Gregory O. Lehn

Optimization of a 48Ca–43Ca double-spike MC-TIMS method for measuring Ca isotope ratios (δ44/40Ca and δ44/42Ca): limitations from filament reservoir mixing

We used a Monte Carlo error model to optimize a 48Ca–43Ca double-spike technique for measuring Ca isotope ratios (δ44/40Ca and δ44/42Ca) by Multi-Collector Thermal Ionization Mass Spectrometry (MC-TIMS). The model considers errors for counting statistics and Johnson noise, as well as changes in collector cup efficiency (drift). For a 20 V 40Ca ion-beam implemented in a three-hop, dynamic multi-collection routine, the model predicts that a wide range of 48Ca/43Ca and spike/sample ratios should yield internal precisions (2σSEM) of 0.015–0.020‰ for δ44/40Ca and 0.025–0.030‰ for δ44/42Ca. Using a Thermo Fisher MC-TIMS (Triton), we tested 48Ca/43Ca = 1.5 [43Ca/(48Ca + 43Ca) = 0.40 mol mol−1] and spike/sample = 0.66 [Cadsp/(Cadsp + Casmp) = 0.40 mol mol−1] by repeatedly analyzing OSIL Atlantic seawater, NIST SRM 915a, NIST SRM 915b, USGS BHVO-1, and CaF2 over 4 sessions spanning 1 month. While the measured internal precisions generally agreed with model predictions, external reproducibility (2σSD) was much worse than expected. For the 81 measurements made, the average external reproducibility was 0.223‰ for δ44/40Ca and 0.126‰ for δ44/42Ca. After processing raw data through the double-spike equations, nearly all fractionation-corrected ratios showed remnant fractionation patterns. Such patterns reflect deviation from ideal exponential mass-fractionation due to mixing of multiple, independently fractionating reservoirs on the filament. Additional model simulations, as well as comparison against δ44/40Ca values determined with a 43Ca–42Ca double-spike, support the concept of an “average mass rule”, which states that inaccuracies in fractionation-corrected data are greater for isotope ratios having an average mass further away from the average mass of the normalizing ratio. Until advancements are made to eliminate filament reservoir effects, 43Ca–42Ca and 46Ca–43Ca double-spikes should yield the most precise δ44/40Ca and δ44/42Ca values, respectively, when using MC-TIMS. Within the limits of the 48Ca–43Ca double-spike technique, we observed no evidence for 40Ca enrichments among the standards analyzed. Finally, we found that sample matrix effects do not influence the quality of Ca isotope measurements by MC-TIMS, and we tentatively propose that the external reproducibility determined from the repeated analysis of standards can represent the uncertainty of a single sample analysis.

On the geochemical heterogeneity of rivers draining into the straits and channels of the Canadian Arctic Archipelago

Ten rivers across northern Canada and the Canadian Arctic Archipelago (CAA) were sampled during spring 2014 and summer 2015 to investigate their geochemical heterogeneity for comparison against larger North American (i.e., Mackenzie and Yukon Rivers) and Siberian rivers. In general, rivers draining the western and/or northern regions of the study area have higher solute concentrations and lower 87Sr/86Sr ratios compared to rivers draining the eastern and/or southern regions. The inorganic geochemical signatures largely reflect the bedrock geology, which is predominately carbonate in the western and/or northern regions and silicate in the eastern and/or southern regions. Riverine δ18O values primarily correlate with latitude, with only a few exceptions. Measurements of total alkalinity (TA) were combined with a regional analysis of bedrock geology and extrapolated to produce a range for the mean characteristic TA of rivers draining into the straits and channels of the CAA (628–819 µeq kg−1). Combining this estimate with contributions from the Mackenzie River yields a revised North American river runoff TA of 935–1182 µeq kg−1, which is much lower than that of the Mackenzie River (1540 µeq kg−1). This lower concentration suggests that TA may not be used to distinguish between North American and Siberian river contributions in regions such as Davis Strait.

Seasonality of dissolved nitrogen from spring melt to fall freezeup in Alaskan Arctic tundra and mountain streams

Predicting the response of dissolved nitrogen export from Arctic watersheds to climate change requires an improved understanding of seasonal nitrogen dynamics. Recent studies of Arctic rivers emphasize the importance of spring thaw as a time when large fluxes of nitrogen are exported from Arctic watersheds, but studies capturing the entire hydrologic year are rare. We examined the temporal variability of dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) concentrations in six streams/rivers in Arctic Alaska from spring melt to fall freeze-up (May through October) in 2009 and 2010. DON concentrations were generally high during snowmelt and declined as runoff decreased. DIN concentrations were low through the spring and summer and increased markedly during the late summer and fall, primarily due to an increase in nitrate. The high DIN concentrations were observed to occur when seasonal soil thaw depths were near maximum extents. Concurrent increases in DIN and DIN-to-chloride ratios suggest that net increases from nitrogen sources contributed to these elevated DIN concentrations. Our stream chemistry data, combined with soil thermistor data, suggest downward penetration of water into seasonally thawed mineral soils, and reduction in biological nitrogen assimilation relative to remineralization, may increase DIN export from Arctic watersheds during the late summer and fall. While this is part of a natural cycle, improved understanding of seasonal nitrogen dynamics is particularly important now because warmer temperatures in the Arctic are causing earlier spring snowmelt and later fall freeze-up in many regions.