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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 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

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

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