Geoscience Reference
In-Depth Information
2010). Their geographical distribution seems to be
constrained in part by the ASH. The present-day
ASH ranges from 50 to 600 m in the North Pacii c
where corals are few, mostly calcitic, and do not
build bioherms (Guinotte
et al
. 2006 ; Tittensor
et al
. 2009). In contrast, the ASH is much deeper
(> 2000 m) in the North Atlantic, which harbours a
much larger coral diversity, including the aragonitic
scleractinian coral
Lophelia pertusa
that builds spec-
tacular bioherms tens of metres high and kilometres
long off the Atlantic coasts of northern Europe. The
shoaling of the ASH implies that deep-sea coral eco-
systems could soon become immersed in seawater
undersaturated with respect to aragonite. Guinotte
et al
. (2006) showed that more than 95% of the deep-
sea, bioherm-forming corals were located in areas
that were supersaturated with respect to aragonite
in 1765 and that only about 30% of coral locations
will remain in supersaturated waters in 2099 (Fig.
7.4 ). Even though
L. pertusa
seems to be able to cal-
cify in moderately undersaturated seawater (Maier
et al
. 2009), it is likely that undersaturation and
increased carbonate dissolution will weaken bio-
herms, decrease their structural complexity, and
greatly reduce their biodiversity. In response to the
shoaling of the ASH, it is likely that the maximum
depth where these corals and other deep-sea calci-
i ers are found will transition to shallower depths.
It is not possible to conclude unequivocally that
the observed difference of deep-sea corals and
bioherms in the Atlantic and Pacii c Oceans are
strictly due to the differences in seawater CO
2
chem-
istry, but based on theory, it is reasonable to assume
that the chemistry is a key factor.
7.3.5 Shallow-ocean environments
Organisms and ecosystems in the surface ocean are
the i rst to experience the direct effects of anthropo-
genic ocean acidii cation as there is a time lag before
the anthropogenic CO
2
reaches the deep ocean.
Nonetheless, anthropogenic CO
2
has already pene-
trated to depths exceeding 1000 m in all major ocean
basins causing shoaling of carbonate saturation
horizons (Feely
et al
. 2004 ; see Chapter 3 ). Even
shallow regions of the oceans that are currently
supersaturated with respect to atmospheric
p
CO
2
and act as sources of CO
2
to the atmosphere due to
net heterotrophy and/or calcii cation (e.g. many
estuaries and coral reefs frequently experience
p
CO
2
well above the
p
CO
2
expected from equilibrium
with the atmosphere) will experience acidii cation
as the pH of the open-ocean source waters continu-
ously decreases.
In considering the effects of ocean acidii cation on
shallow benthic ecosystems, it is important to bear in
mind that a whole new pattern of benthic communi-
ties emerged as sea level rose from the Last Glacial
Maximum (LGM) 18 000 years ago to late pre-indus-
trial time. It is during this time that extensive carbon-
ate reef, bank, platform, and shelf ecosystems were
established amounting to at least 4500 Gt of CaCO
3
accumulation ( Milliman 1993 ; Vecsei and Berger
2004). These ecosystems today account for 26% of
the total CaCO
3
produced and 45% of the CaCO
3
accumulated, annually, in the ocean (Wollast 1994;
Milliman and Droxler 1996 ; Mackenzie
et al
. 2005 ).
These deposits represent the establishment of impor-
tant benthic communities that were not present to a
signii cant degree for 80 000 years and the accumula-
tion of the calcifying shoal-water ecosystems may
have contributed in part to the natural rise in atmos-
pheric CO
2
from the LGM to late pre-industrial times
of approximately 100 μatm (the 'coral reef hypothe-
sis'; Berger 1982 ; Opdyke and Walker 1992 ; Vecsei
and Berger 2004). Importantly, these coastal carbon-
ate systems are dominated by highly soluble benthic
skeletal mineralogies with aragonite and high-
5
Pre-industrial
Year 2099
4
3
2
1
0
-70
-50
-30
-10
10
30
50
70
Latitude
Figure 7.4
Model-projected depth of the aragonite saturation horizon
for globally distributed deep-sea coral locations in pre-industrial time and
in the year 2099 (from Guinotte
et al
. 2006, reprinted by permission of
the Ecological Society of America).
