The evolution of climatically driven weathering inputs into the western Arctic Ocean since the late Miocene: Radiogenic isotope evidence

We present the first continuous records of dissolved radiogenic neodymium, hafnium, and lead isotope compositions of deep waters in the western Arctic Ocean, spanning the time from the late Miocene to the present. The data were obtained from three hydrogenetic ferromanganese (Fe-Mn) crusts recovered from seamounts along the northernmost edge of the Northwind Ridge in the Canada Basin from water depths of 2200, 2400, and 3600 m. Dating the crusts using cosmogenic 10Be documents undisturbed present-day growth surfaces and yields growth rates between 27 and 2.2 mm/Myr. The Nd (Hf) isotope time series of the three crusts show similar evolutions from εNd (εHf) of −8.5 (+4) in the oldest parts to −11.5 (−4) at the surfaces and a pronounced trend to less radiogenic values starting at ∼4 Ma. This coincided with a trend of the Pb isotope evolution towards more radiogenic 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb. It is inferred that climatically controlled changes in weathering regime and sediment transport along the North American continent were responsible for the major change of the radiogenic isotope composition of the Arctic Deep Water (ADW) in the Canada Basin. Based on these records we conclude that weathering inputs from the North American continent linked to enhanced glacial conditions started to increase and to influence the radiogenic isotope composition of ADW ∼4 million years ago and were further intensified at ∼1 Ma. These new time series differ markedly from the radiogenic isotope evolution of Arctic Intermediate Water recorded on the Lomonosov Ridge and suggest that much larger isotopic differences between the water masses of the Arctic Ocean than today prevailed in the past.