A.P. Fleer

Export flux of carbon at the equator during the EqPac time-series cruises estimated from 234Th measurements

Abstract Distributions of 234 Th were determined in three particle-size classes ( > 53, 1–53 and 0.7–1.0 μm) and in filtered seawater during each of the two time-series cruises of the U.S. JGOFS Process Study in the equatorial Pacific. Four vertical profiles were measured on the equator at 140°W from the sea surface to 400 m depth between 24 March and 9 April 1992 (Time-series I) and again between 3 and 18 October 1992 (Time-series II). In addition, both organic and inorganic carbon were measured in each of the particle fractions. The results were used with a one-dimensional model, which includes the equatorial upwelling, to estimate the flux of particulate carbon sinking out of the surface layer. The flux of particulate organic carbon (POC) at the base of the euphotic zone (0.1 % light level, 120 m depth) was estimated to average 1.9 mmol m −2 day −1 during El Nino (Time-series I) and 2.4 mmol m −2 day −1 during the cold period that followed (Time-series II). These values amount to only ∼ 2% of the primary production measured during each of the same periods and are insufficient to balance the new production, estimated previously to be ∼ 17% of primary production. These results are consistent with the hypothesis that the major part of the new production is removed from the region by advection in the form of dissolved organic matter. The POC flux profile indicates a net remineralization below the 1 % light level (80 m depth) such that the flux reaching 200 m depth has been reduced by ∼ 55%, giving a remineralization length scale of ∼ 155 m. For particulate inorganic (carbonate) carbon the flux at 200 m averaged 0.54 mmol m −2 day −1 during Time-series I and 0.71 mmol m −2 day −1 during Time-series II, very similar to the fluxes reported in deep sediment traps deployed at the same time. Estimates of the average large-particle sinking velocity give values −1 in the upper part of the euphotic zone, show a sharp increase near the base of the euphotic zone and level off to values of 30–60 m day −1 at 200 m depth.

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.

Fluxes of 230Th and 231Pa to the deep sea: implications for the interpretation of excess 230Th and 231Pa/230Th profiles in sediments

Abstract Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230 Th scavenged to the seafloor remains close to its production rate from the decay of 234 U in the overlying water column, and generally validates the use of 230 Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230 Th measured in the low flux regions amounts to 90±6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230 Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230 Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231 Pa and the 231 Pa/ 230 Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231 Pa fluxes and 231 Pa/ 230 Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231 Pa/ 230 Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231 Pa/ 230 Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231 Pa/ 230 Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231 Pa/ 230 Th is more difficult and requires a more quantitative understanding of the influence of particle composition.

Hydrothermal scavenging at the Mid-Atlantic Ridge: radionuclide distributions

We have carried out a study of U-series radionuclide distributions in the neutrally buoyant plume overlying the TAG hydrothermal vent field at 26°N, Mid-Atlantic Ridge. Limited uptake of U accompanies the formation of Fe-oxyhydroxide particulate material within the hydrothermal plume. The particulate U concentrations are too small for measurable dissolved U anomalies to be produced within the water column at plume height. However, such uptake, followed by deposition in the underlying sediments, could account for ∼ 10% of the U enrichment that has been reported previously for metalliferous ridge-crest sediments. Particulate 234Th, 230Th and 228Th activities in the TAG hydrothermal plume represent ≥65% of total Th activities typical of the deep ocean. Low dissolved Th activities coincide with the high particulate Th activities, indicating that hydrothermal scavenging strongly affects Th-isotope distributions in the water column close to hydrothermal vents. Much of the scavenging occurs during the initial formation of Fe-rich hydrothermal precipitates but some further scavenging continues as the material is dispersed, away from its source. Distributions of particulate 210Pb and 231Pa activities in the neutrally buoyant plume are also extremely high (7–12 dpm/102 l and 30–160 dpm/106 l, respectively). Like Th, 210Pb and231Pa show evidence for scavenging within the hydrothermal plume. Total Th activities at plume height are ∼ 50% higher than “background”. This “focusing” of Th into the plume is consistent with the entrainment and re-emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume.

Determination of natural 32P and 33P in rainwater, marine particles and plankton by low-level beta counting

Abstract Methods were developed for determining the natural levels of 32P and 33P in samples of rainwater, marine particulate matter and plankton. Preconcentration from rainwater consists of the extraction of radiophosphorus on alumina. Filtration of large volumes of seawater is required to obtain sufficient quantities of suspended particulate matter. The radiochemical scheme, which consists of a series of precipitations and a column separation, results in a pure source containing the two beta emitters. The activities of 32P and 33P in the source are determined separately by an external absorber method on low-level beta counters. The yields are monitored by the addition of stable phosphate in rainwater samples and by the natural levels of stable phosphate in marine particulate matter and plankton. The methods allow the specific activities and the activity ratio to be determined with a 10% precision.