Refined Os isotope stratigraphy of ferromanganese crusts from the Arctic Ocean and implications for polar environmental change since the late Miocene

The long-term environmental history of the Arctic region remains restricted due to the limited number of pre-Quaternary sedimentary records. We present the first Os isotope data of hydrogenetic ferromanganese crusts collected from four locations in the Arctic Ocean: Northwind Ridge, Mendeleev Ridge, Knipovich Ridge and Voring Spur; water depth varied from 1300 to 3851 m. The Os concentrations, ¹⁸⁷Os /¹⁸⁸Os ratios, and major- and trace-element compositions of sublayers (2–5 mm thick) show clear temporal variations. Be isotope ages determined for the Mendeleev Ridge crust were used to guide the ¹⁸⁷Os /¹⁸⁸Os ages of the FeMn crusts, which were obtained by comparison of the ¹⁸⁷Os /¹⁸⁸Os ratios in the crusts with those of the Cenozoic seawater curve. The results greatly improve the Arctic Os isotope temporal record from FeMn crust stratigraphy for the late Miocene and Quaternary.

Several groups of elements show similar behavior spatially (predominantly Ti, Ba, U, REE, Y, Nb, Hf, Cd, Zr, and Ni, Cu) and two groups temporally (Fe, V, As and Ca, Co, Pb, Th). The temporal history is characterized by variations in radiogenic Os isotope composition, which were associated with the long-term (> 10⁵ yr) climatic trends in the region, such as late Miocene cooling, early-to-middle Pliocene warming, and Pliocene-Pleistocene Northern Hemisphere glaciation. The FeMn crust growth rate decreased drastically in the late Miocene possibly due to changes in bottom water conditions associated with the final widening and deepening of the Fram Strait.