Description: Highlights • Seawater Hf–Nd–Pb isotopic evolution in the deep Arctic Ocean of the past 7 Myr. • Climatically driven changes in weathering inputs since 4 Ma. • North American (Laurentide Ice Sheet) runoff controlled the isotopic budget in the Canada Basin. • More congruent Hf release due to glacial weathering conditions. • Past Arctic Ocean water masses show larger isotopic differences than today. Abstract 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 εNdεNd(εHf)(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.

Description: Highlights • Nd isotope data reflect advection and dilution of Mediterranean Outflow Water on its way north in the Bay of Biscay. • Combined Hf and Nd isotopes are a sensitive indicator of inputs from land as well as long distance advection and mixing. • Nd isotope results of this and earlier studies demonstrate the temporally variable flow path of Mediterranean Sea Water. We present dissolved neodymium and hafnium concentrations and isotope compositions of surface and deep-water masses from the Bay of Biscay. Neodymium isotope signatures in surface waters of the Bay of Biscay are mostly dominated by local weathering inputs from the surrounding continental margin. Subsurface Eastern North Atlantic Central Water (ENACW) shows a distinct Nd isotope signature (εNd ≅ −12) at the southwestern-most station and is significantly diluted by mixing with more radiogenic waters or shifted by inputs of relatively radiogenic particulate Nd on its way north along the European margin. Furthermore, the Nd isotope data clearly show a declining fraction of Mediterranean Sea Water (MSW) at intermediate depths on its way north indicating that only 40% to 60% of MSW still present in the mixture at the Galician margin arrive at the stations further north in the Bay of Biscay. An interannual variability of the flow path of MSW is identified when comparing the results of the Nd isotope compositions and salinity data of this study with those of earlier studies from the area. In agreement with Nd isotope and concentration analyses the Hf isotope composition of MSW is set by large-scale inputs of terrigenous material into the Mediterranean as can be deduced from elevated Hf concentrations still observable at the Galician margin. Hf isotope signatures of all water masses of the Bay of Biscay, moreover, are overprinted by local weathering inputs and do not reflect water mass mixing. However, combined dissolved Nd and Hf isotopes serve as indicators of local weathering influences on signatures expected from long distance water mass mixing.

Description: The role of accessory minerals in the incongruent release of Hf and Pb during continental weathering and its implications for the generation of distinct seawater isotope compositions is subject of debate. While it has been suggested that radiogenic Hf and Pb isotope signatures released during the dissolution of rocks are controlled by the relative abundances of minerals with distinct isotope compositions and differences in their resistance to dissolution there has not been a comprehensive experimental investigation of these processes to date. We carried out systematic sequential leaching experiments on fresh and partly weathered granitic rock samples as well as separated zircons from the Central Aar Granite in Switzerland. Combined with major and rare earth element concentrations our new quantitative experimental data reveal systematic preferential release of radiogenic Nd, Hf and Pb isotopes primarily controlled by dissolution characteristics of the host rock's easily dissolvable accessory and major minerals, in particular apatite and sphene, during weak chemical weathering. Moreover, Pb isotope signatures of incipient weathering conditions, contrary to expectations, indicate initial congruent release of Pb from freshly exposed mineral surfaces that becomes subsequently incongruent. During more advanced chemical weathering stages, as well as enhanced physical weathering conditions, the dissolution of major minerals (i.e. feldspars) becomes dominant for Nd and Pb isotope signatures, whereas Hf isotopes are still dominated by contributions from highly radiogenic accessories. Additional leaching experiments of zircon separates were performed to test the specific role of zircons for Hf isotope compositions of riverine runoff. It is demonstrated that zircon is more efficiently dissolved when physical weathering is enhanced. This increased Hf release originating from partial dissolution of zircons, however, is quantitatively not sufficient to explain less radiogenic Hf isotope signatures in seawater during episodes of enhanced mechanical erosion alone. Moreover, the observed addition of Hf from the more congruent dissolution of the zircon-free fractions of the parent rock due to enhanced physical weathering indicate that these minerals also play an important role in controlling Hf isotope signatures released under deglacial conditions.