GREEN ICEBERGS FORMED BY FREEZING OF ORGANIC-RICH SEAWATER TO THE BASE OF ANTARCTIC ICE SHELVES

Although most icebergs are blue, green icebergs are seen occasionally in the Antarctic ocean. Chemical and isotopic analysis of samples from green icebergs indicate that the ice consists of desalinated frozen s eawater, as does the basal ice from the Amery Ice Shelf. Spectral refl ectance of a green iceberg measured near 67-degrees-S, 62-degrees-E, c onfirms that the color is inherent to the ice, not an artifact of the illumination. Pure ice appears blue owing to its absorption of red pho tons. Addition of a constituent that absorbs blue photons can shift th e peak reflectance from blue to green. Such a constituent was identifi ed by spectrophotometric analysis of core samples from this iceberg an d from the Amery basal ice, and of seawater samples from Prydz Bay off the Amery Ice Shelf. Analysis of the samples by fluorescence spectros copy indicates that the blue absorption, and hence the inherent green color, is due to the presence of marine-derived organic matter in the green iceberg, basal ice, and seawater. Thick accumulations of green i ce, in icebergs and at the base of ice shelves, indicate that high con centrations of organic matter exist in seawater for centuries at the d epth of basal freezing.