Light Absorption in Arctic Sea Ice: Chlorophyll and Black Carbon

TL;DR

This study models light absorption in Arctic sea ice, highlighting that chlorophyll from ice algae can rival black carbon in light attenuation, especially in spring, with implications for ice melt and biological activity.

Contribution

It presents the first simulations comparing chlorophyll and black carbon contributions to light absorption in Arctic sea ice during the contemporary period.

Findings

Chlorophyll can cause comparable light attenuation to black carbon during Boreal Spring.

Bottom layer chlorophyll concentrations reach 300-1000 mg/m3 in some regions.

Chlorophyll levels are much lower (>0.1 micro g/m3) in the ice interior north of 75°N.

Abstract

Arctic sea ice extent has declined continuously for the past decade, owing partially to light absorption by black carbon (BC) and other impurities deposited on snow and the underlying pack. We present simulations for the contemporary period showing that the optical depth contributed by Arctic ice algal chlorophyll may be comparable during Boreal Spring to the corresponding values attributable to BC. The largest chlorophyll attenuation is obtained in the bottom layer, which supports pigment concentrations of about 300 to 1000 mg/m3 in the Bering Sea and Sea of Okhotsk. However, chlorophyll concentrations for the ice interior in regions north of 75{\deg} N and across the Canadian Archipelago are less than 0.1 micro g/m3. Freeboard and infiltration communities lead to intermediate levels of light removal. Since BC works its way downward from the atmospheric interface, there will be regions where it appropriates photosynthetic capability. Where ice thicknesses permit significant penetration through the pack column the ice algae may be crucial absorbers. We propose a continuous increase in relative chlorophyll activity and attenuation in the future, as biological activity becomes stronger in thin ice toward the center of the Arctic basin. A shift in relative importance of the two absorber types could occur as total BC mixing ratios are reduced because of environmental advocacy.