Partial reduction and associated micro-structure alterations of vernadite-like phases in central Arctic sediments: new constrains on Mn stratigraphy and early-diagenesis in the Arctic Ocean

Late Quaternary marine sediments in the central parts of the Arctic Ocean are characterized by recurrent brownish Mn-rich layers that are often strongly bioturbated and rich in micro- and nanno-fossils, and are thus believed to be deposited during interstadial/interglacial periods^[1]. Since the brownish Mn-rich layers are widespread and easy to identify, they have been used as stratigraphic tools to correlate past climate events recorded by existing sediment cores across the central Arctic Ocean. Although these Mn-rich layers were assumed to contain abundant Mn oxyhydroxide phases^[1,2] and reported to undergo slow but increasing diagenetic processes with depths^[2], the geochemical nature of these Mn phases and their responses to long-term diagenesis have not been examined on a molecular level. Here, we sampled a large number of thin sediment layers (representing different stratigraphic levels with varying Mn XRF counts) of six cores from different parts of the central Arctic Ocean, and investigated the speciation and average oxidation state (AOS) of Mn in these layers by synchrotron-based Mn K-edge XANES and EXAFS spectroscopy. By linking to the sedimentological/geochemical features (e.g., bulk density, abundances of benthic and planktic communities, micro-XRF profiles) of the same cores, the synchrotron data revealed that (i) Mn pools in nearly all of the sediments (including those deposited recently as well as during the past glacial/interglacial periods) were strongly dominated by phyllomanganate phases, structurally similar to vernadite (a common product of bacterially-mediated Mn oxidation); (ii) the structural alterations of the vernadite-like phases (e.g., the splitting of Mn-O and Mn-Mn inter-atomic distances) in the cores were correlated inversely to the values of Mn AOS, suggesting an active and gentle diagenesis-driven Mn reduction processes; and  (iii) the sediment layers with lowest Mn XRF counts overall had lowest values of Mn AOS, but displayed no sign of other Mn phases. The sediments will be analyzed for total concentrations of Mn and other elements as well as labile and recalcitrant organic matter (via ramped combustion-evolved CO₂ gas analysis). By combining with the additional data (e.g., Co/Mo ratios and availability of labile organic matter), this study will provide new insights into the key factors regulating the deposition and diagenetic alterations/redistribution of Mn in the sediment cores. This will, in turn, add important constrains on the geo-stratigraphic occurrence and diagenetic processes of the Mn-rich layers as well as the interpretation of linked Nd isotopes (important tracers for past water mass) and other biogeochemical processes (e.g., carbon and nutrient cycling) in the central Arctic Ocean during the glacial-interglacial climate cycles.

 

References

[1] Löwemark. L., et al. (2014) Quaternary Science Reviews, 92, 97-111.

[2] März. C., et al. (2011) Geochimica et Cosmochimica Acta, 75, (23), 7668-7687.