Geoscience Reference
In-Depth Information
remineralization of this material and production of
CO
2
, could increase. On the other hand, if the pro-
duction and sinking of biogenic CaCO
3
particles
from the surface ocean decreased, less organic mate-
rial may make it down to the bottom of the deep sea
because CaCO
3
acts as important ballast (Barker
et al
. 2003; see Chapters 6 and 12). In recent years,
declining oxygen concentrations have been detected
in large regions of the oceans, and it has been pro-
posed that the oxygen minimum zones—or 'dead
zones', void of aerobic life—of the oceans could
increase in response to global warming (O
2
becomes
less soluble) as well as increased export and break-
down of organic material (e.g. Oschlies
et al
. 2008 ;
Stramma
et al
. 2008 ; Brewer and Peltzer 2009 ).
Oxygen starvation affects decomposition and recy-
cling of organic material in benthic environments
and has drastic consequences for benthic ecosys-
tems (Stramma
et al
. 2008 ; Brewer and Peltzer 2009 ).
Coastal ocean environments, particularly those
with seasonal thermoclines, are particularly vulner-
able to warming and the development of regions of
hypoxia and anoxia.
mineral structure but calcium ions have been ran-
domly replaced by magnesium ions in the latter (up
to ~30 mol%; Morse and Mackenzie 1990). These dif-
ferences result in somewhat different chemical and
physical properties. For example, Mg-calcite with a
signii cant mol% magnesium in calcite is more solu-
ble than aragonite, which is more soluble than cal-
cite. It is not known in detail why some organisms
and not others favour a certain mineral phase, but it
is most certainly linked to the mechanism and con-
trol of the calcii cation process, which is different in
different organisms. Some organisms deposit differ-
ent mineralogies at different life stages and some
even have multiple mineralogies in different parts of
their calcareous hard parts. Evidence exists from
palaeo-oceanographic records and controlled labo-
ratory experiments that the mineralogy being depos-
ited may change as a function of temperature and
seawater chemical composition including changes
in the Mg-to-Ca ratio and the distribution of inor-
ganic carbon species (e.g. Mackenzie
et al
. 1983 ;
Agegian 1985 ; Ries 2010 ; Stanley
et al
. 2010 ).
Based on thermodynamic and kinetic principles,
as seawater carbonate ion concentration ([CO
3
2-
])
and carbonate mineral saturation state (Ω) decrease
as a result of ocean acidii cation (see Chapter 1 ),
one would expect that the rate of calcii cation of
benthic marine calcii ers as well as other calcii ers
would decrease. Indeed the majority of studies
show a consistent decline in the rate of benthic cal-
cii cation as a result of increasing CO
2
and ocean
acidii cation ( Fig. 7.2 and Table 7.1 ; e.g. Marubini
et al
. 2003 ; Langdon and Atkinson 2005 ; Schneider
and Erez 2006 ; Gazeau
et al
. 2007 ; Anthony
et al
.
2008 ; Jokiel
et al
. 2008), although a few recent stud-
ies show no response or an increase in calcii cation
in a range of different benthic calcii ers exposed to
moderately elevated CO
2
conditions (Ries
et al
.
2009 ; Rodolfo-Metalpa
et al
. 2010 ). However, one
has to be cautious in interpreting these results.
Regardless of whether calcii cation in marine
organisms has been observed to increase or
decrease in response to elevated CO
2
and lower Ω,
deposition of CaCO
3
is thermodynamically less
favourable under such conditions. Wood
et al
.
(2008) proposed that some organisms may be able
to up-regulate their metabolism and calcii cation
to compensate for increased acidity of seawater.
7.2.3 Calcii cation
Many benthic organisms deposit skeletal hard parts
made of CaCO
3
. One hypothesis suggests that the
calcii cation process in marine organisms originally
evolved under conditions of high calcium concen-
tration in the ocean as a detoxii cation mechanism
( Brennan
et al
. 2004), but the explanation may be
much more complex than this. Regardless, calcare-
ous hard parts provide myriad advantages to marine
benthic calcii ers, including protection from preda-
tors, a refuge for intertidal organisms to avoid desic-
cation when exposed to air during low tide, structural
support, increased surface area, a mechanism to
maintain elevation above the sediment-water inter-
face, a way of maintaining close proximity to high
light levels, and as a means of keeping up with sea-
level rise. There are three commonly occurring car-
bonate mineral phases deposited by benthic marine
calcii ers, namely aragonite, calcite, and magnesian
calcite (Mg-calcite). Aragonite and calcite have the
same chemical composition but a different mineral
structure (orthorhombic versus rhombohedral),
whereas calcite and Mg-calcite have the same basic
