CALCIUM CARBONATES
L. C. Peterson, University of Miami, Miami, FL, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The ocean receives a continual input of calcium from
riverine and groundwater sources and from the
hydrothermal alteration of oceanic crust at midocean ridge spreading centers. Balancing this input is
the biological precipitation of calcium carbonate
(CaCO 3 ) by shell-and skeleton-building organisms in
both shallow marine and open-ocean environments.
In the deep sea, the primary contributors to the
carbonate budget of open-ocean sediments are the
skeletal remains of calcareous plankton that have
settled down from the surface after death. Seafloor
sediments consisting of more than 30% by weight
calcium carbonate are traditionally referred to as
calcareous or carbonate ooze; such oozes accumulate
at the rate of 1–4 cm per 1000 years and cover
roughly half of the ocean bottom. Carbonate oozes
are the most widespread biogenous sediments in the
ocean.
While the biological production of calcium carbonate in oversaturated surface waters determines
the input of carbonate to the deep sea, it is the dissolution of carbonate in undersaturated deep waters
that has the dominant control on calcium carbonate
accumulation in the open ocean. Since carbonate
production rates in the surface ocean today greatly
exceed the rate of supply of calcium, this ‘compensation’ through dissolution must occur in order to
keep the system in steady-state. Increased dissolution
at depth is largely a function of the effect of increasing hydrostatic pressure on the solubility of
carbonate. However, superimposed on this bathymetric effect are regional preservation patterns related to differences in carbonate input and the
carbonate chemistry of deep water masses. Carbonate oozes in the deep sea serve as a major reservoir of
calcium and carbon dioxide on the Earth’s surface.
Their spatial and temporal accumulation patterns in
the marine stratigraphic record are thus a primary
source of data about the carbonate chemistry and
circulation of past oceans, as well as of the global
geochemical cycle of CO 2 .
Carbonate Producers
The most important carbonate producers in the open
ocean are planktonic coccolithophorids and foraminifera, unicellular phytoplankton and zooplankton respectively, which inhabit the upper few
hundred meters of the water column (Figure 1).
Coccolithophorids are the dominant carbonateprecipitating organisms on Earth. During part of
their life cycle, they produce a skeletal structure (the
coccosphere) consisting of loosely interlocking
plates, often button-like in appearance, known as
coccoliths. Deep-sea carbonates generally contain
only the individual coccoliths, as the intact coccospheres are rarely preserved. Foraminifera produce a
calcareous shell, or ‘test’, a few hundred microns in
size that sinks after death or reproduction to the sea
floor. Both coccolithophorids and the planktonic
foraminifera construct their skeletal elements out of
the mineral calcite, the more stable polymorph of
CaCO 3 . Calcareous sediments dominated by one or
the other component are termed coccolith oozes or
foraminiferal oozes, although in reality most carbonate-rich sediments are a mixture of both.
Coccolithophorids made their first appearance in
the geological record in the earliest Jurassic, while
planktonic foraminifers evolved somewhat later in
the middle Jurassic. The appearance of these two
dominant pelagic carbonate producers, and their
rapid diversification in the Cretaceous, would have
had major effects upon the carbonate geochemistry of
the oceans. Before this, most carbonate was deposited
in shallow seas, accounting for the high proportion of
limestones among older rocks on the continents. Since
the Mesozoic, deep-ocean basins have become enormous sinks for carbonate deposition.
Smaller contributions to the deep-sea carbonate
budget come from a variety of other sources. Pteropods, free-swimming pelagic gastropods, construct a
relatively large (several millimeters) but delicate shell
out of the metastable form of CaCO 3 known as
aragonite. However, while pteropods can be unusually abundant in certain environments, the increased solubility of aragonite leads to very restricted
preservation of the shells and pteropod oozes are
relatively rare in the ocean. In the vicinity of shallow,
tropical carbonate platforms such as the Bahamas or
Seychelles Bank, shedding of aragonitic bank-top
sediments derived from algal and coral production
can lead to aragonite-rich ‘periplatform oozes’ in
deep waters around the perimeters of the platform.
336
L. C. Peterson, University of Miami, Miami, FL, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The ocean receives a continual input of calcium from
riverine and groundwater sources and from the
hydrothermal alteration of oceanic crust at midocean ridge spreading centers. Balancing this input is
the biological precipitation of calcium carbonate
(CaCO 3 ) by shell-and skeleton-building organisms in
both shallow marine and open-ocean environments.
In the deep sea, the primary contributors to the
carbonate budget of open-ocean sediments are the
skeletal remains of calcareous plankton that have
settled down from the surface after death. Seafloor
sediments consisting of more than 30% by weight
calcium carbonate are traditionally referred to as
calcareous or carbonate ooze; such oozes accumulate
at the rate of 1–4 cm per 1000 years and cover
roughly half of the ocean bottom. Carbonate oozes
are the most widespread biogenous sediments in the
ocean.
While the biological production of calcium carbonate in oversaturated surface waters determines
the input of carbonate to the deep sea, it is the dissolution of carbonate in undersaturated deep waters
that has the dominant control on calcium carbonate
accumulation in the open ocean. Since carbonate
production rates in the surface ocean today greatly
exceed the rate of supply of calcium, this ‘compensation’ through dissolution must occur in order to
keep the system in steady-state. Increased dissolution
at depth is largely a function of the effect of increasing hydrostatic pressure on the solubility of
carbonate. However, superimposed on this bathymetric effect are regional preservation patterns related to differences in carbonate input and the
carbonate chemistry of deep water masses. Carbonate oozes in the deep sea serve as a major reservoir of
calcium and carbon dioxide on the Earth’s surface.
Their spatial and temporal accumulation patterns in
the marine stratigraphic record are thus a primary
source of data about the carbonate chemistry and
circulation of past oceans, as well as of the global
geochemical cycle of CO 2 .
Carbonate Producers
The most important carbonate producers in the open
ocean are planktonic coccolithophorids and foraminifera, unicellular phytoplankton and zooplankton respectively, which inhabit the upper few
hundred meters of the water column (Figure 1).
Coccolithophorids are the dominant carbonateprecipitating organisms on Earth. During part of
their life cycle, they produce a skeletal structure (the
coccosphere) consisting of loosely interlocking
plates, often button-like in appearance, known as
coccoliths. Deep-sea carbonates generally contain
only the individual coccoliths, as the intact coccospheres are rarely preserved. Foraminifera produce a
calcareous shell, or ‘test’, a few hundred microns in
size that sinks after death or reproduction to the sea
floor. Both coccolithophorids and the planktonic
foraminifera construct their skeletal elements out of
the mineral calcite, the more stable polymorph of
CaCO 3 . Calcareous sediments dominated by one or
the other component are termed coccolith oozes or
foraminiferal oozes, although in reality most carbonate-rich sediments are a mixture of both.
Coccolithophorids made their first appearance in
the geological record in the earliest Jurassic, while
planktonic foraminifers evolved somewhat later in
the middle Jurassic. The appearance of these two
dominant pelagic carbonate producers, and their
rapid diversification in the Cretaceous, would have
had major effects upon the carbonate geochemistry of
the oceans. Before this, most carbonate was deposited
in shallow seas, accounting for the high proportion of
limestones among older rocks on the continents. Since
the Mesozoic, deep-ocean basins have become enormous sinks for carbonate deposition.
Smaller contributions to the deep-sea carbonate
budget come from a variety of other sources. Pteropods, free-swimming pelagic gastropods, construct a
relatively large (several millimeters) but delicate shell
out of the metastable form of CaCO 3 known as
aragonite. However, while pteropods can be unusually abundant in certain environments, the increased solubility of aragonite leads to very restricted
preservation of the shells and pteropod oozes are
relatively rare in the ocean. In the vicinity of shallow,
tropical carbonate platforms such as the Bahamas or
Seychelles Bank, shedding of aragonitic bank-top
sediments derived from algal and coral production
can lead to aragonite-rich ‘periplatform oozes’ in
deep waters around the perimeters of the platform.
336