ALBERT

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 indicates that these minerals also play an important role in controlling Hf isotope signatures released under deglacial conditions.

Description: Combined seawater radiogenic hafnium (Hf) and neodymium (Nd) isotope compositions were extracted from bulk sediment leachates and foraminifera of Site 1088, ODP Leg 177, 2082 m water depth on the Agulhas Ridge. The new data provide a continuous reconstruction of long and short-term changes in ocean circulation and continental weathering inputs since the Mid-Miocene. Due to its intermediate water depth the sediments of this core sensitively recorded changes in admixture of North Atlantic Deep Water (NADW) to the Antarctic Circumpolar Current (ACC) as a function of the strength of the Atlantic Meridional Overturning Circulation (AMOC). Nd isotope compositions (εNd) range from -7 to -11 with glacial values generally 1 to 3 units more radiogenic than during the interglacials of the Quaternary. The data reveal episodes of significantly increased AMOC strength during late Miocene and Pliocene warm periods whereas peak radiogenic εNd values mark a strongly diminished AMOC during the major intensification of Northern Hemisphere Glaciation near 2.8 Ma and in the Pleistocene after 1.5 Ma. In contrast, the Hf isotope compositions (εHf) show an essentially continuous evolution from highly radiogenic values of up to +11 during the Miocene to less radiogenic present day values (+2 to +4) during the late Quaternary. The data document a long-term transition in dominant weathering inputs, where inputs from the South America are replaced by those from Southern Africa. Moreover, radiogenic peaks provide evidence for the supply of radiogenic Hf originating from Patagonian rocks to the Atlantic sector of the Southern Ocean via dust inputs.

Description: The past and present variability of ocean circulation and weathering inputs can be reconstructed from the geochemical composition of seawater and marine sediments. Among other proxies, the radiogenic isotope compositions of neodymium (Nd), hafnium (Hf) and lead (Pb) have been applied to reliably trace variations in the ocean that have been related to climate change. However, not all mechanisms and processes governing the geochemical cycling and redistribution of these isotopes are understood in sufficient detail. In order to improve the understanding of some of these mechanisms, the subject of the studies presented in this thesis are the evolution of the radiogenic isotopes of Nd, Hf and Pb in the hydrosphere from the first release into solution during continental weathering via the cycling in present day seawater to the marine sediments, where the information about variations in past ocean circulation and weathering inputs is stored.

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: A first ferromanganese crust record of neodymium, hafnium and lead isotopes, reflecting the deep water evolution of the Arctic Ocean was compiled during this work. The archives were dated using beryllium isotopes and reach back to the late Miocene. The isotopic signatures of three crusts from the flanks of sea mounts at the northernmost edge of the Northwind Ridge in the Canada Basin reflect changes in continental weathering influxes and contributions of inflowing deep water from the North Atlantic on a million year scale. The data show that stable conditions prevailed until about 4 Ma before today. After that, climatically controlled changes in weathering regime and sediment transport on the North American continent have driven the Arctic Deep Water chemistry in the deep Canada Basin until today. From our records we conclude that the first influences of strong glacial-like weathering conditions on the North American continent reached the Arctic Deep Water as early as 4 Ma and have prevailed there ever since.

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.

Description: We present dissolved neodymium and hafnium concentrations and radiogenic 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 dominated by local weathering inputs from the surrounding continental margin. Subsurface Eastern North Atlantic Central Water (ENACW) shows a distinct Nd isotope signature (εNd≅‑12.0) at the southwestern-most station and is significantly diluted by mixing with more radiogenic waters on its way north along the European margin. Furthermore, the Nd isotope data clearly show a declining fraction of Mediterranean Outflow Water (MOW) at intermediate depths on its way north indicating that only 40% to 60% of MOW still present in the mixture at the Galician margin arrive at the stations further north in the Bay of Biscay. A potentially seasonal variability of the flow path of MOW 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 Hf isotope composition of MOW is affected by large-scale inputs of terrigenous material into the Mediterranean as can be deduced from elevated Hf concentrations 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 a useful indicator of local weathering influences on signatures expected from long distance admixture.

Description: Combined seawater radiogenic hafnium (Hf) and neodymium (Nd) isotope compositions were extracted from bulk sediment leachates and foraminifera of Site 1088, ODP Leg 177, 2082 m water depth on the Agulhas Ridge. The new data provide a continuous reconstruction of long and short-term changes in ocean circulation and continental weathering inputs since the Mid-Miocene. Due to its intermediate water depth the sediments of this core sensitively recorded changes in admixture of North Atlantic Deep Water (NADW) to the Antarctic Circumpolar Current (ACC) as a function of the strength of the Atlantic Meridional Overturning Circulation (AMOC). Nd isotope compositions (epsilon-Nd) range from -7 to -11 with glacial values generally 1 to 3 units more radiogenic than during the interglacials of the Quaternary. The data reveal episodes of significantly increased AMOC strength during late Miocene and Pliocene warm periods whereas peak radiogenic epsilon-Nd values mark a strongly diminished AMOC during the major intensification of Northern Hemisphere Glaciation near 2.8 Ma and in the Pleistocene after 1.5 Ma. In contrast, the Hf isotope compositions (epsilon-Hf) show an essentially continuous evolution from highly radiogenic values of up to +11 during the Miocene to less radiogenic present day values (+2 to +4) during the late Quaternary. The data document a long-term transition in dominant weathering inputs, where inputs from the South America are replaced by those from Southern Africa. Moreover, radiogenic peaks provide evidence for the supply of radiogenic Hf originating from Patagonian rocks to the Atlantic sector of the Southern Ocean via dust inputs.