Travertine terracing: patterns and mechanisms - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 347

Fig. 9. Microterracettes in Pamukkale, Turkey, seemingly displaying a characteristic spacing.

processes: enhanced growth at the rim constituting
local positive feedback, and long-range negative
feedback involving both reduced growth in the
pools and upstream inundation. In a very general
sense, travertine terrace patterning can therefore
be included in the local self-activation/lateral inhi-
bition class of pattern formation systems (Gierer &
Meinhardt 1972, 2000). Such systems typically
produce patterns of regularly spaced points, or par-
allel or labyrinthic stripes, as observed in many bio-
logical and geological settings. In the travertine
terrace system, the lateral inhibition proceeds pre-
dominantly upstream, by the pool dammed by the
rim. The possible regular spacing between rims in
the downslope direction (Fig. 9, and Hammer
et al. 2007; but see Viles & Pentecost 1999 and
Veysey & Goldenfeld 2008) may be understood in
terms of this theoretical framework. However, the
mechanism for lateral inhibition involves advection
driven

mutual competition. This unusual pattern formation
regime may be responsible for the surreal impr-
ession invoked by travertine terrace landscapes.

Conclusions

There is probably not a single mechanism respon-
sible for localization of precipitation at the rim in
all circumstances. At small scales with slow,
laminar shallow water flow, we suggest that the
Laplace instability effects suggested for ice by
Ogawa & Furugawa (2002) and Ueno (2003) and
confirmed in a travertine terrace setting by
Hammer et al. (2008) can initiate microterracing.
When the ridge begins to affect bathymetry and
hydrodynamics, advective effects come into play
by dampening small-wavelength features (Ogawa
& Furugawa 2002; Ueno 2003), by bringing
ions to and from the calcite surface (Veysey &
Goldenfeld 2008; Hammer et al. 2008) and by com-
pression of concentration gradients over the rim
(Hammer et al. 2008). At slightly larger scales and
for faster flow, turbulence sets in and diffusion-
limited precipitation becomes dependent on flow
rate by thinning of the laminar boundary layer
(Buhmann & Dreybrodt 1985). At even larger
scales relative to water depth and flow rate, spatial
patterns of outgassing across the water-air interface
can reach the water - calcite interface and open up a
positive feedback loop involving loss of CO 2 .
Across all scales and flow rates, mechanical sticking
of particles on the rim may also play a role. Surface

by complex hydrodynamics rather than

simple isotropic diffusion, producing intricate

morphologies.
High travertine precipitation rates are generally
found in positions of high flow velocity, whether
or not there is a causal relationship between the
two. In this respect, the travertine terrace system is
precisely the opposite of more familiar systems of
erosional flow (Hammer 2008). In high-velocity
locations in rivers and streams the substrate is typi-
cally removed, rather than accreted, leading to
localization of the flow channel. In contrast, water
flow in the travertine terrace system is generally
diverging, and the pools sprawl out laterally in