Peter G. Brewer

Rare earth elements in the Pacific and Atlantic Oceans

Abstract The first profiles of Pr, Tb, Ho, Tm and Lu in the Pacific Ocean, as well as profiles of La, Ce, Nd, Sm, Eu, Gd and Yb, are reported. Concentrations of REE (except Ce) in the deep water are two to three times higher than those observed in the deep Atlantic Ocean. Surface water concentrations are typically lower than in the Atlantic Ocean, especially for the heavier elements Ho. Tm, Yb and Lu. Cerium is strongly depleted in the Pacific water column, but less so in the oxygen minimum zone. The distribution of the REE group is consistent with two simultaneous processes: 1. (1) cycling similar to that of opal and calcium carbonate 2. (2) adsorptive scavenging by settling particles and possibly by uptake at ocean boundaries. However, the first process can probably not be sustained by the low REE contents of shells, unless additional adsorption on surfaces is invoked. The second process, adsorptive scavenging, largely controls the oceanic distribution and typical seawater pattern of the rare earths.

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.

Removal of 230Th and 231Pa at ocean margins

Abstract Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO 2 adsorbers placed in line behind the filters in the in-situ pumping systems. Concentrations of dissolved 230 Th and 231 Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved 230 Th/ 231 Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate 230 Th/ 231 Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained 230 Th/ 231 Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent. Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for 231 Pa relative to 230 Th. Furthermore, the margins are sinks for 230 Th and, to a greater extent, 231 Pa transported by horizontal mixing from the open ocean.

Removal of230Th and231Pa from the open ocean

Concentrations of230Th and231Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S2), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D). Dissolved230Th/231Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported230Th/231Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the230Th produced by decay of234U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the231Pa produced by decay of235U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks.

Measurements of total carbon dioxide and alkalinity by potentiometric titration in the GEOSECS program

Abstract Approximately 6000 determinations of the alkalinity and total carbon dioxide content of seawater have now been made in the Atlantic, Pacific and Indian Oceans as part of the GEOSECS program by a computer-controlled potentiometric titration technique. The equations used to locate the equivalence points of the carbonic acid system on this titration curve were developed in 1971 but have not previously been published. These functions may be represented by: F 1 = ( V 2 − V ) V 0 N [ H + ] / K 1 C + ( V 0 + V ) V 0 ( [ H + ] + [ H S O 4 − ] + [ H F ] − [ B ( O H ) 4 − ] ) × ( 1 + [ H + ] / K 1 C ) F 2 = ( V 0 + V ) V 0 ( [ H + ] + [ H S O 4 − ] + [ H F ] − [ H C O 3 − ] ) Upon inspection, these functions are analogous to the modified Gran functions of Hansson and Jagner [25] with the omission of the contributions of [OH − ] and [CO 3 2− ], and with the contribution of B(OH) 4 − being assessed at a chlorinity of 19‰ for all samples. Reprocessing the original titration e.m.f.-volume data with appropriate corrections and modified Gran functions reveals an error of about +12 μmol/kg in the GEOSECS total carbon dioxide data. In addition, the protonation of dissolved phosphate species during the titration results in a contribution to measured total carbon dioxide equal to the total phosphate concentration. Differences in the application of the GEOSECS functions between the Atlantic and the Pacific-Indian Oceans expeditions are also to be found so that the error deriving from this source for the Atlantic expedition was only +5 μmol/kg. The application of the correct functions increases precision enabling smaller differences, such as those attributable to fossil fuel carbon dioxide, potentially to be observed, and increases accuracy so that the error in titrator total carbon dioxide previously diagnosed by Takahashi [14] can be logically accounted for.

Biological control of the removal of abiogenic particles from the surface ocean

Concurrent measurements of particle concentrations in the near-surface water and of particle fluxes in the deep water of the Sargasso Sea show a close coupling between the two for biogenic components. The concentrations of suspended matter appear to follow an annual cycle similar to that of primary production and deepwater particle flux. Although the concentration of particulate aluminum in the surface water appears to vary randomly with respect to that cycle, the removal of aluminum to deep water is intimately linked to the rapid downward transport of organic matter.

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.

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%.

Occurrence of methane in the near-surface waters of the western subtropical North-Atlantic

Abstract Methane observations from the western subtropical North Atlantic show maxima at about 100 m in a zone of sharp density contrast immediately above the regional salinity maximum. Open ocean surface waters have methane concentrations 48 to 67% higher than predicted from equilibrium with the atmosphere, while concentrations at the maxima reach 1.9 to 2.5 times the equilibrium solubility. A physical model shows that transport processes cannot supply sufficient methane from nearshore reducing environments to account for the observed mixed layer excesses in offshore waters. Instead, several observations suggest that in situ biological production in the oxygenated water column is important in determining the methane distribution. All known methane bacteria are obligate and strict anaerobes; thus some new mode of methane production must be postulated.

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).