Arctic Ocean Mn-stratigraphy: genesis, synthesis and inter-basin correlation

Abstract

Across the Arctic Ocean, late Quaternary deep marine sediments are characterized by the occurrence of brownish layers intercalated with yellowish to olive gray sediments. These layers show enhanced levels of bioturbation, peaks in Mn content, and typically contain elevated abundances of planktonic and benthic micro- and nannofossils. It was early surmised that these layers were deposited under interglacial conditions and that their cyclical downcore occurrence could be correlated to the global benthic oxygen isotope curve. However, the synchronicity of Mn layers with interglacial conditions and the underlying mechanisms responsible for their formation remain controversial. Here we compile and synthesize findings of the last decades with several recent studies that shed light on issues such as the sources of Mn to the Arctic Ocean, the processes and pathways for Mn to the deep sea, the chemical processes active in the sediment, and the spatial and temporal distribution of Mn-rich layers in Arctic deep marine sediments. Budget calculations show that about 90% of Mn input to the Arctic Ocean originates from Arctic rivers or coastal erosion, two sources effectively shut down during mid- to late Quaternary glacial intervals by continental ice sheets blocking or re-directing the rivers and vast subaerial exposure of the shelf areas. Thus, the strong late Quaternary interglacial-glacial cyclicity in Mn content is clearly an input-related signal, and only secondarily influenced by chemical processes in the water column and in the sediment. On the shelves, the Mn undergoes repeated geochemical recycling caused by the high organic carbon content in the sediments before it is ultimately exported to the deep basins where scavenging processes in the water column effectively bring the Mn to the sea floor in the form of Mn (oxyhydr)oxides. The close synchronicity with enhanced bioturbation and elevated micro and nannofossil abundances shows that the Mn peaks are preserved at a stratigraphic level closely corresponding to the interglacial intervals. However, under certain biogeochemical conditions, Mn (oxyhydr)oxides may diagenetically become both dissolved and re-precipitated deep in the sediments, as shown by pore water analyses and X-ray radiograph studies. Dissolution is particularly conspicuous in late Quaternary sediments from the Lomonosov Ridge, where in rapidly deposited coarse grained intervals (diamictons) with elevated total organic carbon (TOC) contents, Mn appears almost completely removed from within the glacial sediments, and also the surrounding interglacial sediments. Correspondingly, bundles of closely spaced, mm-thick, Mn-rich horizontal bands are observed in sediment otherwise devoid of indicators for interglacial conditions, suggesting that these bands were purely formed by diagenetic processes redistributing the Mn from deeper sediment layers. This type of diagenetic Mn redistribution within the sediment can be recognized in XRF-core scanner data combined with sedimentological information from X-ray radiographs, while pore water data are highly promising if clear diagenetic features in the sediment are missing. With this increasing ability to recognize intervals where a diagenetic overprint exists in the Mn record, the recently improved understanding of the Mn cycle in the Arctic Ocean provides a conceptual paleoenvironmental framework in which carefully applied Mn stratigraphy can provide a powerful correlation tool, when combined with other paleoceanographic proxies and sedimentological data.