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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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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 throrium-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

More realistic oceanic particle field improved the thorium-230 and protactinium-231 modeling

Thorium-230 (230Th) and protactinium-231 (231Pa) are two geochemical tracers extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. A key feature in reproducing their distributions by modelling is to understand and constrain as good as possible the scavenging processes, which means: 1) having the good adsorption-desorption kinetic rates and 2) describing the up to date best particle field. The later was challenged by the NEMO-PISCES team who considerably improved the particle field description of the NEMO-PISCES model. This recent development allowed van Hulten and co-workers (2018, see reference below) to propose a new simulation of 230Th and 231Pa using a version called NEMO-ProThorP 0.1 in which the dust lithogenic particles were added. Although nepheloid and hydrothermal particles are still missing to better simulate the particle field this new version provides satisfying distributions of both tracers. Thanks to the GEOTRACES field database, comparison of the model results to the measured ones shows more realistic partition coefficients than what was simulated so far. Although further improvements are still needed, this work is an important step forward in our understanding of these tracer behaviors in the ocean.

18 vanHulten l

Figure: Modelled dissolved thorium-230 activity at four depth level (mBqm−3 ); observations are represented as discs on the same colour scale. Click here to view the figure larger.

Reference: 

van Hulten, M., Dutay, J.-C., & Roy-Barman, M. (2018). A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1). Geoscientific Model Development, 11(9), 3537–3556. DOI: http://doi.org/10.5194/gmd-11-3537-2018

The residence times of trace elements determined in the surface Arctic Ocean during the 2015 US Arctic GEOTRACES expedition

Data collected during the US Arctic GEOTRACES expedition in 2015 (along GEOTRACES section GN01) were used to estimate the mean residence time of dissolved trace elements (iron-Fe, manganese-Mn, nickel-Ni, cadmium-Cd, sinc-Zn, copper-Cu, lead-Pb, vanadium-V) in surface water with respect to atmospheric deposition. The calculations utilized mixed layer trace element (TE) inventories, aerosol solubility determinations, and estimates of the atmospheric trace element flux into the upper ocean. The later was estimated by the product of the beryllium-7 (7Be) flux (determined by the ocean 7Be inventory) and the TE/7Be ratio of aerosols. The breadth of measurements afforded by the GEOTRACES program allowed these data to be assembled.

The distribution of residence times with respect to atmospheric input across the expedition track informs us of additional sources or sinks for each element. For example, the residence time of dissolved Fe was ~ 20–40 y for most stations. However, several stations that display a longer, oceanographically inconsistent apparent Fe residence time of ~300–500 years are likely influenced by additional input from the Transpolar Drift (TPD), which has been shown to convey shelf water properties to the central Arctic. This was seen for Cu, Ni and Zn as well, but contrastingly V and Pb show a decrease in the apparent residence times within the TPD waters, suggesting removal of these elements from the source region of the TPD.

18 Kadko
Figure:
Left column: Residence time plotted against latitude for elements showing enrichments in the TPD (Fe, Cu, Ni, Zn). Right column: Residence time plotted against latitude for elements showing a deficiency in the TPD (V, Pb) and for Mn and Cd which do not display an apparent relationship with the TPD. The shaded area represents the stations influenced by the TPD. The horizontal red dashed lines indicate residence times from prior literature in other ocean basins. Click here to view the figure larger.

Reference:

Kadko, D., Aguilar-Islas, A., Bolt, C., Buck, C. S., Fitzsimmons, J. N., Jensen, L. T., W. M. Landing, C. M. Marsay, R. Rember, A. M. Shiller, L. M. Whitmore, Anderson, R. F. (2018). The residence times of trace elements determined in the surface Arctic Ocean during the 2015 US Arctic GEOTRACES expedition. Marine Chemistry. DOI: http://doi.org/10.1016/J.MARCHEM.2018.10.011

 

Ever wonder how long your favorite element remains in the ocean before it’s gone again?

This timeframe, sometimes called a residence time, ranges from decades for the most reactive trace elements to millions of years for the most unreactive elements such as the major components of sea salt. The residence time is often difficult to constrain and involves estimating how much of an element is presently in the ocean (i.e., the inventory) as well as the magnitude of the total supply rate or removal rate of the element. In the study published by Hayes and co-authors in Global Biogeochemical Cycles (2018, see reference below), a replacement time (or residence time with respect to supply) can be quantified using large synthesized GEOTRACES datasets from the North Atlantic which can precisely define the inventory of trace elements as well as their supply rate using radioactive tracers. In particular, their method suggests an ocean replacement for iron that is only 6 years, meaning this micronutrient element may be cycling much more quickly than previous estimates have suggested and will provide a target for ocean models to understand how this element is removed from the ocean in terms of biological uptake or abiotic scavenging.

18 Hayes2
Figure:
(Right) Replacement time of dissolved Fe across the GEOTRACES cruise section GA03. This replacement time is how long it would take to replace all of the iron in the North Atlantic Ocean with a source of iron derived from the quantifiable delivery of the crustal isotope thorium-232 to the ocean. (Left) Map showing the GEOTRACES section GA03 in the Atlantic Ocean. Click here to view the figure larger.

Reference:

Hayes, C. T., Anderson, R. F., Cheng, H., Conway, T. M., Edwards, R. L., Fleisher, M. Q., Ho, P., Huang, K.-F., John, S., Landing, W.M., Little, S. H. Lu, Y., Morton, P. L., Moran, S. B., Robinson, L. F., Shelley, R. U., Shiller, A. M., Zheng, X.-Y. (2018). Replacement Times of a Spectrum of Elements in the North Atlantic Based on Thorium Supply. Global Biogeochemical Cycles, 32(9), 1294–1311. DOI: http://doi.org/10.1029/2017GB005839

 You can also read the Research Spotlight about this paper published on Eos.org: https://eos.org/research-spotlights/a-novel-approach-reveals-element-cycles-in-the-ocean

Methylmercury subsurface maxima explain mercury accumulation in Canadian Arctic marine mammals

Mercury (Hg) concentrations in Canadian Arctic marine mammals were monitored during the last four decades and found to be highly elevated, frequently exceeding toxicity thresholds. Mercury concentrations in marine biota are also found to be generally higher in the western part of the Canadian Arctic than in the east. Thanks to the Canadian Arctic GEOTRACES cruise, Wang and co-authors (2018, see reference below) carried out a high-resolution total mercury and methylmercury (MeHg) measurements from the Canada Basin in the west to the Labrador Sea in the east. Total Hg concentrations show a distinctive longitudinal gradient along the transect with concentrations increasing from the Canada Basin eastward through the Canadian Arctic Archipelago to Baffin Bay, which is opposite to the spatial gradient in mammal Hg.

What is remarkable is the distribution patterns of MeHg. The authors found that MeHg concentrations are lowest at the surface, peak in a subsurface layer (~100–300 m), and subsequently decrease towards the bottom. Longitudinally, the subsurface MeHg peak value is highest in the western part of the section and decreases towards the east, eventually reaching its lowest values in the Labrador Sea. Given that it is MeHg that accumulates and biomagnifies in marine biota and that the MeHg subsurface maxima lie within the depths where Arctic marine biota reside, this gradient readily explains the spatial distribution of Hg levels observed in Canadian Arctic mammals.

Elucidating the processes that generate and maintain this subsurface MeHg maximum is the next challenge...

18 Wang l
Figure: Mercury (Hg) concentrations in the marine food web and seawater across the Canadian Arctic and Labrador Sea (Wang et al. 2018). Upper panel: Map of Hg (as total Hg or monomethylmercury) concentrations in two zooplankton species, ringed seals and polar bears along the Canadian GEOTRACES transect based on data collected between 1998 and 2012. Lower panel: Methylmercury (MeHg) concentrations in seawater along the same transect as determined during the 2015 Canadian Arctic GEOTRACES.  Click here to view the image larger.

Reference:

Wang, K., Munson, K. M., Beaupré-Laperrière, A., Mucci, A., Macdonald, R. W., & Wang, F. (2018). Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic. Scientific Reports, 8(1), 14465. DOI:  http://doi.org/10.1038/s41598-018-32760-0

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