ingrown 222 Rn from a sample equilibrator to a scintillation cell, we eliminate the use of 222 Rn extraction and transfer boards, and reduce the sample processing time from 35 to 5 min per analysis. We describe the new transfer technique and compare these results with the standard oceanographic method.

Cores drilled into active smokers on Juan de Fuca Ridge

The October 1991 Chronology expedition to the Endeavour Segment of the Juan de Fuca Ridge sought to exploit shipboard and land-based radioisotope analyses based on detailed geological observations and comprehensive sampling to determine the absolute ages of an actively venting ridge-axis hydrothermal field. Samples collected from a 1988 expedition indicated that some of the hydrothermal deposits in this area are unusually rich in barium (Ba) and radium (Ra) and thus extraordinarily amenable to radioisotopic techniques [Stakes and Moore, 1991]. An in situ gamma detector was developed to permit mapping of the distribution of radioactive components in the hydrothermal deposits and selection of subsamples that could easily be dated. To obtain samples that represent progressive mineral growth for the entire age of a chimney, we developed the HSTR drill (Figure 1), a diamond-bit drill to be operated by the submersible ALVIN.

Identification of rain‐freshened plumes in the coastal ocean using Ra isotopes and Si

Office of Global Programs, NOAA, Silver Spring, Maryland Abstract. Two transient plumes of freshened water in the South Atlantic Bight off the coast of South Carolina were measured and mapped during a continental shelf cruise in July 1994. Salinity in the plumes was depressed 1-2 parts per thousand compared to surrounding shelf waters. One plume was at least 160 kIn long, 20 kIn wide, and 5.7 m deep. The other was 75 km long, 25 kIn wide and 5-7 m deep. Both plumes disintegrated almost completely in 6 days due to mixing or advection away from the sampling area. Measurement and analysis of short-lived Ra isotopes and Si indicated that the plumes were not caused by river discharge but were rather due to an intense rain event a few days prior to the sampling cruise. With only salinity measurements we would have incorrectly concluded that the freshened water was a result of river plumes having entered the shelf. The rain was associated with the system that spawned tropical storm Alberto. We estimate that 220-330 mm of rain fell on the coastal ocean immediately before the cruise. Accompanying the rain were pesticides and nutrients. We estimate that the flux of nitrate from the rain was twice the riverine flux of nitrate to the study area in the month preceding the cruise. Plumes of freshened water in the coastal ocean may be derived from river runoff or heavy offshore rain events. Large rivers may cause ocean freshening at scales of over 1000 km (Atkinson and Wallace, 1975; Ortner et ai., 1995; Moore et ai., 1986). Distinguishing the source of the fresh water after the fact is difficult. Here we employ Si and Ra, tracers previously used to distinguish between Amazon-derived water and rainfall in the tropical Atlantic, to identify rainfall as the agent of a freshened plume off the coast of South Carolina. The Estuary-Shelf-Ocean Exchange experiment (ESOE) was conducted to establish techniques to determine the rate of cross-shelf exchange of estuarine and nearshore waters with waters of the middle and outer continental shelf. The experiment was conducted in the South Atlantic Bight (SAB), a misnamed region off the southeast U.S. coast between Cape Hatteras and Cape Canaveral, July 6-17, 1994, during a prolonged period of vertical shelf stratification. ESOE was designed to use the distributions of 223Ra (t 112 = 11.4d) and 224Ra (t 112 = 3.66d) as an index of the exchange time. Five shore-perpendicular transects between the Cape Fear River (CF) and the Savannah River (SV) were sampled from the coastline to the edge of the shelf. Two transects, Myrtle Beach (MB) and Charleston (CH), were resampled two more times, and the Winyah Bay transect (WB) was sampled three more times to determine if the tracer distributions were stable on a 2- week timescale. Figure 1 illustrates these transects and gives the station locations for the first pass. No single river dominates freshwater input to the South Atlantic Bight (SAB). The Savannah, Santee, and Pee Dee are the largest rivers; each discharged 100-500 m3 S.I during the

226Ra in the Black Sea and Sea of Marmara

Water column distributions of226Ra were determined at stations in the Sea of Marmara and the Black Sea as part of the 1988 Joint U.S.—Turkish Black Sea Expedition. Black Sea surface water226Ra concentrations were a factor of three to four lower than measurements made 20 years earlier. The most likely cause is increased removal of226Ra and Ba [35] due to increased surface biological activity; a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments. The amount of226Ra missing from the surface waters of the Black Sea over this period is accounted for in the high-porosity surficial “fluff” sediment layer. Throughout the Black Sea, depth profiles of226Ra exhibited pronounced maxima of approximately 25 dpm/100 L at aboutσθ = 16.2–16.3, in the vicinity of a bacterial maximum, but slightly shallower than the total dissolved Mn and Fe maxima (σθ = 16.4–16.5) reported by Lewis and Landing [38]. While the226Ra maximum may, in part, be linked to the cycling of Mn and Fe oxyhydroxides near theO2H2S interface, its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region. A simple steady-state two-☐ model has been used to obtain a semiquantitative understanding of the behavior of226Ra in the Black Sea. By incorporating reasonable estimates for the input and removal of226Ra in the Black Sea, an excellent agreement between predicted and observed (1988)226Ra concentrations was achieved. The model suggests that the dominant variables controlling the distribution of226Ra in the Black Sea are riverine input and cycling with Ba.