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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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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

 

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

Cadmium to phosphorus ratio in euphotic zone particulates: why does it vary?

Bourne and co-workers examine the particulate cadmium to phosphorus ratio (Cd/P) variations of 3 particle fractions (<1µm, 1-51µm and >51µm) from 50 casts covering spatial and temporal scales never reached so far for these parameters. This impressive data set allows them to study the effects of an El Niño, upwelling, large-scale in situ Fe fertilization, low-oxygen conditions, and seasonal variation on the cadmium to phosphorus (Cd:P) in particles. The authors found seasonal and spatial variation over an order of magnitude in particulate Cd:P ratios. They figure out that Cd:P tends to be higher (~1–2 mmol/mol) in particles gathered in biologically dynamic waters and is much lower (typically ~0.1 mmol/mol) in oligotrophic regions. Using a statistical approach, they find that 3 factors—local dissolved nitrate, silicate concentrations, and euphotic zone depth—can predict 59% of the variation in particulate Cd:P.

GEOTRACES GP15, at sea from September through November 2018 will collect size fractionated particulates along the 152W meridian from the Aleution Islands to Tahiti. In the future, Bourne et al. hope to use Cd:P data from those particles to compare to their predictions.

18 Bourne joint l
Figures:
(A) Map of sample locations. Samples collected using the Multiple Unit Large Volume Filtration System are marked in maroon. Samples collected during GEOTRACES GA03 and GP16 are marked in dark blue.
(B) Left: Euphotic zone average Cd:P in combined <1 and 1-51 μm particles collected during two US-JGOFS Equatorial Pacific cruises in February and August 1992. Blue circles represent the August 1992 cruise when typical upwelling conditions were present. Orange circles represent the February 1992 cruise during El Nino conditions.  
(B) Right: Euphotic zone average Cd: P values along Line P from four cruises in the combined  <1 and the 1-51 μm size fractions. Different shapes represent the different seasons. Day and night profiles were taken at OSP during the May and August 1996 cruises [Bishop et al., 1999].
(C) Seasonal euphotic zone particulate Cd:P prediction for fall (September, October, November). Peach dots represent stations in current GP15 cruise.
Click here to view the figure larger.

Reference:

Bourne, H. L., Bishop, J. K. B., Lam, P. J., & Ohnemus, D. C. (2018). Global Spatial and Temporal Variation of Cd:P in Euphotic Zone Particulates. Global Biogeochemical Cycles, 32(7), 1123–1141. DOI: http://doi.org/10.1029/2017GB005842

Bishop, J. K. B., Calvert, S. E., & Soon, M. Y. S. (1999). Spatial and temporal variability of POC in the northeast subarctic Pacific. Deep Sea Research Part II: Topical Studies in Oceanography, 46(11–12), 2699–2733. DOI: https://doi.org/10.1016/S0967-0645(99)00081-8

 

Helium-3 plumes in the deep Indian Ocean confirm hydrothermal activity

Thanks to samples collected as part of the Japanese GEOTRACES cruise in 2009 – 2010, along section GI04, Takahata and co-workers (2018, see reference below) identified a maximum helium-3 ratios value (δ3He >14%) at mid-depth (2000 - 3000 m) in the northern part (north of 30°S) of the central Indian Ocean, whereas lower ratio was found in the southern part at the same depth. These values identify an hydrothermal helium-3 plume originating from the Central Indian Ridge around 20°S flowing eastward from the ridge as previously reported in WOCE cruises. Another hydrothermal source of helium-3 is observed in the Gulf of Aden, also helping to constrain the deep circulation off the North East African coast.

18 Takahata
Figure: Vertical distribution of excess helium-3 (3He) along 70˚E of the central Indian Ocean. Two hydrothermal plumes are identified at mid-depth; one is from the Central Indian Ridge and the other from Gulf of Aden. Click here to view it larger.

Reference:

Takahata, N., Shirai, K., Ohmori, K., Obata, H., Gamo, T., & Sano, Y. (2018). Distribution of helium-3 plumes and deep-sea circulation in the central Indian Ocean. Terrestrial, Atmospheric and Oceanic Sciences, 29(3), 331–340. http://doi.org/10.3319/TAO.2017.10.21.02

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