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Diffuse hydrothermal venting: A hidden source of iron to the oceans

Diffuse hydrothermal venting: A hidden source of iron to the oceans
Diffuse hydrothermal venting: A hidden source of iron to the oceans
Iron (Fe) limits primary productivity and nitrogen fixation in large regions of the world’s oceans. Hydrothermal supply of Fe to the global deep ocean is extensive; however, most of the previous work has focused on examining high temperature, acidic, focused flow on ridge axes that create “black smoker” plumes. The contribution of other types of venting to the global ocean Fe cycle has received little attention. To thoroughly understand hydrothermal Fe sources to the ocean, different types of vent site must be compared. To examine the role of more diffuse, higher pH sources of venting, a hydrothermal plume above the Von Damm vent field (VDVF) was sampled for Total dissolvable Fe (unfiltered, TDFe), dissolved Fe (<0.2 μm, dFe) and soluble Fe (<0.02 μm, sFe). Plume particles sampled in situ were characterized using scanning electron microscopy and soft X-ray spectromicroscopy. The VDVF vents emit visibly clear fluids with particulate Fe (TDFe-dFe, >0.2 μm) concentrations up to 196 nmol kg–1 comparable to concentrations measured in black smoker plumes on the Mid-Atlantic Ridge. Colloidal Fe (cFe) and sFe increased as a fraction of TDFe with decreasing TDFe concentration. This increase in the percentage of sFe and cFe within the plume cannot be explained by settling of particulates or mixing with background seawater. The creation of new cFe and sFe within the plume from the breakdown of pFe is required to close the Fe budget. We suggest that the proportional increase in cFe and sFe reflects the entrainment, breakdown and recycling of Fe bearing organic particulates near the vents. Fe plume profiles from the VDVF differ significantly from previous studies of “black smoker” vents where formation of new pFe in the plume decreases the amount of cFe. Formation and removal of Fe-rich colloids and particles will control the amount and physico-chemical composition of dFe supplied to the deep ocean from hydrothermal systems. This study highlights the differences in the stabilization of hydrothermal Fe from an off-axis diffuse source compared to black smokers. Off-axis diffuse venting represent a potentially significant and previously overlooked Fe source to the ocean due to the difficulties in detecting and locating such sites.
hydrothermal, GEOTRACES, Iron, BIOGEOCHEMISTRY
2296-7745
Lough, Alastair J. M.
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Connelly, Douglas P.
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Homoky, William B.
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Hawkes, Jeffrey A.
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Chavagnac, Valerie
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Castillo, Alain
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Kazemian, Majid
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Nakamura, Ko-ichi
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Araki, Tohru
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Kaulich, Burkhard
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Mills, Rachel A.
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Lough, Alastair J. M.
64be5e9d-9ca2-438c-b566-cfee88ff1a37
Connelly, Douglas P.
d49131bb-af38-4768-9953-7ae0b43e33c8
Homoky, William B.
c303b5fc-93d0-421b-88dd-6a7eb145fa2f
Hawkes, Jeffrey A.
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Chavagnac, Valerie
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Castillo, Alain
527a5a5f-f364-4e00-8e46-29c3ee7fdd3f
Kazemian, Majid
31d6e5e4-dc28-423d-9f8a-ec896e989f9f
Nakamura, Ko-ichi
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Araki, Tohru
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Kaulich, Burkhard
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Mills, Rachel A.
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Lough, Alastair J. M., Connelly, Douglas P., Homoky, William B., Hawkes, Jeffrey A., Chavagnac, Valerie, Castillo, Alain, Kazemian, Majid, Nakamura, Ko-ichi, Araki, Tohru, Kaulich, Burkhard and Mills, Rachel A. (2019) Diffuse hydrothermal venting: A hidden source of iron to the oceans. Frontiers in Marine Science, 6. (doi:10.3389/fmars.2019.00329).

Record type: Article

Abstract

Iron (Fe) limits primary productivity and nitrogen fixation in large regions of the world’s oceans. Hydrothermal supply of Fe to the global deep ocean is extensive; however, most of the previous work has focused on examining high temperature, acidic, focused flow on ridge axes that create “black smoker” plumes. The contribution of other types of venting to the global ocean Fe cycle has received little attention. To thoroughly understand hydrothermal Fe sources to the ocean, different types of vent site must be compared. To examine the role of more diffuse, higher pH sources of venting, a hydrothermal plume above the Von Damm vent field (VDVF) was sampled for Total dissolvable Fe (unfiltered, TDFe), dissolved Fe (<0.2 μm, dFe) and soluble Fe (<0.02 μm, sFe). Plume particles sampled in situ were characterized using scanning electron microscopy and soft X-ray spectromicroscopy. The VDVF vents emit visibly clear fluids with particulate Fe (TDFe-dFe, >0.2 μm) concentrations up to 196 nmol kg–1 comparable to concentrations measured in black smoker plumes on the Mid-Atlantic Ridge. Colloidal Fe (cFe) and sFe increased as a fraction of TDFe with decreasing TDFe concentration. This increase in the percentage of sFe and cFe within the plume cannot be explained by settling of particulates or mixing with background seawater. The creation of new cFe and sFe within the plume from the breakdown of pFe is required to close the Fe budget. We suggest that the proportional increase in cFe and sFe reflects the entrainment, breakdown and recycling of Fe bearing organic particulates near the vents. Fe plume profiles from the VDVF differ significantly from previous studies of “black smoker” vents where formation of new pFe in the plume decreases the amount of cFe. Formation and removal of Fe-rich colloids and particles will control the amount and physico-chemical composition of dFe supplied to the deep ocean from hydrothermal systems. This study highlights the differences in the stabilization of hydrothermal Fe from an off-axis diffuse source compared to black smokers. Off-axis diffuse venting represent a potentially significant and previously overlooked Fe source to the ocean due to the difficulties in detecting and locating such sites.

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Accepted/In Press date: 28 May 2019
Published date: 12 July 2019
Keywords: hydrothermal, GEOTRACES, Iron, BIOGEOCHEMISTRY

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Local EPrints ID: 433542
URI: https://eprints.soton.ac.uk/id/eprint/433542
ISSN: 2296-7745
PURE UUID: db698b6a-6705-4f88-996f-ca6d39927b74
ORCID for Rachel A. Mills: ORCID iD orcid.org/0000-0002-9811-246X

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Date deposited: 27 Aug 2019 16:30
Last modified: 19 Nov 2019 01:59

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Contributors

Author: Alastair J. M. Lough
Author: Douglas P. Connelly
Author: William B. Homoky
Author: Jeffrey A. Hawkes
Author: Valerie Chavagnac
Author: Alain Castillo
Author: Majid Kazemian
Author: Ko-ichi Nakamura
Author: Tohru Araki
Author: Burkhard Kaulich
Author: Rachel A. Mills ORCID iD

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