All living organisms rely on iron as an essential nutrient. In the ocean, iron's abundance or scarcity means all the difference as it fuels the growth of plankton, the base of the ocean's food web.
A new study by biogeochemists and glaciologists at Woods Hole Oceanographic Institution (WHOI) identifies a unexpectedly large source of iron to the North Atlantic -- meltwater from glaciers and ice sheets, which may stimulate plankton growth during spring and summer. This source is likely to increase as melting of the Greenland ice sheet escalates under a warming climate.
The study was published online in Nature Geoscience on March 10, 2013.
"There's only been one other study looking at the amount of iron that's being released in meltwater runoff itself," says Maya Bhatia, a graduate of the MIT/WHOI Joint Program in Oceanography and Applied Ocean Sciences and Engineering, and the study's lead author, "and that had reported high nanomolar concentrations. So to find iron in concentrations several orders of magnitude higher -- in the micromolar range -- was very surprising."
Iron from wind-blown dust and river runoff fuels annual plankton blooms in the world's ocean. Ice sheets and glaciers are now also thought to contribute iron from sediments on the bottom of calved icebergs and glacially-derived dust. Until now, meltwater runoff from glaciers and ice sheets was considered too dilute to carry much iron, although previous research has shown a strong correlation between the plankton blooms and the runoff from Greenland ice sheet.
"Glacial runoff has only recently been considered a potentially important source of nutrients that are useable, or bioavailable, to downstream ecosystems," says Bhatia. "We believe our study now adds iron to that list of nutrients, and underscores the potential for a unique but as-yet-undetermined chemical impact from increasing ice sheet meltwater runoff."
During the course of two expeditions to the Greenland ice sheet in May and July 2008, Bhatia and her colleagues collected samples from sites at several land-terminating glaciers on the western side of the Greenland ice sheet. The glaciers' meltwater empties into a large lake, which eventually drains into an estuary system before reaching the open ocean. Their study reports levels of dissolved iron orders of magnitude higher than previously found for Greenland glacial runoff rivers. When the WHOI team extrapolated their findings to calculate the contribution of iron from the entire ice sheet, they estimated its value to be within the range of that from dust deposition in the North Atlantic, which is believed to be the primary source of bioavailable iron to this ocean. This value is only an order of magnitude lower than the estimated annual contribution of iron from rivers worldwide.
When an ice sheet or glacier melts, most of the water doesn't simply run off the surface of the ice sheet. Instead it first drains to the bedrock below the ice sheet through cracks and conduits called moulins and then exits in large runoff rivers.
"A lot of people think of a glacier and an ice sheet as a big block of ice," says Bhatia, "but it's actually quite a porous, complicated system underneath a glacier, with lots of moulins and crevasses leading to the bottom. Once you get into the bottom, there are large tunnels that these waters are passing through." The more time the water spends in contact with the bedrock and sediments beneath the glacier, the more nutrients it picks up, including iron.
The WHOI team says further research is needed to determine how much of this iron actually reaches the open ocean, as their study followed the meltwater from the edge of the glaciers to the large lake they empty into. For this study, the team assumed that the amount of iron filtered out as the water moves through estuaries before reaching the marine environment would be roughly the same for glacial systems as it is for river systems.
The researchers hope to do more work to confirm the study's numbers by sampling over a larger geographical area. Additional research could also confirm whether this influx of iron is in a form that can be easily utilized by phytoplankton and therefore stimulates primary production in the ocean.
"We don't have enough historical measurements to say that this iron contribution is an increase over past conditions, but if it is working the way we think it is, the contribution would be greater as meltwater discharge increases," Bhatia says. "It is interesting to think that, as ice sheets melt, there are biogeochemical considerations beyond changing sea level."
Maya Bhatia is currently a postdoctoral fellow in the Department of Microbiology and Immunology at the University of British Columbia in Vancouver, BC, Canada. Her co-authors include Elizabeth B. Kujawinski, Department of Marine Chemistry and Geochemistry; Sarah B. Das, Department of Geology and Geophysics; Crystaline F. Breier, Department of Marine Chemistry and Geochemistry; Paul Henderson, Department of Marine Chemistry and Geochemistry; and Matthew A. Charette, Department of Marine Chemistry and Geochemistry, all of WHOI.
This research was supported by the WHOI Clark Arctic Research Initiative, the National Science Foundation, the WHOI Ocean and Climate Change Institute, and an AGU Horton Hydrology Grant.
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Viumbe wote walio hai hutegemea chuma kuwa virutubisho muhimu. Katika bahari, wingi au upungufu wa chuma ni muhimu sana kwa sababu huchochea ukuaji wa plankton, msingi wa mtandao wa chakula cha bahari. Utafiti wa hivi karibuni uliofanywa na wataalamu wa biogeochemistry na glaciologists katika Taasisi ya Oceanography ya Woods Hole (WHOI) unaonyesha chanzo kikubwa cha chuma katika Atlantiki ya Kaskazini - maji ya kuyeyuka kutoka kwa barafu na barafu, ambayo yanaweza kuchochea ukuaji wa plankton wakati wa majira ya kuchipua na majira ya joto. Chanzo hiki kinaweza kuongezeka kadiri kuyeyuka kwa barafu ya Greenland inavyoongezeka chini ya hali ya hewa ya joto. Utafiti huo ulichapishwa kwenye jarida la Nature Geoscience mnamo Machi 10, 2013. "Hakuna utafiti wowote wa kutosha wa kiasi cha chuma kinachotolewa katika maji ya kuyeyuka, ""Maya Bhatia, mhitimu wa MIT na WHOI Joint Program katika Oceanography na Applied Ocean Sciences na Uhandisi, na mwandishi mkuu wa utafiti, alisema." Kwa hiyo, kugundua chuma katika viwango vya juu zaidi - katika kiwango cha micromolar - kulikuwa jambo lenye kushangaza sana". Chuma kutoka kwa vumbi linalopeperushwa na upepo na maji ya mito huchochea maua ya kila mwaka ya plankton katika bahari za ulimwengu. Sasa inadhaniwa kwamba barafu na barafu za barafu pia huchangia chuma kutoka kwa matope yaliyo chini ya milima ya barafu na vumbi linalotokana na barafu. "Hata hivyo, mpaka sasa, maji ya kuyeyuka kutoka kwa barafu na barafu ya barafu yalionwa kuwa ""yenye umajimaji mkubwa"" na hayawezi kubeba chuma, ingawa utafiti wa awali umeonyesha uhusiano mkubwa kati ya maua ya plankton na maji ya barafu ya Greenland." "Mvua ya barafu imekuwa ikionwa hivi karibuni tu kuwa chanzo muhimu cha virutubisho vinavyoweza kutumiwa, au vinavyopatikana kibiolojia, kwa mifumo ya ikolojia ya chini ya mto", anasema Bhatia. "Tunaamini utafiti wetu sasa unaongeza chuma kwenye orodha hiyo ya virutubisho, na inasisitiza uwezekano wa athari ya kipekee lakini bado haijulikani ya kemikali kutoka kwa kuongezeka kwa maji ya kuyeyuka ya barafu. """ Katika safari mbili za kuchunguza barafu ya Greenland mnamo Mei na Julai 2008, Bhatia na wenzake walikusanya sampuli kutoka maeneo kadhaa ya barafu kwenye upande wa magharibi wa barafu ya Greenland. Maji ya barafu hayo yanapoyeyuka huingia katika ziwa kubwa, ambalo hatimaye huingia katika mfereji wa maji kabla ya kufika baharini. Utafiti wao unaripoti viwango vya chuma kilichomwagika vya ukubwa wa juu kuliko ilivyoonekana hapo awali kwa mito ya maji ya barafu ya Greenland. Wakati timu ya WHO ilichunguza matokeo yao ili kuhesabu mchango wa chuma kutoka kwa barafu nzima, walikadiria thamani yake kuwa ndani ya safu ya ile kutoka kwa kuhifadhi vumbi katika Atlantiki ya Kaskazini, ambayo inaaminika kuwa chanzo kikuu cha chuma cha bioavailable kwa bahari hii. Thamani hii ni ya kiwango kidogo tu kuliko mchango wa kila mwaka wa chuma kutoka kwa mito ulimwenguni pote. Wakati barafu au barafu inapeyuka, maji mengi hayaondoki kwenye barafu. Badala yake, maji hayo huingia kwenye mwamba ulio chini ya barafu kupitia nyufa na mifereji inayoitwa moulins na kisha hutiririka katika mito mikubwa. "Watu wengi huona barafu na barafu kama ""kifungu kikubwa cha barafu,"" lakini kwa kweli ni mfumo wenye kuvu na tata ulio chini ya barafu, wenye miamba na nyufa nyingi zinazoelekea chini." Unapofika chini, kuna mifereji mikubwa ambayo maji hayo hupita." Kadiri maji yanavyotumia muda mwingi katika kuwasiliana na mwamba na matope yaliyo chini ya barafu, ndivyo yanavyopokea virutubisho vingi, kutia ndani chuma. Watafiti wanasema utafiti zaidi unahitajika ili kuamua ni kiasi gani cha chuma hiki kinachofika baharini, kwani utafiti wao ulifuatilia maji ya kuyeyuka kutoka ukingo wa barafu hadi ziwa kubwa ambalo huingia. Kwa ajili ya utafiti huu, timu hiyo ilidhani kwamba kiasi cha chuma kilichopitishwa wakati maji yanapotembea kupitia vijito vya maji kabla ya kufikia mazingira ya baharini itakuwa sawa kwa mifumo ya barafu kama ilivyo kwa mifumo ya mto. Watafiti wanatumaini kufanya kazi zaidi ili kuthibitisha idadi ya utafiti huo kwa kuchukua sampuli katika eneo kubwa la kijiografia. Utafiti wa ziada pia unaweza kuthibitisha kama mtiririko huu wa chuma ni katika fomu ambayo inaweza kwa urahisi kutumika na phytoplankton na hivyo huchochea uzalishaji wa msingi katika bahari. """Hatuna vipimo vya kutosha vya kihistoria vya kusema kwamba mchango huu wa chuma ni ongezeko juu ya hali za zamani, lakini ikiwa inafanya kazi kama tunavyofikiri, mchango utakuwa mkubwa zaidi kama kutolewa kwa maji ya kuyeyuka kunaongezeka,"" anasema Bhatia." "Ni jambo lenye kupendeza kufikiri kwamba, kadiri barafu zinavyeyuka, kuna mambo mengine ya kibiolojia na ya kimaakili yanayohusika mbali na mabadiliko ya kiwango cha bahari". Maya Bhatia ni mwanafunzi wa postdoctoral katika Idara ya Microbiology na Immunology katika Chuo Kikuu cha British Columbia huko Vancouver, BC, Canada. "Wanafunzi wa Chuo Kikuu cha Kimataifa cha Kimataifa cha Kimataifa cha Kimataifa cha Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa ya Kimataifa. """ Utafiti huu ulifadhiliwa na WHOI Clark Arctic Research Initiative, Shirika la Sayansi la Kitaifa, Taasisi ya Bahari na Mabadiliko ya Hali ya Hewa ya WHOI, na Grant ya Hydrology ya AGU Horton. Namba hii ya ukurasa: