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Science Highlights

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

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

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

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

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