alternating conditions of dissolution and precipitation
whereby metastable precursor minerals, such as magnesium calcite, change to more stable phases such as dolomite. Such conditions require large-scale fluctuations in
pH, which is controlled by the pCO 2 . Deelman has
explained how “breaking Ostwald’s Rule,” which governs the course of irreversible geochemical reactions,
can occur when fluctuations of sufficient amplitude, frequency, and duration are capable of crossing the border
between metastable and stable fields, and both metastable and stable phases will nucleate simultaneously. During these fluctuations, conditions opposing the
subsequent growth of the metastable phase will favor
continued growth of the stable phase. This explanation
gets around one aspect of the Dolomite Problem
(Fairbridge, 1957; Hsu, 1966; Land, 1985, 1998; Hardie,
1987; Fowles, 1991; Braithwaite, 1991), which centers
around the observation that dolomite will not precipitate
from normal or oversaturated solutions at low temperature and atmospheric pressure. The second part of the
paradox is that dolomite is a very common carbonate
mineral in sedimentary rocks throughout the geological
record, which contrasts with the relatively limited
amount of modern day surface dolomite (see Wright
and Wacey, 2004). Clearly, there has to be a simple
explanation of the process of dolomitization.
Vasconcelos et al. (1995), Vasconcelos and McKenzie
(1997), and McKenzie and Vasconcelos (2009) have proposed that microbial mediation may overcome the thermodynamic and kinetic barriers to dolomite nucleation in the
natural environment. Anaerobic bacteria can play a role in
primary dolomite precipitation and secondary replacement (Warthmann et al., 2000). Clearly, the presence of
microbes is the indispensible component driving the
reaction.
Dolomite occurrences and hydrological processes
Deelman (2008, Chapter 4) provides a comprehensive
account of the regional occurrences of Recent dolomites.
In addition to the occurrences in coral reefs, atolls, and
guyots which are discussed later, dolomite is forming in
the Coorong region of South Australia, in deep sea sediments with high organic content (organogenic dolomite),
in the carbonate platforms of the Bahamas, in the hot arid
sabkha environments, especially, in the Middle East and in
the supersaturated saline lagoons along the Rio de Janiero
coast of Brazil, namely, Lagoa Vermelha and Brejo do
Espinho. Clearly all these environments, irrespective of
their unique hydrological circumstances, are characterized
by physiochemical conditions that fluctuate “wildly” in
the presence of enhanced microbial activity. The hydrological setting in which dolomite is known to be forming
have been summarized into the following: seepage reflux,
capillary concentration, evaporative pumping or sabkha,
solution cannibalization, groundwater-seawater mixing,
Dorag dolomitization, geothermal springs, storm
recharge, Kohout convection, tidal pumping, coastal zone
mixing, and evaporative mixed water. In all of these settings, seawater in some form of modification flows
through the sediments or sedimentary rocks allowing for
both primary deposition and replacement of preexisting
carbonate particles. Often the resultant dolostone preserves the original depositional textures and “ghosts” of
preexisting grains. Tucker and Wright (1991) provide
a comprehensive account with numerous illustrations of
these hydrological settings where dolomite is forming
present day.
Dolomite in reefs, atolls, and guyots
Montaggioni and Braithwaite (2009) provide a very useful
summary of the occurrence of dolomite in Quaternary
coral reef. The best data on dolomitization comes from
the deep drillings of the Pacific and Indonesian atolls,
especially, Enewetak, Mururoa, and Midway, all of these
having been drilled more than once to pre-reefal basement.
The earliest account of the occurrence of dolomite in
coral reefs is that of Skeats (1903) and Cullis (1904) in
the cores recovered from the 1896 drilling undertaken by
the Royal Society of London at Funafuti Atoll. Here, dolomite crystals were only detected at depths greater than
194 m where dolomite became the predominant carbonate
mineral. Other drillings of Pacific Atolls (Skeats, 1905)
also recorded the occurrence or the absence of dolomite
(Deelman, 2008). Of particular significance were the drillings at Niue (Wheeler and Aharon, 1997; Aharon et al.,
1987), Kita-daito-jima (Suzuki et al., 2005), Bikini (Ladd
et al., 1948), Enewetak (Saller, 1984), Midway (Ladd
et al., 1970), Mururoa and Fangataufa (Aissaoui et al.,
1986; Aissaoui, 1988).
Fairbridge (1950) commented on the fact that drillings
at Bikini, Maratoea (Indonesia), and the Great Barrier
Reef bores of Michaelmas Cay and Heron Island did not
encounter dolomite. He inferred that anoxic conditions
were not present to facilitate dolomitization. More recent
drillings of the Guyots of the Mid-Pacific Mountains and
western Pacific have recorded the occurrence of dolomite
(Flood, 1998). Recent drilling of Boulder Reef and Ribbon Reef No. 5 of the Great Barrier Reef (International
Consortium for Great Barrier Reef Drilling, 2001; Webster and Davies, 2003; Braithwaite et al., 2004) failed to
record the presence of dolomite, whereas deep drillings
on the Queensland Continental Slope (Swart, 1993) did
record the presence of dolomite.
In the Enewetak Atoll bore, Ladd et al. (1953) could not
discern any pattern in the distribution of dolomite. It only
occurred at depths below 1,340 m, whereas in the Kitadaito-jima bore dolomite was found only in the upper
122 m. On Midway Atoll, dolomite was found between
130 and 150 m depth. This variability in depth of the dolomite occurrences would suggest that there is no relationship between dolomite formation and water depth.
Schlanger (1963) reexamined the cores from the atolls of
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DOLOMITIZATION
whereby metastable precursor minerals, such as magnesium calcite, change to more stable phases such as dolomite. Such conditions require large-scale fluctuations in
pH, which is controlled by the pCO 2 . Deelman has
explained how “breaking Ostwald’s Rule,” which governs the course of irreversible geochemical reactions,
can occur when fluctuations of sufficient amplitude, frequency, and duration are capable of crossing the border
between metastable and stable fields, and both metastable and stable phases will nucleate simultaneously. During these fluctuations, conditions opposing the
subsequent growth of the metastable phase will favor
continued growth of the stable phase. This explanation
gets around one aspect of the Dolomite Problem
(Fairbridge, 1957; Hsu, 1966; Land, 1985, 1998; Hardie,
1987; Fowles, 1991; Braithwaite, 1991), which centers
around the observation that dolomite will not precipitate
from normal or oversaturated solutions at low temperature and atmospheric pressure. The second part of the
paradox is that dolomite is a very common carbonate
mineral in sedimentary rocks throughout the geological
record, which contrasts with the relatively limited
amount of modern day surface dolomite (see Wright
and Wacey, 2004). Clearly, there has to be a simple
explanation of the process of dolomitization.
Vasconcelos et al. (1995), Vasconcelos and McKenzie
(1997), and McKenzie and Vasconcelos (2009) have proposed that microbial mediation may overcome the thermodynamic and kinetic barriers to dolomite nucleation in the
natural environment. Anaerobic bacteria can play a role in
primary dolomite precipitation and secondary replacement (Warthmann et al., 2000). Clearly, the presence of
microbes is the indispensible component driving the
reaction.
Dolomite occurrences and hydrological processes
Deelman (2008, Chapter 4) provides a comprehensive
account of the regional occurrences of Recent dolomites.
In addition to the occurrences in coral reefs, atolls, and
guyots which are discussed later, dolomite is forming in
the Coorong region of South Australia, in deep sea sediments with high organic content (organogenic dolomite),
in the carbonate platforms of the Bahamas, in the hot arid
sabkha environments, especially, in the Middle East and in
the supersaturated saline lagoons along the Rio de Janiero
coast of Brazil, namely, Lagoa Vermelha and Brejo do
Espinho. Clearly all these environments, irrespective of
their unique hydrological circumstances, are characterized
by physiochemical conditions that fluctuate “wildly” in
the presence of enhanced microbial activity. The hydrological setting in which dolomite is known to be forming
have been summarized into the following: seepage reflux,
capillary concentration, evaporative pumping or sabkha,
solution cannibalization, groundwater-seawater mixing,
Dorag dolomitization, geothermal springs, storm
recharge, Kohout convection, tidal pumping, coastal zone
mixing, and evaporative mixed water. In all of these settings, seawater in some form of modification flows
through the sediments or sedimentary rocks allowing for
both primary deposition and replacement of preexisting
carbonate particles. Often the resultant dolostone preserves the original depositional textures and “ghosts” of
preexisting grains. Tucker and Wright (1991) provide
a comprehensive account with numerous illustrations of
these hydrological settings where dolomite is forming
present day.
Dolomite in reefs, atolls, and guyots
Montaggioni and Braithwaite (2009) provide a very useful
summary of the occurrence of dolomite in Quaternary
coral reef. The best data on dolomitization comes from
the deep drillings of the Pacific and Indonesian atolls,
especially, Enewetak, Mururoa, and Midway, all of these
having been drilled more than once to pre-reefal basement.
The earliest account of the occurrence of dolomite in
coral reefs is that of Skeats (1903) and Cullis (1904) in
the cores recovered from the 1896 drilling undertaken by
the Royal Society of London at Funafuti Atoll. Here, dolomite crystals were only detected at depths greater than
194 m where dolomite became the predominant carbonate
mineral. Other drillings of Pacific Atolls (Skeats, 1905)
also recorded the occurrence or the absence of dolomite
(Deelman, 2008). Of particular significance were the drillings at Niue (Wheeler and Aharon, 1997; Aharon et al.,
1987), Kita-daito-jima (Suzuki et al., 2005), Bikini (Ladd
et al., 1948), Enewetak (Saller, 1984), Midway (Ladd
et al., 1970), Mururoa and Fangataufa (Aissaoui et al.,
1986; Aissaoui, 1988).
Fairbridge (1950) commented on the fact that drillings
at Bikini, Maratoea (Indonesia), and the Great Barrier
Reef bores of Michaelmas Cay and Heron Island did not
encounter dolomite. He inferred that anoxic conditions
were not present to facilitate dolomitization. More recent
drillings of the Guyots of the Mid-Pacific Mountains and
western Pacific have recorded the occurrence of dolomite
(Flood, 1998). Recent drilling of Boulder Reef and Ribbon Reef No. 5 of the Great Barrier Reef (International
Consortium for Great Barrier Reef Drilling, 2001; Webster and Davies, 2003; Braithwaite et al., 2004) failed to
record the presence of dolomite, whereas deep drillings
on the Queensland Continental Slope (Swart, 1993) did
record the presence of dolomite.
In the Enewetak Atoll bore, Ladd et al. (1953) could not
discern any pattern in the distribution of dolomite. It only
occurred at depths below 1,340 m, whereas in the Kitadaito-jima bore dolomite was found only in the upper
122 m. On Midway Atoll, dolomite was found between
130 and 150 m depth. This variability in depth of the dolomite occurrences would suggest that there is no relationship between dolomite formation and water depth.
Schlanger (1963) reexamined the cores from the atolls of
322
DOLOMITIZATION