Derek W. Spencer

210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter

Abstract The distribution of 210 Po and 210 Po in dissolved ( 0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported 210 Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved 210 Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in 210 Po, with 210 Po/ 210 Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for 210 Pb and a 0.6-year residence time for 210 Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m, 210 Po maxima occur and unsupported 210 Po is frequently present. Calculations indicate that at least 50% of the 210 Po removed from the mixed layer is recycled within the thermocline. Similar calculations for 210 Pb suggest much lower recycling efficiencies. Comparison of the 210 Pb distribution with the reported distribution of 226 Ra at nearby GEOSECS stations has confirmed the widespread existence of a 210 Pb/ 226 Ra disequilibrium in the deep sea. Vertical profiles of particulate 210 Pb were used to test the hypothesis that 210 Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate 210 Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred 210 Pb flux. It is proposed instead that an additional sink for 210 Pb in the deep sea must be sought. Estimates of the dissolved 210 Pb/ 226 Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of 210 Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3−6 × 10 7 cm 2 /sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater, 210 Po is deficient, on the average, by ca. 10% relative to 210 Pb; a corresponding enrichment is found in the particulate phase. Total inventories of 210 Pb and 210 Po over the entire water column, however, show no significant departure from secular equilibrium.

The distribution of particulate iodine in the Atlantic Ocean

The iodine content of marine suspended matter obtained from thirteen stations in the Atlantic between 75°N and 55°S has been measured. The concentration of particulate iodine is high in the surface, up to 127 ng/kg of seawater being observed. Below the euphotic zone, it drops sharply to 1–2 ng/kg. The iodine-containing particles are probably biogenic. A simple box-model calculation shows that only 3% of the particulate iodine produced in the surface water may reach the deep sea and that the residence time of these particles in the surface water is about 0.1 year.

Manganese in seawater and the marine manganese balance

Abstract Surface water samples from the Sargasso Sea, the western North Atlantic, and the northeast Pacific all have manganese concentrations of about 0.1 ppb. Sargasso Sea and northeast Pacific deep water samples have about one-fourth those concentrations; concentrations in the North Atlantic western boundary current appear intermediate. The data suggest that manganese is removed from seawater more rapidly than the general oceanic turnover time of about 1,500 years. Downward transport of manganese in downwelling surface water and falling biogenic debris can supply only a few per cent of the total amount accumulating in pelagic sediments, although it can supply all the manganese accumulating in ferromanganese crusts and nodules. A Sargasso Sea and a northeast Pacific profile show no indication of a deep water manganese maximum, suggesting that submarine volcanism and hydrothermal input is not a major source of manganese to either deep ocean water or normal pelagic sediments. Because the sedimentary manganese is supplied neither by dissolved manganese in seawater nor by hydrothermal inputs, we conclude that the ‘excess’ manganese in pelagic sediments comes from terrigenous particles.

An oceanic calcium problem

Abstract Recent published data on dissolved calcium in seawater reveal an apparent excess of calcium over that predicted from the changes in alkalinity. In the South Pacific this excess calcium is approximately 40 μmoles/kg. We suggest that this arises from an in-situ titration of some of the alkalinity by protons derived from the redox changes associated with oxidative decomposition of organic matter. This postulates an effective flux of nitric and phosphoric acids into the deep water. Other redox changes, such as in the oxidation of reduced sulfur, may also contribute protons, but these are more difficult to evaluate. This concept changes current thinking on the oceanic CO 2 -carbonate system. It increases the amount of calcium carbonate believed to have dissolved in the ocean by ca. 25%; and alters the proportions of abyssal CO 2 believed to be derived from respiration versus carbonate dissolution by about 10%.

Aspects of the distribution and trace element composition of suspended matter in the Black Sea

During cruise #49 of the R/V Atlantis II to the Black Sea in March and April of 1969, samples of suspended matter were collected by filtration of 8 to 101. water samples through 0·45 μ membrane filters. The distribution of the total suspended matter at this time was dominated by detrital silicate particles from the major rivers of Russia, Romania and Bulgaria but a significant influence from the Sea of Azov was detectable. The concentrations of Mn, Fe, Co, Hg, Sc, Zn, La and Sb in the suspended matter have been determined by instrumental neutron activation analysis. From these data it is apparent that the distribution of these elements in vertical profiles is influenced by four processes: 1. (1) The presence of detrital silicates. 2. (2) Precipitation as sulphides in the deep water. 3. (3) Coprecipitation with or adsorption by MnO2 that is precipitated just above the oxygen zero boundary. This is due to the upward flux of dissolved Mn(II) by advection and diffusion. 4. (4) Concentration by marine organisms in the surface waters. The profiles of scandium, lanthanum and iron are dominated by process (1) but processes (2), (3) and (4) also can be shown to be significant for iron. The manganese profile is dominated by process (3) and the zinc profile by process (2) while the profiles of cabalt, antimony and mercury are influenced by processes (1), (2) and (3).

Paleobiochemistry of molluscan shell proteins

Abstract The amino acid composition in calcified and uncalcified tissues of mollucs can be described by a limited number of independent factors which are related to both phylogeny and environment.

Lead-210, polonium-210, manganese and iron in the Cariaco Trench

Measurements of temperature, salinity, dissolved oxygen, sulfide, 226Ra, Mn, Fe, 210Pb, 210Po, and total suspended matter from three cruises of R.V. Atlantis II in the Cariaco Trench are reported. Distributions of dissolved and particulate Mn and Fe are similar to those for the Black Sea and reflect similar diffusion and redox-potential controls. Results for 210Pb and 226Ra in the anoxic deep water indicate a residence time for 210Pb in solution of less than two years. Particulate 210Pb results suggest that removal occurs largely by incorporation into metal sulfide phases forming in the water column. There is also evidence for precipitation of 210Pb with manganese oxide or iron hydroxide just above the O2-H2S interface. In the upper 300 m, 210Po is enriched by 55% over radioactive equilibrium with 210Pb. Possible sources of this excess include the bottom sediments and the water upwelling along the Venezuelan coast.

Deep Advective Transport of Lithogenic Particles in Panama Basin

Sediment traps were deployed at several depths between 660 and 3800 meters in the Panama Basin. The flux of lithogenic particles increased with increasing depth. This increase was due primarily to particles of beidellite (a smectite clay), which was identical to the clay occurring in bottom sediment on the continental slope to the west of the sediment trap mooring. The beidellite vertical flux at the Panama Basin station increased when an easterly current prevailed and decreased when the current reversed, indicating that a major portion of smectite was transported horizontally at mid-water depth to the mooring site from the nearby continental slope.

Some aspects of the distribution, chemistry, and mineralogy of suspended matter in the Gulf of Maine

Abstract Suspended matter in the Gulf of Maine decreases from the surface to about 50 m and then increases exponentially as the bottom is approached. Mineralogical and chemical data show that, from 50 m down, the suspended matter is dominantly layer silicates which are interpreted as resuspended bottom material. The chemical data show that in the upper 50 m particulate phosphorus, copper, and zinc occur principally in organisms. There is, however, little evidence that significant amounts of these elements are reaching the bottom in an organic particulate form, and most appear to be regenerated in the water column. The distributions of particulate Al and Fe are controlled largely by the suspended silicates, and there is no evidence that a free ferric hydroxide phase is important.

Lead-210 and polonium-210 in ocean water profiles of the continental shelf and slope south of New England

Concentration profiles of 210Pb and 210Po were measured on a traverse of the continental shelf and slope during the first phase of the SEEP (Shelf Edge Exchange Processes) program. Results from the upper 100 m show low concentrations of dissolved 210Pb over the shelf (<2dpm/100 kg) and increasing concentrations progressing into the slope water, where concentrations 10dpm/100 kg are found. This trend reflects the more efficient removal of 210Pb from the water column over the shelf. The deep waters over the slope show the commonly observed 210Pb deficiency, relative to 226Ra, due to removal by chemical scavenging. Vertical profiles indicate somewhat intensified scavenging of 210Pb near the sediment-water interface. Flux calculations for the shelf show an imbalance such that the input of atmospheric 210Pb must be compensated by an export of 210Pb off the shelf. This provides an important constraint on models that would predict the fate of materials introduced to the continental shelf. Over the slope, on the other hand, a good vertical flux balance is maintained, indicating that inputs of 210Pb to the slope water by cross-frontal particle transport or by exchange across the Gulf Stream may be unimportant. Results for 210Po in the surface waters show strong depletion, relative to 210Pb, in the dissolved form and enrichment in the particulate form, indicative of particulate transport out of the surface layer. Evidence for release of 210Po is seen in a sub-surface layer, centered at 50–100 m, where 210Po is enriched in the dissolved form and depleted in the particles.