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Home → March, 2019

Monthly Archives: March 2019 − News & Stories

Mys­tery of the green ice­bergs sol­ved

Green ice­bergs, also cal­led jade ice­bergs, are rare. Actual­ly so rare that they have some­ti­mes been taken for a cock-and-bull sto­ry. But they do exist. I have seen seve­ral ones mys­elf. So long ago that I have not been able to take pro­per pho­tos, with the stone-age equip­ment that I had back then. So I do not have any pho­tos that show the colour real­ly well. Ok, next time.

Green iceberg (jade iceberg), Bransfield Strait

Green ice­berg (jade ice­berg) in Brans­field Strait, March 2003. Pho­to taken with pre-his­to­ri­cal equip­ment (dia­film, scan­ned), so not much remains of the beau­tiful ori­gi­nal colour.

But if you have the rare luck to see such an ice­berg, then the green colour is very pro­mi­nent inde­ed. It is not just a hint of green within the usu­al blue of an ice­berg. It is real­ly a dif­fe­rent colour.

The­re has been a lot of spe­cu­la­ti­on regar­ding the reason for the green colour. Accor­ding to the most com­mon theo­ry, the green ice is mari­ne ice: sea­wa­ter fro­zen to the bot­tom of an ice shelf. Not ter­restri­al gla­cier ice, but fro­zen sea­wa­ter. Not fro­zen on the sur­face of the sea, so it is not what is com­mon­ly refer­red to as sea ice. Hence the spe­cial term mari­ne ice. The colour, now, was said to be due to a high con­tent of orga­nic mat­ter of the fro­zen sea­wa­ter. Phy­to­plank­ton trap­ped in the ice.

A team of sci­en­tists (Ste­phen G. War­ren, Col­lin S. Roes­ler, Richard E. Brandt and Mark Cur­ran) have now come up with a new theo­ry which they descri­be in an artic­le published in the Jour­nal of Geo­phy­si­cal Rese­arch: Oce­ans. The good news: not ever­y­thing we pre­vious­ly belie­ved is wrong now. Also this recent publi­ca­ti­on con­firms the mari­ne ori­gin of the ice in ques­ti­on. It is fro­zen sea­wa­ter, fro­zen to the bot­tom of an ice shelf, that is respon­si­ble for the green colour of the jade ice­bergs.

A num­ber of con­di­ti­ons have to be met to enable sea­wa­ter to free­ze to the bot­tom of an ice shelf. This hap­pens at con­sidera­ble depths of seve­ral hundred to more than thousand met­res. And the water has to be very cold. The Ame­ry Ice Shelf in east Ant­ar­c­ti­ca has a groun­ding line depth (whe­re the ice shelf rests on the sea flo­or) of 2400 met­res. The­re, the free­zing point of sea water is -3.7°C. If this water hap­pens to move to hig­her levels under the ice shelf, then it is super­coo­led.

Green iceberg (jade-iceberg) near the South Orkney Islands

Green ice­berg (jade ice­berg) near the South Ork­ney Islands, Janu­ary 2009. The mari­ne ice has for­med within cracks at the bot­tom of the ice shelf, which gives the green ice the appearance of being work­ed into the blu­eish-white, ter­restri­al gla­cier ice.

The dif­fe­rence to older theo­ries is the ques­ti­on of what in the sea­wa­ter exact­ly brings the green colour. War­ren and his col­le­agues cla­im that the sub­s­tance in ques­ti­on is not orga­nic, but iron com­pounds. Mea­su­re­ments of an ice core from the Ame­ry Ice Shelf that includes basal sec­tions of mari­ne ice do not show high con­cen­tra­ti­ons of orga­nic mat­ter. Ins­tead, the iron con­tent was hig­her than expec­ted.

The iron is part of various che­mi­cal com­pounds, but altog­e­ther of anor­ga­nic ori­gin. The main mine­ral is Goe­thi­te, an iron-bea­ring hydr­o­xi­de which is a com­mon mine­ral in the upper crust. The source is rock mate­ri­al ero­ded at the gla­cier base.

The opti­cal pro­per­ties (absorp­ti­on spec­tra) of Goe­thi­te dust tend to give a yel­lo­wish colou­ra­ti­on. But the inter­play of this brow­nish-yel­lo­wish colour with the blue of den­se (wit­hout air bubbles) sea ice can pro­du­ce exact­ly the jade-colour in ques­ti­on.

Com­plex mat­ter! Not sur­pri­sin­gly, the aut­hors of the stu­dy con­clude that fur­ther rese­arch is nee­ded: regar­ding the exact com­po­si­ti­on of orga­nic (and anor­ga­nic) sub­s­tances mixed with mari­ne ice and their opti­cal cha­rac­te­ristics. Final­ly, such know­ledge might be used to gain infor­ma­ti­on about the che­mi­cal com­po­si­ti­on of ice­bergs from light spec­tra that can be mea­su­red effi­ci­ent­ly by remo­te sens­ing, using air­craft or dro­nes or even satel­li­tes.

And the che­mi­cal com­po­si­ti­on of ice­bergs, espe­ci­al­ly the iron con­tent, is actual­ly very important for the mari­ne eco­lo­gy of the Sou­thern Oce­an: mine­rals trans­por­ted by ice­bergs can fer­ti­li­se sea­wa­ter that is other­wi­se poor in such nut­ri­ents. Such fer­ti­li­sa­ti­on can have signi­fi­cant effects on the bio­lo­gi­cal pro­duc­ti­vi­ty of the oce­an. Hence, the beau­tiful “jade ice­bergs” gain unex­pec­ted importance for the eco­lo­gi­cal sys­tem of the Sou­thern Oce­an.

The volu­me of green ice is pro­ba­b­ly lar­ger than one might belie­ve, con­side­ring the rari­ty of green ice­bergs. But only smal­ler ice­bergs are actual­ly able to turn over, expo­sing the green bot­tom. Lar­ger tabu­lar ice­bergs may trans­port much grea­ter quan­ti­ties of green ice but this remains then hid­den at gre­at depth. And of cour­se the light has to be right to see it and you have to bee around … and not every ice shelf pro­du­ces jade ice­bergs in quan­ti­ties. The Ame­ry Ice Shelf is so remo­te that peo­p­le hard­ly every get the­re, other than sci­en­tists who work on the sta­ti­ons Maw­son and Davis (both belong to the Aus­tra­li­an Ant­ar­c­tic Divi­si­on).

So, if you have every seen a green ice­berg in rea­li­ty, then you are one of a lucky few!


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