Dissolved Mn(II) in the hydrothermal plume is known to be microbially oxidized to form Mn(III/IV) oxides, and the Mn oxides scavenge other trace elements in seawater. In the GEOTRACES GP16 cruise, dissolved Mn (dMn) and particulate Mn (pMn) were found to be transported over 4000 km westwards from the Southern Eastern Pacific Rise. Previous studies in this plume showed different removal rates of dMn and pMn as well as pMn size distribution between the near-field (<80 km from the ridge axis) and far-field (>80 km) plumes. In order to understand Mn cycling in these plumes, spatial distribution, oxidation states, and mineral structures of Mn in small size fraction (SSF; 0.8–51 μm) and large size fraction (LSF; >51 μm) particles from the near-field and far-field plumes were examined using micro X-ray fluorescence spectrometry (μ-XRF), X-ray absorption near-edge structure spectroscopy (XANES), chemical species mapping, and extended X-ray absorption fine-structure spectroscopy (EXAFS). In the near-field plume, pMn in the SSF is dominated by oxidized Mn with Mn(III) fractions of ∼30%. They are a mixture of δ-MnO2 and triclinic birnessites that is known to be formed as a result of autocatalytic Mn(II) oxidation at the surface of freshly-formed δ-MnO2, suggesting that both microbial and autocatalytic Mn oxidation occur in the near-field plume. The LSF pMn in the near-field plume is also oxidized and often found in large aggregates several hundreds of μm in size. These aggregates settle out in the near-field and during transport, and are not found in the far-field plume. In the far-field plume where Mn oxides are not newly formed, pMn in the SSF is oxidized, but their Mn(III) fractions are smaller than in the near-field pMn. Unlike the SSF, the far-field plume LSF pMn is dominated by reduced Mn, implying very slow aggregation of pMn in the far-field plume. The different characteristics of pMn between the near-field and far-field plumes affect its scavenging of other trace elements. In the near-field plume, Co, Mo, 231Pa are associated with pMn, but not in the far-field plume. 231Pa is adsorbed to pFe rather than pMn in the far-field plume, and Pb is adsorbed to pFe in the entire plume. The result shows that freshly-formed Mn oxides in the near-field plume have higher scavenging capacity than the far-field plume pMn. Our findings suggest that the mineralogical age of Mn oxides may be an important parameter that controls the scavenging of many other trace elements and isotopes.
Bibliographical noteFunding Information:
We would like to thank Dan Ohnemus, Erin Black, Sarah Nicholas and S. Pike for operating in-situ pumps and processing particle samples on board during the GP16 cruise. The authors also thank Sirine Fakra and Sharon Bone for their support at the ALS and SSRL facilities and Martin Fleisher for operating the LDEO ICP-MS facility. Portions of this research were carried out at the Advanced Light Source, a U.S. DOE Office of Science User Facility under contract no. DE-AC02-05CH11231, and at the Stanford Synchrotron Radiation Lightsource, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (P30GM133894). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH. This research was supported by the Chemical Oceanography program through the National Science Foundation under grant number NSF OCE-1518110 to PJL, NSF OCE-1233688 to RFA and MQF, and NSF OCE-1233903 to RLE and HC. The particulate trace elements (pTE) and 231 Pa (p 231 Pa) concentrations data from the U.S. GEOTRACES EPZT cruise can be found at the Biological and Chemical Oceanography Data Management Office (BCO-DMO): https://www.bco-dmo.org/dataset/668083 for pTE and http://www.bco-dmo.org/dataset/676231 for p 231 Pa.
© 2021 The Author(s)
- East Pacific Rise
- Eastern Pacific Zonal Transect
- Extended X-ray absorption fine-structure (EXAFS) spectroscopy
- GEOTRACES GP16
- Hydrothermal plume particles
- Micro X-ray fluorescence (μ-XRF) spectrometry
- Particle chemistry
- X-ray absorption near edge structure (XANES) spectroscopy
- X-ray microprobe chemical speciation mapping