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SUMMARY:Why and how does organic matter in sediment matter to climate chan
 ge science? - Dr Albert Galy\, Department of Earth Sciences\, University o
 f Cambridge
DTSTART:20111027T103000Z
DTEND:20111027T113000Z
UID:TALK33597@talks.cam.ac.uk
CONTACT:Helen Mawdsley
DESCRIPTION:At geological time scale (Ma and beyond)\, the Earth’s surfa
 ce has recorded temperature changes in the order of a couple of 10ºC.  Th
 e strong negative feedback that is responsible for such a narrow change in
  the Earth’s surface temperature (by comparison with what is known for V
 enus or Mars) is the temperature sensitivity of the kinetic of atmospheric
  CO2 consumption. For an inorganic reaction\, such as the chemical weather
 ing of silicate rocks and the precipitation of carbonate\, such temperatur
 e sensitivity is the Arrhenius’ law. Initiated by Robert Garrels in the 
 1960’s\, the modelling of such reaction at global scale led to a first o
 rder understanding of climatic changes found in the geological record for 
 the Phanerozoic. However\, a pure inorganic reaction fails explaining some
  of the key periods\, like the sudden cooling during the upper Paleozoic (
 almost 300 Myrs ago) or the cooling during the Neogene (the last 25 Myrs).
  Most recent model are linking global rates of silicate chemical weatherin
 g to biologically enhanced weathering through the health of terrestrial ec
 osystems. However\, we do not have direct evidences for this.  The cycling
  of element by the vegetation can be monitored using stable isotopes. For 
 instance\, the vegetation preferentially selects the light isotopes of Ca 
 leading to a measurable effect of the biological cycling of Ca in the 4
 4/42Ca of soils under forest. In some cases\, the extremely low 44/42Ca
  values witness the recycling of the Ca from the litter in the upper soil 
 horizons. However inorganic processes can also led to Ca isotopic fraction
 ation.  For instance\, the precipitation of secondary carbonate scavenges 
 more of the light isotopes of Ca. The biological cycling of Ca becomes onl
 y witnessed by 44/42Ca in river water at small scale and during wet per
 iods. At a larger scale than few km2\, the biological cycling of Mg and Ca
  have no measurable impact on the isotopic composition of the dissolved lo
 ad.  The biological cycling is simply too fast to leave a mark on the diss
 olved load of sizeable rivers.  An alternative feedback mechanism involves
  the rate of burial of organic material that is sensitive to the temperatu
 re (complex effect\, with both positive and negative feedbacks) but even m
 ore directly to atmospheric CO2 levels (negative feedback). If the organic
  sub-cycle of the carbon cycle has been taken in consideration in any mode
 ling attempt (primarily via the modeling of the 13C of carbonate)\, its
  direct study (both production and fate in the sediments) remained to be p
 roperly done at global scale. Recent studies in active tectonic areas (Tai
 wan\, Himalaya) suggest a significant recycling of fossil organic matter a
 ssociated with the burial of newly photosynthetised organic matter and bot
 h are climatically controlled\, adding another negative feedback that part
 ly accounts for the cooling during the Carboniferous but could also for th
 e recent icehouse period.
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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