Introduction to tufas and speleothems - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 1

Introduction to tufas and speleothems

H. MARTYN PEDLEY* & MIKE ROGERSON
Department of Geography, University of Hull, Cottingham Road, Hull, HU6 7RX, UK
*Corresponding author (e-mail: [e-mail])

Ambient temperature freshwater carbonates
precipitate as surface deposits within karstic
stream, lake and swamp environments (tufas) and
in subterranean situations (speleothems), where
they line vadose caves and fracture systems.
Although physico-chemical mineral precipitation
contributes significantly to both kinds of deposit,
there is a clear spatial association between the
development of tufa deposits and the occurrence
of microbial biofilms. This fact, and the recent dis-
covery that the occurrence of certain heterotrophic
bacteria promote precipitation onto the surface of
stalactites (Cacchio et al. 2004), strongly implicates
a degree of microbial influence in the calcite pre-
cipitation process, regardless of the environmental
context. To add to the inherent complexity of
these systems, there is considerable interplay
between biological and physical processes to con-
sider. Water velocity and turbulence will strongly
affect biofilm colonization and may damage the
community, thereby affecting carbonate precipi-
tation rates, in addition to regulating important
kinetic limitations on precipitation via modifi-
cations of the calcium ion delivery rate. Exchange
of CO 2 gas at the air-water interface is an important
conditioner for precipitation in vadose systems but
will also occur within surface systems as a conse-
quence of photosynthesis. It is only by considering
karst hydrological systems holistically that these
processes can be untangled.
Tufas and speleothems share the same soil-
derived meteoric water supply, represent zones of
deposition of calcium ions chemically eroded from
the same geological sources and produce laminated
deposits which are superficially similar. In passing
from cave environments via resurgences (Fig. 1)
into surface waterways, individual packages of
water pass down an interconnected hydrological
system at any point in which the conditions necess-
ary for calcite precipitation may be achieved.
Within the deposits that this precipitation creates, it
is apparent that there is a progressive gradation
from massive, laminated speleothems fabrics into
stromatolitic, biofilm dominated tufas fabrics. In
fact, speleothems and tufa represent two end
members within a continuum of freshwater carbon-
ate reflecting different balances of physico-
chemically and biologically controlled precipitation.

On a regional scale the occurrences of tufas and
speleothems are both controlled by water table fluc-
tuations. Typically, tufa deposition is associated
with predominantly high water tables and although
tufas enjoy global distributions from the tropics
to polar regions, they are most effective as bio-
constructors where spring fed streams are not
subjected to spate conditions. Similarly, tufa devel-
opments are severely impaired by fluctuating water
tables associated with increasingly arid climatic
cycles. Limitation on surface carbonate precipi-
tation is consequently derived from the necessity
for biofilm development combined with the equal
necessity of adequate supply of dissolved calcium
and carbon, which must be present at least in part
as carbonate. The latter requirement of sufficient
Ca 2þ (aq) and CO 3 22 (aq) ionic activity demands that
these ions are not lost from solution before resur-
gence, making it likely that tufas will develop best
where caves are flooded, thereby minimizing the
distribution of the subterranean vadose environment
where speleothems develop most abundantly.
Curiously, these elevated tables are frequently
encouraged by the tufa growth itself as a conse-
quence of the valley bottom ponding and back flood-
ing caused by barrage development. Conversely, as
lower water tables become established and the
subterranean vadose environment becomes more
important, speleothems will become established.
Conceptually, the occurrence of abundant tufa or
speleothem deposition simply reflects the position
of a hydrochemical 'knick-point', which occurs
when sufficient CO 2 has been lost from solution for
carbonate ions to become abundant, for example
when the thermodynamic gradient promoting pre-
cipitation (Gibbs Free Energy) exceeds the barrier
presented by the activation energy. This knick-point
may occur either above or below ground depending
on the height of the water table.
As part of the same hydrochemical system, tufas
and speleothems offer an inseparable duo when
exploring the climatic archive, and will reflect the
same processes within the catchment. Much
palaeo-environmental information in tufas and
speleothems can be extracted from geochemical
time series created from these deposits. However,
one of the greatest obstacles to collective use of
these materials in 'climate' reconstruction is the