Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Marine Chemistry 201 (2018): 183-197, doi:10.1016/j.marchem.2018.01.011.
Description:
Iron (Fe)-poor surface waters limit phytoplankton growth and their ability to remove carbon (C)
from the atmosphere and surface ocean. Over the past few decades, research has focused on
constraining the global Fe cycle and its impacts on the global C cycle. Hydrothermal vents have
become a highly debated potential source of Fe to the surface ocean. Two main mechanisms for
transport of Fe over long distances have been proposed: Fe-bearing nanoparticles and organic C
complexation with Fe in the dissolved (dFe) and particulate (pFe) pools. However, the ubiquity
and importance of these processes is unknown at present, and very few vents have been
investigated for Fe-Corg interactions or the transport of such materials away from the vent. Here
we describe the near-field contributions (first ~100 km from ridge) of pFe and Corg to the
Southern East Pacific Rise (SEPR) plume, one of the largest known hydrothermal plume features
in the global ocean. Plume particles (〉 0.2 μm) were collected as part of the U.S. GEOTRACES
Eastern Pacific Zonal Transect cruise (GP16) by in-situ filtration. Sediment cores were also
collected to investigate the properties of settling particles. In this study, X-ray absorption near
edge structure (XANES) spectroscopy was used in two complementary X-ray synchrotron
approaches, scanning transmission X-ray microscopy (STXM) and X-ray microprobe, to
investigate the Fe and C speciation of particles within the near-field non-buoyant SEPR plume.
When used in concert, STXM and X-ray microprobe provide fine-scale and representative
information on particle morphology, elemental co-location, and chemical speciation. Bulk
chemistry depth profiles for particulate Corg (POC), particulate manganese (pMn), and pFe
indicated that the source of these materials to the non-buoyant plume is hydrothermal in origin.
The plume particles at stations within the first ~100 km down-stream of the ridge were
composites of mineral (oxidized Fe) and biological materials (organic C, Corg). Iron chemistry
in the plume and in the core-top suspended sediment fluff layer were both dominated by Fe(III)
phases, such as Fe(III) oxyhydroxides and Fe(III) phyllosilicates. Particulate sulfur (pS) was a
rare component of our plume and sediment samples. When pS was detected, it was in the form of
an Fe sulfide mineral phase, composing ≤ 0.4% of the Fe on a per atom basis. The resuspended
sediment fluff layer contained a mixture of inorganic (coccolith fragments) and Corg bearing
(lipid-rich biofilm-like) materials. The particle morphology and co-location of C and Fe in the
sediment was different from that in plume particles. This indicates that if the Fe-Corg composite
particles settle rapidly to the sediments, then they experience strong alteration during settling and/or within the sediments. Overall, our observations indicate that the particles within the first ~
100 km of the laterally advected plume are S-depleted, Fe(III)-Corg composites indicative of a
chemically oxidizing plume with strong biological modification. These findings confirm that the
Fe-Corg relationships observed for non-buoyant plume particles within ~ 100 m of the vent site
are representative of particles within this region of the non-buoyant plume (~100 km). These
findings also point to dynamic alteration of Fe-Corg bearing particles during transport and
settling. The specific biogeochemical processes at play, and the implications for nutrient cycling
in the ocean are currently unknown and represent an area of future investigation.
Keywords:
Scanning transmission X-ray microscopy (STXM)
;
X-ray absorption near edge structure (XANES) spectroscopy
;
X-ray microprobe
;
GEOTRACES GP16
;
Eastern Pacific Zonal Transect
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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