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Lundi 24 Septembre - GEOTRACES seminars

Par SEMSOU Dernière modification 16/09/2018 11:22
Quand ? Le 24/09/2018,
de 10:00 à 12:00
Où ? salle Lyot
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Robert F. Anderson

Lamont-Doherty Earth Observatory of Columbia University, Palisades NY, USA

 

Deep-sea oxygen depletion and ocean carbon sequestration during the last ice age

  

Abstract: Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels.  Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean’s biological pump, in which case carbon storage in the deep sea would have been accompanied by a stoichiometric reduction in dissolved oxygen.  We exploit the sensitivity of biomarker preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep equatorial Pacific Ocean during the last glacial period to have been 20 - 50 µmol/kg, much less than modern values of ca. 168 µmol/kg.  We further demonstrate that reduced oxygen levels characterized the water column below depths of ~1000 m.  Converting the ice-age oxygen level to a stoichiometrically equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep-sea CO2 storage during the last ice age exceeded modern values by ca. 850±110 PgC, sufficient to balance the loss of carbon from the atmosphere (ca. 200 PgC) and from the terrestrial biosphere (ca. 600 PgC).

 

 

William M. Landing

Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, Florida, USA

 

Atmospheric Deposition to the Oceans Controls Biological Productivity


Abstract: The atmospheric flux of trace elements and isotopes (TEIs) to the oceans can be extremely important in marine biogeochemical cycles, especially for bioactive trace elements such as Mn, Fe, Co, Ni, Cu, Zn, and Cd. However, there are large uncertainties with converting aerosol TEI concentrations into atmospheric deposition fluxes so we need an aerosol tracer whose atmospheric deposition can be tracked as a function of time. By measuring upper ocean inventories and aerosol concentrations of Be-7, we can derive a “bulk deposition velocity” that accounts for both wet and dry deposition. The bulk deposition velocities can then be applied to any other aerosol species to calculate its flux. Where the fractional solubility of aerosol TEIs has been measured, the bulk deposition velocities can be used to calculate the atmospheric flux of soluble TEIs. This is especially useful for estimating the impact of atmospheric deposition on biological productivity since soluble TEIs are expected to be intrinsically bioavailable to phytoplankton. We will present results from field campaigns in the Sargasso Sea (Bermuda) and the Arctic Ocean to test this approach.

 

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