Utility of dissolved barium in distinguishing North American from Eurasian runoff in the Arctic Ocean

Abstract

Dissolved barium has been shown to have the potential to distinguish Eurasian from North American (NA) river runoff. As part of the ARK-XXII/2 Polarstern expedition in summer 2007, Ba was analyzed in the Barents, Kara, Laptev seas, and the Eurasian Basins as well as the Makarov Basin up to the Alpha and Mendeleyev Ridges. By combining salinity, δ 18 O and initial phosphate corrected for mineralization with oxygen (PO 4 *) or N/P ratios we identified the water mass fractions of meteoric water, sea ice meltwater, and marine waters of Atlantic as well as Pacific origin in the upper water column. In all basins inside the lower halocline layer and the Arctic intermediate waters we find Ba concentrations close to those of the Fram Strait branch of the lower halocline (41-45 nM), reflecting the composition of the incoming Atlantic water. A layer of upper halocline water (UHW) with higher Ba concentrations (45-55 nM) is identified in the Makarov Basin. Atop of the UHW, the Surface Mixed Layer (SML), including the summer and winter mixed layers, has high concentrations of Ba (58-67 nM). In the SML of the investigated area of the central Arctic the meteoric fraction can be identified by assuming a conservative behavior of Ba to be primarily of Eurasian river origin. However, in productive coastal regions biological removal compromises the use of Ba to distinguish between Eurasian and NA rivers. As a consequence, the NA river water fraction is underestimated in productive surface waters or waters that have passed a productive region, whereas this fraction is overestimated in subsurface waters containing remineralised Ba, particularly when these waters have passed productive shelf regions. Especially in the Laptev Sea and small regions in the Barents Sea, Ba concentrations are low in surface waters. In the Laptev Sea exceptionally high Ba concentrations in shelf bottom waters indicate that Ba is removed from surface waters to deep waters by biological activity enhanced by increasing ice-free conditions as well as by scavenging by organic matter of terrestrial origin. We interpret high Ba concentrations in the UHW of the Makarov Basin to result from enrichment by remineralisation in bottom waters on the shelf of the Chukchi Sea and therefore the calculated NA runoff is an artefact. We conclude that no NA runoff can be demonstrated unequivocally anywhere during our expedition with the set of tracers considered here. Small contributions of NA runoff may have been masked by Ba depletion and could only be resolved by supportive tracers on the uptake history. We thus suggest that Ba has to be used with care as it can put limits but not yield quantitative water mass distributions. Only if the extra Ba inputs exceed the cumulative biological uptake the signal can be unequivocally attributed to NA runoff.

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

“…This biological cycling is closely linked to the cycling of Ba [cf. Guay and Falkner , 1997; Abrahamsen et al , 2009; Roeske et al , 2012] (Figure 2, middle). Both Ba and 226 Ra are depleted in the Laptev and Barents (Station 239) surface waters, while they show enhanced values in Laptev bottom waters.…”

“…Moreover, both Ba and 226 Ra show an offset between Eurasian and Pacific surface waters in the central basins [cf. Roeske et al , 2012].…”

“…Moreover, the procedure corrects for biological uptake/release. On the Laptev shelf, biological uptake removed 25–48% of 226 Ra and 31% of Ba [ Roeske et al , 2012] in the surface water (Figure 2). On the other hand, one should always be careful applying a mixing plot using ARs because ratios do not mix linearly [ Kadko and Muench , 2005] (compare Figure 8).…”

The Impact of Surface Mixing on the Arctic River Water Distribution and Stratification in a Global Ice–Ocean Model

“…5b-d). Estimates of river water fraction based on ship observations (Guay et al 2009;Roeske et al 2012) indicate that the river water is concentrated within the top 50 m, particularly in the Canadian Basin, and that the fraction exceeds 10% in some regions. Thus, the river water distribution in LSFC, LSNW, and LALL is more realistic than that in CTRL, with LALL giving the best results.…”

Circulation patterns in the lower Arctic Ocean derived from geochemical data

“…6b). Just as Roeske et al (2012) used Ba to track freshwater from a specific river, a particular geochemical parameter could be used to track a particular source water.…”