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SUMMARY:Back to basics: Trace metal evidence for a more alkaline glacial o
 cean - Ros Rickaby (Oxford University)
DTSTART:20080805T100000Z
DTEND:20080805T110000Z
UID:TALK12393@talks.cam.ac.uk
CONTACT:Deb Shoosmith
DESCRIPTION:The Southern Ocean has long been recognised as the major playe
 r in driving the climate oscillations of the Pleistocene. Reconstructing t
 he biogeochemistry of the past Southern Ocean is crucial for understanding
  the partitioning of atmospheric carbon dioxide between the atmosphere and
  the ocean. Hypotheses to account for glacially reduced atmospheric carbon
  dioxide (pCO2) have focussed on increased stratification of the water col
 umn or enhanced efficiency of nutrient utilisation in the surface waters o
 f the Southern Ocean. But this region\, and the Weddell Sea in particular\
 , is also responsible for the formation of the major carbon and nutrient r
 ich deepwater mass of the global oceans\, Antarctic Bottom Water (AABW)\, 
 which ultimately determines the carbonate chemistry of the deep ocean. An 
 alternative means of titrating pCO2 into the ocean during glacial times co
 uld be to increase the alkalinity of the global deep ocean. The lack of ca
 rbonate preservation in the Southern Ocean area\, however\, has largely pr
 evented probing of the past palaeochemistry of the Southern Ocean with tra
 ditional foraminiferal proxies. Here we present an 800 kyr record of trace
  metal contents from benthic foraminifera from PS1506 (67.8oS 5.8oW\, 2426
 m\, Weddell Sea) to investigate how the biogeochemistry of Southern Ocean 
 has evolved during Pleistocene glacial-interglacial cycles. Our results in
 dicate that at the extreme low temperatures of the high latitude Southern 
 Ocean\, the downcore variability in all trace metals in foraminifera is la
 rgely controlled by variations in the carbonate ion content of the ambient
  water. For instance\, benthic B/Ca and Mg/Ca correlate closely and show a
  strong 100 kyr cyclicity but with higher values during the glacial period
 s. Our glacial increase in trace metal content of the benthic foraminifera
  is paralleled by an increase in the %carbonate content of PS1506\; a patt
 ern which is reproduced in a number of cores from around the Antarctic mar
 gin and for all glacial periods of the last 800 kyrs. Using existing calib
 rations\, our trace metal values correspond to elevated carbonate ion conc
 entrations in glacial southern sourced deepwaters of 10-20 µmol/mol. This
  elevated alkalinity may derive from a glacial decrease in the flux of low
  alkalinity northern sourced water to the Weddell Sea\, but could also be 
 due to local changes in the carbonate cycle associated with sea ice and po
 lynyas of the glacial Weddell Sea. We propose that an increase in the carb
 onate ion of southern sourced deepwaters is responsible for the “Pacific
 -style” carbonate cycles\, and accounts for at least part of the 90 ppmv
  drawdown of pCO2 during glacial periods. 
LOCATION:British Antarctic Survey\, Room 187
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