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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.).

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Marco van Hulten and co-workers (2017, see reference below) ran a global ocean model to understand manganese (Mn), a biologically essential element. The model shows that:

(i) in the deep ocean, dissolved [Mn] is mostly homogeneous ~0.10—0.15 nM. The model reproduces this with a threshold on MnO₂ of 25 pM, suggesting a minimal particle concentration is needed before aggregation and removal become efficient.

(ii) The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vent.

17 vanHulten l2Figure: (A) The modelled dissolved [Mn] (nM) at the Zero-Meridian section component of the GIPY5 cruise dataset, and the West Atlantic GA02 GEOTRACES section cruise (annual average). Observations at the transects are presented as coloured dots. (B) Worldmap showing cruise transects for GA02 (red) and GIPY5 (green, in the Atlantic sector of the Southern Ocean). Please click here to view the figure larger. Modified from Biogeosciences.

Reference:

van Hulten, M., Middag, R., Dutay, J.-C., de Baar, H., Roy-Barman, M., Gehlen, M., Tagliabue, A., and Sterl, A. (2017) Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese, Biogeosciences, 14, 1123-1152. DOI: 10.5194/bg-14-1123-2017.

Shelf sediment dissolved iron source via non-reductive dissolution in the Gulf of Alaska

Crusius and co-workers (2017, see reference below), reveal temporal and spatial variability in the sources of iron (Fe) to the northern Gulf of Alaska, based on data from cruises from three different seasons from the Copper River (AK) mouth to beyond the shelf break.  April data are the first to describe late winter Fe behavior before surface-water nitrate depletion began.  Sediment resuspension during winter and spring storms generated high “total dissolvable Fe” (TDFe) concentrations of ~1000 nmol kg-1 along the entire continental shelf, which decreased beyond the shelf break.  In July, high TDFe concentrations were similar on the shelf, but more spatially variable, and driven by low-salinity glacial meltwater.  Conversely, dissolved Fe (DFe) concentrations in surface waters were far lower and more seasonally consistent, ranging from ~4 nmol kg-1 in nearshore waters to ~0.6-1.5 nmol kg-1 seaward of the shelf break during April and July, despite dramatic depletion of nitrate over that period. The April DFe data can be simulated using a simple numerical model that assumes a DFe flux from shelf sediments, horizontal transport by eddy diffusion, and removal by scavenging.  Calculations suggest dust is an important Fe source beyond the shelf break.

17 Crusius lFigure:  Seasonal and spatial variability in Fe in the northern Gulf of Alaska: a) Sampling region in the northern Gulf of Alaska extending from the Copper River Mouth to ~50 km beyond the shelf break.  The surface water transect was carried out along the line defined by the green dots (which define sampling stations).  This is superimposed upon a MODIS image from 9 April, 2010 that shows resuspended sediments (light blue) landward of the 500-m depth contour (orange line).  b) Surface water total dissolvable Fe (TDFe) concentrations and salinity plotted versus distance from shore during April, May and July.  c) Dissolved Fe (DFe) data (blue squares) from April, along with several time-dependent model simulations that bracket the data, with varying flux of DFe from the shelf sediments, horizontal eddy diffusion, and removal by chemical scavenging. Click here to view the figure larger.

Reference:

Crusius, J., A. W. Schroth, J. A. Resing, J. Cullen, and R. W. Campbell (2017), Seasonal and spatial variabilities in northern Gulf of Alaska surface-water iron concentrations driven by shelf sediment resuspension, glacial meltwater, a Yakutat eddy, and dust, Global Biogeochem. Cycles, 31, doi:10.1002/2016GB005493.

Surprising cadmium isotope results north of the Subantarctic Front in the South West Atlantic Ocean

Xie and co-workers (2017, see reference below) report cadmium (Cd) isotopic compositions from five stations and 15 tow-Fish surface waters from 50ºS to the equator along GEOTRACES GA02 Leg 3. Along this transect, the coupled Cd concentrations and Cd isotopes reflect classical behaviour dominated by preferential uptake of light Cd by the biological species at the surface, release in the twilight zone and water mass mixing deeper. Surprisingly, ε112/110Cd displays a "flattening off" pattern in the surface and subsurface waters of stations north of the Subantarctic Front, while Cd concentrations decrease to low levels; this observation can be extended to the global Cd isotope dataset at hand for Cd concentrations below a nominal value of 0.1 nmol kg-1. Two explanations are proposed for this behaviour: 1) either Cd is bound by organic detritus, colloids or ligands and passes the 0.2μm filtration of the samples, products which could dominate ε112/110Cd over that of the dissolved pool; or 2) the ε112/110Cd values result from a simple open system, steady-state model for the (sub)surface layer, fed with an in-flux of Cd from deeper waters.

17 Xie
Figure:
Map of the five super stations (color circles) and tow-Fish surface sites (crosses) for Cd isotopes along GA02 Leg 3 (Left), and Cd isotope systematics in the western South Atlantic (Middle) and the global ocean (right). Color circles for profile samples, and open circles for tow-Fish seawater samples (this study). In the right-hand panel: grey diamonds – North Atlantic (Boyle et al., 2012; Conway and John 2015a; John and Conway, 2014); open triangles – North (Conway and John, 2015b; Ripperger et al., 2007) and South Pacific (New Zealand (Gault-Ringold et al., 2012), South China Sea and Philippine Sea (Yang et al., 2012, 2014)); open squares – Southern Ocean (Abouchami et al., 2011, 2014; Xue et al., 2013). Red dashed lines in the middle and right-hand panels schematically highlight the evolution of seawater e112/110Cd toward low Cd concentrations. Error bars (2s) are shown. Click here to view the figure larger.

Reference:

Xie, R. C., Galer, S. J. G., Abouchami, W., Rijkenberg, M. J. A., de Baar, H. J. W., De Jong, J., & Andreae, M. O. (2017). Non-Rayleigh control of upper-ocean Cd isotope fractionation in the western South Atlantic. Earth and Planetary Science Letters (Vol. 471). DOI: 10.1016/j.epsl.2017.04.024

Low iron sulfide precipitation rate in hydrothermal fluids during the early stage of mixing

Waeles and co-authors (2017, see reference below) report for the first time on the dissolved-particulate partition of iron (Fe) after in situ filtration at the early stage of mixing of hydrothermal fluids with seawater. This study was performed at three hydrothermal fields on the Mid-Atlantic Ridge (Lucky Strike, TAG and Snakepit). For the different vents examined, Fe predominantly occurred (>90%) in the dissolved fraction and dissolved Fe showed a strictly conservative behavior, arguing for low iron-bearing sulfide precipitation in basalt-hosted systems with low Fe:H2S ratios. The small part of Fe being precipitated as sulfides in the mixing gradient (<10%) is restricted to the inclusion of Fe in minerals of high copper (Cu) and zinc (Zn) content because the kinetic of pyrite formation is slow compared to the time scale of mixing processes. Their works also show that secondary venting, i.e. lower temperature clear smokers and diffusive venting, is a source of Fe-depleted hydrothermal fluids and provide new constrains on Fe fluxes from hydrothermal venting one of the main present issue of the GEOTRACES programme.

17 Waeles l
Figure:
 Dissolved Fe (dFe), particulate Fe (pFe) and other chemical species concentrations measured at Aisics, a black smoker vent on the Lucky Strike vent field. Concentrations are given as a function of temperature and dissolved Mn (dMn) which is used as the conservative tracer. a) Dissolved Fe occurs essentially as Fe(II) species and coexists with sulfide until the coldest part of the mixing gradient due to the kinetically limited formation of pyrite particles. b) As opposed to Fe, Zn and Cu precipitate quantitatively before venting and/or during the very early stage of mixing (T > 150°C); Zn and Cu were mainly found as particulate rather than as dissolved species over the studied gradients. c) The data also showed that secondary venting, i.e. lower temperature auxiliary smokers and diffusive venting, is a source of Fe-depleted fluids. Click here to view the figure larger.

Reference:

Waeles, M., L. Cotte, B. Pernet‐Coudrier, V. Chavagnac, C. Cathalot, T. Leleu, A. Laës‐Huon, A. Perhirin, R. Riso, and P. Sarradin (2017), On the early fate of hydrothermal iron at deep‐sea vents: a reassessment after in‐situ filtration, Geophysical Research Letters. DOI: 10.1002/2017GL073315.

 

Complex cobalt story in the Mediterranean Sea

Dulaquais and co-authors (2017, see reference below) propose the first comprehensive study of cobalt behaviour in the Mediterranean Sea, work conducted in the framework of MedBlack GEOTRACES cruise (GA04N). They measured the following cobalt (Co) fractions: soluble (sCo<0.02 μm), dissolved (DCo<0.2 μm), colloidal (cCo, as DCo minus sCo), and particulate (pCo>0.2 μm).

While soluble Co is the predominant form (90%) of the dissolved Co in the Mediterranean Sea, colloidal Co and particulate Co show a close distribution, yielding the authors to suspect a biogeochemical link between these two fractions.

More striking is the scavenged-like profile observed everywhere, with up to 350 nM dissolved Co concentrations in the surface waters dropping to 45 nM at depth. Such behaviour results from several mechanisms. High-surface Co inputs at Gibraltar Strait are horizontally transported by the Mediterranean circulation, surface dissolved Co is stabilized in a soluble form and biogenic particulate Co is very rapidly regenerated: all these processes concur to the accumulation of dissolved Co in the surface layers. Conversely, low particulate Co export, low remineralization of biogenic particulate Co at depth, and removal of dissolved Co by scavenging prevented its accumulation in the intermediate and deep sea.

17 Dulaquais l

Figure: Distribution and partitioning of dissolved cobalt (DCo) in the Mediterranean Sea. We measured DCo along the GA04N section (a) and observed a scavenged like profile in all the different sub-basins of the Mediterranean Sea (b). In the Med, DCo was almost entirely composed of soluble cobalt (sCo) and colloids represented only 10% of the DCo pool (c). Resulting from high recycling rate and its stabilization under a soluble form, surface DCo concentrations increased eastward with ageing of surface waters (d). Differently accumulation of DCo by remineralization in the intermediate water was not discernable (d) and surprisingly the zonal distribution of DCo in the deep sea showed homogenous concentrations (d). We related these features to scavenging rates depth dependents and of different magnitude in the two Mediterranean basins as well as to the fast Mediterranean circulation that homogenize concentrations in the deep sea. Click here to view the figure larger.

Reference:

Dulaquais, G., Planquette, H., L’Helguen, S., Rijkenberg, M. J. A., & Boye, M. (2017). The biogeochemistry of cobalt in the Mediterranean Sea. Global Biogeochemical Cycles, 31(2), 377–399. DOI: 10.1002/2016GB005478

 

 

A new method for simultaneous analysis of nickel, copper and zinc isotopes in seawater

Takano and co-workers (2017, see reference below) have developed a new method to determine nickel (Ni), copper (Cu) and zinc (Zn) isotopes in seawater. This method is very simple and rapid only using single chelating extraction and single anion exchange. First, target metals are extracted from seawater by NOBIAS Chelate PA-1 resin. Then, target metals are purified by anion exchange. Finally, isotope ratios are measured by MC-ICPMS. The analyses of GEOTRACES reference samples showed this method is precise and accurate. Vertical profiles of δ60Ni, δ65Cu, and δ66Zn in the South Pacific Ocean were revealed using this method.

This method is expected to accelerate isotopic research and contribute to our understanding of biogeochemical cycling in the ocean.

17 Takano l

Figure: A schematic diagram of the procedure for isotopic analysis of dissolved Ni, Cu, and Zn in seawater (upper panel). Depth profiles of dissolved δ60Ni, δ65Cu, and δ66Zn at GR15 station (30.00°S, 170.00°W) in the subtropical South Pacific Ocean. The error bars represent 2 standard errors for the MC-ICPMS measurement (lower panel). Click here to view the figure larger.

Reference:

Takano, S., Tanimizu, M., Hirata, T., Shin, K.-C., Fukami, Y., Suzuki, K., & Sohrin, Y. (2017). A simple and rapid method for isotopic analysis of nickel, copper, and zinc in seawater using chelating extraction and anion exchange. Analytica Chimica Acta, 967, 1–11. DOI: 10.1016/j.aca.2017.03.010

 

GEOTRACES intercalibration of the stable silicon isotope composition of dissolved silicic acid in seawater

Dissolved silicon (Si) is a major oceanic nutrient and variations of its stable isotope values are reflecting the intensity of surface primary production. As for other isotopes, agreement between the different laboratories is crucial. Here, the first intercalibration study of the stable silicon (Si) isotopes in seawater (δ30Si(OH)4) is presented as a contribution to the international GEOTRACES programme. 

Eleven laboratories from seven countries participated in the study. Si isotope measurements were performed on three different mass spectrometer types Neptune MC-ICP-MS, Nu Plasma MC-ICP-MS and a MAT 252 IRMS.

Two seawater samples from the North Pacific subtropical gyre (Station ALOHA) collected at 300 m (9 μmol Si L-1; ALOHA300) and at 1000 m (113 μmol Si L -1; ALOHA1000) water depth were analyzed.

Agreement among laboratories is considered very good with mean values for δ30Si(OH)4:

  • ALOHA300 = +1.68 ± 0.35 (2 s.d.; Median: +1.66 ± 0.13)
  • ALOHA1000 = +1.24 ± 0.20 (2 s.d.; Median: +1.25 ± 0.06)

For future studies analyzing δ30Si(OH)4 in seawater it is recommended to analyze ALOHA300 and ALOHA1000 and report these results to facilitate and evaluate comparability of data between laboratories.

17 Grasse l

Figure: δ30Si(OH)4 results from all groups for ALOHA300 (red circles) and ALOHA1000 (blue circles). The vertical lines indicates the mean value of all measurements for ALOHA1000 (blue) and for ALOHA300 (red). The data points represent the individual δ30Si(OH)4 values for Si isotopes measurements. Short vertical solid lines are the means obtained by individual laboratories for the two samples. Uncertainty in the mean for all measurements (2 s.d.) is indicated by the horizontal bars at the top of the figure (Modified from Grasse et al. 2017, JAAS). Click here to view the figure larger.

Reference:

Grasse, P., Brzezinski, M. A., Cardinal, D., de Souza, G. F., Andersson, P., Closset, I., et al. (2017). GEOTRACES inter-calibration of the stable silicon isotope composition of dissolved silicic acid in seawater. Journal of Analytical Atomic Spectrometry, 32, 562–578. DOI: 10.1039/C6JA00302H

Important external dissolved iron inputs, HNLC water formation and strong biological seasonality explained in the North Pacific Ocean

Subarctic Pacific is known as High Nutrient, Low Chlorophyll (HNLC) area, where phytoplankton growth is limited by dissolved iron (DFe) availability. The biological activity in the surface waters is also characterized by a marked seasonality. Nishioka and Obata (2017, see reference below) propose a detailed DFe zonal section across the North Pacific (~47°N), realized in the framework of Japan-GEOTRACES programme. Their data reveal important external Fe sources at mid-depth from the Sea of Okhotsk, and the continental margin followed by long range transport in the formation of Fe-rich intermediate water, although these enriched waters do not reach the Alaskan Gyre. This work also enlights why surface macronutrient consumption differ between the western and eastern gyre. Both HNLC water formation in the subarctic Pacific and high amplitude of seasonal variation in biogeochemical parameters in the western subarctic gyre are explained.

17 Nishioka

Figure: Dissolved Fe-rich intermediate water is transported laterally and distributed across the western subarctic gyre, over 2000 km (upper panel). The spatial pattern of Fe to nutrient stoichiometry supplied from the intermediate water to the surface (lower panel). Line P data is cited by Martin et al. 1989 and Nishioka et al. 2001. (OKTZ: Oyasio-Kuroshio transition zone, WSG: western subarctic gyre, CNP: central North Pacific, AG: Alaskan Gyre, AS: Alaskan stream). Click here to view the figure larger.

References:

Nishioka, J. and H. Obata, 2017, Dissolved iron distribution in the western and central subarctic Pacific: HNLC water formation and biogeochemical processes, Limnology and Oceanography, doi: 10.1002/lno.10548

Nishioka, J., S. Takeda, C. S. Wong, and W. K. Johnson. 2001. Size-fractionated iron concentrations in the northeast Pacific Ocean: distribution of soluble and small colloidal iron. Mar. Chem. 74: 157-179. doi:10.1016/S0304-4203(01)00013-5

Martin, J. H., R. M. Gordon, S. Fitzwater, and W.W. Broenkow. 1989. VERTEX: Phytoplankton/iron studies in the Gulf of Alaska. Deep Sea Res. Part I Oceanogr. Res. Pap. 36(5): 649–680. doi:10.1016/0198-0149(89)90144-1

 

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