[1] Using multiple lines of evidence, we demonstrate that volcanic ash deposition in August 2008 initiated one of the largest phytoplankton blooms observed in the subarctic North Pacific. Unusually widespread transport from a volcanic eruption in the Aleutian Islands, Alaska deposited ash over much of the subarctic NE Pacific, followed by large increases in satellite chlorophyll. Surface ocean pCO 2 , pH, and fluorescence reveal that the bloom started a few days after ashfall. Ship-based measurements showed increased dominance by diatoms. This evidence points toward fertilization of this normally iron-limited region by ash, a relatively new mechanism proposed for iron supply to the ocean. The observations do not support other possible mechanisms. Extrapolation of the pCO 2 data to the area of the bloom suggests a modest ∼0.01 Pg carbon export from this event, implying that even large-scale iron fertilization at an optimum time of year is not very efficient at sequestering atmospheric CO 2 . Citation: Hamme, R. C., et al. (2010), Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific, Geophys.

The silicon isotopic composition of silicic acid (δ 30 Si(OH) 4 ) and biogenic silica (δ 30 Si-bSiO 2 ) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ 30 Si(OH) 4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30 ε = À0.97 ± 0.17‰, agrees well with laboratory and global-ocean estimates. Nevertheless, the δ 30 Si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30 ε, between À0.33‰ and À0.41‰, depending on how the contribution of sea-ice diatoms is incorporated. In the upper 400 m, δ 30 Si-bSiO 2 values were among the heaviest ever measured in marine suspended bSiO 2 (+2.03‰ to +3.51‰). A positive correlation between δ 30 Si-bSiO 2 and sea-ice cover implies that heavy signals can result from isotopically heavy sea-ice diatoms introduced to pelagic assemblages. Below the surface bSiO 2 production zone, the δ 30 Si(OH) 4 distribution followed that of major water masses. Vertical δ 30 Si(OH) 4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125-200 m) composed of modified Pacific water and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300-500 m deep). In the Canada Basin Deep Water (below 2000 m), δ 30 Si(OH) 4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH) 4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ 30 Si(OH) 4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box model simulation of the global marine δ 30 Si(OH) 4 distribution successfully reproduced the observed patterns, with the δ 30 Si(OH) 4 of the simulated deep Arctic Ocean being the heaviest of all deep-ocean basins.

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[1] We compared net community production determined from an in situ O 2 /Ar mass balance (O 2 /Ar-NCP) with incubation measurements of new and primary production in the subarctic northeast Pacific. In situ O 2 /Ar-NCP was strongly correlated to new production from 24-h 15 NO 3 À uptake integrated over the mixed layer ( 15 N-NewP), if measurements were separated into high and low-productivity conditions. Under high-productivity conditions, O 2 /Ar-NCP estimates were similar to 15 N-NewP, whereas under low productivity conditions O 2 /Ar-NCP was up to two times higher than 15 N-NewP. The relationship between O 2 /Ar-NCP and 24-h 13 C primary production ( 13 C-PP) was more variable, but with a consistent mean O 2 /Ar-NCP: 13 C-PP ratio of 0.52 AE 0.17 when only low-productivity, summer measurements were considered. This relationship with primary production is perturbed by high productivity events such as a late-summer, iron-stimulated bloom observed at the offshore stations. Finally, we show that diapycnal mixing usually dominates the O 2 /Ar mass balance in winter in the subarctic Pacific, preventing the determination of NCP by the O 2 /Ar method at that time, except for one unusual stratification event in February 2007.Citation: Giesbrecht, K. E., R. C. Hamme, and S. R. Emerson (2012), Biological productivity along Line P in the subarctic northeast Pacific: In situ versus incubation-based methods, Global Biogeochem. Cycles, 26, GB3028,

A recently recovered and compiled set of inorganic carbon data collected in the Canadian Arctic since the 1970s has revealed substantial change, as well as variability, in the carbonate system of the Beaufort Sea and Canada Basin. Whereas the role of this area as a net atmospheric carbon sink has been confirmed, high pCO 2 values in the upper halocline underscore the potential for CO 2 outgassing as sea ice retreats and upwelling increases. In addition, increasing total inorganic carbon and decreasing alkalinity are increasing pCO 2 and decreasing CaCO 3 saturation states, such that undersaturation with respect to aragonite now occurs regularly in both deep waters and the upper halocline.