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The Arctic Ocean has turned from a perennial ice‐covered ocean into a seasonally ice‐free ocean in recent decades. Such a shift in the air‐ice‐sea interface has resulted in substantial changes in the Arctic carbon cycle and related biogeochemical processes. To quantitatively evaluate how the oceanic CO2 sink responds to rapid sea ice loss and to provide a mechanistic explanation, here we examined the air‐sea CO2 flux and the regional CO2 sink in the western Arctic Ocean from 1994 to 2019 by two complementary approaches: observation‐based estimation and a data‐driven box model evaluation. The pCO2 observations and model results showed that summer CO2 uptake significantly increased by about 1.4 ± 0.6 Tg C decade−1 in the Chukchi Sea, primarily due to a longer ice‐free period, a larger open area, and an increased primary production. However, no statistically significant increase in CO2 sink was found in the Canada Basin and the Beaufort Sea based on both observations and modeled results. The reduced sea ice coverage in summer in the Canada Basin and the enhanced wind speed in the Beaufort Sea potentially promoted CO2 uptake, which was, however, counteracted by a rapidly decreased air‐sea pCO2 gradient therein. Therefore, the current and future Arctic Ocean CO2 uptake trends cannot be sufficiently reflected by the air‐sea pCO2 gradient alone because of the sea ice variations and other environmental factors. Key Points: Both observations and model results conclude that summertime CO2 sink increased significantly by 1.4 ± 0.6 Tg C decade−1 in the Chukchi SeaModel results suggest that increased CO2 sink in the Chukchi Sea is driven by the reduced sea ice and increased primary productionBoth observations and model results suggest no significant trend of CO2 sink in the Beaufort Sea and the Canada Basin between 1994 and 2019 [ABSTRACT FROM AUTHOR]

Hydrographic data collected from research cruises, bottom‐anchored moorings, drifting Ice‐Tethered Profilers, and satellite altimetry in the Beaufort Gyre region of the Arctic Ocean document an increase of more than 6,400 km3 of liquid freshwater content from 2003 to 2018: a 40% growth relative to the climatology of the 1970s. This fresh water accumulation is shown to result from persistent anticyclonic atmospheric wind forcing (1997–2018) accompanied by sea ice melt, a wind‐forced redirection of Mackenzie River discharge from predominantly eastward to westward flow, and a contribution of low salinity waters of Pacific Ocean origin via Bering Strait. Despite significant uncertainties in the different observations, this study has demonstrated the synergistic value of having multiple diverse datasets to obtain a more comprehensive understanding of Beaufort Gyre freshwater content variability. For example, Beaufort Gyre Observational System (BGOS) surveys clearly show the interannual increase in freshwater content, but without satellite or Ice‐Tethered Profiler measurements, it is not possible to resolve the seasonal cycle of freshwater content, which in fact is larger than the year‐to‐year variability, or the more subtle interannual variations. The Beaufort Gyre centered in the Canada Basin of the Arctic Ocean is the major reservoir of fresh water in the Arctic. The primary focus of this study is on quantifying variability and trends in liquid (water) and solid (sea ice) freshwater content in this region. The Beaufort Gyre Exploration Program was initiated in 2003 to synthesize results of historical data analysis, design and conduct long‐term observations, and to provide information for numerical modeling under the umbrella of the FAMOS (Forum for Arctic Observing and Modeling Synthesis) project. The data collected from research cruises, moorings, Ice‐Tethered Profiler observations, and satellite altimetry document an increase of more than 6,400 km3 of liquid freshwater content from 2003 to 2018, a 40% growth relative to the climatology of the 1970s. This fresh water volume is comparable to the fresh water volume released to the sub‐arctic seas during the Great Salinity Anomaly episode of the 1970s. Thus, since the 2000s, the stage has been set for another possible release of fresh water to lower latitudes with accompanying climate impacts, including changes to sea ice conditions, ocean circulation, and ecosystems of the Sub‐Arctic similar to the influence of the Great Salinity Anomaly observed in the 1970s. Key Points: Beaufort Gyre freshwater content time series (2003–2018) from different data sets are updated, compared, and analyzedQualitative and quantitative estimates of factors and mechanisms driving freshwater content changes are providedIn 2003–2018, the major sources of accumulated fresh water were sea ice melt, Mackenzie River runoff, and Bering Strait transport [ABSTRACT FROM AUTHOR]

Abstract: The concentration of dissolved iron (DFe) and suspended leachable particulate iron (LPFe) in the water column of the western Beaufort Sea were investigated during the late summer of 2010. Elevated concentrations of surface DFe (0.49–1.42 nM) were similar to those reported in resent studies, likely reflecting input from melting sea ice and river discharge. The rapid decrease in DFe (5.20–0.48 nM) and LPFe (88.2–1.83 nM) values observed from inshore to offshore in Pacific influenced waters, suggest scavenging processes limit the input of DFe from the shelf to the deep basin. However, frequent eddies found in this region are likely important in promoting lateral advection, as suggested by higher surface DFe concentrations at an offshore station in the vicinity of a warm-core eddy. Within the Atlantic layer, relatively homogeneous DFe (0.69–0.80 nM) and LPFe (1.18–2.13 nM) concentrations were observed at all the stations, reflecting a balance in the interplay between input and removal processes within this watermass. An input of DFe east of the Lomonosov Ridge was inferred by comparing DFe values within the core of Atlantic water between the Eastern and Western Arctic. [Copyright &y& Elsevier]

This study examined possible environmental factors that affect prokaryote variables in surface waters (upper 100 m of water column) in the Canada Basin, western Arctic Ocean. We collected data on prokaryote abundance and heterotrophic production ([H]leucine incorporation) at eight stations deployed along a slope-to-offshore transect during September 2009. Prokaryote production and growth tended to increase with increasing chlorophyll a (Chl. a) and temperature and with decreasing salinity. The combination of Chl. a, temperature, and salinity accounted for a large fraction (74%) of the variability in prokaryote production, with the highest contribution made by Chl. a ( r = 0.56), followed by salinity ( r = 0.14) and temperature ( r = 0.03). Similarly, the variability in prokaryote growth rate was largely accounted for by the combination of the three environmental variables (overall r of 0.64), with Chl. a making the largest contribution to variability ( r = 0.33), followed by salinity ( r = 0.27) and temperature ( r = 0.05). These data are consistent with the notion that organic matter supply associated with freshwater inputs to surface layers can result in enhanced prokaryote production and growth in the Canada Basin. Our results provide insights into the regulation of the microbial loop in the Canada Basin where freshening has been proceeding rapidly due to increasing river discharge and sea-ice melting. [ABSTRACT FROM AUTHOR]

In general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice‐free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitored a fixed‐point observation (FPO) station in the Canada Basin for about 3 weeks in the fall of 2014 to examine the oceanic and biological responses to strong winds. At the FPO site, oceanic microstructure measurements, hydrographic surveys, and water sampling were performed with high temporal resolution, recording internal wave propagation, eddy passage, and water mass changes. Strong winds and internal wave propagation significantly enhanced the mixing above and at the seasonal pycnocline, but their effects were diminished at the nutricline, which was much deeper than the seasonal pycnocline. Therefore, wind‐induced mixing did not increase the upward nutrient supply from the nutricline and did not impact phytoplankton (chlorophyll a) distribution in the surface layer of the FPO site. The temporal evolution of the chlorophyll a concentration was most closely related to water mass changes. We also observed prominent subsurface chlorophyll a maxima with abundant large‐sized phytoplankton that were likely carried by warm‐core eddies to the FPO site. Phytoplankton biomass may have been sustained by the high concentration of ammonium within the eddy and ammonium regeneration at the seasonal pycnocline, where particulate organic matter likely accumulated. Plain Language Summary: The Arctic basins, once covered with multiyear ice, are gaining ice‐free areas in summer and fall as an effect of global warming. An ice‐free upper ocean is more susceptible to wind forcing than ice‐covered waters. Here, we studied the oceanic and biological responses to strong winds in the ice‐free Canada Basin, where a density gap (pycnocline) was seasonally formed at a depth of about 20 m between the sea surface layer containing sea ice meltwater and the layer below. Although significant wind‐induced mixing was observed above and at this seasonal pycnocline, mixing was attenuated at nutricline depths (~60 m), where nutrient concentrations increased strongly with depth. Thus, nutrients were not supplied from the nutricline to the sea surface layer through wind‐induced mixing, and phytoplankton biomass did not increase in response to strong winds. However, phytoplankton biomass sometimes increased around the seasonal pycnocline, where ammonium was available for use in photosynthesis. This ammonium was likely supplied by warm‐core eddies transporting shelf water into the Canada Basin. Another conceivable source of ammonium was particulate organic matter suspended at the seasonal pycnocline that decomposed, producing ammonium. Key Points: A nutricline deeper than the seasonal pycnocline in the Canada Basin makes nutrient and phytoplankton distributions unresponsive to windThe increase in phytoplankton in the Canada Basin is partly explained by the effects of warm‐core eddies with high ammonium concentrationsParticulate organic matter suspended at the seasonal pycnocline depth in the Canada Basin may be an ammonium source for photosynthesis [ABSTRACT FROM AUTHOR]

We examined vertical profiles of methane (CH 4) concentrations inside and outside of a warm-core eddy during a cruise of the R/V Mirai cruise (MR15-03 Leg. 1) in the Arctic Ocean in late summer/early fall 2015. Because a coherent mesoscale eddy is one of the possible mechanisms for transporting shelf water from the Chukchi Sea into the Canada Basin interior, we hypothesized that the warm-core eddy transported dissolved CH 4 as well as nutrients and impacted the distribution of CH 4. Seawater samples were therefore collected to obtain detailed vertical profiles of CH 4 concentrations in the southwestern Canada Basin. We found high CH 4 concentrations in bottom water on the shelf, which was located outside the warm-core eddy. The percent saturation in that water was as high as 947% of atmospheric CH 4. We also found that sub-surface CH 4 peaks within the radius of the maximum velocity of the eddy were broader at depth than those outside of the eddy. Lateral transport of the eddy was likely the controlling factor responsible for subsurface CH 4 peaks within the warm-core eddy. Our study indicated that eddies play a crucial role in controlling and transporting CH 4 laterally and vertically. [ABSTRACT FROM AUTHOR]

Abstract: The vertical distributions of prokaryote heterotrophic production (3H-leucine incorporation rate) and abundance were investigated in the meso- and bathy-pelagic layers of the Canada Basin, western Arctic Ocean, during September 2009. Prokaryote production and abundance were high in the Pacific-origin water mass located in the upper mesopelagic layer (depth, 100–200m). Below the halocline layer (depth, 300–3000m), both the production and abundance decreased with depth, with log–log regression slopes of −1.33 and −0.77, respectively. Depth-integrated production and biomass in the meso- and bathy-pelagic layers was three- to five-fold lower than the corresponding values reported in the subpolar regions, whereas they were close to or lower than the corresponding values in oligotrophic subtropical regions. Prokaryote turnover times were estimated to be 1.1 and 6.1 years for meso- and bathy-pelagic layers, respectively, with the latter being among the longest turnover times reported for oceanic basins. We estimated prokaryote carbon demand in the water column (100–3000m) to be on the order of 11mgCm−2 d−1, which largely exceeds (by 38-fold) the sinking particulate organic carbon flux at depths of 120–200m reported in the literature. This large carbon imbalance may be partly explained by organic carbon delivery by lateral intrusion of the Pacific-origin water mass into the upper mesopelagic layer. [Copyright &y& Elsevier]

129I concentrations were measured in surface waters and for two depth profiles in the Chukchi Borderland (74.5°N 162.0°W) and the southern Canada Basin (72.5°N 155.4°W) during the R/V Mirai cruises in 2014 and 2015. The 129I concentrations in the surface waters of the North Pacific Ocean and the Bering and Chukchi seas in 2014 and 2015 were 1.35 ± 0.19 and 1.18 ± 0.17 × 107 atoms L−1, respectively. The depth profiles of 129I were almost identical in 2014 and 2015. 129I concentrations were 10–20 × 107 atoms L−1 in the surface layer, less than 10 × 107 atoms L−1 (i.e., the minimum value) around 100–150 m, and 65–120 × 107 atoms L−1 below 300 m (below 200 m in 2015). Although the peak 129I concentrations exhibited slightly fluctuating values of 90–120 × 107 atoms L−1, the average 129I concentrations at water depths in ranging from 300 to 800 m were an almost constant (83–86 × 107 atoms L−1) from 2014 to 2015. Both the low 129I concentration level relative to other areas of the Arctic Ocean and the constant 129I concentrations over time may correspond to the ages of low discharge rate, until the mid-1990s, from the nuclear fuel-reprocessing facilities at Sellafield (U.K.) and La Hague (France) to the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Winter subsurface transport of the Pacific-origin warm water toward the Arctic Chukchi Borderland located west of the Canada Basin was investigated by mooring measurements and modeling analyses. In mid-winter or spring of 2011–2014, subsurface warming signals under sea ice were detected by the multi-year bottom-tethered mooring data in the Chukchi Abyssal Plain (CAP) of the western Chukchi Borderland. Lateral advection of shelf-origin ocean heat is a key process for the subsurface warming. To address the detailed pathways and processes of subsurface warm water transport, which have not been deeply explored, an interannual experiment for 2001–2014 was performed using a pan-Arctic sea ice-ocean model configured in a high-resolution framework. The horizontal grid size was set to approximately 5 km so that narrow intense currents along complex sharp topography could be resolved. The model result captured the similar seasonality of subsurface temperature in the CAP region and produced interannual variability in the ocean heat content associated with the shelf-origin water distribution around the Chukchi Borderland. In addition to the Barrow Canyon throughflow, westward jets along the steep flank of the Chukchi shelf break constituted a primary pathway for the subsurface warm water transport toward the Chukchi Borderland in the model experiment. Since the simulated shelf break jet was much faster than main streams of the Beaufort Gyre, its role in ocean heat transport should be considered separately. Whereas ocean heat in the Chukchi shelf break region was partly lost via wind-driven turbulent mixing into upper halocline depths of approximately 20 m, a substantial amount of the subsurface warm water remained even after mid-winter. The highly stratified condition due to anomalous sea ice meltwater assisted the winter heat transport. [ABSTRACT FROM AUTHOR]

Abstract: Distributions of colored dissolved organic matter (CDOM) in the upper 400m of the southern Canada Basin and East Siberian Sea were determined using an in situ WETStar fluorometer and fluorescence spectroscopy during cruises in 2008 as part of the Canada/US Joint Ocean Ice Study and Japan''s International Polar Year program. Despite the low CDOM range (0.009–0.069r.u.) observed in the upper 400m of the study area, our results show that CDOM can be quantified from in situ DOM fluorescence sensor measurements. Unlike DOC concentrations, which are known to decrease with increasing depth, a pronounced mid-depth CDOM maximum was associated with the Pacific-derived winter water throughout our study area. Using parallel factor analysis (PARAFAC) to resolve dominant fluorophore components in fluorescence excitation–emission matrices (EEM), we identified three humic-like and two proteinaceous components. The nature and origin of these five fluorophores were investigated based on their fluorescent characteristics as well as their vertical and geographical distributions. The lowest terrestrial humic-like signals in the surface waters were mostly due to photochemical processes, whereas the highest microbial/marine humic-like signal revealed interactions with sediment during the formation of Pacific-origin haloclines over the Arctic shelves. The humic-like fluorophores dominated DOM fluorescence in the Westernmost region in the East Siberian Sea whereas the contribution of protein-like fluorophores was predominant elsewhere. The significant difference in CDOM composition between East and West of the 180° meridian suggests the presence of a front that divides our study area into the Eastern Chukchi—Beaufort and East Siberian sides. This indicates a change in water circulation, and that more than one DOM source affects our study area. Unlike proteinaceous material, the humic-like compounds varied significantly in the halocline. Ten to 20 percent enrichment was observed in terrestrially-derived DOM in the two Pacific-derived haloclines relative to the Atlantic-derived lower halocline. The application of PARAFAC modeling on fluorescent DOM is shown to be an important tool to investigate the dynamics and transport of allochthonous DOM in the Arctic Ocean. [Copyright &y& Elsevier]

The increase in anthropogenic carbon dioxide emissions and attendant increase in ocean acidification and sea ice melt act together to decrease the saturation state of calcium carbonate in the Canada Basin of the Arctic Ocean. In 2008, surface waters were undersaturated with respect to aragonite, a relatively soluble form of calcium carbonate found in plankton and invertebrates. Undersaturation was found to be a direct consequence of the recent extensive melting of sea ice in the Canada Basin. In addition, the retreat of the ice edge well past the shelf-break has produced conditions favorable to enhanced upwelling of subsurface, aragonite-undersaturated water onto the Arctic continental shelf. Undersaturation will affect both planktonic and benthic calcifying biota and therefore the composition of the Arctic ecosystem. [ABSTRACT FROM AUTHOR]