Some recent GEOTRACES science findings are reported below.
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Icebergs have been speculated to constitute one of the largest fluxes of iron (Fe) into the polar oceans since the 1930s and thus recent increases in ice discharge around the world could potentially change Fe availability in the ocean. But how much Fe is in an iceberg? As part of an international collaboration involving several cruises over the past 5 years including the GEOTRACES Fram Strait GN05 section Hopwood et al., (2019, see reference below) report the concentrations of Fe in ice from over 10 glaciated regions around the world. The global mean iceberg Fe content is found to be similar to, or slightly higher than, limited earlier estimates. However, a critical insight is the highly uneven distribution of this Fe with the ‘dirtiest’ 4% of samples collected accounting for over 90% of the cumulative Fe. Investigating how these ‘dirty’ layers are formed and their fate in the ocean is therefore essential to determining the significance of icebergs for marine primary production.
Figure: Ice from around the world is found to have a highly variable total dissolvable Fe content ranging from 2 nM to 2 mM. Click here to view the figure larger.
Mark J. Hopwood, Dustin Carroll, Juan Höfer, Eric P. Achterberg, Lorenz Meire, Frédéric A. C. Le Moigne, Lennart T. Bach, Charlotte Eich, David A. Sutherland & Humberto E. González, (2019) High variability is evident even within individual geographic regions. Reference: Highly variable iron content modulates iceberg-ocean fertilisation and potential carbon export, Nature Communications, 10, 5261 DOI: https://doi.org/10.1038/s41467-019-13231-0
In the frame of GEOTRACES cruise GA08, Rahlf et al. (2020, see reference below) determined seawater profiles of dissolved radiogenic neodymium isotope signatures (εNd) and neodymium concentrations across the restricted western Angola Basin and along the northern Cape Basin to investigate mixing and provenance of water masses. Nd isotope compositions of deep water masses in both basins primarily reflect conservative water mass mixing, except the central Angola Basin, which is significantly overprinted by terrestrial inputs. Bottom waters of the Cape Basin show some excess Nd concentrations released from detrital sediments, however, without significantly changing their Nd isotopic compositions ranging between εNd -9.6 and 10.5. Highly unradiogenic εNd signatures of up to -17.6 in the upper water column originate from a west African coastal plume in the Angola Basin and from Mozambique Channel surface waters advected into the Cape Basin.
Figure: Nd isotope distributions in the western Angola Basin (left section) and the northern Cape Basin (right section) along the cruise track of GA08, combined with isotope data obtained further south from cruise ANT-XXIIV/3 (Stichel et al., 2012). Dashed lines indicate approximate boundaries of the prevailing water masses. The figure is adopted from Rahlf et al. (2020). Click here to view the figure in new window.
Rahlf, P., Hathorne, Ed., Laukert, G., Gutjahr, M., Weldeab, S., Frank, M. (2020). Tracing water mass mixing and continental inputs in the southeastern Atlantic Ocean with dissolved neodymium isotopes. Earth Planet. Sci. Lett., in press, DOI: 10.1016/j.epsl.2019.115944.
Stichel, T., Frank, M., Rickli, J., Haley, B.A. (2012). The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean. Earth Planet. Sci. Lett. 317–318, 282–294.
The importance of the cycle and speciation of nitrate and its isotopes (δ15N) in the ocean does not have to be demonstrated anymore. In an attempt to overcome the difficulty to compare the results of N/δ15N cycle models to a sparse set of data, Rafter and co-workers propose an original approach, based on artificial intelligence (AI) methods.
They use a compilation of 12,277 published δ15N measurements together with climatological maps of physical and biogeochemical tracers to create a surface to-seafloor map of δ15N using an ensemble of artificial neural networks (EANN). In other words, they train the seawater parameters to deduce a δ15N value at a given location and depth taking into accounts the climatological values. The strong correlation (R2 > 0.87) and small mean difference (< 0:05 ‰) between EANN-estimated and observed nitrate δ15N indicate that the EANN provides a good estimate of climatological nitrate δ15N without a significant bias. This climatology reveals large-scale spatial patterns in nitrate δ15N and allows the quantification of regional and basin-average oceanic values of nitrate δ15N. This work demonstrates how AI tools could help to address the unavoidable deficiency of data inherent to oceanic studies, keeping in mind that they require ab initio reasonable data coverage and mostly a good understanding of the parameter fate.
Figure: (Top) Available nitrate δ15N (N isotopic composition) measurements at the time of publication. (Bottom) View of nitrate δ15N at 3500 m from two perspectives: the observed value (circles) and the model value (the contours). Click here to view the figure larger.
Rafter, P. A., Bagnell, A., Marconi, D., & DeVries, T. (2019). Global trends in marine nitrate N isotopes from observations and a neural network-based climatology. Biogeosciences, 16(13), 2617–2633. https://doi.org/10.5194/bg-16-2617-2019
The Solomon Sea is surrounded by high and intensively weathered islands. Its complex topography and hydrography make this area a potential source of trace elements to the waters flowing through it. Dissolved rare earth elements (DREE) analysis of more than 150 samples collected in the framework of the PANDORA cruise (along GEOTRACES GP12 section) showed that DREE maxima reveal significant coastal effects at some locations, consistent with dissolved aluminum (Al) and manganese (Mn) data, enrichments mostly observed in the exiting Straits and along the island coasts. A box model allows calculating and discussing a net enrichment of the dissolved REE in the lower thermocline that further partially feeds the Equatorial Under Current cold tongue. Positive Europium (Eu) anomalies* allow identifying the basaltic nature of the lithogenic enrichments.
Figure: Vertical profiles of the anomalies of Eu (Eu/Eu*) after normalization to the PAAS values, calculated following Bau et al. (2004), Bau et al. (1996). The red circles correspond the Coral Sea and Southern entrance of the Solomon Sea. Samples collected in waters flowing through four straits (Indispensable, Vitiaz, St George’s and Solomon), are colored with dark violet, black, green and dark blue, respectively.
*Eu anomaly reflects the relative variation of the Eu abundance compared to its REE neighbors: it is calculated a the ratio between the measured Eu concentration and the interpolated one between these neighbors. In this paper, positive Eu anomalies (≥1) were found whatever water mass type and sample location. The higher dEu anomalies (~1.3) at the surface likely reflect recent enrichments of basaltic material origin from surrounding volcanic islands (Grenier et al., 2013). Click here to view the figure larger.
Pham, V. Q., Grenier, M., Cravatte, S., Michael, S., Jacquet, S., Belhadj, M., Nachez, Y., Germineaud, C., Jeandel, C. (2019). Dissolved rare earth elements distribution in the Solomon Sea. Chemical Geology, 524, 11–36. DOI: https://doi.org/10.1016/J.CHEMGEO.2019.05.012
Bau, M., Koschinsky, A., Dulski, P., Heinz, J.R. (1996) Comparison of the Partitioning Behaviours of Yttrium, Rare Earth Elements, and Titanium between Hydrogenetic Marine Ferromanganese Crusts and Seawate. Geochimica et Cosmochimica Acta, Volume 60, Issue 10, May 1996, Pages 1709-1725 DOI: https://doi.org/10.1016/0016-7037(96)00063-4
Bau, M., Alexander, B., Chesley, J.T., Dulski, P., Brantley, S.L. (2004). Mineral dissolution in the Cape Cod aquifer, Massachusetts, USA: I. Reaction stoichiometry and impact of accessory feldspar and glauconite on strontium isotopes, solute concentrations, and REY distribution. Geochim. Cosmochim. Acta, DOI: http://doi.org/10.1016/j.gca.2003.08.015
Grenier, M., Jeandel, J., Lacan, L., Vance, D., Venchiarutti, C., Cros, A., Cravatte, S. (2013) From the subtropics to the central equatorial Pacific Ocean: Neodymium isotopic composition and rare earth element concentration variations. Journal of Geophysical Research: Oceans DOI: http://doi.org/10.1029/2012JC008239
Thanks to the newly launched research vessel (R/V) Isabu of the Korea Institute of Ocean Science and Technology (KIOST), and the acquisition of a contamination-free PRISTINE (NIOZ, NL) ultraclean seawater sampling system for trace elements, the Korean marine geochemists are pleased to published their first reliable trace metal (TM) results. Two cruises conducted in the Indian Ocean together with an intercalibration conducted at a GEOTRACES cross over station allowed them to assess their data quality. Thanks to these very positive results, researchers from KIOST and other academic institutes of Korea are currently conducting and planning R/V Isabu-based long-term research in offshore areas (Korean marginal seas) and the open ocean. Welcome to GEOTRACES!
Figure: A) Photographs of operating the PRISTINE ultra-clean sampler at sea and of subsampling (Upper left). B) Sampling station in the Indian Ocean in Apr. 2018 (yellow dots of lower left). Yellow star (station 19) indicates the GEOTRACES crossover station (69.54°E–5.16°S) where samples were also collected in 2017. Yellow dotted arrow line denotes the cruise track. C) Contour maps of some dissolved trace element along the western Indian Oceans (60°E and 68°S). The direction of contour (left to right) is the same as the cruise track in Fig. B. Modified from Ocean Science Journal. Click here to view the figure larger.
Kim, S. H., Ra, K., Kim, K.-T., Jeong, H., Lee, J., Kang, D.-J., Rho, T., Kim, I. (2019). R/V Isabu-Based First Ultraclean Seawater Sampling for Ocean Trace Elements in Korea. Ocean Science Journal, 1–12. https://doi.org/10.1007/s12601-019-0030-x
As part of the GEOTRACES process study HEOBI (GIpr05) van der Merwe and co-workers (2019, see reference below) conducted a thorough characterization of the labile and refractory iron phases of the particles discharged to the local seawater by the glacial erosion and rivers of Heard and Mc Donald islands (Southern Indian Ocean). They demonstrate that, together with their specific lithology, the fraction of labile iron is significantly larger for particles that experienced glacial weathering processes than for particles of submarine hydrothermal origin. Moreover, they estimate that this labile particulate iron supplied from Heard and to a lesser extent, McDonald Island, is likely the missing iron required meeting biological demand over the plateau, downstream of the islands where traditionally an intense seasonal bloom develops.
Figure: Mixed layer, mean labile fraction of pFe (ratio of labile to refractory pFe) at each of the contrasting regions within the study. Heard Island displayed a significantly higher mean labile fraction compared to all other sites, including the reference station (one-way ANOVA, Games-Howell post hoc, p < 0.01). Furthermore, in addition to the ratio of labile to refractory Fe being higher at Heard Island, the absolute concentration of labile particulate Fe (pFelab) was also significantly higher at Heard Island (115 ± 34 nM, n = 9) compared to McDonald Island (79 ± 20 nM, n = 12) (one-way ANOVA, Games-Howell post hoc, p < 0.01). Standard boxplot median (centre black line), 25th and 75th percentiles (upper and lower box limits) and 95% confidence intervals (inner fences) are indicated. Outliers are shown as open circles and defined as less than 1.5 times the interquartile range. The number of independent data points for the reference, plateau, McDonald and Heard regions were 3, 2, 8 and 9 respectively. Click here to view the figure larger.
van der Merwe, P., Wuttig, K., Holmes, T., Trull, T. W., Chase, Z., Townsend, A. T., Goemann, K., Bowie, A. R. (2019). High Lability Fe Particles Sourced From Glacial Erosion Can Meet Previously Unaccounted Biological Demand: Heard Island, Southern Ocean. Frontiers in Marine Science, 6, 332. DOI: https://doi.org/10.3389/fmars.2019.00332
Conway and co-authors (2019, see reference below) present the first evidence that anthropogenic iron (Fe) from combustion sources is visible at the basin scale, using iron isotopic composition (δ56Fe) analysis of the soluble aerosol phases collected during GEOTRACES cruise GA03 in the North Atlantic Ocean. Off Sahara, soluble aerosol samples have near-crustal δ56Fe whereas those from near North America and Europe display δ56Fe values as light as −1.6‰. Coupled to aerosol deposition modeling these results reveal that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated.
Figure. Tracing anthropogenic iron with iron isotopes (adapted from Conway et al., 2019). Panels a and b show that aerosols collected from near the Sahara have low solubility, a near-crustal iron isotope composition (beige circle) and a near-crustal Pb/Al composition (beige diamond). In contrast, those collected from near North America or Western Europe have very soluble iron, very light iron isotopes and are very enriched in Pb, indicating pollution from humans. When sampling points are overlain on output from dust modelling, it can be seen that the light iron isotopes correspond to where fossil fuel iron is expected to be important, and the crustal iron isotopes correspond to where natural dust iron is most important (panel c).
Conway, T. M., Hamilton, D. S., Shelley, R. U., Aguilar-Islas, A. M., Landing, W. M., Mahowald, N. M., & John, S. G. (2019). Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes. Nature Communications, 10(1), 2628. DOI: https://doi.org/10.1038/s41467-019-10457-w
Joint Science Highlight with US-Ocean Carbon & Biogeochemistry (US-OCB).
In a recent study, Ardyna et al (2019, see reference below) combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron (Fe) of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.
Figure: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key). Adapted from Ardyna, et al., 2019. Click here to view the figure larger.
Ardyna, M., Lacour, L., Sergi, S., d’Ovidio, F., Sallée, J.-B., Rembauville, M., Blain, S., Tagliabue, A., Schlitzer, R., Jeandel, C., Arrigo, K.R., Claustre, H. (2019). Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nature Communications, 10(1), 2451. DOI: https://doi.org/10.1038/s41467-019-09973-6