ALBERT

Description: The two pairs of geochemical twins, Zr–Hf and Nb–Ta, have similar chemical properties, leading to their limited fractionation throughout the igneous processes and thus useful and widely used to elucidate rock and mineral formation. In contrast to the analysis of solid samples (e.g. igneous rocks), however, reports of these elements from aquatic samples (e.g. seawater) are very limited due to difficulties in analyzing their very low concentrations in seawater compared to those in solid samples (up to 6 orders of magnitude different). Recent developments of clean sampling techniques coupled with pre-concentration and ICP-MS determination have made trace elements analysis in seawater reliable. Here we report the first vertical distribution of dissolved Zr, Hf, Nb and Ta in the Indian Ocean in addition to those in the Atlantic Ocean, Andaman Sea and Gulf of Thailand. In the Atlantic and northeastern Indian Ocean, Zr, Hf, Nb and Ta show surface depletion and deep water enrichment. The average deepwater Zr/Hf molar ratios in the western North Atlantic, eastern North Atlantic and northeastern Indian Ocean were 270, 315 and 280, respectively. Compared to North Pacific Ocean Zr/Hf ratios of ~500, strong intra- and inter-ocean fractionation, a term that describe a difference between concentration of trace metals in deep Atlantic and deep Pacific seawater, is observed to occur in the global ocean. However, the inter-ocean fractionation of Nb/Ta is weaker due to a more uniform distribution of Nb and Ta in seawater. In contrast to open ocean seawater, Zr, Hf, Nb and Ta concentration at stations close to the continent in the Andaman Sea and Gulf of Thailand were highest in surface water decreasing through deep water, with Zr/Hf and Nb/Ta closer to continental crust ratios indicating significant terrestrial inputs of these elements to seawater. Results suggest that, in spite of the similar chemical properties of these geochemical twin pairs generating coherent fractionation in igneous rocks, strong fractionations of Zr–Hf and Nb–Ta takes place in aquatic environments such as seawater.

Description: The two pairs of geochemical twins, Zr–Hf and Nb–Ta, have similar chemical properties, leading to their limited fractionation throughout the igneous processes and thus useful and widely used to elucidate rock and mineral formation. In contrast to the analysis of solid samples (e.g. igneous rocks), however, reports of these elements from aquatic samples (e.g. seawater) are very limited due to difficulties in analyzing their very low concentrations in seawater compared to those in solid samples (up to 6 orders of magnitude different). Recent developments of clean sampling techniques coupled with pre-concentration and ICP-MS determination have made trace elements analysis in seawater reliable. Here we report the first vertical distribution of dissolved Zr, Hf, Nb and Ta in the Indian Ocean in addition to those in the Atlantic Ocean, Andaman Sea and Gulf of Thailand. In the Atlantic and northeastern Indian Ocean, Zr, Hf, Nb and Ta show surface depletion and deep water enrichment. The average deepwater Zr/Hf molar ratios in the western North Atlantic, eastern North Atlantic and northeastern Indian Ocean were 270, 315 and 280, respectively. Compared to North Pacific Ocean Zr/Hf ratios of ~500, strong intra- and inter-ocean fractionation, a term that describe a difference between concentration of trace metals in deep Atlantic and deep Pacific seawater, is observed to occur in the global ocean. However, the inter-ocean fractionation of Nb/Ta is weaker due to a more uniform distribution of Nb and Ta in seawater. In contrast to open ocean seawater, Zr, Hf, Nb and Ta concentration at stations close to the continent in the Andaman Sea and Gulf of Thailand were highest in surface water decreasing through deep water, with Zr/Hf and Nb/Ta closer to continental crust ratios indicating significant terrestrial inputs of these elements to seawater. Results suggest that, in spite of the similar chemical properties of these geochemical twin pairs generating coherent fractionation in igneous rocks, strong fractionations of Zr–Hf and Nb–Ta takes place in aquatic environments such as seawater.

Description: Dissolved iron (DFe) and manganese (DMn) are essential micronutrients involved in vital phytoplankton physiological pathways, and their deficit can limit primary production in otherwise nutrient-replete surface ocean waters. In this work we present the spatial distributions and biogeochemical cycling of these metals across the Canadian GEOTRACES transect in the Canadian Arctic Ocean during the summer and autumn of 2015. Surface concentrations are dominated by freshwater inputs showing a strong negative correlation with salinity, especially for DMn which behaves more conservatively than DFe. The highest surface concentrations were measured in the Canadian Arctic Archipelago (Fe: 0.401–1.91 and Mn: 4.33–9.54 nmol kg−1) and the Canada Basin (Fe: 0.225–0.479 and Mn: 3.93–7.02 nmol kg−1), regions highly influenced by riverine inputs, whereas the lowest values were found in the Labrador Sea (Fe: 0.106–0.362 and Mn: 0.450–1.09 nmol kg−1) where freshwater inputs diminished and phytoplankton uptake increased. Subsurface and deep water distributions for both metals are largely controlled by a complex balance between sources (advective inputs and organic matter remineralization) and removal processes. The subsurface peaks (∼100–300 m) observed in the Canada Basin (Fe: 0.541 ± 0.060 and Mn: 1.38 ± 0.42 nmol kg−1) and Baffin Bay (Fe: 0.753–1.03 nmol kg−1) were advected from the Chukchi Sea and the Canadian Arctic Archipelago respectively, where DFe and DMn are released from the benthic boundary layer in these shelf-dominated environments. Advective sources associated with the Arctic Circumpolar Boundary Current, rather than vertical fluxes of DFe and DMn in sinking particles, dominate metal distributions in the deep Canada Basin waters (〉300 m). In the highly productive Baffin Bay and the Labrador Sea, organic matter remineralization is a notable source of DFe and DMn to deep waters. In the deepest waters (〉1000 m), scavenging of DFe and DMn govern their vertical distributions; a pseudo-first order scavenging model explained the continuous removal of DMn in the Canada Basin, where the concentrations reach uniformly low concentrations (0.150 ± 0.004 nmol kg−1) after ∼400 years. Applying this DMn scavenging model we were able to estimate the age (120–190 years) of deep Baffin Bay waters, a topic of discussion for many years.