A Model‐Based Analysis of Physical and Biogeochemical Controls on Carbon Exchange in the Upper Water Column, Sea Ice, and Atmosphere in a Seasonally Ice‐Covered Arctic Strait

Abstract

In this study, we consider a 1-D model incorporating both sea ice and pelagic systems in order to assess the importance of various processes on the vertical transport and exchange of carbon in the seasonally ice-covered marine Arctic. The model includes a coupled ice-ocean ecosystem, a parameterization of ikaite precipitation and dissolution, a formulation for ice-air carbon exchange, and a formulation for brine rejection and freshwater dilution of dissolved inorganic carbon (DIC) and total alkalinity (TA) associated with ice growth and melt. Sensitivity analyses illustrate that (1) the pelagic ecosystem accounts for more than half of the net ocean carbon uptake, but ice algae have little effect on the air-sea exchange in the standard run;(2) inclusion of ikaite precipitation and dissolution do not strongly affect the net ocean carbon uptake for concentrations within the observed range but can become important for larger concentrations; (3) varying DIC and TA in the ice by equal amounts, or varying brine deposition depth, does not affect the net ocean carbon uptake, because the coincident changes in TA and DIC concentrations at the sea surface serve to counteract one another with respect to sea surface pCO 2 ; and (4) the proportions of carbon released to the water column (versus to the atmosphere) during ice growth and melt are important quantities to constrain in order to determine the contribution of the combined ice-ocean system to oceanic uptake of atmospheric carbon. Plain Language SummaryCarbon dioxide is being taken up by the global oceans, especially at the poles. Although sea ice has been treated as a barrier to air-sea exchange of gases, recent observations have shown that it is not so simple. This study uses a model to represent loss, gain, and movement of inorganic carbon due to sea ice growth and melt, ice and water column ecosystems, and air-sea exchange in the open-water season. Results from this study indicate that the ecosystem is a major player in the uptake of carbon dioxide by the Arctic Ocean and that fundamental processes relating to the sea ice need to be further studied, in particular, (a) how ice (low in inorganic carbon) is created from seawater (high in inorganic carbon) and (b) the formation of carbon-containing crystals in sea ice.

Modeled Impacts of Sea Ice Exchange Processes on Arctic Ocean Carbon Uptake and Acidification (1980–2015)

“…The ice algae take up nutrients and DIC within the ice skeletal layer during photosynthetic growth, and release both detritus and organic carbon into the water column during senescence. Also, consistent with the 1-D model described in Mortenson et al (2017Mortenson et al ( , 2018, ice algal detritus contributes to the pelagic large phytoplankton through seeding. The detrital organic carbon is converted to DIC through remineralization as the detritus sinks through the water column.…”

“…CanOE has been modified to include an ice carbon pump with nonlocal vertical transport by brine plumes (high in DIC and TA relative to in situ sea water concentrations) during ice growth, and the dilution of near-surface waters during ice melt. The depth of deposition of brine-rejected DIC and TA during ice formation in the present study and Mortenson et al (2018) is based on results from Jin et al (2015), in which subgrid scale brine-rejected salt in a simulation best represents observed salinity profiles when deposited at the bottom of the mixed layer. The scenario with brine-rejected tracers at the mixed layer depth can be viewed as one extreme scenario among the likely deposition depths, as opposed to surface release as the other extreme assumed in Grimm et al (2016) and Moreau et al (2016) and an intermediate scenario of mixing throughout the mixed layer as described in DeGrandpre et al (2019).…”

“…where Δh ice is the change in ice thickness over a model time step, Δt, and DIC ref ice and DIC ref SW are the constant reference concentrations of DIC in the ice and seawater (Table 1 in Mortenson et al, 2018). The proportion of DIC released to the water column is denoted P SW and is set at 99% as in the 1-D study described in Mortenson et al (2018). The remaining proportion 1-P SW is released from the ice directly into the atmosphere.…”

“…A novel treatment in the present study is the nonlocal transport of the DIC and TA during ice growth, associated with dense brine rejection, which is implemented by depositing both DIC and TA at the bottom of the mixed layer rather than at the surface. The development and analysis of this parameterized ice carbon pump in a 1-D model study are described in detail in Mortenson et al (2018). Here, we apply the parameterization into a regional coupled sea ice and ocean biogeochemical model of the Arctic Ocean and evaluate its regional and pan-Arctic impact.…”

New insights into aerosol and climate in the Arctic

“…configuration (Mortenson et al, 2017). Subsequently, sulfur and inorganic carbon cycling were developed and implemented into the model Mortenson et al, 2018). The simulated Arctic sea ice ecosystem and sulfur cycle were next incorporated into a three-dimensional (3-D) regional configuration (Hayashida et al, 2018a(Hayashida et al, , 2018b.…”