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Fig. 7. Cross section of travertine at Rapolano Terme, Italy. Flow was from right to left. Note the initiation and upwards
coarsening of rim A, leading to the upstream drowning and termination of rim B.
with time. Rims folded into lobes and migrated
downstream with differential rates. In such a
model, high flow rates are found not only on steep
slopes, but also in shallow regions over rims
where velocity must increase to maintain the flux.
This provides a positive feedback mechanism
between hydrodynamics and precipitation on the
rim, leading to localization of precipitation and the
stabilization and growth of rims.
Goldenfeld et al. (2006) and Veysey & Golden-
feld (2008) developed a much more detailed cellular
model, including simplified rules for water flow,
surface tension, water chemistry and outgassing of
CO
2
. Their precipitation rule included idealized
terms incorporating oversaturation level, flux
normal to the surface, and flow velocity. The simu-
lated travertine terraces were natural-looking and
reproduced statistical properties observed in the
field (see below). Considering the work of
Hammer et al. (2007) it is likely that the flow rate
term in the precipitation rule is the most important
cause of terrace formation in the simulations of
Veysey & Goldenfeld (2008).
Mechanisms for precipitation localization
The models described above assumed a relationship
between flow rate and precipitation, but did not
address in detail the mechanism for such a relation-
ship. In addition, it is conceivable that the relation-
ship is not causal, but that both quantities reflect
other, underlying causes. An extraordinary variety
of mechanisms have been proposed to explain the
higher precipitation rates at terrace rims.
A popular explanation has been increased out-
gassing of CO
2
at rims due to agitation and shallow-
ing, the latter increasing the local surface to volume
ratio (Varnedoe 1965; Chen et al. 2004). Consider-
ing the travertine system as a whole, under normal
(e.g. not hyperalkaline) conditions calcite precipi-
tation depends on loss of CO
2
from aqueous solution
to the atmosphere (e.g. Dreybrodt et al. 1992). An
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