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This study presents uranium (U) and lithium (Li) isotope and major and trace element data for the dissolved load, suspended particles and bedload from rivers draining dominantly basaltic catchments on the island of Sao Miguel in the Azores Archipelago. Uranium activity ratios are at secular equilibrium in the bedload, but are higher for suspended material, suggesting sorption of uranium from the solution onto particle surfaces. The (234U/238U) of the riverine dissolved phase varies between 1.02 and 1.86, and lies on a mixing trend between the values expected from chemical weathering of basalt (high [U]; (234U/238U) at secular equilibrium) and an endmember with low [U] and high (234U/238U), which probably reflects α-recoil effects associated with physical weathering and/or soil formation. Lithium isotope ratios are consistently lighter in suspended material than the corresponding bedload, suggesting preferential retention of 6Li in the suspended load. In turn, the δ7Li of the dissolved load is always isotopically heavier, ranging from 5.9 to 36.2‰, although the lightest values are affected by hydrothermal input. The lithium isotopic composition of the dissolved load reflects the balance of primary mineral dissolution to secondary mineral formation. Comparison of the behaviour of U and Li isotopes between Sao Miguel, Iceland, and other basaltic terrains suggests that whilst U isotopes are dominated by weathering regimes dependent on factors such as weathering rates, temperature, runoff and climate, neither weathering intensity nor climate exerts a direct influence on Li isotope behaviour; rather the formation of secondary minerals, which is indirectly controlled by climate, weathering, primary mineralogy and biology, appears to play the dominant role.

An annually laminated, uranium-series dated, Holocene stalagmite from southeast Ethiopia has been analysed for growth rate and δ13C and δ18O variations at annual to biennial resolution, in order to provide the first long duration proxy record of decadal-scale rainfall variability in this climatically sensitive region. Our study site (10°N) is climatically influenced by both summer (June—August) and spring (March—May) rainfall caused by the annual movement of the Inter-Tropical Convergence Zone (ITCZ) and modulated by large-scale anomalies in the atmospheric circulation and in ocean temperatures. Here we show that stalagmite growth, episodic throughout the last 7800 years, demonstrates decadal-scale (8—25 yr) variability in both growth rate and δ 18O. A hydrological model was employed and indicates that this decadal variability is due to variations in the relative amounts of rainfall in the two rain seasons. Our record, unique in its combination of length (a total of ~1000 years), annual chronology and high resolution δ18O, shows for the first time that such decadal-scale variability in rainfall in this region has occurred through the Holocene, which implies persistent decadal-scale variability for the large-scale atmospheric and oceanic driving factors.

This study presents major and trace elements and uranium (U), lithium (Li) and magnesium (Mg) isotope data for dissolved and suspended particulate material from estuaries draining dominantly basaltic terrains in Iceland (Borgarfjörður) and Sao Miguel (Povoação) in the Azores archipelago. The concentrations of dissolved sodium (Na), calcium (Ca), potassium (K), sulphate (SO4), Mg and Li, and 7Li/6Li and 26Mg/24Mg isotope ratios vary directly as a function of the dissolved chloride concentration [Cl], indicating that these species are non-reactive in these estuaries. However, U appears to be affected either by colloid flocculation or the formation of Fe-oxyhydroxides in Borgarfjörður. The major element composition of suspended particulate material in the estuary at Borgarfjörður shows that the detrital silicates delivered to the estuarine mixing zone are relatively unweathered. Accordingly, the δ26Mg isotope composition of the suspended material is indistinguishable from that of the parent basalts. However, the (234U/238U) activity ratios of the suspended material are significantly different from the parent basalt indicating rapid equilibration of the U isotopes between particles and seawater. The Li concentration and δ7Li isotope composition of the suspended load are also observed to increase with increasing [Cl], consistent with ongoing weathering of detrital material and the formation of secondary minerals in seawater. Depending on the particle residence time in the estuary, this process could remove as much as 15–25% of the global riverine input of Li to the ocean.

This study presents major-, trace-element and Mg isotope data for the dissolved load and suspended particulates of Icelandic rivers draining dominantly basaltic catchments, including both glacier-fed and direct-runoff rivers. These samples provide the opportunity to understand the behaviour of Mg isotopes during chemical weathering, where variations due to lithology are not extant. Given the significant role of Mg in the carbon cycle, such variations may provide important information on the regulation of Earth's climate. Hydrothermal waters, groundwater, precipitation (glacial ice), basalt glass, olivine and representative soils have also been analysed. The dissolved load shows a wide range of δ26Mg compositions, compared to the parent basaltic glass (δ26Mg = − 0.29‰), ranging from − 0.96 to + 0.64‰, while precipitation and hydrothermal waters possess δ26Mg values of − 0.83‰ and + 0.85‰, respectively, with lower Mg concentrations than the dissolved load. Biomass activity in vegetation and organic material in soils and rivers (colloids) appear to have little effect on the Mg isotope compositions. Rather, the data suggest that Mg elemental and isotopic variations are largely controlled by the formation and stability of secondary phases in response to differing hydrological conditions. In some samples seawater, in the form of direct precipitation or glacial runoff, also appears to be an important source of Mg. Glacier-fed rivers, groundwaters, and some direct-runoff rivers, with a high pH, have higher δ26Mg than basalt, which is most likely due to the incorporation of light Mg isotopes in secondary minerals. In contrast, those direct-runoff rivers which have a relatively low pH, have low δ26Mg (relative to basalt), consistent with preferential incorporation of heavy Mg isotopes into secondary phases, although it is not possible to rule out some contribution from precipitation. Riverine suspended particulates are depleted in mobile elements, and have δ26Mg compositions

This study presents U and Li isotope and major and trace element data for the dissolved load, suspended particulates and bedload for Icelandic rivers draining predominantly basaltic catchments. Physical erosion rates range from 920 to 2084 t/km2/yr, with the higher values associated with glacier-fed rivers. Chemical erosion rates range from 45 to 91 t/km2/yr, with lower rates being associated with glacier-fed rivers. Uranium activity ratios, (234U/238U), are close to secular equilibrium in the suspended and bedloads, but all dissolved load samples show values greater than unity, ranging from 1.13 to 2.41. The highest (234U/238U) values are found in glacier-fed rivers, and can be attributed to α-recoil effects, as grinding by glaciers locally enhances rates of physical weathering. Activity ratios in glacial rivers decrease with distance from the glacial source due to input from non-glacial tributaries which have high levels of dissolved uranium and lower activity ratios. In contrast, in non-glacial rivers, uranium activity ratios increase with distance downstream due to continued weathering in soils and of bedrock. The δ7Li value of the suspended load is always lower than that of the bedload due to preferential retention of 6Li in secondary minerals during weathering. In turn, the δ7Li value of the dissolved load is always greater than that of the bedload, ranging from 17.0 to 43.7‰. δ7Li for the dissolved load decreases with increasing levels of dissolved silicon, and the saturation index of secondary minerals, which suggests that δ7Li decreases with increasing chemical weathering. There is no correlation between δ7Li values and uranium activity ratios for the dissolved load for non-glacial rivers, but for glacier-fed rivers there is an increase in δ7Li with increasing (234U/238U), suggesting that where physical comminution of mineral grains by glaciers is high, chemical weathering is suppressed, and vice-versa.