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Biogeochemical processes in land-ocean interfaces, surface estuaries, subterranean estuaries and sediment-water interface
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The chemical speciation of particulate manganese in the Arctic ocean and potential implications on scavenging of other trace elements
P-C2-10-S
Yang Xiang* , University of California, Santa Cruz
Phoebe J. Lam, University of California, Santa Cruz
Presenter Email: yaxiang@ucsc.edu |
| The Arctic Ocean is characterized by extremely broad continental shelves, consisting of 53% of its overall area (Jakobsson et al., 2003). Shelf sediments have long been recognized as a source of manganese (Mn) to the ocean (Johnson et al., 1992). We find that the Arctic Ocean has relatively high observed concentrations and fractions of Mn oxides compared to other ocean basins, and this is because of the importance of the broad continental shelves. Particle-reactive trace metals have a very high affinity for Mn oxides, therefore, information about the chemical speciation of different types of Mn oxides, which varies in the adsorption capability, may help understand the total scavenging of particle-reactive trace elements and their isotopes (TEIs). Here, we present the chemical speciation of particulate manganese derived from synchrotron-based X-ray absorption spectroscopy (XAS) in the upper 250 m from the U.S. GEOTRACES Arctic cruise (GN01) in 2015. An evident basin and shelf difference in the oxidation state of pMn reflects two different pools in the total Mn reservoir: pMn on the Arctic shelf is mostly Mn (II), and XAS spectra indicate that this is predominantly lithogenic material; in contrast, pMn in the basin at halocline depths (~100 - 250 m) is significantly more oxidized, consistent with authigenic Mn (III/IV) oxides. XAS analyses of pMn from both Pacific- and Atlantic-derived halocline waters in the Amerasian Basin show that pMn appears to be biogenic hexagonal birnessite, a highly reactive layer-structure Mn (IV) oxide with hexagonal symmetry. Furthermore, there also seems to be inter-basin variability of pMn speciation in the Arctic Ocean. A higher fraction of reduced Mn, possibly as sorbed Mn (II), is found in the upper water column of the North Pole in the Eurasian Basin when compared to Amerasian Basin stations, suggesting a possible difference in sorption capacity by Mn oxides in the two sub-basins. Further study is necessary to disentangle the Mn cycling in the Arctic Ocean with such intense shelf-basin interactions and quantify scavenging of other TEIs by different types of Mn oxides. We hope to understand the baseline cycling of Mn before climate change fully affects the Arctic. |
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