Robert M. Sherrell

Precise determination of element/calcium ratios in calcareous samples using sector field inductively coupled plasma mass spectrometry

A new method was developed for rapid and precise simultaneous determination of Mg/Ca, Sr/Ca, Mn/Ca, Cd/Ca, Ba/Ca and U/Ca ratios in foraminiferal shells using sector field inductively coupled plasma mass spectrometry (ICPMS). Element/calcium ratios were determined directly from intensity ratios using external, matrix-matched standard to correct for instrumental mass discrimination. Because of large differences in the abundance of chemical constituents of the foraminiferal shell, major elemental ratios were determined in analog mode (using (24)Mg, (43)Ca, (44)Ca, (55)Mn, and (88)Sr) whereas trace elemental ratios were determined in pulse-counting mode (using (111)Cd, (138)Ba, (238)U, and the low-abundance (46)Ca isotope). Matrix-induced variations in mass discrimination over a calcium concentration range of 2.0-24.5 mM were observed only for Mg/Ca and Cd/Ca ratios. However, these effects are negligible if the samples and standard calcium concentration are within a factor of 2-3. Multiratio method reproducibility was better than previously reported for other ICPMS methods yielding precision (1σ) of Sr/Ca = 0.45%; Mg/Ca = 0.45%, Mn/Ca = 0.8%, Cd/Ca = 1.7%, Ba/Ca = 0.7%, and U/Ca = 1.4% for foraminifera samples as small as 25 μg. Using this approach for a single-ratio analysis, Sr/Ca ratios were determined with precision of 0.06% (1σ) on carbonate samples as small as a single foraminifera shell (<10 μg). The new method is more sensitive, more precise, and simpler to use than previously available ICPMS techniques. It provides an efficient tool for simultaneous determination of several elemental ratios of paleoceanographic interest in a single foraminiferal sample, thereby reducing overall sample size requirement and analysis time.

Controls on tropical Pacific Ocean productivity revealed through nutrient stress diagnostics

In situ enrichment experiments have shown that the growth of bloom-forming diatoms in the major high-nitrate low-chlorophyll (HNLC) regions of the worlds oceans is limited by the availability of iron. Yet even the largest of these manipulative experiments represents only a small fraction of an ocean basin, and the responses observed are strongly influenced by the proliferation of rare species rather than the growth of naturally dominant populations. Here we link unique fluorescence attributes of phytoplankton to specific physiological responses to nutrient stress, and use these relationships to evaluate the factors that constrain phytoplankton growth in the tropical Pacific Ocean on an unprecedented spatial scale. On the basis of fluorescence measurements taken over 12 years, we delineate three major ecophysiological regimes in this region. We find that iron has a key function in regulating phytoplankton growth in both HNLC and oligotrophic waters near the Equator and further south, whereas nitrogen and zooplankton grazing are the primary factors that regulate biomass production in the north. Application of our findings to the interpretation of satellite chlorophyll fields shows that productivity in the tropical Pacific basin may be 1.2–2.5 Pg C yr-1 lower than previous estimates have suggested, a difference that is comparable to the global change in ocean production that accompanied the largest El Niño to La Niña transition on record.

Modulation of cadmium uptake in phytoplankton by seawater CO2 concentration

The vertical distribution of cadmium in the ocean is characteristic of an algal nutrient, although an underlying physiological basis remains undiscovered. The strong correlation between dissolved cadmium and phosphorus concentrations in sea water has nevertheless been exploited for reconstructing past nutrient distributions in the ocean. In culture experiments, the addition of cadmium accelerates the growth of some marine phytoplankton and increases the activity of carbonic anhydrase, normally a zinc-based metalloenzyme that is involved in inorganic carbon acquisition. Here we show that the concentration of a Cd-carbonic-anhydrase—distinct from Zn-carbonic-anhydrases—in a marine diatom is regulated by the CO2 partial pressure (pCO2) as well as by the zinc concentration. Field studies in intensely productive coastal waters off central California demonstrate that cadmium content in natural phytoplankton populations similarly increases as surface-water pCO2 decreases. Incubation experiments confirm that cadmium uptake by natural phytoplankton is inversely related to seawater pCO2 and zinc concentration. We thus propose that biological removal of cadmium from ocean surface waters is related to its utilization in carbonic anhydrase, and is regulated by dissolved CO2 and zinc concentrations. The dissolved seawater Cd/P ratio would therefore vary with atmospheric pCO2, complicating the use of cadmium as a tracer of past nutrient concentrations in the upper ocean.

True fractional calcium absorption is decreased after Roux-en-Y gastric bypass surgery.

Objective: Roux‐en‐Y gastric bypass (RYGB) is considered to be the gold standard alternative treatment for severe obesity. Weight loss after RYGB results primarily from decreased food intake. Inadequate calcium (Ca) intake and metabolic bone disease can occur after gastric bypass. To our knowledge, whether malabsorption of Ca contributes to an altered Ca metabolism in the RYGB patient has not been addressed previously.

Dissolved and particulate Fe in a hydrothermal plume at 9°45′N, East Pacific Rise:: Slow Fe (II) oxidation kinetics in Pacific plumes

Production of Fe(III) particles in hydrothermal plumes is of fundamental importance to the long-term effect of hydrothermal circulation on seawater composition. To elucidate the fundamental controls on Fe redox kinetics and solution/particle partitioning in neutrally buoyant plumes, we sampled near-field (<3 km) plume particles at 9°45′N on the East Pacific Rise in 1996, returning in 1997 to sample both particulate and dissolved phases (0.40 μm filter). Concentrations of dissolved Fe varied from 320 to 20 nM in proximal (<0.3 km from vent site) to distal samples (1–3 km downfield), constituting ∼85–50% of total Fe, respectively. Based on vent fluid dilution factors calculated from dissolved Mn, a mass balance for vent fluid Fe at this site indicates that ∼65% of Fe is lost to particulate sulfide settling in the buoyant plume, and that particulate Fe in distal (1–3 km) samples is twice as concentrated as predicted from dilution of particles in proximal plume water. These observations are consistent with a calculated Fe(II) oxidation half-time of 3.3 h, long enough that Fe(III) colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions, preventing generation of high particulate Fe concentrations (11–56 nM observed). A general investigation of Fe(II) oxidation rates in plumes worldwide gives Fe(II) oxidation half-lives as short as 17 min at some Atlantic sites, and as long as 6 h at some Pacific sites. The calculations indicate that the distribution of Fe particles in plumes depends chiefly on inter-basin differences in ambient deep water chemistry (primarily pH and dissolved O2) and on local currents driving plume dilution, and to a much lesser extent on variations in primary vent fluid composition. Long-term changes in thermohaline circulation or ocean biogeochemistry may therefore alter Fe dynamics and minor element fluxes associated with global hydrothermal activity, independent of variations in crustal production rates.

Uptake and fractionation of rare earth elements on hydrothermal plume particles at 9°45′N, East Pacific Rise

Abstract Particulate samples (>0.45 μm) from a neutrally buoyant hydrothermal plume at 9°45′N on the northern East Pacific Rise were collected using large volume in situ filtration and analyzed for Fe, Al, Mn, Ni, and fourteen rare earth elements (REE). The Sm/Fe ratio (a proxy for overall REE/Fe) and Nd/Er (light/heavy REE fractionation) increased moderately with decreasing particulate Fe. Chemically, the sense of these relationships matched that documented previously in the TAG plume on the Mid-Atlantic Ridge (German et al., 1990) , although particulate Fe was about 10 fold lower at 9°45′N. Spatial trends relative to the vent source, however, were opposite of expectation because slow Fe(II) oxidation and Fe(III) colloid aggregation over this interval led to increased particulate Fe (10–26 nM) with distance from source (Field and Sherrell, submitted) . After subtraction of non-plume background particle composition, plume particles at 9°45′N and TAG had indistinguishable ranges of light REE-enriched fractionation relative to ambient seawater and had very similar Sm/Fe (therefore Kd for Fe oxyhydroxides), demonstrating that plume particles in both oceans reflect to a first degree the local seawater REE composition. Within-plume REE variations at 9°45′N were investigated using a simple mixing model which accounts for the bulk Fe-Al-Mn variations in the plume using two endmembers: fresh hydrothermal oxyhydroxide precipitates and ridge-crest background particles (composed largely of locally resuspended sediment). Sm/Fe and Nd/Er plot linearly with mixing ratio (R > 0.96), implying that the observed REE trends result from mixing of these two endmembers. Extrapolation to the composition of pure hydrothermal precipitates suggests that Nd/Er is fractionated relative to seawater by a factor of 1.8 during adsorption onto fresh Fe oxyhydroxide particles. The ridge-crest background particles are 5 fold higher in Sm/Fe and Nd/Er is 2.49 relative to seawater, partly a result of enriched terrigenous component in the resuspended matter. A reinterpretation of REE at TAG reveals that positive curvature in REE vs. Fe plots, argued previously to reflect continuous REE uptake (i.e., increasing Kd; German et al., 1990 ), may result from local depletion of the dissolved REE pool by partitioning onto Fe particles at Fe > 100 nM. Similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater-source particle-reactive species in plumes that are sampled at high particulate Fe concentration. In sum, REE behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types, than with kinetic uptake control. Precise measurements of REEs in modern ridge-crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long-term changes in REE of the hydrothermal component.

Direct determination of 10 trace metals in 50 µL samples of coastal seawater using desolvating micronebulization sector field ICP-MS

Understanding the trace metal marine geochemistry of temporally variable coastal systems requires intensive sampling programs with attendant analytical burdens. Most established techniques for multi-element trace metal determinations are slow, require a skilled chemist, and are not easily automated. Advances in sample introduction systems and ICP-MS instrumentation now provide marine chemists with the sensitivity and mass resolution necessary to determine many trace metals at natural concentrations in coastal seawater. A new method has been developed for the rapid (10 samples h –1 ) determination of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in diluted seawater, requiring just 50 µL of seawater and no reagents other than pure nitric acid. A sensitivity of 800 000-1 200 000 cps ppb –1 86 Sr in a 10% sea water matrix is obtained when microconcentric desolvating nebulization is combined with a shielded torch and hot plasma high resolution ICP-MS. Analyses are standardized by a matrix-matched external calibration curve with variations in sensitivity corrected by normalizing to the natural internal standard Sr, a conservative ion in seawater. The method thus depends on mass bias stability for each analyte relative to Sr, which was examined as a function of forward power and matrix and found to be optimized at 1100-1350 W. Precision and accuracy are limited by appropriate correction for blanks, which derive mainly from the ICP-MS introduction system, and are equivalent to about 10% of typical coastal seawater concentrations for these metals. Preliminary evaluation of a new low-flow nebulizer (µFlow, Elemental Scientific, Omaha, NE, USA) suggested lower blanks and compatibility with solutions high in total dissolved solids compared with standard microconcentric designs. Determination of dissolved concentrations in reference seawater (CASS-3) demonstrate very good agreement with certified values (within 95% confidence limit) and a precision of 3-12% (1σ) for all elements except Cr (15%). The utility of the method is demonstrated by the determination of spatial trends for these metals in a transect of seawater samples from shelf waters off southern New Jersey, USA. The new technique is sufficiently sensitive to determine some of these metals in open ocean seawater and, with minor modifications, should be applicable to a larger suite of analytes in a wide variety of natural waters.

Techniques for determination of trace metals in small samples of size-fractionated particulate matter: phytoplankton metals off central California

The oceanic biogeochemical cycles of many trace elements are dominated by their association with the growth, death, consumption and sinking of phytoplankton. The trace element content of marine phytoplankton reflects nutritional status, species composition, surface area to volume ratios, and interactions with bioactive and toxic elements in the ambient seawater. Despite the ecological and environmental importance of trace element assimilation by autotrophs, there are few modern measurements of trace elements in phytoplankton assemblages from the natural environment. Here we introduce a new method for collection and analysis of size-fractionated particulate samples from practical seawater volumes. We pay particular attention to accurate determination of trace element filter blanks which are typically the limiting factor for analysis of such samples. Metals were determined at very low detection limits by high resolution inductively coupled plasma mass . spectrometry HR-ICP-MS for 11 elements Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, U, Zn and P, which is used as a biomass . . . normalizer in three types of polymer filters 0.45, 5.0, and 53 mm pore size and a quartz fiber filter 0.8 mm pore size . To place these new determinations in a practical context, results are presented for a vertical profile of samples filtered from 1-4 . l of coastal seawater 0.3-1.0 mg total solid dry weight at a station off central California. The results demonstrate that the blanks of the evaluated filter types, precleaned appropriately, are sufficiently low to allow accurate determination of the trace . metal content of three size-classes of phytoplankton. At the Pacific station, measured phytoplankton Zn content as Zn rP agrees with values predicted from single-species culture studies growing at seawater Zn concentrations expected for coastal waters. The new method has utility as a generally applicable and simple size fractionation technique, and allows determination of natural and pollutant elements in small samples of phytoplankton and particles in coastal, estuarine and offshore marine regimes. q 1999 Elsevier Science B.V. All rights reserved.

Increasing stoichiometric imbalance in North America's largest lake: Nitrification in Lake Superior

more than 600 times the mean requirement ratio for primary producers. We examine the rate of [NO3 ] increase relative to budgets for NO3 and fixed N. Nitrate in Lake Superior has continued to rise since 1980, though possibly at a reduced rate. We constructed whole-lake NO3 and N budgets and found that NO3 must be generated in the lake at significant rates. Stable O isotope results indicate that most NO3 in the lake originated by in-lake oxidation. Nitrate in the lake is responding not just to NO3 loading but also to oxidation of reduced forms of nitrogen delivered to the lake. The increasing [NO 3 ]:[PO4 ]s toichiometric imbalance in this large lake is largely determined by these in-situ processes. Citation: Sterner, R. W., E. Anagnostou, S. Brovold, G. S. Bullerjahn, J. C. Finlay, S. Kumar, R. M. L. McKay, and R. M. Sherrell (2007), Increasing stoichiometric imbalance in North America’s largest lake: Nitrification in Lake Superior, Geophys. Res. Lett., 34, L10406, doi:10.1029/ 2006GL028861.

Isotopic equilibration between dissolved and suspended particulate lead in the Atlantic Ocean: Evidence from 210Pb and stable Pb isotopes

Decreased consumption of leaded gasoline in the United States over the past two decades has drastically altered the flux and isotopic composition of Pb entering the western North Atlantic from the atmosphere. Here we exploit the resulting temporal changes in the distribution and isotopic composition of oceanic Pb to investigate interactions between dissolved and particulate Pb in the oceanic water column. Measurements of dissolved Pb isotopic composition on samples collected in 1987 in the upper water column near Bermuda demonstrate that surface water 206Pb/207Pb decreased from ∼1.203 to ∼1.192 since 1983 and that a measurable change propagated to 300–500 m since the 1984 profile of Shen and Boyle (1988). The first accurate measurements of suspended particulate Pb in an open ocean profile show concentrations of 1–3 pmol/L, equal to 2–4% of total Pb. Vertical profiles of (1) the stable lead isotopic composition and (2) the ratio of total Pb to 210Pb in suspended particles closely track contemporaneous depth variations in these ratios for dissolved Pb throughout the upper 2000 m of the Sargasso Sea near Bermuda. Thus suspended particles reach isotopic equilibrium with ambient sea water Pb on a time scale which is shorter than their residence time with respect to vertical removal, in agreement with equilibrium scavenging hypotheses based on interpretations of Th isotope distributions. A simple flux model suggests that the effect of deep ocean scavenging processes on the flux and isotopic composition of Pb sinking on large particles was minor throughout the preanthropogenic and most of the anthropogenic era but has become important as surface inputs decrease to pre-leaded gasoline levels and may exceed the contribution of surface-derived Pb flux in the next decade.