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Science Highlights

Lithogenic influence from the Hawaiian Islands detectable up to Station ALOHA surface waters

By coupling neodymium (Nd) and radium (Ra) isotopes and Rare Earth Element (REE) signals, Henning Fröllje and co-workers (2016, see reference below) show that the coastal Hawaiian waters are affected by a prominent lithogenic influence from the Hawaiian Islands. Rare earth patterns, radiogenic εNd signatures and high 228Ra levels are clearly tracing this influence. Moreover, it is perceptible as far as ALOHA station (100 km north). This influence at ALOHA is most pronounced in February, when precipitation on the islands is highest and dust input from Asia is low. In summer, however, Nd isotopes are shifted towards the Asian dust endmember (εNd = -10), indicating seasonal dust influence overlying the Hawaiian background signal.

A close study of the rare earth distribution and speciation confirm that these elements are truly dissolved in seawater and that they are following water mass advection and mixing in the intermediate and deep central North Pacific Ocean.

16 FrolijelFigure: Location of station ALOHA and sampling stations around Oahu, Hawaii, along with εNd and Nd concentration profiles. ALOHA full water column: εNd, Nd concentrations, and Radium-228 at ALOHA. Seasonal: seasonal εNd of the upper 800m, showing a shift to higher dust influence in June-August compared to February. Oahu: εNd and Nd concentration profiles at coastal sites around Oahu. Click here to view the figure larger.

Reference:

Fröllje, H., Pahnke, K., Schnetger, B., Brumsack, H.-J., Dulai, H., & Fitzsimmons, J. N. (2016). Hawaiian imprint on dissolved Nd and Ra isotopes and rare earth elements in the central North Pacific: Local survey and seasonal variability. Geochimica et Cosmochimica Acta, 189, 110–131. doi:10.1016/j.gca.2016.06.001

Inference about rates of thorium and particle cycling in the ocean water column

Insoluble thorium (Th) isotopes are particle reactive while their radioactive parents are fairly soluble. This difference of behaviour has allowed chemical oceanographers to use Th isotopes to develop understanding about the scavenging of particle reactive elements and the cycling of particles in the water column. However, many prior models rely on numerous assumptions. Among these assumptions is vertical uniformity of the rate parameters governing sorption reactions and particle processes in the mesopelagic zone and in the deeper regions of the water column (Th adsorption and desorption, particle remineralization and settling speed, etc.).

In this work, Paul Lerner and co-authors (2016, see reference below) use Th and particle data collected at station GT11-22 of the GEOTRACES North Atlantic transect (section GA03) in order to test two models of Th and particle cycling: a conventional one that assumes uniform rate parameters and another one that allows the rate parameters to vary with depth. They show that the second model leads to a significantly better fit to the data, thereby challenging the assumption of parameter uniformity in the conventional model. Moreover, by combining the second model with the data, they diagnose the various terms in the depth-dependent Th isotope budgets at GT11-22, showing in particular that the behaviour of 230Th is dominated by a balance between adsorption and desorption over most of the water column.

16 Lernerl

Figure: Inversion of radiochemical and particle data for station GT11-22 of section GA03 (19º26’ N, 29º 22’ W). The data used include measurements of dissolved and particulate 228,230,234Th, 228Ra, particle concentration, and observational estimates of 234,238U. Panel (a) shows the budget of dissolved 230Th (left box) and particulate 230Th (right box) in terms of vertical averages (dpm m-3 yr-1) along the water column (below 125 m). The numbers are estimated fluxes and their estimated errors. Panels (b) and (c) show the vertical distribution of the 230Th fluxes.

Reference:

Lerner, P., Marchal, O., Lam, P. J., Anderson, R. F., Buesseler, K., Charette, M. A., Edwards, R. L., Hayes, C. T., Huang, K-F., Lu, Ya., Robinson, L F., Solow, A. (2016). Testing models of thorium and particle cycling in the ocean using data from station GT11-22 of the U.S. GEOTRACES North Atlantic section. Deep Sea Research Part I: Oceanographic Research Papers, 113, 57–79. doi:10.1016/j.dsr.2016.03.008

 

 

Decreasing of the industrial lead contamination in the Amundsen Sea area

Since the phasing-out of leaded automobile gasoline, the imprint of industrial lead is decreasing everywhere. Antarctica snow and ice are no exception to this general rule, as shown by measurements of lead (Pb) concentration and isotopic composition in these environments as well as in the Amundsen Sea Polynya. Up to 95% of the Pb at the time of sampling is natural in source, allowing tracing weathering from Antarctic rocks....

16 Ndungu lPhoto: Silke Severmann, Rutgers University.

Read more: Decreasing of the industrial lead contamination in the Amundsen Sea area

Hydrothermalism: a major contributor to the oceanic inventory of dissolved zinc

High-quality dataset of dissolved zinc (Zn) from the US GEOTRACES EPZT cruise (GP16 section) allows Roshan and co-workers (2016, see reference below) to revisit the oceanic budget of dissolved Zn. These investigators have observed a basin-scale transport of mantle-derived dissolved Zn from the hydrothermal vents of the East Pacific Rise to the farfield regions (>4000 km away) across the South Pacific at depths 2300–2800 m (see Figure). The strong correlation (R2 = 0.9) between hydrothermal-controlled component of dissolved Zn (i.e. Excess Zn) and hydrothermal activity tracer, 3He leads to a global hydrothermal zinc flux of 1.75 ± 0.35 G mol yr− 1 that is many-fold higher than the input fluxes estimated by other studies. These results suggest that mantle-derived dissolved Zn dominates the oceanic Zn inventory and that dissolved Zn residence time is much shorter (3000 ± 600 years) than previous input-based estimates (11,000 and 50,000 years) and more consistent with previous removal-based estimate (3000–6000 years)...

16 Roshan lFigure: The top panel shows the dispersion of hydrothermal-controlled component of dissolved Zn (Excess Zn) from the East Pacific Rise (elevated topography at the east of the transect and shown in the bottom left panel) to the west of the transect (>4000 km). Excess Zn shows a strong correlation with hydrothermal tracer, 3He which allows for the accurate estimation of hydrothermal dissolved Zn input rate of 1.75 G mole/year.

Reference:

Roshan, S., Wu, J., & Jenkins, W. J. (2016). Long-range transport of hydrothermal dissolved Zn in the tropical South Pacific. Marine Chemistry, 183, 25–32. doi:10.1016/j.marchem.2016.05.005

Scandium: a new oceanic tracer with surprising properties

The first modern data set of dissolved scandium (Sc) in the open ocean is presented. Thanks to three GEOTRACES cruises, its distribution is compared between the different oceanic basins. This work also compares the reactivity of Sc with its trivalent periodic table group mates: yttrium (Y) and lanthanum (La) on the one hand, and aluminium (Al) and gallium (Ga) on the other hand.

These distributions and comparisons reveal that scandium is a hybrid-type metal with a residence time on the order of 1000 years. In addition, Sc displays similar particle reactivity to iron, which makes the authors suggest that Sc could help to constrain the non-biogenic part of the iron cycle.

16 Parkeretal l
Figure: Comparing the concentrations of elements from deep water in different ocean basins is a classic way to categorize their reactivity and understand something about how they cycle through the ocean. Scandium, yttrium and lanthanum all occupy the same column in the periodic table, meaning that they are likely to have similar reactivity. However, this figure shows that while yttrium and lanthanum have higher concentrations in the North Pacific deep water than in the North Atlantic, scandium has similar deep-water concentrations in both ocean basins. From this we conclude that scandium is more reactive than yttrium and lanthanum in the ocean, and that scandium is in fact similar to iron, an important element that perhaps can be better understood through its similarity to scandium. Please click here to view the figure larger.

Reference:

Parker, C. E., Brown, M. T., & Bruland, K. W. (2016). Scandium in the open ocean: A comparison with other group 3 trivalent metals. Geophysical Research Letters, 43, 2758–2764. doi:10.1002/2016GL067827

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