A rapid procedure for isotopic purification of copper and nickel from seawater using an automated chromatography system.

BACKGROUND: Trace metals such as iron, nickel, copper, zinc, and cadmium (Fe, Ni, Cu, Zn, and Cd) are essential micronutrients (and sometimes toxins) for phytoplankton, and the analysis of trace-metal stable isotopes in seawater is a valuable tool for exploring the biogeochemical cycling of these elements in the ocean. However, the complex and often time-consuming chromatography process required to purify these elements from seawater has limited the number of trace-metal isotope samples which can be easily processed in biogeochemical studies. To facilitate the trace-metal stable isotope analysis, here, we describe a new rapid procedure that utilizes automated chromatography for extracting and purifying Ni and Cu from seawater for isotope analysis using a prepFAST-MC™ system (Elemental Scientific Inc.). RESULTS: We have tested the matrix removal effectiveness, recoveries, and procedural blanks of the new purification procedure with satisfactory results. A nearly complete recovery of Ni and a quantitative recovery of Cu are achieved. The total procedural blanks are 0.33 ± 0.24 ng for Ni and 0.42 ± 0.18 ng for Cu, which is negligible for natural seawater samples. The new procedure cleanly separates Ni and Cu from key seawater matrix elements that may cause interferences during mass spectrometry analysis. When the new procedure was used to purify seawater samples for Ni and Cu stable isotope analysis by multi-collector ICP-MS, we achieved an overall uncertainty of 0.07 ‰ for δ60Ni and 0.09 ‰ for δ65Cu (2 SD). The new purification procedure was also tested using natural seawater samples from the South Pacific, for comparison of δ60Ni and δ65Cu achieved in the same samples purified by traditional hand columns. Both methods produced similar results, and the results from both methods are consistent with analyses of δ60Ni and δ65Cu from other ocean locations as reported by other laboratories. SIGNIFICANCE: This study presents a new rapid procedure for seawater stable-metal isotope analysis by automating the chromatography step. We anticipate that the automated chromatography described here will facilitate the rapid and accurate analysis of seawater δ60Ni and δ65Cu in future studies, and may be adapted in the future to automate chromatographic purification of Fe, Zn, and Cd isotopes from seawater.

Link of the short-term temporal trends of Sr and Nd isotopic composition of aeolian dust over the Arabian Sea with the source emissions.

Aeolian transport of continental dust from the Middle East and South Asia to the Arabian Sea (AS) is an important route for delivering key trace metals and nutrients. Despite being surrounded by several deserts, it is not clear which dust source is most likely contributing to mineral aerosols over this marine basin in winter. Substantial information on dust source emissions and transport pathways over the AS is, thus, needed for better constraining the biogeochemical effects in the sunlit surface waters. Here, we investigated the Sr and Nd isotopic composition (87Sr/86Sr and εNd(0)), respectively) of dust samples collected over the AS during a GEOTRACES-India expedition (GI-10: 13 January-10 February 2020). Both tracers, 87Sr/86Sr (0.70957-0.72495) and εNd(0) (-24.0 to -9.3), showed pronounced spatial variability. These proxies were further tagged with their source profiles of surrounding land masses based on the origin of air mass back trajectories (AMBTs). We also encountered two dust storms (DS), one on 27 January 2020 (87Sr/86Sr: 0.70957; εNd(0): -9.3) and the second one on 10 February 2020 (87Sr/86Sr: 0.71474, εNd(0):-12.5), which showed distinct isotopic signatures. AMBTs and satellite imagery together revealed that DS1 is from the Arabian Peninsula and DS2 is from Iran and/or the Indo-Gangetic Plain. Notably, the Sr and Nd isotope composition of DS1 is further consistent with other dust samples collected over the pelagic waters, suggesting the impact of dust outbreaks from the Arabian Peninsula during winter season. Such documentation based on the 87Sr/86Sr and εNd(0) over the Arabian Sea, hitherto, is lacking in literature and, thus, highlights the need for more measurements.

Isotopes illustrate vertical transport of anthropogenic Pb by reversible scavenging within Pacific Ocean particle veils.

Reversible scavenging, the oceanographic process by which dissolved metals exchange onto and off sinking particles and are thereby transported to deeper depths, has been well established for the metal thorium for decades. Reversible scavenging both deepens the elemental distribution of adsorptive elements and shortens their oceanic residence times in the ocean compared to nonadsorptive metals, and scavenging ultimately removes elements from the ocean via sedimentation. Thus, it is important to understand which metals undergo reversible scavenging and under what conditions. Recently, reversible scavenging has been invoked in global biogeochemical models of a range of metals including lead, iron, copper, and zinc to fit modeled data to observations of oceanic dissolved metal distributions. Nonetheless, the effects of reversible scavenging remain difficult to visualize in ocean sections of dissolved metals and to distinguish from other processes such as biological regeneration. Here, we show that particle-rich "veils" descending from high-productivity zones in the equatorial and North Pacific provide idealized illustrations of reversible scavenging of dissolved lead (Pb). A meridional section of dissolved Pb isotope ratios across the central Pacific shows that where particle concentrations are sufficiently high, such as within particle veils, vertical transport of anthropogenic surface-dissolved Pb isotope ratios toward the deep ocean is manifested as columnar isotope anomalies. Modeling of this effect shows that reversible scavenging within particle-rich waters allows anthropogenic Pb isotope ratios from the surface to penetrate ancient deep waters on timescales sufficiently rapid to overcome horizontal mixing of deep water Pb isotope ratios along abyssal isopycnals.

Decline of anthropogenic lead in South Atlantic Ocean surface waters from 1990 to 2011: New constraints from concentration and isotope data.

Anthropogenic emissions have severely perturbed the marine biogeochemical cycle of lead (Pb). Here, we present new Pb concentration and isotope data for surface seawater from GEOTRACES section GA02, sampled in the western South Atlantic in 2011. The South Atlantic is divided into three hydrographic zones: equatorial (0-20°S), subtropical (20-40°S), and subantarctic (40-60°S). The equatorial zone is dominated by previously deposited Pb transported by surface currents. The subtropical zone largely reflects anthropogenic Pb emissions from South America, whilst the subantarctic zone presents a mixture of South American anthropogenic Pb and natural Pb from Patagonian dust. The mean Pb concentration of 16.7 ± 3.8 pmol/kg is 34 % lower than in the 1990s, mostly driven by changes in the subtropical zone, with the fraction of natural Pb increasing from 24 % to 36 % between 1996 and 2011. Although anthropogenic Pb remains predominant, these findings demonstrate the effectiveness of policies that banned leaded gasoline.

Automated determination of gallium in seawater using seaFAST pre-concentration and high-resolution inductively-coupled plasma mass spectrometry.

There has been increased interest in dissolved gallium (Ga) in natural waters due to its long residence time and its usefulness in tracking water masses; however, current analytical approaches are time consuming and labor intensive (e.g., magnesium hydroxide co-precipitation method, (Mg(OH)2)) or have concerns such as carryover and sample recovery (automated resin column extraction). Ocean observing programs, such as GEOTRACES, recover hundreds of samples per expedition. There are both logistical (sample volume) and analytical (person-hour) demands to economically collect and analyze Ga. We present an automated isotope dilution method (using 99.8% enriched 71Ga) to determine Ga in seawater utilizing commercially available equipment while addressing the challenges of a) sample volume and sample pre-concentration factor, b) instrumental interferences, c) sample-sample carryover, d) sample recovery variability, and e) improving sample detection limits, accuracy and precision. A seaFAST SC-4DXS pico (Elemental Scientific, Inc.; ESI) was used to pre-concentrate 20 mL of sample on a Nobias PA1 resin column 67-fold before analysis in medium resolution on a ThermoFisher high-resolution inductively-coupled plasma mass spectrometer (HR-ICP-MS) equipped with an APEX Q FAST enabled spray chamber (ESI) to increase signal intensity and decrease instrument interferences. The new automated seaFAST method reproduced Ga concentrations determined by the Mg(OH)2 method, but with greater precision (RSD <4%) and a lower detection limit (0.10 pmol L-1). This method is ideal for high throughput applications and can be easily implemented using commercially available equipment.

The unaccounted dissolved iron (II) sink: Insights from dFe(II) concentrations in the deep Atlantic Ocean.

Hydrothermal vent sites found along mid-ocean ridges are sources of numerous reduced chemical species and trace elements. To establish dissolved iron (II) (dFe(II)) variability along the Mid Atlantic Ridge (between 39.5°N and 26°N), dFe(II) concentrations were measured above six hydrothermal vent sites, as well as at stations with no active hydrothermal activity. The dFe(II) concentrations ranged from 0.00 to 0.12 nmol L-1 (detection limit = 0.02 ± 0.02 nmol L-1) in non-hydrothermally affected regions to values as high as 12.8 nmol L-1 within hydrothermal plumes. Iron (II) in seawater is oxidised over a period of minutes to hours, which is on average two times faster than the time required to collect the sample from the deep ocean and its analysis in the onboard laboratory. A multiparametric equation was used to estimate the original dFe(II) concentration in the deep ocean. The in-situ temperature, pH, salinity and delay between sample collection and its analysis were considered. The results showed that dFe(II) plays a more significant role in the iron pool than previously accounted for, constituting a fraction >20 % of the dissolved iron pool, in contrast to <10 % of the iron pool formerly reported. This discrepancy is caused by Fe(II) loss during sampling when between 35 and 90 % of the dFe(II) gets oxidised. In-situ dFe(II) concentrations are therefore significantly higher than values reported in sedimentary and hydrothermal settings where Fe is added to the ocean in its reduced form. Consequently, the high dynamism of dFe(II) in hydrothermal environments masks the magnitude of dFe(II) sourced within the deep ocean.

Novel Insights into Marine Iron Biogeochemistry from Iron Isotopes.

The micronutrient iron plays a major role in setting the magnitude and distribution of primary production across the global ocean. As such, an understanding of the sources, sinks, and internal cycling processes that drive the oceanic distribution of iron is key to unlocking iron's role in the global carbon cycle and climate, both today and in the geologic past. Iron isotopic analyses of seawater have emerged as a transformative tool for diagnosing iron sources to the ocean and tracing biogeochemical processes. In this review, we summarize the end-member isotope signatures of different iron source fluxes and highlight the novel insights into iron provenance gained using this tracer. We also review ways in which iron isotope fractionation might be used to understand internal oceanic cycling of iron, including speciation changes, biological uptake, and particle scavenging. We conclude with an overview of future research needed to expand the utilization of this cutting-edge tracer.

Strong Margin Influence on the Arctic Ocean Barium Cycle Revealed by Pan-Arctic Synthesis.

Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 µmol m-2 day-1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.

Perfluoroalkyl Substances in the Western Tropical Atlantic Ocean.

The dispersion of perfluoroalkyl substances (PFAS) in surface and deep-water profiles (down to 5845 m deep) was evaluated through the Western Tropical Atlantic Ocean (TAO) between 15°N and 23°S. The sum concentrations for eight quantifiable PFAS (∑8PFAS) in surface waters ranged from 11 to 69 pg/L, which is lower than previously reported in the same area as well as in higher latitudes. Perfluoroalkyl carboxylic acids (PFCAs) were the predominant PFASs present in the Western TAO. The 16 surface samples showed variable PFAS distributions, with the predominance of perfluorooctanoic acid (PFOA) along the transect (67%; 11 ± 8 pg/L) and detection of perfluoroalkyl sulfonic acids (PFSAs) only in the Southern TAO. Perfluoroheptanoic acid (PFHpA) was often detected in the vertical profiles. PFAS distribution patterns (i.e., profiles and concentrations) varied with depth throughout the TAO latitudinal sectors (North, Equator, South Atlantic, and in the Brazilian coastal zone). Vertical profiles in coastal samples displayed decreasing PFAS concentrations with increasing depth, whereas offshore samples displayed higher PFAS detection frequencies in the intermediate water masses. Together with the surface currents and coastal upwelling, the origin of the water masses was an important factor in explaining PFAS concentrations and profiles in the TAO.

A new method to extract 232Th, 230Th and 231Pa from seawater using a bulk-extraction technique with Nobias PA-1 chelating resin.

The long-lived radioisotopes of Th and Pa are unique tracers for quantifying rates of biogeochemical processes in the ocean. However, their generally low concentrations (sub-fg/kg for 230Th and 231Pa and pg/kg for 232Th) in seawater make them difficult to measure. Here, we present a new approach to determine 232Th and 230Th using Nobias PA-1 chelating resin following a bulk-extraction technique, and report for the first time the use of this resin to measure 231Pa concentrations. This method has high extraction efficiency (>80%) at pH of 4.4 ± 0.2 and the lowest procedural blanks reported in the literature: 1.0 ± 0.2 pg, 0.10 ± 0.03 fg, and 0.02 ± 0.01 fg for 232Th, 230Th, and 231Pa, respectively, representing 3%, 0.02%, and 0.01% of the total dissolved 232Th, 230Th, and 231Pa found in 5 L of a typical low-concentration surface seawater sample from the subtropical Pacific Ocean. The procedure yields data with high precision for all three isotopes (0.76% for 232Th, 0.89% for 230Th, and 0.96% for 231Pa, 2σ), allowing us to reliably measure Th and Pa in the oceans even at concentrations as low as those found in surface waters of the South Pacific Ocean. The accuracy of this method was confirmed by the analysis of well-characterized standard solutions (SW STD 2010-1 and SW STD 2015-1) and seawater samples collected aboard the FS Sonne (cruise SO245) during the UltraPac cruise in the South Pacific Ocean. Simultaneous and rapid extraction of 232Th, 230Th and 231Pa from seawater, as well as the high precision and accuracy of this method makes it ideal for both spatially and temporally high-resolution studies.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

While chromium stable isotopes (δ53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ53Cr is still lacking. Here we present total dissolved δ53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and δ53Cr suggests that δ53Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor (∆53Cr) of -1.08‰ ± 0.25 relative to total dissolved δ53Cr, in agreement with the global δ53Cr-Cr fractionation factor (-0.82‰ ± 0.05). No perturbation to δ53Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 µmol/kg), suggesting no strong control by O2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and δ53Cr in the modern ocean. Consequently, δ53Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that δ53Cr records may be a useful tracer for biological productivity.

210Pb dating of recent sediments from the continental shelf of western India: factors influencing sedimentation rates.

Several cores from the continental shelf of the Arabian Sea along the Indian Coast were investigated for change in sedimentation rates duly constrained for chronology based on 210Pb and 137Cs dating techniques. The emphasis was to look for spatial and temporal variation in the sedimentation rate along the continental shelf of eastern Arabian Sea between Goa to Kochi for water depth ≤ 150 m. This study showed varying sedimentation rates in coastal and distant sediment cores. Both 210Pb and 137Cs dating techniques showed comparable sedimentation rate in most sediment cores. The sedimentation in the continental shelf region of the coastal Arabian Sea is primarily controlled by discharge of sediments from rivers during Indian summer monsoon. Increased sedimentation rate from the north (off Goa) to the south (off Kochi) was triggered by high riverine flux and longshore sediment transport.

Submarine karstic springs as a source of nutrients and bioactive trace metals for the oligotrophic Northwest Mediterranean Sea.

Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3- was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2- and NH4+ was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m3 s-1 for the entire investigated coastline, determined from simultaneous 224Ra and 226Ra mass balances. The contribution of groundwater PO43-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

Subpolar marginal seas fuel the North Pacific through the intermediate water at the termination of the global ocean circulation.

The mechanism by which nutrients in the deep ocean are uplifted to maintain nutrient-rich surface waters in the subarctic Pacific has not been properly described. The iron (Fe) supply processes that control biological production in the nutrient-rich waters are also still under debate. Here, we report the processes that determine the chemical properties of intermediate water and the uplift of Fe and nutrients to the main thermocline, which eventually maintains surface biological productivity. Extremely nutrient-rich water is pooled in intermediate water (26.8 to 27.6 σθ) in the western subarctic area, especially in the Bering Sea basin. Increases of two to four orders in the upward turbulent fluxes of nutrients were observed around the marginal sea island chains, indicating that nutrients are uplifted to the surface and are returned to the subarctic intermediate nutrient pool as sinking particles through the biological production and microbial degradation of organic substances. This nutrient circulation coupled with the dissolved Fe in upper-intermediate water (26.6 to 27.0 σθ) derived from the Okhotsk Sea evidently constructs an area that has one of the largest biological CO2 drawdowns in the world ocean. These results highlight the pivotal roles of the marginal seas and the formation of intermediate water at the end of the ocean conveyor belt.

A global perspective on mercury cycling in the ocean.

Mercury (Hg) is a ubiquitous metal in the ocean that undergoes in situ chemical transformations in seawater and marine sediment. Most relevant to public health is the production of monomethyl-Hg, a neurotoxin to humans that accumulates in marine fish and mammals. Here we synthesize 30 years of Hg measurements in the ocean to discuss sources, sinks, and internal cycling of this toxic metal. Global-scale oceanographic survey programs (i.e. CLIVAR and GEOTRACES), refined protocols for clean sampling, and analytical advancements have produced over 200 high-resolution, full-depth profiles of total Hg, methylated Hg, and gaseous elemental Hg throughout the Atlantic, Pacific, Arctic, and Southern Oceans. Vertical maxima of methylated Hg were found in surface waters, near the subsurface chlorophyll maximum, and in low-oxygen thermocline waters. The greatest concentration of Hg in deep water was measured in Antarctic Bottom Water, and in newly formed Labrador Sea Water, Hg showed a decreasing trend over the past 20 years. Distribution of Hg in polar oceans was unique relative to lower latitudes with higher concentrations of total Hg near the surface and vertical trends of Hg speciation driven by water column stratification and seasonal ice cover. Global models of Hg in the ocean require a better understanding of biogeochemical controls on Hg speciation and improved accuracy of methylated Hg measurements within the international community.

GEOTRACES: Accelerating Research on the Marine Biogeochemical Cycles of Trace Elements and Their Isotopes.

The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs-the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings.

Underground gamma-ray measurements of radium isotopes from hydrothermal plumes in the deep Pacific Ocean.

The radium isotopes 226Ra and 228Ra can provide important data on the dynamics of deep-sea hydrothermal plumes that travel the oceans for decades and have great impact on the ocean chemistry. This study focuses on parameters important for obtaining low detection limits for 228Ra using gamma-ray spectrometry. It is present at mBq-levels in samples collected during the US GEOTRACES 2013 cruise to the Southeast Pacific Ocean.

Regulation of the Phytoplankton Heme b Iron Pool During the North Atlantic Spring Bloom.

Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L-1) in iron limited phytoplankton in cultures and in the field. Here, we determined heme b in marine particulate material (>0.7 µm) from the North Atlantic Ocean (GEOVIDE cruise - GEOTRACES section GA01), which spanned several biogeochemical regimes. We examined the relationship between heme b abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme b concentrations ranged from 0.16 to 5.1 pmol L-1 (median = 2.0 pmol L-1, n = 62) in the surface mixed layer (SML) along the cruise track, driven mainly by variability in biomass. However, in the Irminger Basin, the lowest heme b levels (SML: median = 0.53 pmol L-1, n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 µmol L-1, n = 25; chlorophyll a: median = 2.0 nmol L-1, n = 23) pointing to regulatory mechanisms of the heme b pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L-1, n = 11) in the Irminger Basin, but large diatoms (Rhizosolenia sp.) dominated. Hence, heme b depletion and regulation is likely to occur during bloom progression when phytoplankton class-dependent absolute iron requirements exceed the available ambient concentration of DFe. Furthermore, high heme b concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L-1, n = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme b provides a snapshot of the cellular activity in situ and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.

Pre-concentration of thorium and neodymium isotopes using Nobias chelating resin: Method development and application to chromatographic separation.

The isotopes of thorium (Th) and neodymium (Nd) are used as tracers in oceanography, and are key parameters in the international GEOTRACES program. The very low concentrations of Th and Nd as well as the reactive nature of Th isotopes makes the analysis of seawater samples a complex process. Analysis requires time-consuming pre-concentration from over 5 L of seawater. We describe a method to simultaneously pre-concentrate dissolved Th and Nd from acidified seawater samples using the Nobias® PA1L chelating resin. Prior to pre-concentration, hydrofluoric acid is added to the sample to stabilise Th, ammonium acetate buffer added (0.05 M), pH adjusted to 4.75, and then finally the prepared sample is pumped through the Nobias resin at a rate of 15 ml min-1. Up to 6 samples can be processed simultaneously. Following elution in 3 M HNO3, both elements are chromatographically separated and determined using Inductively Coupled Plasma Mass Spectrometry. Oxidation of the sample between all column separation steps, including after the initial Nobias resin, is important for obtaining maximum elemental recoveries. The method has >90% recovery with blank levels typically <10 pg for 232Th and <70 pg for Nd. Accuracy is excellent, as our reported values generally agree within 1% of the GEOTRACES intercalibration standards. The long-term analysis of these materials also indicates excellent reproducibility. The pre-concentration of Th and Nd using the Nobias resin is a time saving option compared to the widely used iron co-precipitation technique. Sample handling is also reduced, decreasing the risk of sample contamination. The simplicity of our suggested pre-concentration procedure makes it possible to be applied at sea.

Shallow particulate organic carbon regeneration in the South Pacific Ocean.

Particulate organic carbon (POC) produced in the surface ocean sinks through the water column and is respired at depth, acting as a primary vector sequestering carbon in the abyssal ocean. Atmospheric carbon dioxide levels are sensitive to the length (depth) scale over which respiration converts POC back to inorganic carbon, because shallower waters exchange with the atmosphere more rapidly than deeper ones. However, estimates of this carbon regeneration length scale and its spatiotemporal variability are limited, hindering the ability to characterize its sensitivity to environmental conditions. Here, we present a zonal section of POC fluxes at high vertical and spatial resolution from the GEOTRACES GP16 transect in the eastern tropical South Pacific, based on normalization to the radiogenic thorium isotope 230Th. We find shallower carbon regeneration length scales than previous estimates for the oligotrophic South Pacific gyre, indicating less efficient carbon transfer to the deep ocean. Carbon regeneration is strongly inhibited within suboxic waters near the Peru coast. Canonical Martin curve power laws inadequately capture POC flux profiles at suboxic stations. We instead fit these profiles using an exponential function with flux preserved at depth, finding shallow regeneration but high POC sequestration below 1,000 m. Both regeneration length scales and POC flux at depth closely track the depths at which oxygen concentrations approach zero. Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modeling and future work to disentangle.