More Like this

1. Quantifying Drivers of Seasonal and Interannual Variability of Dissolved Oxygen in the Canada Basin Mixed Layer

   Arroyo, Ashley; Timmermans, Mary‐Louise; DeGrandpre, Mike (July 2024, Journal of Geophysical Research: Oceans)

   Abstract Analysis of dissolved oxygen (O₂) in the Arctic's surface ocean provides insights into gas transfer between the atmosphere‐ice‐ocean system, water mass dynamics, and biogeochemical processes. In the Arctic Ocean's Canada Basin mixed layer, higher O₂concentrations are generally observed under sea ice compared to open water regions. Annual cycles of O₂and O₂saturation, increasing from summer through spring and then sharply declining to late summer, are tightly linked to sea ice cover. The primary fluxes that influence seasonal variability of O₂are modeled and compared to Ice‐Tethered Profiler O₂observations to understand the relative role of each flux in the annual cycle. Findings suggest that sea ice melt/growth dominates seasonal variations in mixed layer O₂, with minor contributions from vertical entrainment and atmospheric exchange. While the influence of biological activity on O₂variability cannot be directly assessed, indirect evidence suggests relatively minor contributions, although with significant uncertainty. Past studies show that O₂molecules are expelled from sea ice during brine rejection; sea ice cover can then inhibit air‐sea gas exchange resulting in winter mixed layers that are super‐saturated. Decreasing mixed layer O₂concentrations and saturation levels are observed during winter months between 2007 and 2019 in the Canada Basin. Only a minor portion of the decreasing trend in wintertime O₂can be attributed to decreased solubility. This suggests the O₂decline may be linked to more efficient air‐sea exchange associated with increased open water areas in the winter sea ice pack that are not necessarily detectable via satellite observations. 

   more » « less
2. Summertime Evolution of Net Community Production and CO ₂ Flux in the Western Arctic Ocean

   https://doi.org/10.1029/2020GB006651  

   Ouyang, Zhangxian; Qi, Di; Zhong, Wenli; Chen, Liqi; Gao, Zhongyong; Lin, Hongmei; Sun, Heng; Li, Tao; Cai, Wei‐Jun (March 2021, Global Biogeochemical Cycles)

   Abstract To examine seasonal and regional variabilities in metabolic status and the coupling of net community production (NCP) and air‐sea CO₂fluxes in the western Arctic Ocean, we collected underway measurements of surface O₂/Ar and partial pressure of CO₂(pCO₂) in the summers of 2016 and 2018. With a box‐model, we demonstrate that accounting for local sea ice history (in addition to wind history) is important in estimating NCP from biological oxygen saturation (Δ(O₂/Ar)) in polar regions. Incorporating this sea ice history correction, we found that most of the western Arctic exhibited positive Δ(O₂/Ar) and negativepCO₂saturation, Δ(pCO₂), indicative of net autotrophy but with the relationship between the two parameters varying regionally. In the heavy ice‐covered areas, where air‐sea gas exchange was suppressed, even minor NCP resulted in relatively high Δ(O₂/Ar) and lowpCO₂in water due to limited gas exchange. Within the marginal ice zone, NCP and CO₂flux magnitudes were strongly inversely correlated, suggesting an air to sea CO₂flux induced primarily by biological CO₂removal from surface waters. Within ice‐free waters, the coupling of NCP and CO₂flux varied according to nutrient supply. In the oligotrophic Canada Basin, NCP and CO₂flux were both small, controlled mainly by air‐sea gas exchange. On the nutrient‐rich Chukchi Shelf, NCP was strong, resulting in great O₂release and CO₂uptake. This regional overview of NCP and CO₂flux in the western Arctic Ocean, in its various stages of ice‐melt and nutrient status, provides useful insight into the possible biogeochemical evolution of rapidly changing polar oceans. 

   more » « less
3. Weakening of Cold Halocline Layer Exposes Sea Ice to Oceanic Heat in the Eastern Arctic Ocean

   https://doi.org/10.1175/JCLI-D-19-0976.1  

   Polyakov, Igor V.; Rippeth, Tom P.; Fer, Ilker; Alkire, Matthew B.; Baumann, Till M.; Carmack, Eddy C.; Ingvaldsen, Randi; Ivanov, Vladimir V.; Janout, Markus; Lind, Sigrid; et al (September 2020, Journal of Climate)

   Abstract A 15-yr duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (~150–900 m) warm Atlantic Water (AW) to the surface mixed layer and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017–18 showing AW at only 80 m depth, just below the wintertime surface mixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3–4 W m −2 in 2007–08 to >10 W m −2 in 2016–18. This seasonal AW heat loss in the eastern EB is equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback. 

   more » « less
4. Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera

   https://doi.org/10.1029/2020PA004095  

   Lund, David C.; Chase, Zanna; Kohfeld, Karen E.; Wilson, Earle A. (May 2021, Paleoceanography and Paleoclimatology)

   Abstract Southern Ocean sea ice plays a central role in the oceanic meridional overturning circulation, transforming globally prevalent watermasses through surface buoyancy loss and gain. Buoyancy loss due to surface cooling and sea ice growth promotes the formation of bottom water that flows into the Atlantic, Indian, and Pacific basins, while buoyancy gain due to sea ice melt helps transform the returning deep flow into intermediate and mode waters. Because northward expansion of Southern Ocean sea ice during the Last Glacial Maximum (LGM; 19–23 kyr BP) may have enhanced deep ocean stratification and contributed to lower atmospheric CO₂levels, reconstructions of sea ice extent are critical to understanding the LGM climate state. Here, we present a new sea ice proxy based on the¹⁸O/¹⁶O ratio of foraminifera (δ¹⁸O_c). In the seasonal sea ice zone, sea ice formation during austral winter creates a cold surface mixed layer that persists in the sub‐surface during spring and summer. The cold sub‐surface layer, known as winter water, sits above relatively warm deep water, creating an inverted temperature profile. The unique surface‐to‐deep temperature contrast is reflected in estimates of equilibrium δ¹⁸O_c, implying that paired analysis of planktonic and benthic foraminifera can be used to infer sea ice extent. To demonstrate the feasibility of the δ¹⁸O_cmethod, we present a compilation ofN. pachydermaandCibicidoidesspp. results from the Atlantic sector that yields an estimate of winter sea ice extent consistent with modern observations. 

   more » « less
5. Inorganic Carbon and p CO ₂ Variability During Ice Formation in the Beaufort Gyre of the Canada Basin

   https://doi.org/10.1029/2019JC015109  

   DeGrandpre, Michael D.; Lai, Chun‐Ze; Timmermans, Mary‐Louise; Krishfield, Richard A.; Proshutinsky, Andrey; Torres, Daniel (June 2019, Journal of Geophysical Research: Oceans)

   Abstract Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (A_T). The brine sinks, potentially exporting DIC andA_Tto deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 yearpCO₂mooring time series where a ~35‐μatm increase inpCO₂was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resultingpCO₂is sensitive to the ratio ofA_Tand DIC retained in the ice and the mixed layer depth, which controls dilution of the ice‐derivedA_Tand DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing thepCO₂at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC andpCO₂in the surface of the Arctic Ocean as seasonal ice production and wind‐driven mixing increase with diminishing ice cover. 

   more » « less