Extinction of ultraviolet-A, visible and near-infrared wavelength light in snow and antarctic sea ice

Abstract

The optical properties of sea ice are important in the understanding of sea ice thermodynamics, growth and decay processes, polar climates, and remote sensing. The optical properties of ice have been fairly well described, but most studies have focused on wavelengths longer than 400 nm, and on Arctic sea ice. With increased interest in the effects of changing ultraviolet light levels from Antarctic ozone depletion, there has been recent work done on the extinction of ultraviolet light in Antarctic sea ice, including the study reported here. A spectrometer was modified, and taken on two trips to Antarctica to measure the extinction of light in sea ice. Extinction in summer and in winter sea ice were measured in the wavelength range 320 to 900 nm with the resolution range of 1 to 6 nm. This wavelength range covers the ultraviolet A band (320-400 nm), photosynthetically active region (PAR, 400-700 nm), and a small part of the near infrared (NIR, 700-1000 nm). The extinction coefficient in the middle of melting sea ice had a minimum of 0.6 m$\sp{-1}$ in the range of 450-500 nm, rising to 1 m$\sp{-1}$ at 350 nm, and 1.8 at 700 nm. Winter sea ice had spectrally flat extinction between 320-600 nm with values ranging from 1.7-5 m$\sp{-1}$ for different ice floes. This is in contrast to an expected minimum around 450 nm from absorption by ice and water. Extinction increased to 10-15 m$\sp{-1}$ at 900 nm in the winter sea ice. Both warm and cold ice had increased extinction at all wavelengths near the bottom of the floes. Algae were identified in several ice floes by the chlorophyll absorption peaks at 330, 430 and 680 nm. The presence of algae in the ice was associated with an increase in the extinction by a factor of up to 5 for wavelengths shorter than 600 nm, with a much smaller effect on longer wavelengths. Absorption by algae was evident at nearly all depths in some of the ice, but was mostly concentrated in the bottom 15% to 35%.