The Arctic is one of the most important locations for past and present global environmental change, containing arguably the largest concentration of critical components in the Earth’s climate system. In the past, the Arctic region experienced abrupt changes in both climate and ocean circulation. Neodymium isotope ratios (εNd) have been used extensively as a tracer of continental crustal evolution and in sediment and seawater provenance studies. In this thesis εNd is measured on a suite of marine and terrestrial sediment and waters, broadly focused on the circum-Arctic region, providing insights into both past and current water and sediment sources. A large compilation of εNd in authigenic phases of deep-sea sediments is used to investigate past changes in deep water sourcing and formation in the Nordic Seas from the last glacial to modern. During the Last Glacial Maximum, εNd in the Nordic Seas is found to have a similar homogeneity to the late Holocene and modern, suggesting similar advective circulation during both periods. Heterogeneous εNd during the last deglaciation indicates a lack of vigorous deep-water formation. A high temporal resolution εNd record on authigenic phases from a marine sediment core in the southern Norwegian Sea is used to investigate abrupt climate shifts and sediment sourcing in this region over the last 40 ka. It is concluded that authigenic εNd at this core site is controlled predominantly by release of Nd from reactive basaltic particulates, giving εNd values which are distinct from the rest of the Nordic Seas. Changes in authigenic εNd at this site are large and abrupt correlating with millennial scale climate changes and existing proxy records of sediment provenance and bottom water temperature. Rare earth elements (REE), εNd, and a range of elemental concentration data on paired suspended sediments and waters in a major Arctic river (the Mackenzie) are used to investigate sediment reactivity and source. Sequential leaching of riverine suspended particulate matter (SPM) shows that river sediments contain labile iron (Fe) phases that host a substantial amount of REEs, including Nd. The εNd in the Fe oxide phases in SPM and the dissolved Nd pool are similar and are systematically higher than the silicate fraction of the SPM, up to a maximum of 8 epsilon units. This is interpreted this as evidence for dynamic cycling between Fe oxide phases in SPM and the river water, demonstrating the reactivity of the labile Fe phase. Neodymium isotope and Nd/Fe molar ratios suggest that a significant amount of labile Fe and Nd phases are derived from an isotopically distinct reservoir within the sedimentary source rock. Finally, Arctic weathering environments are put in the context of global riverine εNd behaviour, using a compilation of new and literature data from global rivers. Global riverine εNd trends suggest that highly weather-able phases and rock types provide an important source of dissolved and reactive Nd.