Green Icebergs Revisited

Abstract

Ice crystals form in supercooled seawater beneath several Antarctic ice shelves; as they rise to the ice‐shelf base they scavenge particles from the water and incorporate them into the growing basal ice. The resulting marine ice can be ~100 m thick; it differs from sea ice in that it is clear, desalinated, and bubble‐free. Icebergs of marine ice vary in color from blue to green, depending on the nature and abundance of foreign constituents in the seawater that became trapped in the ice as it grew. A red or yellow material (i.e., one that absorbs blue light), in combination with the blue of ice, can shift the wavelength of minimum absorption to green. Previously, dissolved organic carbon (DOC) had been proposed to be responsible for the green color. Subsequent measurements of low DOC values in green icebergs, together with the recent finding of large concentrations of iron in marine ice from the Amery Ice Shelf, suggest that the color of green icebergs is caused more by iron‐oxide minerals than by DOC. These icebergs travel great distances from their origin; when they melt they can deliver iron as a nutrient to the Southern Ocean.

Deposition of brown carbon onto snow: changes in snow optical and radiative properties

“…As BrC particles from smoldering combustion are mostly homogeneous (Sumlin et al, 2018a) and spherical (Chakrabarty et al, 2010), BrC aerosol optics can be described by the BrC complex refractive index and size distribution using Mie theory (Mie, 1908;Moosmüller et al, 2011;Sumlin et al, 2018b). A variety of fuels emit BrC during open combustion (Laskin et al, 2015), but peat is of particular interest for its common physical proximity to snow and ice at high latitudes of the Northern Hemisphere (Joosten and Clarke, 2002) and due to its strong tendency to burn in the smoldering combustion phase (Watts and Kobziar, 2013). Peatlands are a land surface primarily found in the Northern Hemisphere composed of organic soil and decomposing plant material.…”

“…However, measured albedos at 350 nm for the natural snowpack are appreciably lower, near ∼ 0.84. Along with BrC already present in the snow, other impurities are likely a mix of mineral dust and BC (Hadley et al, 2010;Sterle et al, 2013); however, it is BrC and mineral dust that are responsible for greater albedo decreases (compared to pure snow) at these lower wavelengths (Laskin et al, 2015;Lu et al, 2015;Moosmüller et al, 2009;Skiles et al, 2017;Warren et al, 2019;Wu et al, 2016). While we can account for the presence of BrC already in the snowpack by assigning the laboratorymeasured UV-Vis absorption to the measured TOC concentrations of the natural snowpack samples, dust concentrations were not measured and therefore remain unknown.…”

Deposition of brown carbon onto snow: changes of snow optical and radiative properties

“…The absorption is well-explained (R 2 > 0.9) by TOC in the meltwater throughout the UV, but above 353 nm, the confidence in that relationship drops quickly (R 2 < 0.5). If we consider just the natural snow samples, the calculated R 2 doesn't suggest high confidence (R 2 < 0.7 across all wavelengths), likely due the low sample size (n=6 snowpack is two orders of magnitude greater than values inferred by Warren et al (2019) for Alaskan sea ice at 400 nm, but closer to that of snow and ice in the northern Tibetan Plateau (Yan et al, 2016).…”

“…However, measured albedos at 350 nm for the natural snowpack are appreciably lower, near ~0.84. Along with BrC already present in the snow, other impurities are likely a mix of mineral dust and BC (Hadley et al, 2010;Sterle et al, 2013); however, it is BrC and mineral dust that are responsible for greater albedo decreases (compared to pure snow) at these lower wavelengths (Laskin et al, 2015;Lu et al, 2015;Moosmüller et al, 2009;Skiles et al, 2017;Warren et al, 2019;Wu et al, 2016). While we can account for the presence of BrC already in the snowpack by assigning the laboratory-measured UV-vis absorption to the measured TOC concentrations of the natural snowpack samples, dust concentrations were not measured and therefore remain unknown.…”

“…The slope of the linear regression between absorption coefficient (m -1 ) at wavelength and TOC concentrations give the (mass) specific absorbance, -(m 2 g -1 ), which is an indicator of the contribution by OC to the absorption coefficient in snow and ice (Warren et al, 2019;Zhang et al, 2019). --sometimes referred to as specific absorbance -at various wavelengths is used to describe other properties, such as aromaticity of the OC (Hansen et al, 2016), drinking water quality (Potter and Wimsatt, 2012), or used to help describe different processes in rivers, lakes, and oceans (Fichot and Benner, 2011;Twardowski et al, 2004;Yacobi et al, 2003).…”