156
C. N. Wold· G. J. Schwartz· C. Morrill
Model (LSX; Pollard and Thompson 1995) that accounts for the physical effects
of vegetation. The land surface grid has a resolution of 2° x 2°. Up to two vegetation layers (trees and grass) can be specified for each 2° x 2° grid cell. Within
LSX is a soil model with the same horizontal resolution and 6 layers in the vertical extending to a depth of 4.25 m. Heat is diffused linearly, soil moisture nonlinearly, and ice is predicted for each soil layer. LSX also includes a snow model
with three layers. Inland lakes, snow cover, and the land-sea distribution can all
be represented as fractional data on the 2° x 2° grid cells. A 50 m mixed-layer
slab ocean model captures the oceans thermal capacity and transports heat
poleward. GENESIS also includes a three-layer dynamic and thermodynamic
sea ice model.
We used the results from GENESIS (2.0) simulation of present-day climate to
drive the PFM for evaporites. The results from the GENESIS (2.0) simulation
were stored as data files on the mass-storage system at the National Center for
Atmospheric Research (Boulder, Colorado). These files were then later retrieved
and used to run the PFM for evaporites.
5
The Proxy Formation Model for Evaporites and its Prediction of the Modern
Evaporite Distribution
The PFM for evaporites (Pollard and Schulz 1994) consists of a 1 x 1 m 2 column
of water that is 50 m deep and assumed to be well mixed. The PFM is initialized
with a salt-content sufficient to precipitate gypsum or halite. Positive values of
evaporation minus precipitation (E-P) over the brine surface indicate the potential for precipitation of evaporite minerals. The PFM for evaporites is driven by
monthly-mean meteorological forcing fields including: (1) atmospheric precipitation; (2) surface air temperature; (3) upward sensible heat flux; (4) surface-incident solar radiation; (5) air density; (6) surface-incident infrared radiation; (7)
wind speed; and (8) relative humidity over the brine surface. The reader is referred to Pollard and Schulz (1994) for a more detailed description of the PFM
for evaporites.
When seawater evaporates completely, a typical sequence of minerals precipitates. Calcite (CaC0 3 ) precipitates when the brine has been concentrated to approximately three times (105) the normal salinity (35) of seawater. Gypsum
(CaS0 4 • 2H z O) precipitates at 5 times (175) the normal concentration of seawater. Then halite (NaCl) begins to precipitate at 11 times (385) the normal concentration of seawater. Finally, the potassium salts precipitate at about 100 times
the normal concentration of seawater (Anonymous 1985; Warren 1988). These
are the four major evaporite minerals that precipitate from seawater. There are
a host of other minerals that precipitate at different concentrations of the brine
depending on the original composition and subsequent mixing with other waters.
The output from the PFM is the rate of evaporation from water at the Earth's
surface. Two simulations were performed using the PFM, one where the brine
C. N. Wold· G. J. Schwartz· C. Morrill
Model (LSX; Pollard and Thompson 1995) that accounts for the physical effects
of vegetation. The land surface grid has a resolution of 2° x 2°. Up to two vegetation layers (trees and grass) can be specified for each 2° x 2° grid cell. Within
LSX is a soil model with the same horizontal resolution and 6 layers in the vertical extending to a depth of 4.25 m. Heat is diffused linearly, soil moisture nonlinearly, and ice is predicted for each soil layer. LSX also includes a snow model
with three layers. Inland lakes, snow cover, and the land-sea distribution can all
be represented as fractional data on the 2° x 2° grid cells. A 50 m mixed-layer
slab ocean model captures the oceans thermal capacity and transports heat
poleward. GENESIS also includes a three-layer dynamic and thermodynamic
sea ice model.
We used the results from GENESIS (2.0) simulation of present-day climate to
drive the PFM for evaporites. The results from the GENESIS (2.0) simulation
were stored as data files on the mass-storage system at the National Center for
Atmospheric Research (Boulder, Colorado). These files were then later retrieved
and used to run the PFM for evaporites.
5
The Proxy Formation Model for Evaporites and its Prediction of the Modern
Evaporite Distribution
The PFM for evaporites (Pollard and Schulz 1994) consists of a 1 x 1 m 2 column
of water that is 50 m deep and assumed to be well mixed. The PFM is initialized
with a salt-content sufficient to precipitate gypsum or halite. Positive values of
evaporation minus precipitation (E-P) over the brine surface indicate the potential for precipitation of evaporite minerals. The PFM for evaporites is driven by
monthly-mean meteorological forcing fields including: (1) atmospheric precipitation; (2) surface air temperature; (3) upward sensible heat flux; (4) surface-incident solar radiation; (5) air density; (6) surface-incident infrared radiation; (7)
wind speed; and (8) relative humidity over the brine surface. The reader is referred to Pollard and Schulz (1994) for a more detailed description of the PFM
for evaporites.
When seawater evaporates completely, a typical sequence of minerals precipitates. Calcite (CaC0 3 ) precipitates when the brine has been concentrated to approximately three times (105) the normal salinity (35) of seawater. Gypsum
(CaS0 4 • 2H z O) precipitates at 5 times (175) the normal concentration of seawater. Then halite (NaCl) begins to precipitate at 11 times (385) the normal concentration of seawater. Finally, the potassium salts precipitate at about 100 times
the normal concentration of seawater (Anonymous 1985; Warren 1988). These
are the four major evaporite minerals that precipitate from seawater. There are
a host of other minerals that precipitate at different concentrations of the brine
depending on the original composition and subsequent mixing with other waters.
The output from the PFM is the rate of evaporation from water at the Earth's
surface. Two simulations were performed using the PFM, one where the brine