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SUMMARY:Banded Iron Formation and Ancient Life - Professor Kurt Konhauser\
 , University of Alberta
DTSTART:20130122T163000Z
DTEND:20130122T173000Z
UID:TALK42118@talks.cam.ac.uk
CONTACT:Dr. Alex Liu
DESCRIPTION:Iron formations (IF) are iron rich (20-40% Fe) and siliceous (
 40-50% SiO2) sedimentary deposits that precipitated throughout much of the
  Precambrian. Their trace element composition is now being used as a proxy
  for ancient seawater chemistry\, with the view of better understanding nu
 trient availability for the ancient marine biosphere. Three examples are p
 rovided here. \n\nFirst\, it has been proposed that low P concentrations i
 n Archean and Paleoproterozoic IF reflected limited marine phosphorous ava
 ilability at that time\, which would have reduced levels of photosynthesis
  and carbon burial\, thereby inhibiting long-term oxygen production on the
  early Earth1\, although this has been questioned2. A subsequent increase 
 in P content of IF in the Neoproterozoic\, following Snowball Earth deglac
 iations\, may then have led to enhanced cyanobacterial photosynthesis\, wh
 ich in turn\, produced enough oxygen to facilitate the evolution of animal
  life3. \n\nSecond\, it has been shown that the nickel content in IF has c
 hanged dramatically over time\, and that a drop in Ni availability in the 
 oceans around 2.7 billion years ago would have had profound consequences f
 or microorganisms that depended on it\, that being methane-producing bacte
 ria called methanogens4. These bacteria have a unique Ni requirement for t
 heir methane-producing enzymes\, and crucially\, these bacteria have been 
 implicated in controlling oxygen levels on the ancient Earth as the methan
 e they produced was reactive with oxygen and kept atmospheric oxygen level
 s low. It is possible that a Ni famine eventually led to a cascade of even
 ts that began with reduced methane production\, the expansion of cyanobact
 eria into shallow-water settings previously occupied by methanogens\, and 
 ultimately increased oxygenic photosynthesis that tipped the atmospheric b
 alance in favour of oxygen\, the so-called Great Oxidation Event (GOE) at 
 around 2.5 Gyr. \n\nThird\, a recent compilation of Cr enrichment in IF sh
 ows a profound enrichment coincident with the GOE5. Given the insolubility
  of Cr minerals\, its mobilization and incorporation into IF indicates enh
 anced chemical weathering at that time\, most likely associated with the e
 volution of aerobic continental pyrite oxidation. If we accept that IF can
  serve as useful proxies for the composition of ancient seawater\, the que
 stion then becomes what do other trace elements in IF tell us about the an
 cient biosphere? Prospectives for the future include evaluating marine tra
 ce element evolution as recorded by IF in relation to trace element record
 s from other lithologies\, such as black shales or carbonates\, as well as
  in relation to other biological proxies\, such as stable isotope records.
 \n\n1.	Bjerrum\, C.J. & Canfield\, D.E. Ocean productivity before about 1.
 9 Gyr limited by phosphorous adsorption onto iron oxides. Nature 417\, 159
 -162 (2002).\n\n2.	Konhauser\, K.O.\, Lalonde\, S.\, Amskold\, L.\, and Ho
 lland\, H.D. Was there really an Archean phosphate crisis? Science 315\, 1
 234 (2007).\n\n3.	Planavsky\, N.J.\, Rouxel\, O.\, Bekker\, A.\, Lalonde\,
  S.V.\, Konhauser\, K.O.\, Reinhard\, C.T.\, and Lyons\, T.\, 2010. The ev
 olution of the marine phosphate reservoir. Nature\, 467:1088-1090.\n\n4.	K
 onhauser\, K.O.\, Pecoits\, E.\, Lalonde\, S.V.\, Papineau\, D.\, Nisbet\,
  E.G.\, Barley\, M.A.\, Arndt\, N.T.\, Zahnle\, K.\, and Kamber\, B.S. Oce
 anic nickel depletion and a methanogen famine before the Great Oxidation E
 vent. Nature 458\, 750-753 (2009).\n\n5.	Konhauser\, K.O.\, Lalonde\, S.V.
 \, Planavsky\, N.\, Pecoits\, E.\, Lyons\, T.\, Mojzsis\, S.\, Rouxel\, O.
 J.\, Barley\, M.\, Rosiere\, C.\, Fralick\, P.W.\, Kump\, L.R.\, and Bekke
 r\, A.\, 2011. Aerobic bacterial pyrite oxidation and acid rock drainage d
 uring the Great Oxidation Event. Nature\, 478:369-374.\n
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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