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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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All the bioactive elements are not affected by the land-ocean gradient of the atmospheric deposition along the Eastern Pacific Zonal Transect

Atmospheric dust is considered an important source of trace elements to the ocean. As part of the Eastern Pacific Zonal Transect GEOTRACES cruise (EPZT GP16), Buck and co-workers collected 17 (3-day integrated) aerosol samples along this transect known for its low dust input. Chemical composition and elemental ratios indicate crustal sources for aluminium (Al), titanium (Ti), vanadium (V), manganese (Mn), and iron (Fe), while the analyses suggest that copper (Cu), cadmium (Cd), and lead (Pb) originate from anthropogenic emission. The concentrations of the crustal elements show sharp decreasing gradients within approximately 750 km moving west from the coast of South America. This trend was also observed in the anthropogenic elements. Interestingly, although the highest aerosol concentrations were observed over the Peru upwelling zone, fluxes estimated using a beryllium-7 (7Be) method demonstrate that atmospheric deposition was a minor source of bioavailable iron in this area, while further offshore the relative input from the atmosphere had a greater impact on surface trace metal concentrations. This work also underlines that elemental ratios were more consistent with estimates in average Andesitic crust than in bulk upper continental crust, reinforcing the necessity to carefully consider the source material used to assess trace element enrichment.

19 Buck

Figure: (Top) This map shows the GP16 cruise track from coastal South America to French Polynesia completed during October – December 2013 and indicates the spatial coverage of each collection (alternating black and white line segments). (Bottom) We can assess whether the aerosol material originated from lithogenic sources or anthropogenic sources, i.e. from blowing soil dust or from industry, by normalizing the observed concentrations of aerosol elements to the concentration of aerosol titanium (Ti). For comparison, the ratio of upper continental crust (solid line) and Andesitic crust (dashed line) are included (Taylor and McLennan, 1995). The ratio of aerosol cadmium (Cd) to Ti was two to three orders of magnitude greater than the crustal ratio throughout the cruise section with the highest ratios observed near the coast indicating an anthropogenic source. Aerosol iron (Fe) to Ti ratios displayed a crustal character in the 750 km region characterized by relatively high dust transport but decreased with distance from the continent.

Reference:

Buck, C. S., Aguilar-Islas, A., Marsay, C., Kadko, D., & Landing, W. M. (2019). Trace element concentrations, elemental ratios, and enrichment factors observed in aerosol samples collected during the US GEOTRACES eastern Pacific Ocean transect (GP16). Chemical Geology. http://doi.org/10.1016/J.CHEMGEO.2019.01.002

Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of continental crust. Reviews of Geophysics, 33: 241-265.

Using the three thorium isotope toolbox to probe the particle dynamic within an East Pacific Rise hydrothermal plume

The insoluble radiogenic isotopes of thorium (Th) are produced at a known rate in the water column via the decay of soluble uranium (234Th, 230Th) and radium (228Th) isotopes. These three isotopes are radioactive and their half-lives vary from days (234Th) to years (228Th) to tens of thousands of years (230Th). Combining their known production and decay rates with their insolubility makes them excellent tools to study the particle dynamics on a wide range of timescales.

This toolbox was successfully used by Pavia and co-workers (2019, see reference below) to study particle-dissolved exchange within the hydrothermal plume detected during the GEOTRACES GP16 cruise in the southeast Pacific Ocean. The goal of these authors was to unravel how hydrothermal activity affects the different steps characterizing the scavenging processes, i.e. adsorption and desorption onto particles, particle aggregation, sinking, and eventual sedimentation.

Their main conclusions are that: 1) particle aggregation was occurring much more rapidly in the plume, 2) hydrothermal scavenging is partially irreversible, 3) off-axis hydrothermal Th scavenging rate of 0.15yr−1, value deduced from a modelling and 4) 230Th is surprisingly more depleted than the two other isotopes. This likely reflects progressive scavenging in this region of intense hydrothermal activity and underlines the complexity of interpreting the GP16 hydrothermal plume as being solely a local phenomenon.

19 Pavia

Figure: Depletion observed in three thorium isotopes in the hydrothermal plume observed downstream of the East Pacific Rise on the GEOTRACES GP16 section in the South Pacific Ocean. Plots A), B), and C) show the depletion in each thorium isotope at stations 18 (closest to the ridge axis) to station 21 (furthest from the ridge axis). The depletion increases with increasing half-life of thorium isotope, going from 234Th (half-life = 24.1 days) showing the least depletion, followed by 228Th (half-life = 1.91 years), with 230Th (half-life = 75,587 years) the most depleted. D) Shows the map of the study area, with solid white arrows proportional to current speeds at the plume depth of 2500m, and the white dashed arrow displaying the proposed flowpath of the hydrothermal plume observed in the study, along which thorium is progressively removed from the deep ocean. Click here to view the figure larger.

Reference:

Pavia, F. J., Anderson, R. F., Black, E. E., Kipp, L. E., Vivancos, S. M., Fleisher, M. Q., Charette, M. A., Sanial, V., Moore, W. S., Hult, M., Lu, Y., Cheng, H., Zhang, P., Edwards, R. L. (2019). Timescales of hydrothermal scavenging in the South Pacific Ocean from 234Th, 230Th, and 228Th. Earth and Planetary Science Letters, 506, 146–156. DOI: http://doi.org/10.1016/J.EPSL.2018.10.038

The circulation loop in the North Atlantic and Arctic oceans depicted by the artificial radionuclides

Three articles from three cruises highlighted here!

Atlantic waters have been recently recognized to play an increasing role in reducing sea-ice extent in the Arctic Ocean at a rate now comparable to losses from atmospheric thermodynamic forcing. Beyond the Arctic Ocean, the water mass transport and transformation processes in the North Atlantic Ocean substantially contribute to the Atlantic meridional overturning circulation (AMOC). Artificial radionuclides can be used as transient tracers that provide crucial information on pathways, timescales and processes of key water masses that cannot be obtained from hydrographic properties alone. In particular, radionuclides released from the two European Nuclear Reprocessing Plants, have proven to be specifically useful to trace the circulation of Atlantic waters into the Arctic and sub-Arctic oceans. Within this context, the three recent articles by Castrillejo et al. (2018), Wefing et al. (2019) and Casacuberta et al. (2018, see references below) describe the journey of the two long-lived anthropogenic radionuclides iodine-129 (129I; T1/2=15.7 · 106 y) and uranium-236 (236U; T1/2=23.4 · 106 y) from their sources through the Arctic Ocean and into the North Atlantic Ocean. Each paper corresponds to one GEOTRACES expedition that took place between 2014 and 2016 in the North Atlantic Ocean (GA01 section), Arctic Ocean (GN04 section) and Fram Strait (GN05 section). Main results show that the combination of 129I and 236U serves very well to identify the different Atlantic branches entering the Arctic Ocean: Barents Sea Branch Water (BSBW) and Fram Strait Branch Water (FSBW). Due to the uneven mixing of 129I and 236U from the two European Reprocessing Plants of Sellafield and La Hague in the North Sea, each branch brings a different 129I/236U ratio. Furthermore, this ratio allowed identifying a third Atlantic branch evolving from the Norwegian Coastal Current (NCC), that stays within the upper Polar Mixed Layer and carries a significantly larger proportion of 129I and 236U releases from the European reprocessing plants compared to the FSBW and the BSBW. The evolution of the NCC with a strong 129I and 236U signal is further observed when it returns to the Atlantic Ocean as Polar Surface Water (PSW) in the Fram Strait. This allowed estimating a transit time of 15-22 years for the PSW flowing through the Arctic Ocean. In the subpolar North Atlantic Ocean (SPNA), an increase of 129I was observed in the deep overflow waters in the Labrador and Irminger Seas, confirming the major pathways of Atlantic Waters in the SPNA that were previously suggested by other authors: a short loop through the Nordic seas into the SPNA (8-10 years) and a longer one, which includes all the way through the Arctic Ocean (>16 years). The output of these works proves the potential of using 129I and 236U as a tool for investigations on the circulation within and exchanges between the Arctic and sub-Arctic Seas.

19 Casacuberta

Figure: (Left) Map showing the main Atlantic water circulation in the North Atlantic and Arctic oceans (black arrows). Dashed lines represent the three GEOTRACES sections sampled between 2014 and 2016: North Atlantic Ocean (GA01), Arctic Ocean (GN04) and Fram Strait (GN05). Both 129I and 236U are released from the two European Reprocessing Plants of Sellafield and La Hague (purple stars). Blue triangles represent the 129I/236U atom ratios (in red) at sampling time and the transit time of Atlantic waters (in blue) from their source in the North Sea, to the sampling location. (Right) Section plots of 129I/236U atom ratio in the three GEOTRACES sections, with black contour lines representing potential temperature. Click here to view the image larger.

References:

Casacuberta, N., Christl, M., Vockenhuber, C., Wefing, A.-M., Wacker, L., Masqué, P., Synal, H.-A., Rutgers van der Loeff, M. (2018). Tracing the Three Atlantic Branches Entering the Arctic Ocean With 129I and 236U. Journal of Geophysical Research: Oceans, 123(9), 6909–6921. DOI: http://doi.org/10.1029/2018JC014168

Castrillejo, M., Casacuberta, N., Christl, M., Vockenhuber, C., Synal, H.-A., García-Ibáñez, M. I., Lherminier, P., Sarthou, G., Garcia-Orellana, J., Masqué, P. (2018). Tracing water masses with 129I and 236U in the subpolar North Atlantic along the GEOTRACES GA01 section. Biogeosciences, 15(18), 5545–5564. DOI: http://doi.org/10.5194/bg-15-5545-2018

Wefing, A.-M., Christl, M., Vockenhuber, C., van der Loeff, M. R., & Casacuberta, N. (2019). Tracing Atlantic waters using 129 I and 236 U in the Fram Strait in 2016. Journal of Geophysical Research: Oceans. DOI: http://doi.org/10.1029/2018JC014399

Artificial intelligence helps investigate the oceanic zinc cycle

What explains the hitherto mysterious correlation between zinc (Zn) and silicon, an element not involved in the Zn cycle?

Roshan and co-workers (2018, see reference below) used an artificial neural network (ANN, a machine learning technique inspired by biological neural systems) to produce a global climatology of dissolved Zn concentration, the first such global climatology of a trace metal. They first used an ensemble of ANNs to produce climatological maps of dissolved Zn with the same spatial resolution as the World Ocean Atlas 2013 (WOA13) and then coupled these dissolved Zn maps, and those of phosphate (PO43-) and silicate (SiO44-) from WOA13, to a data-constrained ocean circulation model. They then employed a restoring model to compute the biogeochemical sources and sinks of dissolved Zn, PO43- and SiO44. The main results are:

  • The Zn: PO43- uptake ratio varies by approximately tenfold across latitude and is modulated by Fe availability;
  • Zn remineralizes like PO43- in the upper ocean, but its accumulation in deep waters exceeds that of PO43-;
  • The strong Zn-SiO44- correlation is caused by a combination of surface uptake, desorption from particles, and hydrothermal input, and is therefore completely fortuitous.

19 Roshan

Figure: This schematic shows the reconstructed internal particle-associated cycling of zinc (Zn) in the ocean, as well as some recent estimates of the external sources and sinks of Zn. Funnels represent fluxes of particulate zinc (pink; in giga mol/yr), silicon (green; in tera mol/yr) and phosphorous (cyan; in tera mol/yr), which are biologically-produced in the sunlit surface ocean and exported to the subsurface. In the subsurface, the fluxes gradually attenuate due to degradation/dissolution. Particulate zinc flux attenuates quickly like particulate phosphorus, meaning that these two compounds are associated with labile soft tissues of plankton and re-enter water column at shallower depths than silicon, which is a hard-tissue compound. However, a significant amount of dissolved zinc is supplied to the deep ocean (below 2,000 m; 0.1-2.5 giga mol/yr), which is most likely resulted from a combination of seafloor hydrothermal input and desorption of the zinc ions that are passively adsorbed on the particles at shallower depths. Circles represent the mean dissolved concentrations of the above three compounds at depths below 2,000 m of different regions, which indicate that the mentioned excess input of zinc makes its deep ocean increasing trend (according to water flow arrows) more similar to silicon than phosphorous, and eventually leads to a coincidental zinc-silicon correlation in the ocean. Also annotated are some estimates of the zinc input from rivers and dust, and those of removal to deep and shelf sediments. Click here to view the figure larger.

Reference:

Roshan, S., DeVries, T., Wu, J., & Chen, G. (2018). The Internal Cycling of Zinc in the Ocean. Global Biogeochemical Cycles, 32(12), 1833-1849. DOI:  http://doi.org/10.1029/2018GB006045

Cadmium isotopes, tracers of the cadmium sequestration as cadmium sulphide in oxygen minimum zone?

The linear relationship between the seawater cadmium and phosphate dissolved concentrations lead to use the cadmium/calcium (Cd/Ca) imprinted in calcareous archives to reconstruct the past phosphate (PO4) distributions. However, variations in the Cd/PO4 ratio between different water masses and within vertical oceanic profiles were recently identified. Among the processes that could explain these variations, sequestration of Cd into sulphide phases in microenvironments within sinking biogenic particles has been suggested as a mechanism for Cd depletion (Figure C). Guinoiseau and co-workers (2018, see reference below) experimentally tested if the cadmium sulphide (CdS) precipitation results in a fractionation of Cd isotopes. These experiments were conducted under low oxygen condition, in fresh and salty water, with variable cadmium/sulphide ratios… and they demonstrate, for the first time, an enrichment of light Cd isotopes in the precipitated CdS (Figure A) and a decrease in the fractionation factor (αCdsolution–CdS) with increasing salinity. The fractionation factor between CdS and the seawater matches remarkably the Cd isotope shift observed in modern oceanic oxygen minimum zone (Figure B). In other words, this work proposes that Cd isotopes are interesting tracers of the sequestration of Cd as CdS in low oxygen environment.

18 Guinoiseau

Figure: Identification of cadmium sulphide (CdS) precipitates as an important Cd sequestration process in the ocean. A) Determination of Cd isotope fractionation (αCdsolution-CdS in the figure) during precipitation of CdS in seawater matrix. B) Agreement between the experimental fractionation factor and the seawater isotope data recorded in oxygen minimum zone (OMZ) where CdS is prone to precipitate. C) Schematic view of CdS process occurring within sinking biogenic particles. Click here to view the figure larger.

Reference:

Guinoiseau, D., Galer, S. J. G., & Abouchami, W. (2018). Effect of cadmium sulphide precipitation on the partitioning of Cd isotopes: Implications for the oceanic Cd cycle. Earth and Planetary Science Letters, 498, 300–308. DOI: http://doi.org/10.1016/J.EPSL.2018.06.039

New BioGEOTRACES data sets: Connecting pieces of the microbial biogeochemical puzzle

Microorganisms play a central role in the transfer of matter and energy in the marine food web. Microbes depend on micronutrients (e.g. iron, cobalt, zinc, and a host of other trace metals) to catalyze key biogeochemical reactions, and their metabolisms, in turn, directly affect the cycling, speciation, and bioavailability of these compounds. One might therefore expect that marine microbial community structure and the functions encoded within their genomes might be related to trace metal availability in the ocean. The overall productivity of marine ecosystems—i.e. the amount of carbon fixed through photosynthesis—could in turn be influenced by trace metal concentrations.

For over a decade, the international GEOTRACES programme has been mapping the distribution and speciation of trace metals across vast ocean regions. Given the important relationship between trace metals and the function of marine ecosystems, biological oceanographers collaborate with GEOTRACES scientists to simultaneously probe the biotic communities at some sampling sites, allowing these biological data to be interpreted in the context of detailed chemical and physical measurements.

Two recent papers published in Scientific Data (see references below) describes two new, large-scale biological data sets that will facilitate studies aimed at understanding how microbes and metals relate to one another. Collected on four different sets of GEOTRACES cruises (see figure below), these papers report the public availability of hundreds of single cell genomes and microbial community metagenomes from the Pacific and Atlantic Oceans. The single cell genomes focus on the marine photosynthetic bacteria Prochlorococcus and Synechococcus and how they and other community members vary in different regions of the ocean. The metagenomic sequences provide snapshots of the entire microbial community found in each of these samples, yielding a broad overview of which microbes—and which genes, including those important for understanding nutrient cycling—are found in each sample. These two datasets are complementary and further enhanced by the wealth of chemical and physical data collected by GEOTRACES scientists on the same water samples. In particular, iron is of key interest, since it often limits primary productivity. These data sets can directly link iron availability with microbial community structure and gene content across ocean basins.

With these data, researchers can now ask questions such as how microbes have evolved in response to the availability or limitation of key nutrients and explore which organisms may be contributing to biogeochemical cycles in different parts of the global ocean. The extensive suite of chemical and physical measurements associated with these sequence data underscore their potential to reveal important relationships between trace metals and the microbial communities that drive biogeochemical cycles. These data sets also encourage cross-disciplinary collaborations and provide baseline information as society faces the challenges and uncertainties of a changing climate.

18 BerubeFigure: Locations and depths of samples. (a) Global map of sample locations. Single cell genomes are represented by miniaturized stacked dot-plots (each dot represents one single cell genome), with organism group indicated by color, and cells categorized as “undetermined” if robust placement within known phylogenetic groups failed due to low assembly completeness/quality or missing close references. Larger points correspond to stations on associated GEOTRACES sections where metagenomes were also collected. (b) Depth distribution of metagenome samples along each of the four GEOTRACES sections. Transect distances are calculated relative to the first station sampled in the indicated orientation. For clarity, the depth distribution of samples collected below 250 m are not shown to scale (ranging from 281–5601 m). Adapted from Berube et al. (2018) Sci. Data 5:180154 and Biller et al. (2018) Sci. Data 5:180176. Click here to view the figure larger.

Authors:  Paul M. Berube (Massachusetts Institute of Technology), Steven J. Biller (Massachusetts Institute of Technology; current affiliation: Wellesley College) and Sallie W. Chisholm (Massachusetts Institute of Technology).

Published on Ocean Carbon & Biogeochemistry (OCB)  December 2018 Newsletter.

References:

Berube, P. M. et al. (2018). Single cell genomes of Prochlorococcus, Synechococcus, and sympatric microbes from diverse marine environments. Scientific Data, 5, 180154. http://doi.org/10.1038/sdata.2018.154

Biller, S. J.,et al. (2018). Marine microbial metagenomes sampled across space and time. Scientific Data, 5, 180176. http://doi.org/10.1038/sdata.2018.176

Important spatial variation of the Particulate Organic Carbon export along the GEOVIDE section in the North Atlantic Ocean

Based on the thorium-234 (234Th) isotopes and the Particulate Organic Carbon/Thorium (POC/Th) ratios measured in small and large particles collected at 11 stations along the GEOVIDE section (GA01) using in situ pumps, exported POC flux relative the surface primary production were determined by Lemaitre and colleagues (2018, see reference below). While a factor of 9 characterizes the spatial variability of the exported flux, comparison with results obtained from other studies in the North Atlantic range from similar to up to 27 times larger values, with rapid changes over a 1-month duration, underlining the large temporal variability of the POC export fluxes in this area. The authors demonstrate significant links between this export, the stage of the bloom and the phytoplankton communities: (1) minimal fluxes when sampling occurred close to bloom peak or where picophytoplankton dominated the community, (2) high POC export fluxes in post-bloom periods and where micro- and nanophytoplankton dominated and (3) the export efficiency is mostly below 14%, in agreement with the global value of this parameter and the highest transfer efficiencies (70-80%) are found at stations where coccolithophorids dominated, thereby confirming their ballasting properties.

18 Lemaitre2 lFigures: (A) The map figure highlights the strong spatial variability of the POC export fluxes within the North Atlantic, ranging from 0.7 to 52 mmol C m-2 d-1. Export fluxes deduced during the GEOVIDE cruise (this study, diamond symbols with black borders on the map) either compare well or are in the lower range of values published in the literature. (B) The scatter figure shows the links between POC export fluxes, the stage of the bloom (illustrated by the %max. seasonal primary productivity: a value of 100% corresponds to a sampling time at the bloom peak) and the phytoplankton communities. The bloom intensity at sampling time is also indicated with the colors, indicating in-situ primary productivities. Click here to view the image larger.

Reference:

Lemaitre, N., Planchon, F., Planquette, H., Dehairs, F., Fonseca-Batista, D., Roukaerts, A., Deman, F., Tang, Y., Mariez, C., Sarthou, G. (2018). High variability of particulate organic carbon export along the North Atlantic GEOTRACES section GA01 as deduced from 234Th fluxes. Biogeosciences, 15(21), 6417–6437. DOI: http://doi.org/10.5194/bg-15-6417-2018

Local geologies imprint the Antarctic Bottom Water neodymium isotopic signatures

Dissolved neodymium (Nd) isotopes and concentrations were measured at six stations in the Australian sector of the Southern Ocean, targeting the study of the Adelie Land Bottom Water (ALBW), a variety of Antarctic Bottom Water formed off the Adélie Land coast of East Antarctica. Lambelet and co-authors (2018, see reference below) present the first dissolved neodymium (Nd) isotope and concentration measurements for ALBW. Summertime ALBW Nd isotopic composition display εNd values of -8.9 ± 1.0, while Adélie Land Shelf Water, the precursor water mass for wintertime ALBW, displays the most negative Nd fingerprint observed around Antarctica so far (εNd = -9.9). The summertime signature of ALBW is distinct from Ross Sea Bottom Water and similar to Weddell Sea Bottom Water. This underlines that Antarctic Bottom waters are not uniform around the continent and carry Nd isotope fingerprints characteristic of their formation area (local geology). This makes these water masses traceable back in time and is hence important for paleoceanography and for the study of past climate change.

18 Lambelet l

Figures: a) Map of the sampling area, with the major fronts crossing the section at the time of the survey depicted in dark grey. b) Histogram representing εNd for bottom waters in the different sector of the Southern Ocean, underlining that Antarctic Bottom waters are not uniform around the continent and carry Nd isotope fingerprints characteristic of their formation area. Click here to view the figure larger.

Reference:

Lambelet, M., van de Flierdt, T., Butler, E. C. V., Bowie, A. R., Rintoul, S. R., Watson, R. J., Remenyi, T., Lannuzel, D., Warner, M., Robinson, L. F., Bostock, H. C., Bradtmiller, L. I. (2018). The Neodymium Isotope Fingerprint of Adélie Coast Bottom Water. Geophysical Research Letters. http://doi.org/10.1029/2018GL080074

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