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Science Highlights


Some recent GEOTRACES science findings are reported below.  
When getting older they are compiled in the Science Highlights Archive where the "Title Filter" search box will allow you to filter them by words in title (please note that only one-word search queries are allowed e.g. iron, Atlantic, etc.).

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About the decoupled fates of aluminium, manganese, cobalt and lead in the North Pacific Ocean

Did you know that each of these tracers could follow its own marine story, quite decoupled from the others?

This is what is shown and discussed by Zheng and co-workers (2019, see reference below) after having analysed about 500 samples for aluminium (Al), manganese (Mn), lead (Pb) and cobalt (Co) along three sections in the North Pacific Ocean. They demonstrate that the distribution of each element is uniquely related to ocean circulation; that the subsurface Pb maximum has been sustained in the North Pacific Ocean through the growth of anthropogenic sources in Asia and Russia, contrasting with the decrease observed in the Atlantic Ocean (please also read the science highlight from Bridgestock et al., 2016); that the labile fraction of particulate Al is larger than that of particulate lead; and finally that while the Pb enrichment factor confirms its predominant atmospheric origin, those of Mn and Co clearly attest that sources other than the aerosol deposition are more significant contributors to the concentrations of these two tracers.

19 Zheng l

Figure: Sectional distributions of dissolved metals (dM) and potential density anomaly at depths of 0–1200 m along 160°W (section highlighted in red in the map). Dissolved aluminium (dAl) is high in Equatorial Under Current (EQ, 175 m depth) and North Equatorial Current (20°N, surface). Although dissolved manganese (dMn) and dissolved cobalt (dCo) have a concurrent source at the continental shelf of the Aleutian Islands, dCo is more widely distributed via North Pacific Intermediate Water (NPIW, ~600 m). Dissolved lead (dPb) is concentrated in Subtropical Mode Water and Central Mode Water above the NPIW. Adapted from Zheng et al., 2019. Click here to view the figure larger.

Reference:

Zheng, L., Minami, T., Konagaya, W., Chan, C.-Y., Tsujisaka, M., Takano, S., Norisuye, K., Sohrin, Y. (2019). Distinct basin-scale-distributions of aluminum, manganese, cobalt, and lead in the North Pacific Ocean. Geochimica et Cosmochimica Acta, 254, 102–121. DOI: http://doi.org/10.1016/J.GCA.2019.03.038

Bridgestock, L., van de Flierdt, T., Rehkämper, M., Paul, M., Middag, R., Milne, A., Lohan, M.C., Baker, A.R., Chance, R.,, Khondoker, R., Strekopytov, S., Humphreys-Williams, E., Achterberg, E.P., Rijkenberg, M.J.A., Gerringa, L. J.A., de Baar, H. J. W. (2016). Return of naturally sourced Pb to Atlantic surface waters. Nature Communications, 7, 12921. doi: http://doi.org/10.1038/ncomms12921

On the optimal use of the seaFAST system

Do you wish to improve your recoveries, blanks or any other parameter of your seawater preconcentration system (seaFAST)? Or are you simply curious about it? Wuttig and co-workers (2019, see reference below) propose a critical evaluation of this system’s capabilities. They perform an impressive list of tests including system conditioning, improving blank levels, finding the optimal pH of the buffer, improving preconcentration factors for different sample matrices, estimating memory effects, the initial sample salinity and UV oxidation effects on trace element concentrations. These tests considered an array of trace elements (cadmium, cobalt, copper, iron, gallium, manganese, nickel, lead, titanium and zinc) using SF-ICP-MS with data validation for some of these trace elements by flow injection analysis (iron, manganese) and/or GEOTRACES (n=42 for GSP and GSC) reference samples. They eventually make a long and useful list of recommendations for an optimal use of the system.

19 Wuttig

Figure: During this work, a commercially available seaFAST preconcentration system combined with sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) was utilised to measure six reference seawaters (SAFe S, D1 and D2; GEOTRACES GD, GSC and GSP) 3-42 times each. In this figure, our measured values were compared to the consensus values for copper (Cu), iron (Fe), manganese (Mn) and Titanium (Ti). Titanium is a novel element for this system with limited consensus values available and was compared to values determined with voltammetry by Croot, 2011. Errors are presented as 1 standard deviation (σ) for both consensus and the measured values. Note different scales for Ti. 

Reference:

Wuttig, K., Townsend, A. T., van der Merwe, P., Gault-Ringold, M., Holmes, T., Schallenberg, C., Latour, P., Tonnard, M., Rijkenberg, M. J.A., Lannuzel, D., Bowie, A. R. (2019). Critical evaluation of a seaFAST system for the analysis of trace metals in marine samples. Talanta, 197, 653–668. DOI: http://doi.org/10.1016/J.TALANTA.2019.01.047

Croot, P.L., Rapid determination of picomolar titanium in seawater with catalytic cathodic stripping voltammetry, Anal Chem 83(16) (2011) 6395-400.

A treasure of geochemical data to trace ocean circulation, ventilation, mixing, biogeochemical and hydrothermal processes

This treasure is made of approximately 60,000 valid tritium measurements, 63,000 valid helium isotope determinations, 57,000 dissolved helium concentrations, and 34,000 dissolved neon concentrations, including their metadata (geographic location, date and sample depth). It was compiled by Bill Jenkins and co-workers (2019, see reference below) who describe the nature of the data, discuss their quality, list the contributors and pioneers, and of course are giving free access to this huge dataset (https://doi.org/10.25921/c1sn-9631). They also provide some figures illustrating how powerful this new tool is as for example the figure below. 

Authors invite anyone with knowledge of additional tritium, helium, or neon data that has not been included, to please contact wjenkins@whoi.edu with details for inclusion in future versions of the data set.

19 Jenkins
Figure:
(top) A map of helium values at approximately 2500 m depth. (bottom) A map of helium values at approximately 4000 m depth. The values plotted are simply an average of all measurements within a 1’ square between 3750 and 4250 dbar. Depths shallower than 4000 m are masked in gray, and sampling locations are indicated by light gray dots. Click here to view the figure larger.
3He is an extremely rare isotope that is a sensitive tracer of hydrothermal processes. Since it is both stable and chemically inert, it is detectable over great distances in the ocean. The two maps shown above are of the distribution of δ3He, a tracer of hydrothermal activity, at two levels in the deep ocean. The shallower one roughly corresponds to the depth of the mid-ocean ridge system, where the bulk of this hydrothermal injection takes place. One can see the dominant role of the fast-spreading ridges in the eastern Pacific, which drive two massive, westward reaching plumes north and south of the equator. The deeper horizon shows the spreading of δ3He-impoverished bottom waters from the northern and southern polar regions into the deep ocean basins.

Reference:

Jenkins, W. J., Doney, S. C., Fendrock, M., Fine, R., Gamo, T., Jean-Baptiste, P., Key, R., Klein, B., Lupton, J. E., Newton, R., Rhein, M., Roether, W., Sano, Y., Schlitzer, R., Schlosser, P. Swift, J. (2019). A comprehensive global oceanic dataset of helium isotope and tritium measurements. Earth System Science Data, 11(2), 441–454. DOI: http://doi.org/10.5194/essd-11-441-2019

Fate of different iron phases along the Lena river plume in the Arctic Ocean

Separation of the particulate (> 0:22 μm), colloidal (0.22 μm–1 kDa), and truly dissolved (< 1 kDa) fractions of iron (Fe) was carried out along the 600 km mixing between the Lena river and the Laptev seawater. While 99% of the particulate Fe and 90% of the colloidal one are disappearing across the shelf, the truly dissolved Fe stays relatively constant along the Lena River plume. This could indicate that this truly dissolved fraction is an important source of bioavailable Fe, along with colloidal Fe, for the local Arctic phytoplankton. Conrad and co-workers (2019, see reference below) also determined the Fe isotopes (δ56Fe) on these fractions. Negative colloidal δ56Fe values close to the river mouth are evolving to positive values at the outermost stations (see figure below). The shelf is thus interpreted as a sink for Fe, the negative values representing reduced ferrihydrites. Contrastingly, the positive values would correspond to oxidized Fe oxyhydroxydes, that are remaining in the water column and tag the Fe isotopic signature of the Arctic Ocean.

19 Conrad

Figure: A) Sampling stations in the Arctic Ocean: The black dots mark the sampling stations in the Arctic Ocean. Along the Lena River–Laptev Sea transect (next to the map) we filtered the water samples with different techniques to separate the particles from the colloids and the truly dissolved phase. Iron concentrations and isotope ratios were measured on the different sizes, as well as in the sediments along the East Siberian Sea Shelf. B) The colloidal (CFe) and particulate (PFe) iron concentrations plotted vs. salinity: Particulate Fe is dominating the Fe system close to the river mouth at low salinities, but the increasing salinity along the freshwater plume promotes the flocculation and settling of particles and colloids. Therefore, the concentrations of PFe and CFe are equal at higher salinities. C) Iron-isotope values along the Lena River freshwater plume and the uppermost sediment of the East Siberian Arctic Shelf (ESAS): Negative particulate and colloidal Fe-isotope values are lost during estuarine mixing and buried in the sediment, which shows a similar range of Fe isotope ratios. The positive colloidal Fe isotope values found in the outer plume are resistant to estuarine mixing and can be found in the Arctic Ocean. (Reprinted from Conrad, et al, 2019. Click here to view the image larger. 

Reference:

Conrad, S., Ingri, J., Gelting, J., Nordblad, F., Engström, E., Rodushkin, I., Andersson, P. S., Porcelli, D., Gustafsson, Ö., Semiletov, I., Öhlander, B. (2019). Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea. Biogeosciences, 16(6), 1305–1319. DOI: http://doi.org/10.5194/bg-16-1305-2019

A new brick to the zinc-silicate-phosphate oceanic story

Middag and co-workers (2019, see reference below) established the distributions of dissolved zinc (Zn), silicate (Si) and phosphate (PO4) at high resolution along the GEOTRACES transect GA02, in other words, covering the North and South Atlantic Ocean from Greenland to the tip of South America. This section track allowed them to unravel the influence of various water masses and local biogeochemical processes on these 3 tracers in the Atlantic Ocean. Although they confirm that the Southern Ocean water masses play a driving role in the distributions of these elements, the authors also show that subsurface water masses from northern origin are depleted in Zn, implying that the depletion of Zn occurs not only in the source region of Subantarctic Mode Waters as previously suggested. They also demonstrate that Atlantic remineralization has a profound influence on the PO4 distribution, but not on the distribution of Zn and Si. These 2 elements have a more limited supply to the surface ocean from underlying water masses and their fates are mostly governed by mixing of the various water masses along this transect. Both northern and southern high-latitude waters display a relatively high Zn:PO4 uptake and remineralization ratio, implying it is Zn availability and not only Fe limitation that leads to increased Zn uptake in the high-latitude regions. A close look at the Zn-Si relationships, allowed by the high resolution of the data, also show that it varies between the different water masses (see figure below)…the variations are also discussed in this paper!

19 Middag - Zn versus Si distribution

Figure: The Zn versus Si distribution. (a) The distribution along the transect in the North Atlantic only (north of the equator). (b) The distribution along the entire transect. (c) The distribution in the South Atlantic only (south of 20°S). This figure shows the varying relationships in different water masses with the color scale indicating the deficit of oxygen to represent remineralisation that occurred in the study region. From this figure it is clear there is no universal Zn-Si relationship, but rather that it is the mixing of various water masses that results in an array of mixing lines. Remineralisation along the section is most profound around the equator due to regional upwelling processes that stimulate productivity. Such remineralisation has an apparent effect, as the samples with most remineralisation (warm colors) plot below the majority of the observations. However, this remineralisation effect is very modest and most variation can be explained by the mixing of water masses with radically different concentrations.

Green line: NASPMW mixing with NADW; Black line: mixing of various components of NADW; Blue line: AABW mixing with overlying water masses; Gray line: SACW mixing with AAIW. AABW = Antarctic Bottom Water; NADW = North Atlantic Deep Water; uCDW = upper circumpolar deep water; AAIW = Antarctic Intermediate Water; SACW = South Atlantic Central Water; NASPMW = North Atlantic Sub-Polar Mode Water; NASTMW = North Atlantic Sub-Tropical Mode Water. Reprinted from Middag, et al., 2019. Please click here to view the image larger.

Reference:

Middag, R., de Baar, H. J. W., & Bruland, K. W. (2019). The Relationships Between Dissolved Zinc and Major Nutrients Phosphate and Silicate Along the GEOTRACES GA02 Transect in the West Atlantic Ocean. Global Biogeochemical Cycles, 33(1), 63–84. DOI: http://doi.org/10.1029/2018GB006034

More insights on the natural fertilization processes of the KERGUELEN plateau

Rare earth elements (REE) patterns and neodymium (Nd) isotopic signatures of filtered and unfiltered waters collected as part of KEOPS GEOTRACES process study (mostly KEOPS 2, data compared to KEOPS 1 ones, acquired 6 years earlier) enabled Grenier and co-workers (2018, see reference below) to trace the transport of waters 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of lithogenic inputs from Heard Island, was also revealed. Both confirm that lithogenic material released by intensive aerial and submarine weathering of Heard and Kerguelen islands are naturally fertilizing this area of the Southern Ocean. The authors also discuss that variations in REE concentrations and fractionations are also partly driven by authigenic processes, both inorganically and biologically mediated...

18 Grenier

Figure: Oceanic transports of continental inputs from the Kerguelen Plateau, supplying the downstream annual spring bloom, one of the largest of the High-nutrient, low-chlorophyll (HNLC) Southern Ocean. Chlorophyll-a surface concentration of the KEOPS2 cruise period (colors), overlaid in by mean surface velocity field over the same period (arrows), shows the complexity of the bloom shape north and south of the Polar Front (PF; thick black line; Park et al., 2014). The schematized routes of continental supplies are suggested from the combination of the REE results (stations: white dots and names) with the regional oceanic circulation. These routes underline different geographical origins and transports of continental supplies: 1) inputs from Kerguelen Islands laterally transported north of the PF (red arrow); 2) inputs from Heard Islands laterally redistributed south of the PF (light pink arrow), fuelled by vertical inputs from the Central Plateau sediments (light pink arrowhead viewed from above); 3) exchanges of these sources across the PF (yellow arrow).

References:

Grenier, M., Garcia-Solsona, E., Lemaitre, N., Trull, T. W., Bouvier, V., Nonnotte, P., van Beek, P., Souhaut, M., Lacan, F., Jeandel, C. (2018). Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions. Frontiers in Marine Science, 5. DOI : http://doi.org/10.3389/fmars.2018.00426

Park, Y.-H., Durand, I., Kestenare, E., Rougier, G., Zhou, M., d'Ovidio, F., et al. (2014). Polar Front around the Kerguelen Islands: an up-to-date determination and associated circulation of surface/subsurface waters. J. Geophys. Res. 119, 6575–6592. doi: http://doi.org/10.1002/2014JC010061

Isotopic chromium variations do not always reflect the occurrence of low oxygenated waters

Dissolved chromium (Cr) in the ocean is present under two oxidation states: The oxidized and soluble Cr (VI) and the reduced more reactive Cr (III). Reduction of Cr (VI) to Cr (III) is favored by the occurrence of biological particles, reducing conditions in the sediments or the water column (at least in the Pacific Oxygen Minimum Zones). In addition, the isotopic signature of Cr (δ53Cr representing the variation of the abundance of 53Cr relative to the lighter 52Cr) also reflects redox reactions in the water column. Based on the analyses of Cr speciation and isotopic composition of 5 profiles sampled off the Senegalese coast along the GEOTRACES transect GA06, Goring-Harford and co-workers (2018, see reference below) show that Cr(VI) is unlikely to be reduced under the low oxic conditions characterizing this area (minimal values above 44 µmol/kg). Contrastingly, they reveal that total Cr concentrations and δ53Cr are affected by biological processes and inputs from sediments on the shelf whereas deep waters are relatively unaffected by internal Cr cycling. Moreover, the authors establish that on a world basis, δ53Cr is independent from the concentration of dissolved oxygen, at least for the oxygen ranges as encountered off the Senegalese coast. This result weakens the use of δ53Cr values of ancient marine authigenic precipitates to reconstruct past changes in levels of dissolved oxygen in seawater.

18 Goring f
Figure 1:
 δ53Cr, dFe and dissolved oxygen profiles on one of the stations sampled. Low oxygen concentrations correlate with relatively high dFe concentrations, which are linked to high rates of organic matter remineralisation and high δ53Cr values, related to benthic supply of Cr (Adapted from Goring-Harford et al., 2018, by J. Klar).

Dissolved iron (dFe) and iron (Fe) isotopes were also collected off the same Senegalese coast as the Cr study above contributing to identify the processes leading to enhanced dFe concentrations (up to 2 nM) in the tropical North Atlantic Oxygen Minimum Zone. Negative δ56Fe values observed on the shelf could be attributed to input of dFe from both reductive and non-reductive dissolution of sediments. Contrastingly, when these benthic inputs are upwelled to surface waters, uptaken by the biological activity and remineralized in the twilight zone, they display high and positive δ56Fe and the proportion of this remineralized Fe to the total dFe pool increases with distance from the shelf. The difference of sensitivity to redox conditions between Cr and Fe isotopes is also underlined by this work. Thanks to the simultaneous aluminium concentrations, the authors also demonstrate that dust inputs were low at the time of the GEOTRACES GA06 section, strengthening the relative role of the benthic flux in the context of this study, although located next to the Saharan desert. 

18 3 Klar

Figure 2: δ53Cr and dFe profiles on one of the stations sampled (Adapted from Goring-Harford et al., 2018), B) Schematic interpretation of the Fe cycle in the study area. Shelf sediments supply dFe with a light isotopic composition (↓ δ56Fe) to bottom waters. dFe is supplied to the surface mixed layer (SML) by atmospheric dust deposition and upwelled bottom waters, where phytoplankton takes up dFe with a relatively heavy isotopic composition (↑ δ56Fe). Remineralisation of sinking organic material leads to the release of dFe with a relatively heavy isotopic composition, which is mixed with benthic dFe inputs and upwelled to the SML, where it is mixed with atmospheric dFe inputs. The flux of benthic dFe decreases with distance from the coast. The continuous recycling of dFe by biological uptake and remineralisation leads to increasingly heavy isotopic compositions of dFe in the water column with distance from the shelf. Atmospheric dust inputs (fluxes in μmol dFe m−2 d−1, in brown) to the SML, calculated from dAl concentrations, were low at the time of sampling but are potentially higher at other times of the year (Croot et al., 2004). Fluxes of vertical transport to the SML (white) and horizontal transport between the bottom of the SML and 500 m depth (yellow) are from Milne et al. (2017) and are in μmol dFe m−2 d−1 (Reprinted from Klar et al., 2018). Click here to view the figure larger.

Reference:

Goring-Harford, H. J., Klar, J. K., Pearce, C. R., Connelly, D. P., Achterberg, E. P., & James, R. H. (2018). Behaviour of chromium isotopes in the eastern sub-tropical Atlantic Oxygen Minimum Zone. Geochimica et Cosmochimica Acta, 236, 41–59. https://doi.org/10.1016/j.gca.2018.03.004

Klar, J. K., Schlosser, C., Milton, J. A., Woodward, E. M. S., Lacan, F., Parkinson, I. J., Achterberg, E.P., James, R. H. (2018). Sources of dissolved iron to oxygen minimum zone waters on the Senegalese continental margin in the tropical North Atlantic Ocean: Insights from iron isotopes. Geochimica et Cosmochimica Acta, 236, 60–78. https://doi.org/10.1016/j.gca.2018.02.031

Croot, P. L., Streu, P., Baker A.R. (2004) Short residence time for iron in surface seawater impacted by atmospheric dry deposition from Saharan dust events, Geophys. Res. Lett., 31 (2004), p. L23S08 https://doi.org/10.1029/2004GL020153

Milne, A., Schlosser, C., Wake, B.D., Achterberg, E.P., Chance, R., Baker, A.R., Forryan, A., Lohan, M.C. (2017) Particulate phases are key in controlling dissolved iron concentrations in the (sub)tropical North Atlantic Geophys. Res. Lett., 44, pp. 2377-238 https://doi.org/10.1002/2016GL072314

The Scottish shelf break is not a significant source of iron to North Atlantic surface waters

A high resolution survey of the distribution of dissolved iron (dFe) over the Hebridean (Scottish) shelf break was conducted as part of the U.K. Shelf Sea Biogeochemistry programme, a GEOTRACES process study (GApr04). Despite the close proximity to shelf sediments, which are known to supply large quantities of dFe to overlying water column, the results revealed surprisingly low concentrations of dFe (<0.1 nM) in surface waters overlying the shelf break. Birchill and colleagues (2019, see reference below) relate this to the prevailing physical circulation of the region, which limits off shelf transport in surface waters, and conclude that this shelf system is not a significant source dFe to high latitude North Atlantic surface waters. It is therefore suggested that the conditions leading to seasonal iron limitation of phytoplankton in the Iceland and Irminger basins extend much further eastwards than previously identified.

19 Birchill

Figure: (A) Map of the survey region with sampling locations. (B) Example of cross shelf transect of dFe distribution, detailing the contrast between shelf waters with high dFe concentrations (>2 nM) and surface oceanic waters with remarkably low dFe concentrations (<0.1 nM). (C) Depth profile of dFe: NO3-, oceanic stations close to Hebridean shelf have similar values to those previously reported for the seasonally iron limited Icelandic Basin. Dashed line denotes 0.05 dFe:NO3 (nM:μM), the lower limit observed in Fe replete cultured phytoplankton. Click here to view the figure larger.

Reference:

Birchill, A. J., Hartner, N. T., Kunde, K., Siemering, B., Daniels, C., González-Santana, D., Milne, A. Ussher, S. J., Worsfold, P. J., Leopold, K., Painter, S. C., Lohan, M. C. (2019). The eastern extent of seasonal iron limitation in the high latitude North Atlantic Ocean. Scientific Reports, 9(1), 1435. DOI: http://doi.org/10.1038/s41598-018-37436-3

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