The summer hydrographic structure of the Hanna Shoal region on the northeastern Chukchi Sea shelf: 2011–2013

Significant Biologically Mediated CO2 Uptake in the Pacific Arctic During the Late Open Water Season

“…Alternatively, a recent increase in sea level pressure of ~50 mbar or a S decrease of 7 could also result in a ∆Ar of −5%. Likely, some combination of the above factors contribute to observed ∆Ar , but a recent 2 °C cooling for much of this region is consistent with T differences between our study and observations reported for mid‐September 2011 and 2012 (Danielson et al, ; Weingartner et al, ) as well as NOAA AVHRR Blended Daily high‐resolution sea surface temperature throughout September/October (Reynolds et al, ; see animated Figures S5 and S6).…”

“…Downstream of Bering Strait, water mass properties are subsequently modified to varying degrees by warming/cooling, sea ice melt, mixing, and biological uptake/removal during months of transit over the broad, shallow (~50 m) Chukchi shelf (Pickart et al, ; Shroyer & Pickart, ). The extent and timing of sea ice melt in particular plays a significant role in water column stratification, which in turn impacts potential for surface phytoplankton communities to access inorganic nutrient reservoirs beneath the halocline/pycnocline after surface reservoirs are exhausted (e.g., Danielson et al, ; Lowry et al, ; Weingartner et al, ). For example, an anomalously early sea ice retreat in 2011 led to a significantly reduced meltwater (MW) presence throughout the Chukchi shelf, while in 2012 a lingering ice presence resulted in substantial MW‐influenced layer of 20 m capping the majority of the NE Chukchi, with stratification nearly twice as strong in 2012 versus 2011 (Weingartner et al, ).…”

“…The extent and timing of sea ice melt in particular plays a significant role in water column stratification, which in turn impacts potential for surface phytoplankton communities to access inorganic nutrient reservoirs beneath the halocline/pycnocline after surface reservoirs are exhausted (e.g., Danielson et al, ; Lowry et al, ; Weingartner et al, ). For example, an anomalously early sea ice retreat in 2011 led to a significantly reduced meltwater (MW) presence throughout the Chukchi shelf, while in 2012 a lingering ice presence resulted in substantial MW‐influenced layer of 20 m capping the majority of the NE Chukchi, with stratification nearly twice as strong in 2012 versus 2011 (Weingartner et al, ). Wind strength, direction, and persistence also play a significant role in residence time and physical modification of water masses in this region (Danielson et al, ).…”

“…MW is typically regarded to be low in productivity and nutrient‐deplete, and presence of MW enhances vertical stratification, reducing the prospect for diffusive mixing of nutrients from nutrient‐replete subsurface waters into the surface. However, in September 2012, Weingartner et al () found variance in surface density fields increased twofold to fivefold in high‐resolution transects in the MW‐influenced region in the northern Chukchi compared to transects just south of the MW front. The authors attributed the enhanced density variance to (1) lateral processes involving MW that generate eddies and cross‐frontal exchange in the pycnocline (Lu et al, ) and (2) spatial heterogeneity in sea ice floes, with corresponding variability in ice drag, melt rates buoyancy plumes.…”

“…Sea ice melt is depleted in O 2 and Ar since both are excluded from the sea ice matrix during freezing; taking a pure sea ice melt endmember ∆Ar of −45% (Top et al, , ), a ∆Ar of −5% could be achieved by a 10% fractional contribution of melt in the surface layer (i.e., 2 m melt in a typical 20 m mixed layer). However, sea ice melt is an unlikely culprit for ∆Ar undersaturations observed in 2011 because sea ice retreat occurred very early (i.e., mid‐June, Weingartner et al, ) and any disequilibrium imparted by melt would be equilibrated with the atmosphere in the intervening time. In 2012, ice lingered over the NE Chukchi into mid‐September, but the most undersaturated ∆Ar values were observed in the southern Chukchi, a region clear of ice by July.…”