The Colloidal Hourglass: North Atlantic Iron Distribution Controlled by Dynamic Colloidal Phase

Science Highlights

7 February 2020

K. Kunde and colleagues (2019, reference below) show that a highly dynamic colloidal iron phase in the upper ocean and at the seafloor boundary controls the distribution of dissolved iron at key sources and sinks across the subtropical North Atlantic Ocean, while the ocean interior is marked by an equilibrium partitioning between the soluble and colloidal phases. These processes result in size-fractionated iron distribution resembling an hourglass shape of the colloidal iron fraction (%cFe) of the dissolved pool against depth. In addition, high-resolution surface sampling revealed a strong, dust-derived gradient of surface dissolved iron concentrations that resulted in a “high dust, high %cFe” and “low dust, low %cFe” pattern across the basin. A global comparison of colloidal and dissolved iron reveals that this effect extends to the mesopelagic, where the lithogenic nature of a region is imprinted on the colloidal partitioning signature.

Kunde et al.

Figure: (Left) The fraction of colloidal Fe to the dissolved Fe pool across the GApr08 section forms an hourglass shape against depth. Dynamic Fe cycling in the upper ocean and near the seafloor is reflected in the large range of %cFe, while the tight partitioning in the less dynamic ocean interior forms the neck of the hourglass. The four data points outside this pattern are from the Snakepit hydrothermal source on the Mid-Atlantic Ridge (Station 4) with almost 100% cFe. (Right) Map of the subtropical North Atlantic showing the cruise track and station locations of the GApr08 section (research cruise JC150).

Reference:

Kunde, K., Wyatt, N. J., González‐ Santana, D., Tagliabue, A., Mahaffey, C., & Lohan, M. C. (2019). Iron distribution in the subtropical north Atlantic: the pivotal role of colloidal iron. Global Biogeochemical Cycles, 33, 1532 1547. https://doi.org/10.1029/2019GB006326

Latest highlights

East-West contrasting fate and anthropogenic inputs for dissolved trace metals in the Subarctic Pacific Ocean

Chan and co-authors report the full-depth distribution of dissolved nickel, copper, zinc, and cadmium in the North Pacific Ocean.

Comprehensive quantification of the rare earth element cycle in the northwest Pacific Ocean

Cao and co-authors investigate dissolved rare earth elements and the factors controlling their distributions in the northwest Pacific Ocean.

Iron and zinc isotopes disentangle the anthropogenic, natural and wildfire sources of aerosols over the North and Equatorial Pacific Ocean

Bunnell and co-authors analysed aerosol iron and zinc isotopic compositions along the North Pacific GEOTRACES GP15 section (Alaska-Tahiti).

Contribution of sandy beaches to the oceanic silica cycle

This paper calls into question the commonly accepted idea of an oceanic silicon cycle in equilibrium.

Rechercher