Quantifying the Dolomite problem and its impacts on Mg/Ca change through time

Opdyke, Dr Bradley1

1The Australian National University, Australia

Global dolomite deposition has declined globally throughout the Cenozoic.  While the volumes of other sedimentary rocks increase from the Paleocene to the Holocene.  This anomaly has been called the ‘dolomite problem’ and recognized since the time of Darwin.  Recently my team discovered that crustose coralline algae  (CCA) does precipitate calcium-magnesium carbonate with a dolomite chemistry.  CCA is not as abundant on immature reefs as mature reefs, in fact CCA ‘crusts’ only become thick and widespread once a significant portion of the reef flat has been located within the tidal zone for many thousands of years.  In a world where sea level is moving up and down like a Milankovitch driven yo-yo it is rare for these algal facies to become established, hence dolomite precipitation is rare. Recent sea level studies and compilations of stable isotope records from the Eocene to the Holocene allow us to model the probable trajectory of dolomite deposition over this time interval and calculate the probable Magnesium-Calcium ratio change in sea water.  At the present time we only have a few ‘tie points’ for the Mg/Ca over this time.  Using the stability of sea level as a driver of dolomite production we can predict more precisely how the Mg/Ca ratio has increased from 2.5 at the Eocene/Oligocene boundary to 5.1 in modern sea water.


Biography

AB Geochemistry Columbia University 1984

MS Geology The University of Michigan 1987

PhD Geology The University of Michigan 1990

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