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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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The role of melting-ice in driving the slowdown of circulation in the western Atlantic Ocean revealed by protactinium-thorium ratio

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. Knowing the exact timing and magnitude of the AMOC shift is important to understand the driving mechanism of such climate variability. After a thorough selection of 13 sediment cores, the authors show that the proxy Protactinium-231-Thorium-230 (231Pa/230Th) exhibits remarkably consistent changes both in timing and amplitude over the last 25 thousand years (kyr) in the West and deep high-latitude North Atlantic. This consistent signal reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, an early slowdown of circulation in the western Atlantic is observed consistent with the timing of accelerated Eurasian ice melting, followed by a persistence of this weak AMOC for another millennium, corresponding to the substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown. This work also emphasizes that 231Pa/230Th, under thorough criteria, could hold as pertinent proxy of ocean circulation. 

2018 Ng

Figure: Use of sedimentary 231Pa/230Th to interpret changes in Atlantic Meridional Overturning Circulation (AMOC) strength and its link to climate variations over the past 25 thousand years. (a) Location map of 231Pa/230Th records [1]–[13] and ice melting proxy records [A]–[C] presented in this study, (b) North Atlantic ice rafting records (IRD) and a proxy record of Eurasian meltwater discharge (BIT index), (c) selected West and high-latitude North Atlantic 231Pa/230Th records, (d) Northern Greenland temperature proxy record. The AMOC slowdown observed (c) is consistent with the timing of an increased Eurasian ice melting (b). Click here to view the figure larger.

Reference:

Ng, H. C., Robinson, L. F., McManus, J. F., Mohamed, K. J., Jacobel, A. W., Ivanovic, R. F., Gregoire, L. J., Chen, T. (2018). Coherent deglacial changes in western Atlantic Ocean circulation. Nature Communications, 9(1), 2947. http://doi.org/10.1038/s41467-018-05312-3

52 years of Benthic Nepheloid Layer data!

A data base of 2412 profiles collected using the Lamont Thorndike nephelometer from 1964 to 1984 is used to globally map turbid nepheloid layers by Gardner and co-workers (2018, see reference below). The authors compare maps from that period with maps based on data from 6392 profiles measured using transmissometers from 1979 to 2016 (see GEOTRACES science highlight about this paper ). Beyond this comparison, the final goal is to gain insight about the factors creating/sustaining Benthic Nepheloid Layers (BNLs). Eleven maps, including mean surface Kinetic Energy (KE), are discussed here. The similarity between general locations of high and low particle concentration BNLs during the two time periods indicates that the driving forces of erosion and resuspension of bottom sediments are spatially persistent during recent decadal time spans, though in areas of strong BNLs, intensity is highly episodic. This work confirms that topography, well-developed current systems, and surface KE and EKE play a role in generating and maintaining BNLs.

18 Gardner3 lFigure:  A) Excess particulate matter in “strong” nepheloid layers (> 20 μg l-1) based on transmissometer (cp) and nephelometer (E/ED) profiles.   B) Mean Kinetic Energy per unit mass, cm2 s-2, in surface waters, derived from four years of satellite altimetric data and using the geostrophic relationship (adapted from Wunsch, 2015). Black contours superimposed are Excess particulate matter in “strong” nepheloid layers (> 20 μg l-1 from Figure A). Click here to view the figure larger.

Reference:

Gardner, W. D., Richardson, M. J., Mishonov, A. V., & Biscaye, P. E. (2018). Global comparison of benthic nepheloid layers based on 52 years of nephelometer and transmissometer measurements. Progress in Oceanography, 168(May), 100–111. http://doi.org/10.1016/j.pocean.2018.09.008

Environmental changes in the Arctic Ocean are occurring now!

This is what reveals the first full transarctic section of radium-228 (228Ra) in surface waters measured during Arctic cruises along GEOTRACES transects GN04 (cruise PS94) and GN01 (cruise HLY1502) proposed by Rutgers van der Loeff and colleagues (2018, see reference below). 228Ra activities in the central Arctic have increased from 2007 through 2011 to 2015 (Kipp, et al. 2018), reflecting increased 228Ra input attributed to stronger wave action on shelves resulting from a longer ice-free season (in other words to climate change). However, the authors are going further, associating thorium-228, iodine-129, SF6, thorium-234 and polonium-210 data to their own Ra results to better disentangle the vertical (mostly biogenic) from the advected fluxes. They estimate a ventilation time of 480 years for the deep Makarov-Canada Basin, in good agreement with previous estimates using other tracers.

18 RutgersFigure: Two GEOTRACES expeditions in 2015 provided together a full section across the Arctic Ocean, crossing in surface waters the Transpolar Drift (TPD) identified by the high fraction of river water derived from Siberian rivers. 228Ra is added to the TPD from the sediments on the wide Siberian shelves. 228Ra data in surface waters measured on Healy (GN01, blue) and Polarstern (GN04, red) are in good agreement and show that 228Ra in the TPD has about doubled since earlier sections in 2011 (black circles, track in black) and 2007 (GIPY11, black squares, track not shown). Click here to view the figure larger.

Reference:

Rutgers van der Loeff, M., Kipp, L., Charette, M. A., Moore, W. S., Black, E., Stimac, I., Charkin, A., Bauch, D., Valk, O., Karcher, M., Krumpen, T., Casacuberta, N., Rember, R. (2018). Radium Isotopes Across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs. Journal of Geophysical Research: Oceans, 123(7), 4853–4873. DOI : http://doi.org/10.1029/2018JC013888

Kipp, L.E., Charette, M.A., Moore, W.S., Henderson, P.B., Rigor, I.G., 2018. Increased fluxes of shelf-derived materials to the central Arctic Ocean. Science Advances 4, DOI: http://dx.doi.org/10.1126/sciadv.aao1302

You can also read the EOS.org magazine Editors' highlight "Increased Release Rates of Radium Isotopes on Arctic Shelves": https://eos.org/editor-highlights/increased-release-rates-of-radium-isotopes-on-arctic-shelves

Rare Earth Elements are less and less natural tracers in the ocean

This verdict is well illustrated by the recent study of Rodrigo Pedreira (2018, see reference below) off the North East Brazilian coast. His Rare Earth Elements (REE) data reveal marked positive Gadolinium (Gd) anomaly which reflects the release of Gd in hospital and domestic effluents. Indeed, this element is used as contrasting agent in magnetic resonance imaging (MRI) to enhance clarity of diagnosis. The authors estimated that between 700 and 2000 g Gd d-1 are discharged into Tropical and South Atlantic waters due to submarine outfalls. While the Gd complex behaves conservatively and can be used as a new tracer for sewage discharges from submarine outfalls in ocean waters, it is also clear that high technology wastes are distorting the use of REE as "natural" tracers.

 GEOTRACES Highlights Opcao2 HR

Figure : Sampling took place (a) off the northeastern coast of Brazil, whereas discharges of submarine outfalls located along the coast of Brazil (a insert) were used to estimate order-of-magnitude emissions of anthropogenic Gd to the Atlantic Ocean. A plume of Gd anomalies (Gdsn/Gdsn*) can be clearly identified for surface waters (b). Positive Gd anthropogenic anomalies are observed in shale (PAAS)-normalized REE patterns (c) for surface waters (S) in most stations in the proximity of submarine outfalls (ERVS and EBVS). Click here to view the figure larger.

Reference :

Pedreira, R. M. A., Pahnke, K., Böning, P., & Hatje, V. (2018). Tracking hospital effluent-derived gadolinium in Atlantic coastal waters off Brazil. Water Research, 145, 62–72. DOI : http://doi.org/10.1016/j.watres.2018.08.005

Has the role of atmospheric dust as a control on productivity in oligotrophic regions been overestimated?

Dust particles settling into the surface of open ocean environments are for years assumed to provide nutrients to these distant nutrient-limited areas.

Torfstein and Kienast (2018, see reference below) present a unique high-resolution coupling between dust concentrations (hourly resolution) and chlorophyll-a concentrations (daily time scale resolution) across a 4-year period in the deep, nutrient-poor water column of the north Red Sea, which seriously questions this hypothesis.

This long time series study reveals that there is no correlation between dust and surface chlorophyll-a concentrations, regardless of the time of year, or the possible lags between the dust settling and the oceanic response.

The authors conclude that the role of atmospheric dust as a control on productivity could have been previously overestimated.

18 Torfstein

Figure: The study took place in (a) the Gulf of Aqaba, northern Red Sea, and combined monthly and daily resolved records of  chlorophyl-a concentrations sampled at (b) the Interuniversity Institute for Marine Sciences (IUI) and station A (29°280N, 34°560E, water depth 700 m), respectively. The distance between the two sites is approximately 4 km. Dust time series were recorded at the IUI and its vicinity at weekly, daily and hourly resolution.  (c) A comparison between water temperatures and vertical chlorophyll-a (chl-a) concentrations at station A (monthly resolution), daily and monthly chl-a surface concentrations (μg/L), and dust concentrations (μg/m3) at a weekly, 6 hour and 1 hour time resolution, between January 2012 and August 2016, imply that no statistically significant correlation exists between dust patterns and chl-a concentrations. Click here to view the figure larger.

Reference:

Torfstein, A., & Kienast, S. S. (2018). No correlation between atmospheric dust and surface ocean chlorophyll-a in the oligotrophic Gulf of Aqaba, northern Red Sea. Journal of Geophysical Research: Biogeosciences, 123. https://doi.org/10.1002/2017JG004063

Influence of particle composition on the rate constants of thorium adsorption

Chemical species are constantly exchanged between seawater (solution, D) and particles (solid material, P). This continuous D-P exchange is a key process determining the chemical composition of the ocean. Particles are heterogeneous materials, made of (i) biological material from the surface ocean, (ii) lithogenic material from external inputs to the ocean, and (iii) authigenic (oxyhydr)oxides precipitation in the water column. Understanding the role of each of these phases in driving the D-P exchange is therefore a major issue.

Lerner and co-workers (2018, see reference below) propose to disentangle the particle composition effect on the thorium adsorption rate constant k1 using two different regression models. Model I considers biogenic particles, lithogenic particles, Mn (oxyhydr)oxides, and Fe (oxyhydr)oxides as regressors, and k1 as the regressand. Model II considers ln(biogenic particles), ln(lithogenic particles), ln(Mn (oxyhydr)oxides), and ln(Fe (oxyhydr)oxides) as regressors, and ln(k1) as the regressand, where ln() denotes the natural logarithm. Thus, models I and II posit that the effects of particle phases on k1 are, respectively, additive and multiplicative. Regressions are considered separately in two regions of the North Atlantic: an upwelling region off the western margin of Mauritania, and an open-ocean region east of Bermuda.

The authors find that model II better describes the effect of particle composition on k1. Based on this regression model, the authors find that Mn (oxyhydr)oxides have a stronger effect on k1 in the open-ocean region, and biogenic particles have a stronger effect on k1 in the upwelling region.

18 Lerner

Figure: Relative Importance (RI) of particle phases for influencing the thorium adsorption rate constant, k1, under the additive model (upper panels) and the multiplicative model (lower panels). Results shown at all stations (a,d), open-ocean stations (b,e), and Mauritanian upwelling stations (c,f). The red and blue bars show two different methods to obtain RI values. Biogenic particles and Mn (oxyhydr)oxides have the strongest relationship to k1, depending on the model and stations considered. Click here to view the figure larger.

Reference:

Lerner, P., Marchal, O., Lam, P. J., & Solow, A. (2018). Effects of particle composition on thorium scavenging in the North Atlantic. Geochimica et Cosmochimica Acta, 233, 115–134. http://doi.org/10.1016/J.GCA.2018.04.035

Coupling and decoupling of barium and radium-226 along the GEOVIDE GEOTRACES section

Because radium-226 (226Ra) and barium (Ba) are both alkaline earth metals and thus display similar chemical behaviours, studying their fate in contrasting environments is of prime interest. With this aim, Le Roy and co-workers (2018, see reference below) realised a high resolution description of the distribution of these tracers along the GEOVIDE section (GA01, see section map here). Using an optimum multi parameter analysis of their data, they figured out that:

- both tracers can be considered as conservative of water mass transport in the deep open ocean part of the section;

- non conservative and decoupled behaviours are observed at the ocean boundaries, namely:

  • in the eastern part of the section, the deepest waters (North East Atlantic Deep Water) display high Ba and 226Ra concentrations, which could reflect either an accumulation in this "old" water mass or diffusion from the sediment below;
  • contrastingly, depletion of both tracers in the upper layers of the West European basin likely reflects biological stripping;
  • at the land-ocean contact, as for example close to the Greenland and Newfoundland coasts, marked decoupled behaviours reflect the different main input sources of both tracers (rivers for Ba, sediment for 226Ra).

18 LeRoy
Figure:
Distribution of dissolved 226Ra/Ba ratio (a) measured along the GEOVIDE section and the difference between the measured concentrations and those calculated by the optimum multiparameter (OMP) analysis for 226Ra (b), Ba (c) and  226Ra/Ba ratio (d) along the GA01 section. Positive anomalies (in Red) reflect recent tracer addition, while negative ones (in blue) reflect recent tracer removal. Station numbers are found above the panels. Click here to view the figure larger.

Reference:

Le Roy, E., Sanial, V., Charette, M. A., van Beek, P., Lacan, F., Jacquet, S. H. M., Henderson, P. B., Souhaut, M,. García-Ibáñez, Maribel I., Jeandel, C.,Pérez, F., F., Sarthou, G. (2018). 226Ra–Ba relationship in the North Atlantic during GEOTRACES-GA01. Biogeosciences, 15(9), 3027–3048. http://doi.org/10.5194/bg-15-3027-2018

A new model for protactinium/thorium couple in the Atlantic ocean

The authors present an ocean model of the natural radioactive isotopes thorium-230 and protactinium-231. They are compared with many concentration measurements, both dissolved and particulate; most are from the GEOTRACES transects GA02, GA03 (Atlantic) and GIPY5 (Southern Ocean). The figure below shows the modelled dissolved thorium-230 activity at four depth levels; the discs present the observations. The activities are simulated well, based on an improved model of the scavengers, biogenic and lithogenic particles. This is an important result, because these isotopes are often used to investigate past ocean circulation and particle transport. The model may be used by anyone to make such investigations.

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

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