Geoscience Reference
In-Depth Information
CO 2 chemistry. Naturally, it is very difi cult to con-
duct rigorous ecological experiments to address
these questions as these changes occur over rela-
tively long periods of time. Hence, in addressing
the question about the effects of ocean acidii cation
on benthic ecosystems, it is useful to look at envi-
ronments that naturally experience high-CO 2 condi-
tions at the present time or did so in the past. One
way is to compare the benthic environments of the
Atlantic and Pacii c oceans, which experience sig-
nii cantly different CO 2 chemistry as a function of
depth (Section 7.3.4). There are also local or regional
shallow environments exposed to high-CO 2 and
low-pH conditions resulting from volcanic vents
( Hall-Spencer et al . 2008 ; Martin et al . 2008 ;
Rodolfo-Metalpa et al . 2010 ), seasonal stratii ca-
tion (Andersson et al . 2007), or upwelling of deep
corrosive water on to the continental shelf (Feely
et al . 2008 ; Manzello et al . 2008 ; Section 7.3.5) that
may provide important clues in terms of the effects
of ocean acidii cation on benthic ecosystems.
observed differences in compensation depths
between the ocean basins are mostly a result of dif-
ferent seawater chemistry and the greater extent of
carbonate dissolution in the Pacii c than in the
Atlantic (Morse and Mackenzie 1990). Thus, as the
saturation horizons continue to shoal throughout
the global ocean as a result of anthropogenic CO 2
emissions (Feely et al . 2004 ; see Chapter 3 ), carbon-
ate sediment dissolution will increase and the CCD
and ACD are likely to shoal assuming that the inl ux
of carbonate particles does not increase. However,
this process is slow and there is likely to be a sub-
stantial time lag relative to the shoaling of the satu-
ration horizon before signii cant shoaling of the
compensation depths can be detected in most places
(Chapter 2). The rate of shoaling of the saturation
horizon is also different between different regions,
depending on the uptake and subduction of anthro-
pogenic CO 2 ( Feely et al . 2004). As a result of the
formation of North Atlantic Deep Water (NADW)
and the associated uptake and subduction of CO 2
from the atmosphere (solubility pump) in this
region, anthropogenic CO 2 can be detected at depths
exceeding 2500 m in the North Atlantic Ocean com-
pared with ~1500 m in the South Atlantic (Feely
et al . 2004). Thus, this region may serve as an impor-
tant indicator of the effect of ocean acidii cation on
deep-sea benthic ecosystems. For example, in the
waters surrounding Iceland, Olafsson et al . ( 2009 )
reported that the aragonite saturation horizon
(ASH) between 1985 and 2008 shoaled by 4 m yr -1 ,
immersing 800 km 2 of seal oor and associated ben-
thic communities in seawater undersaturated with
respect to aragonite every year. No data currently
exist on how this might have affected the communi-
ties or biogenic carbonate material present in the
sediments.
Many benthic marine calcii ers found in shallow
seas are also found in the deep sea, including echi-
noderms, molluscs, crustaceans, foraminifera, and
scleractinian corals. Deep-sea coral ecosystems,
sometimes improperly called 'deep-sea coral reefs',
are found in shallow to intermediate-depth waters
(50-1000 m) at high latitudes and at greater depths
at low latitudes (Roberts et al . 2006 ). They are prob-
ably the most three-dimensionally complex habitat
in the deep sea, providing numerous ecological
niches and hosting a high biodiversity (Clark et al .
7.3.4 Deep-ocean environments
As illustrated in Chapter 3, seawater of the Pacii c
Ocean becomes increasingly less alkaline (decreas-
ing pH) and increasingly corrosive with respect to
carbonate minerals at much shallower depths than
seawater in the Atlantic Ocean. This raises the ques-
tion of whether there are differences between the
benthic ecosystems of the Atlantic and Pacii c
oceans that rel ect this observed difference in sea-
water acidity and may provide clues in terms of the
effects of future ocean acidii cation on benthic
ecosystems?
It is well known that the aragonite and the calcite
compensation depths (ACD, CCD), i.e. the depths
at which the rates of sedimentation of these phases
equal their rates of dissolution and thus where
aragonite and calcite generally are no longer found
in the sediments, are located much deeper in the
Atlantic than in the Pacii c (e.g. Morse and
Mackenzie 1990). The compensation depths are
located below the saturation horizons (Ω = 1), which
are thermodynamic boundaries (see Chapter 1), and
below the lysoclines (a kinetic boundary) where
these phases initially begin to undergo signii cant
dissolution in the water column. Nevertheless, the
 
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