What controls hydrothermal plume transport of iron over 4000 km in the deep Pacific Ocean?
The striking extension of the dissolved iron and manganese plumes over more than 4000 km from their hydrothermal sources along the US GEOTRACES East Pacific Zonal Transect (EPZT) cruise (GP16) has challenged our understanding of these element cycles (Resing et al., 2015 see GEOTRACES science highlight).
Fitzsimmons and co-workers (2017, see reference below) analysed the particulate iron and manganese (Mn) in the same plume and showed that they also exceed background concentrations, even 4,000 km from the vent source, despite anticipated gravitational settling losses. Both dissolved and particulate Fe plumes deepen by more than 350 m relative to the conservative helium-3 (3He) one, while the Mn plumes do not show such descent.
Based on Fe speciation and isotope data, the authors suggest that dissolved iron fluxes and geospatial positioning may depend on the balance between stabilization in the dissolved phase by organic ligands and the reversibility of exchange onto sinking particles.
Figure: Interpolated concentrations and station map along the US GEOTRACES EPZT (GP16) section. a, Map of the station locations (colours corresponds to bathymetry; green hues shallower) b, Excess 3He concentrations in fmol kg−1. c, Dissolved Fe concentrations (<0.2 µm, in nM). d, Dissolved Mn concentrations (<0.2 µm, in nM). e, Particulate Fe (>0.45µm, in nM). f, Particulate Mn (>0.45µm, in pM). The black reference line at 2,500m in each panel highlights the deepening of the Fe plumes. Ocean Data View was used to carry out the simulations. Click here to view the figure larger.
Fitzsimmons, J. N., John, S. G., Marsay, C. M., Hoffman, C. L., Nicholas, S. L., Toner, B. M., German, C. R., Sherrell, R. M. (2017). Iron persistence in a distal hydrothermal plume supported by dissolved-particulate exchange. Nature Geoscience. DOI: 10.1038/ngeo2900
Resing, J. A., Sedwick, P. N., German, C. R., Jenkins, W. J., Moffett, J. W., Sohst, B. M., & Tagliabue, A. (2015). Basin-scale transport of hydrothermal dissolved metals across the South Pacific Ocean. Nature, 523(7559), 200–203. DOI: 10.1038/nature14577
Enlighten why macro and micronutrients display different remineralization length scales
Boyd and co-workers (2017, see reference below) explore the abiotic and biotic mechanisms that underpin internal metal cycling. Although they are focusing on iron (Fe) as the best-characterized metal, they are also discussing zinc (Zn), nickel (Ni) and copper (Cu) behaviors. Based on synchrotron X-ray fluorescence (SXRF) mapping and case studies in different biogeochemical areas of the ocean studied in the framework of GEOTRACES (productive Kerguelen plateau, seasonally oligotrophic subtropical waters, oligotrophic Bermuda and Hawaii waters), they reveal contrasting recycling patterns between trace- and macronutrients, explaining why remineralization length scales differ between elements. They also underline that external supply mechanisms of metals are required to complete their biogeochemical cycles.
Figure: Processes that set the vertical length scales for the remineralization of elements within sinking particles. a, Hypothetical remineralization mechanisms for trace and major elements associated with a sinking diatom (based on SXRF element mapping). Preferential subsurface regeneration of elements is linked to their association with structural/biochemical cellular components (for example, membranes) and elemental requirements of microbes (circles). b,c, Idealized processes acting on sinking heterogeneous particles (lithogenic/biogenic components with different labilities). Particle transformations drive both remineralization (b, highlighted terms are metal specific) and depth-dependent changes in particle aggregate surface area (c, bio-optical profiling float data, courtesy of George Jackson), which influences local chemistry and microbial processes. Click here to view the figure larger. (Modified from Boyd et al., 2017, Nature Geoscience)
Boyd, P. W., Ellwood, M. J., Tagliabue, A., & Twining, B. S. (2017). Biotic and abiotic retention, recycling and remineralization of metals in the ocean. Nature Geoscience, 10(3), 167–173. DOI: 10.1038/ngeo2876