Michael P. Bacon

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.

Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria

Abstract The disequilibrium between the particle-reactive tracer 234 Th ( t 1 2 = 24.1 days) and its soluble parent, 238 U, was used to examine Th scavenging and export fluxes during the U.S. JGOFS North Atlantic Bloom Experiment (24 April–30 May 1989) at ∼47°N, 20°W. Four profiles of dissolved and particulate 234 Th in the upper 300 m and a non-steady box model were used to quantify dissolved 234 Th uptake and particle export rates. The highest export fluxes occured during the first half of May. From POC/ 234 Th and PON/ 234 Th ratios, particulate organic C and N fluxes were calculated. Results were 5–41 mmol C m −2 day −1 and 0.9–6.5 mmol N m −2 day −1 from the 0–35 m layer. The ratio of POC export flux to primary production ranged from 0.05 to 0.42, peaking in the first half of May. The estimated fluxes agree with the observed losses of total C and N from the upper ocean during the bloom, but yield significantly higher fluxes than were measured by floating traps at 150 and 300 m.

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.

Physical and biological controls on carbon cycling in the equatorial pacific.

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere and has been proposed to be a major site of organic carbon export to the deep sea. Study of the chemistry and biology of this area from 170� to 95�W suggests that variability of remote winds in the western Pacific and tropical instability waves are the major factors controlling chemical and biological variability. The reason is that most of the biological production is based on recycled nutrients; only a few of the nutrients transported to the surface by upwelling are taken up by photosynthesis. Biological cycling within the euphotic zone is efficient, and the export of carbon fixed by photosynthesis is small. The fluxes of carbon dioxide to the atmosphere and particulate organic carbon to the deep sea were about 0.3 gigatons per year, and the production of dissolved organic carbon was about three times as large. The data establish El Ni�o events as the main source of interannual variability.

230Th normalization: An essential tool for interpreting sedimentary fluxes during the late Quaternary

There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.

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.

Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230Th measurements

Abstract The cause of the climatically controlled fluctuations in the carbonate content of deep-sea sediments remains the subject of uncertainty and debate. Three variables are involved: supply of biogenic carbonate, loss by dissolution, and dilution by non-carbonate phases. It is suggested that 230 Th, which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale. Results of a preliminary analysis based on published data indicate that, for depths at and above the lysocline, the carbonate fluctuations observed in cores from the North Atlantic Ocean are due primarily to variations in the terrigenous clay input, which was 2–5 times higher during glacials than during interglacials. Carbonate deposition appears to have been somewhat reduced during glacials, but probably not by more than a factor of 2. From published 230 Th 232 Th profiles it appears that the South Atlantic Ocean also received increased inputs of terrigenous clay during glacial periods.

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.

Glacial/interglacial changes in sediment rain rate in the SW Indian Sector of subantarctic Waters as recorded by 230Th, 231Pa, U, and δ15N

High-resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high-sedimentation rate core (MD84-527: 43°50′S; 51°19;′E; 3269 m) by normalization to 230Th. This method estimates paleofluxes to the seafloor on a point-by-point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment δ15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on 14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate-induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa-231/Th-230 ratios, suggesting increased scavenging from the water column. Sediment δ15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply rates to surface waters.